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Working Group I Fifth Assessment Report
WORKING GROUP I CONTRIBUTION TO THE IPCC FIFTH ASSESSMENT REPORT
CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS
Final Draft Underlying Scientific-Technical Assessment
A report accepted by Working Group I of the IPCC but not approved in detail.
Note:
The final draft Report, dated 7 June 2013, of the Working Group I contribution to the IPCC 5th Assessment
Report "Climate Change 2013: The Physical Science Basis" was accepted but not approved in detail by the
12th Session of Working Group I and the 36th Session of the IPCC on 26 September 2013 in Stockholm,
Sweden. It consists of the full scientific and technical assessment undertaken by Working Group I.
The Report has to be read in conjunction with the document entitled “Climate Change 2013: The Physical Science Basis.
Working Group I Contribution to the IPCC 5th Assessment Report - Changes to the underlying Scientific/Technical
Assessment” to ensure consistency with the approved Summary for Policymakers (IPCC-XXVI/Doc.4) and presented to the
Panel at its 36th Session. This document lists the changes necessary to ensure consistency between the full Report and the
Summary for Policymakers, which was approved line-by-line by Working Group I and accepted by the Panel at the abovementioned Sessions.
Before publication the Report will undergo final copyediting as well as any error correction as necessary, consistent with the
IPCC Protocol for Addressing Possible Errors. Publication of the Report is foreseen in January 2014.
Disclaimer:
The designations employed and the presentation of material on maps do not imply the expression of any opinion
whatsoever on the part of the Intergovernmental Panel on Climate Change concerning the legal status of any country,
territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
30 September 2013
WORKING GROUP I – TWELFTH SESSION
Stockholm, 23-26 September 2013
th
WG-I: 12 /Doc. 2b, CH01
(12.VIII.2013)
Agenda Item: 5
ENGLISH ONLY
WORKING GROUP I CONTRIBUTION TO THE IPCC FIFTH ASSESSMENT
REPORT (AR5), CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS
Chapter 1: Introduction - Final Draft Underlying Scientific-Technical Assessment
(Submitted by the Co-Chairs of Working Group I)
Confidential – This document is being made available in preparation of
WGI-12 only and should not be cited, quoted, or distributed
NOTE:
The Final Draft Underlying Scientific-Technical Assessment is submitted to the Twelfth Session of Working
Group I for acceptance. The IPCC at its Thirty-sixth Session (Stockholm, 26 September 2013) will be informed
of the actions of the Twelfth Session of Working Group I in this regard.
IPCC Secretariat
c/o WMO • 7bis, Avenue de la Paix • C.P. 2300 • 1211 Geneva 2 • Switzerland
telephone : +41 (0) 22 730 8208 / 54 / 84 • fax : +41 (0) 22 730 8025 / 13 • email : [email protected] • www.ipcc.ch
Final Draft (7 June 2013)
Chapter 1
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Chapter 1: Introduction
Coordinating Lead Authors: Ulrich Cubasch (Germany), Donald Wuebbles (USA)
Lead Authors: Deliang Chen (Sweden), Maria Cristina Facchini (Italy), David Frame (UK/New Zealand),
Natalie Mahowald (USA), Jan-Gunnar Winther (Norway)
Contributing Authors: Achim Brauer (Germany), Valérie Masson-Delmotte (France), Frank Kaspar
(Germany), Janina Körper (Germany), Malte Meinshausen (Australia/Germany), Matthew Menne (USA),
Carolin Richter (Switzerland), Michael Schulz (Germany), Bjorn Stevens (Germany/USA), Rowan Sutton
(UK), Kevin Trenberth (USA), Murat Türkeş (Turkey), Daniel S. Ward (USA)
Review Editors: Yihui Ding (China), Linda Mearns (USA), Peter Wadhams (UK)
Date of Draft: 7 June 2013
Table of Contents
Executive Summary......................................................................................................................................... 2
1.1 Chapter Preview ...................................................................................................................................... 4
1.2 Rationale and Key Concepts of the WGI Contribution ....................................................................... 4
1.2.1 Setting the Stage for the Assessment .............................................................................................. 4
1.2.2 Key Concepts in Climate Science................................................................................................... 5
1.2.3 Multiple Lines of Evidence for Climate Change ............................................................................ 8
1.3 Indicators of Climate Change................................................................................................................. 9
1.3.1 Global and Regional Surface Temperatures ................................................................................ 10
1.3.2 Greenhouse Gas Concentrations ................................................................................................. 11
1.3.3 Extreme Events............................................................................................................................. 12
1.3.4 Climate Change Indicators .......................................................................................................... 13
1.4 Treatment of Uncertainties................................................................................................................... 15
1.4.1 Uncertainty in Environmental Science ........................................................................................ 15
1.4.2 Characterizing Uncertainty ......................................................................................................... 15
1.4.3 Treatment of Uncertainty in IPCC ............................................................................................... 17
1.4.4 Uncertainty Treatment in This Assessment .................................................................................. 17
1.5 Advances in Measurement and Modelling Capabilities ..................................................................... 19
1.5.1 Capabilities of Observations........................................................................................................ 19
1.5.2 Capabilities in Global Climate Modelling ................................................................................... 20
Box 1.1: Description of Future Scenarios .................................................................................................... 22
1.6 Overview and Road Map to the Rest of the Report ........................................................................... 24
1.6.1 Topical Issues .............................................................................................................................. 24
FAQ 1.1: If Understanding of the Climate System Has Increased, Why Hasn’t the Range of
Temperature Projections Been Reduced? ........................................................................................... 25
References ...................................................................................................................................................... 28
Appendix 1.A: Notes and Technical Details on Figures Displayed in Chapter 1..................................... 33
Tables .............................................................................................................................................................. 39
Figures ............................................................................................................................................................ 42
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Executive Summary
Human Effects on Climate
Human activities are continuing to affect the Earth’s energy budget by changing the emissions and
resulting atmospheric concentrations of radiatively important gases and aerosols and by changing
land surface properties. Previous assessments have already shown through multiple lines of evidence that
the climate is changing across our planet, largely as a result of human activities. The most compelling
evidence of climate change derives from observations of the atmosphere, land, oceans, and cryosphere.
Unequivocal evidence from in situ observations and ice core records shows that the atmospheric
concentrations of important greenhouse gases such as carbon dioxide, methane, and nitrous oxides have
increased over the last few centuries. [1.2.2, 1.2.3]
The processes affecting climate can exhibit considerable natural variability. Even in the absence of
external forcing, periodic and chaotic variations on a vast range of spatial and temporal scales are
observed. Much of this variability can be represented by simple (e.g., unimodal or power law) distributions,
but many components of the climate system also exhibit multiple states – for instance, the glacial-interglacial
cycles and certain modes of internal variability such as El Niño-Southern Oscillation (ENSO). Movement
between states can occur as a result of natural variability, or in response to external forcing. The relationship
between variability, forcing and response reveals the complexity of the dynamics of the climate system: the
relationship between forcing and response for some parts of the system seems reasonably linear; in other
cases this relationship is much more complex. [1.2.2]
Multiple Lines of Evidence for Climate Change
Global mean surface air temperatures over land and oceans have increased over the last 100 years.
The temperature measurements in the oceans show a continuing increase in the heat content of the oceans.
Analyses based on measurements of the Earth's radiative budget suggest a small positive energy imbalance
that serves to increase the global heat content of the Earth system. Observations from satellites and in situ
measurements show a trend of significant reductions in the mass balance of most land ice masses and in
Arctic sea ice. The ocean's uptake of carbon dioxide is having a significant effect on the chemistry of sea
water. Paleoclimatic reconstructions have helped place ongoing climate change in the perspective of natural
climate variability. [1.2.3; Figure 1.3]
Observations of carbon dioxide (CO2) concentrations, globally-averaged temperature and sea level rise
are generally well within the range of the extent of the earlier IPCC projections. The recently observed
increases in methane (CH4) and nitrous oxide (N2O) concentrations are smaller than those assumed in
the scenarios in the previous assessments. Each IPCC assessment has used new projections of future
climate change that have become more detailed as the models have become more advanced. Similarly, the
scenarios themselves used in the IPCC assessments have changed over time to reflect the state of knowledge.
The range of climate projections from model results provided and assessed in the first IPCC assessment in
1990 to those in the 2007 AR4 provides an opportunity to compare the projections with the actually observed
changes, thereby examining the deviations of the projections from the observations over time. [1.3.1, 1.3.2,
1.3.4, Figure 1.4, Figure 1.5, Figure 1.6, Figure 1.7, Figure 1.10]
Climate change, whether driven by natural or human forcing, can lead to changes in the likelihood of
the occurrence or strength of extreme weather and climate events or both. Since the AR4, the
observational basis has increased substantially, so that some extremes are now examined over most land
areas. Furthermore, more models with higher resolution and more regional models have been used in the
simulations and projections of extremes. [1.3.3, Figure 1.9]
Treatment of Uncertainties
For AR5, the three IPCC Working Groups use two metrics to communicate the degree of certainty in
key findings: (1) Confidence is a qualitative measure of the validity of a finding, based on the type, amount,
quality, and consistency of evidence (e.g., mechanistic understanding, theory, data, expert judgment) and the
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degree of agreement1; and, (2) Likelihood provides a quantified measure of uncertainty in a finding
expressed probabilistically (e.g., based on statistical analysis of observations or model results)2. [1.4, Figure
1.11]
Advances in Measurement and Modelling Capabilities
Over the last few decades, new observational systems, especially satellite-based systems, have
increased the number of observations of the Earth’s climate by orders of magnitude. Tools to analyse
and process these data have been developed or enhanced to cope with this large increase in information, and
more climate proxy data have been acquired to improve our knowledge of past changes in climate. Since the
Earth's climate system is characterized on multiple spatial and temporal scales, new observations may reduce
the uncertainties surrounding the understanding of short timescale processes quite rapidly. However,
processes that occur over longer timescales may require very long observational baselines before much
progress can be made. [1.5.1, Figure 1.12]
Increases in computing speed and memory have led to the development of more sophisticated models,
which describe physical, chemical, and biological processes in greater detail. Modelling strategies have
been extended to provide better estimates of the uncertainty in climate change projections. The model
comparisons with observations have pushed the analysis and development of the models. The incorporation
of "long-term" simulations has allowed incorporation of information from paleoclimate data to inform
projections. Within uncertainties associated with reconstructions of past climate variables from proxy record
and forcings, paleoclimate information from the Mid Holocene, Last Glacial Maximum, and Last
Millennium have been used to test the ability of models to simulate realistically the magnitude and large
scale patterns of past changes. [1.5.2, Figure 1.13, Figure 1.14]
As part of the process of getting model analyses for a range of alternative images of how the future may
unfold, four new scenarios for future emissions of important gases and aerosols have been developed for the
AR5, referred to as Representative Concentration Pathways (RCPs). [Box 1.1]
1
In this Report, the following summary terms are used to describe the available evidence: limited, medium, or robust;
and for the degree of agreement: low, medium, or high. A level of confidence is expressed using five qualifiers: very
low, low, medium, high, and very high, and typeset in italics, e.g., medium confidence. For a given evidence and
agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of
agreement are correlated with increasing confidence (see Section 1.4 and Box TS.1 for more details).
2
In this Report, the following terms have been used to indicate the assessed likelihood of an outcome or a result:
Virtually certain 99–100% probability, Very likely 90–100%, Likely 66–100%, About as likely as not 33–66%,
Unlikely 0–33%, Very unlikely 0–10%, Exceptionally unlikely 0–1%. Additional terms (Extremely likely: 95–100%,
More likely than not >50–100%, and Extremely unlikely 0–5%) may also be used when appropriate. Assessed
likelihood is typeset in italics, e.g., very likely (see Section 1.4 and Box TS.1 for more details).
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Chapter Preview
This introductory chapter serves as a lead-in to the science presented in the Working Group I (WGI)
contribution to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5).
Chapter 1 in the IPCC Fourth Assessment Report (AR4) (Le Treut et al., 2007) provided a historical
perspective on the understanding of climate science and the evidence regarding human influence on the
Earth’s climate system. Since the last assessment, the scientific knowledge gained through observations,
theoretical analyses, and modelling studies has continued to increase and to further strengthen the evidence
linking human activities to the ongoing climate change. In AR5, Chapter 1 focuses on the concepts and
definitions applied in the discussions of new findings in the other chapters. It also examines several of the
key indicators for a changing climate, and shows how the current knowledge of those indicators compares
with the projections made in previous assessments. The new scenarios for projected human-related emissions
used in this assessment are also introduced. Finally, the chapter discusses the directions and capabilities of
current climate science, while the detailed discussion of new findings is covered in the remainder of the WGI
contribution to the AR5.
1.2
1.2.1
Rationale and Key Concepts of the WGI Contribution
Setting the Stage for the Assessment
The IPCC was set up in 1988 by the World Meteorological Organization and the United Nations
Environment Programme to provide governments with a clear view of the current state of knowledge about
the science of climate change, potential impacts, and options for adaptation and mitigation through regular
assessments of the most recent information published in the scientific, technical and socio-economic
literature worldwide. The WGI contribution to the IPCC AR5 assesses the current state of the physical
sciences with respect to climate change. This report presents an assessment of the current state of research
results and is not a discussion of all relevant papers as would be included in a review. It thus seeks to make
sure that the range of scientific views, as represented in the peer-reviewed literature, is considered and
evaluated in the assessment, and that the state of the science is concisely and accurately presented. A
transparent review process ensures that disparate views are included (IPCC, 2012a).
As an overview, Table 1.1 shows a selection of key findings from earlier IPCC assessments. This table
provides a non-comprehensive selection of key assessment statements from previous assessment reports
(IPCC First Assessment Report (FAR, IPCC, 1990), IPCC Second Assessment Report (SAR, IPCC, 1996),
IPCC Third Assessment Report (TAR, IPCC, 2001) and IPCC Fourth Assessment Report (AR4, IPCC,
2007) with a focus on policy relevant quantities that have been evaluated in each of the IPCC assessments.
Scientific hypotheses are contingent and always open to revision in the light of new evidence and theory. In
this sense the distinguishing features of scientific enquiry are the search for truth and the willingness to
subject itself to critical re-examination. Modern research science conducts this critical revision through
processes such as the peer review. At conferences and in the procedures that surround publication in peerreviewed journals, scientific claims about environmental processes are analysed and held up to scrutiny.
Even after publication, findings are further analysed and evaluated. That is the self-correcting nature of the
scientific process (more details are given in AR4 Chapter 1; Le Treut et al., 2007).
Science strives for objectivity, but inevitably also involves choices and judgments. Scientists make choices
regarding data and models, which processes to include and which to leave out. Usually these choices are
uncontroversial and play only a minor role in the production of research. Sometimes, however, the choices
scientists make are sources of disagreement and uncertainty. These are usually resolved by further scientific
enquiry into the sources of disagreement. In some cases, experts cannot agree on a consensus view.
Examples in climate science include how best to evaluate climate models relative to observations, how best
to evaluate potential sea level rise, and how to evaluate probabilistic projections of climate change. In many
cases there may be no definitive solution to these questions. The IPCC process is aimed at assessing the
literature as it stands, and attempts to reflect the level of reasonable scientific consensus as well as
disagreement.
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In order to assess areas of scientific controversy, the peer-reviewed literature is considered and evaluated.
Not all papers on a controversial point can be discussed individually in an assessment, but every effort has
been made here to ensure that all views represented in the peer-reviewed literature are considered in the
assessment process. A list of topical issues is given in Table 1.3.
The Earth sciences study the multitude of processes that shape our environment. Some of these processes can
be understood through idealized laboratory experiments, altering a single element and then tracing through
the effects of that controlled change. However, in common with other natural and the social sciences, the
openness of environmental systems, in terms of our lack of control of the boundaries of the system, their
spatially and temporally multi-scale character and the complexity of interactions, often hamper scientists’
ability to definitively isolate causal links. This in turn places important limits on the understanding of many
of the inferences in the Earth sciences (e.g., Oreskes et al., 1994). There are many cases where scientists are
able to make inferences using statistical tools with considerable evidential support and with high degrees of
confidence and conceptual and numerical modelling can assist in forming understanding and intuition about
the interaction of dynamic processes.
[INSERT TABLE 1.1 HERE]
Table 1.1: Historical overview of major conclusions of previous IPCC assessment reports. The table provides a noncomprehensive selection of key statements from previous assessment reports (IPCC First Assessment Report (FAR;
IPCC, 1990), IPCC Second Assessment Report (SAR; IPCC, 1996), IPCC Third Assessment Report (TAR; IPCC,
2001) and IPCC Fourth Assessment Report (AR4; IPCC, 2007)) with a focus on global mean surface air temperature
and sea level change as two policy relevant quantities which have been covered in IPCC since the first assessment
report.
1.2.2
Key Concepts in Climate Science
Here, some of the key concepts in climate science are briefly described; many of these were summarized
more comprehensively in earlier IPCC assessments (Baede et al., 2001). We only focus on several of them to
facilitate discussions in this assessment.
First of all, it is important to distinguish the meaning of weather from climate. Weather describes the
conditions of the atmosphere at a certain place and time with reference to temperature, pressure, humidity,
wind, and other key parameters (meteorological elements), the presence of clouds, precipitation, and the
occurrence of special phenomena, such as thunderstorms, dust storms, tornados, etc. Climate in a narrow
sense is usually defined as the average weather, or more rigorously, as the statistical description in terms of
the mean and variability of relevant quantities over a period of time ranging from months to thousands or
millions of years. The relevant quantities are most often surface variables such as temperature, precipitation
and wind. The classical period for averaging these variables is 30 years, as defined by the World
Meteorological Organization. Climate in a wider sense also includes not just the mean conditions, but also
the associated statistics (frequency, magnitude, persistence, trends, etc.), often combining parameters to
describe phenomena such as droughts. Climate change refers to a change in the state of the climate that can
be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties,
and that persists for an extended period, typically decades or longer.
The Earth’s climate system is powered by solar radiation (Figure 1.1). Approximately half of the energy
from the Sun is supplied in the visible part of the electromagnetic spectrum. As the Earth’s temperature has
been relatively constant over many centuries, the incoming solar energy must be nearly in balance with
outgoing radiation. Of the incoming solar shortwave radiation (SWR), about half is absorbed by the Earth’s
surface. The fraction of SWR reflected back to space by gases and aerosols, clouds and by the Earth’s
surface (albedo) is approximately 30%, and about 20% is absorbed in the atmosphere. Based on the
temperature of the Earth’s surface the majority of the outgoing energy flux from the Earth is in the infrared
part of the spectrum. The longwave radiation (LWR, also referred to as infrared radiation) emitted from the
Earth’s surface is largely absorbed by certain atmospheric constituents (water vapour, carbon dioxide; CO2),
methane (CH4), nitrous oxide (N2O) and other greenhouse gases (GHG; see Annex III for Glossary)) and
clouds, which themselves emit longwave radiation into all directions. The downward directed component of
this LWR adds heat to the lower layers of the atmosphere and to the Earth’s surface (greenhouse effect). The
dominant energy loss of the infrared radiation from the Earth is from higher layers of the troposphere. The
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Sun primarily provides its energy to the Earth in the tropics and the subtropics; this energy is then partially
redistributed to middle and high latitudes by atmospheric and oceanic transport processes.
Changes in the global energy budget derive from either changes in the net incoming solar radiation or
changes in the outgoing longwave radiation (OLR). Changes in the net incoming solar radiation derive from
changes in the Sun’s output of energy or changes in the Earth’s albedo. Reliable measurements of total solar
irradiance (TSI) can be made only from space and the precise record extends back only to 1978. The
generally accepted mean value of the TSI is about 1361 W m−2 (Kopp and Lean, 2011; see Chapter 8 for a
detailed discussion on the TSI); this is lower than the previous value of 1365 W m−2 used in the earlier
assessments. Short-term variations of a few tenths of a percent are common during the approximately 11year sunspot solar cycle (see Section 5.2 and Section 8.4 for further details). Changes in the outgoing LWR
can result from changes in the temperature of the Earth’s surface or atmosphere or changes in the emissivity
(measure of emission efficiency) of LWR from either the atmosphere or the Earth’s surface. For the
atmosphere, these changes in emissivity are due predominantly to changes in cloud cover and cloud
properties, in greenhouse gases, and in aerosol concentrations. The radiative energy budget of the Earth is
almost in balance (Figure 1.1), but ocean heat content and satellite measurements indicate a small positive
imbalance (Murphy et al., 2009; Trenberth et al., 2009; Hansen et al., 2011) that is consistent with the rapid
changes in the atmospheric composition.
In addition, some aerosols increase atmospheric reflectivity, while others (e.g., particulate black carbon) are
strong absorbers and also modify SWR (see Section 7.3 for a detailed assessment). Indirectly, aerosols also
affect cloud albedo, because many aerosols serve as cloud condensation nuclei or ice nuclei. This means that
changes in aerosol types and distribution can result in small but important changes in cloud albedo and
lifetime (Section 7.4). Clouds play a critical role in climate, since they can not only increase albedo, thereby
cooling the planet, but they are also important because of their warming effects through infrared radiative
transfer. Whether the net radiative effect of a cloud is one of cooling or of warming depends on its physical
properties (level of occurrence, vertical extent, water path and effective cloud particle size) as well as on the
nature of the cloud condensation nuclei population (Section 7.3). Humans enhance the greenhouse effect
directly by emitting greenhouse gases such as CO2, CH4, N2O, and chlorofluorocarbons (CFC) (Figure 1.1).
In addition, pollutants such as carbon monoxide (CO), volatile organic compounds (VOC), nitrogen oxides
(NOx) and sulfur dioxide (SO2), which by themselves are negligible GHGs, have an indirect effect on the
greenhouse effect by altering, through atmospheric chemical reactions, the abundance of important gases to
the amount of outgoing LWR such as CH4 and ozone (O3), and/or by acting as precursors of secondary
aerosols. Since anthropogenic emission sources simultaneously can emit some chemicals that affect climate
and others that affect air pollution, including some that affect both, atmospheric chemistry and climate
science are intrinsically linked.
In addition to changing the atmospheric concentrations of gases and aerosols, humans are affecting both the
energy and water budget of the planet by changing the land surface including redistributing the balance
between latent and sensible heat fluxes (Sections 2.5, 7.2, 7.6 and 8.2). Land use changes, such as the
conversion of forests to cultivated land, change the characteristics of vegetation, including its colour,
seasonal growth and carbon content (Houghton, 2003; Foley et al., 2005). For example, clearing and burning
a forest to prepare agricultural land reduces carbon storage in the vegetation, adds CO2 to the atmosphere,
and changes the reflectivity of the land (surface albedo), rates of evapotranspiration and longwave emissions
(Figure 1.1).
Changes in the atmosphere, land, ocean, biosphere and cryosphere—both natural and anthropogenic—can
perturb the Earth's radiation budget, producing a radiative forcing (RF) that affects climate. RF is a measure
of the net change in the energy balance in response to an external perturbation. The drivers of changes in
climate can include, for example, changes in the solar irradiance and changes in atmospheric trace gas and
aerosol concentrations (Figure 1.1). The concept of RF cannot capture the interactions of anthropogenic
aerosols and clouds, for example, and thus in addition to the RF as used in previous assessments, Sections
7.4 and 8.1 introduce a new concept, effective radiative forcing (ERF), that accounts for rapid response in
the climate system. ERF is defined as the change in net downward flux at the top of the atmosphere after
allowing for atmospheric temperatures, water vapour, clouds and land albedo to adjust, but with either sea
surface temperatures (SST) and sea ice cover unchanged or with global mean surface temperature
unchanged.
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[INSERT FIGURE 1.1 HERE]
Figure 1.1: Main drivers of climate change. The radiative balance between incoming solar shortwave radiation (SWR)
and outgoing longwave radiation (LWR) is influenced by global climate “drivers”. Natural fluctuations in solar output
(solar cycles) can cause changes in the energy balance (through fluctuations in the amount of incoming SWR) (Section
2.3). Human activity changes the emissions of gases and aerosols, which are involved in atmospheric chemical
reactions, resulting in modified O3 and aerosol amounts (Section 2.2). O3 and aerosol particles absorb, scatter and
reflect SWR, changing the energy balance. Some aerosols act as cloud condensation nuclei modifying the properties of
cloud droplets and possibly affecting precipitation (Section 7.4). Since cloud interactions with SWR and LWR are
large, small changes in the properties of clouds have important implications for the radiative budget (Section 7.4).
Anthropogenic changes in greenhouse gases (e.g., CO2, CH4, N2O, O3, CFCs), and large aerosols (>2.5 μm in size)
modify the amount of outgoing LWR by absorbing outgoing LWR and re-emitting less energy at a lower temperature
(Section 2.2). Surface albedo is changed by changes in vegetation or land surface properties, snow or ice cover and
ocean colour (Section 2.3). These changes are driven by natural seasonal and diurnal changes (e.g., snow cover), as well
as human influence (e.g., changes in vegetation types) (Forster et al., 2007).
Once a forcing is applied, complex internal feedbacks determine the eventual response of the climate system,
and will in general cause this response to differ from a simple linear one (IPCC, 2001, 2007). There are
many feedback mechanisms in the climate system that can either amplify (‘positive feedback’) or diminish
(‘negative feedback’) the effects of a change in climate forcing (Le Treut et al., 2007) (see Figure 1.2 for a
representation of some of the key feedbacks). An example of a positive feedback is the water vapour
feedback whereby an increase in surface temperature enhances the amount of water vapour present in the
atmosphere. Water vapour is a powerful greenhouse gas: increasing its atmospheric concentration enhances
the greenhouse effect and leads to further surface warming. Another example is the ice albedo feedback,
where the albedo decreases as highly reflective ice and snow surfaces melt, exposing the darker and more
absorbing surfaces below. The dominant negative feedback is the increased emission of energy through
longwave radiation as surface temperature increases (sometimes also referred to as blackbody radiation
feedback). Some feedbacks operate quickly (hours), while others develop over decades to centuries; in order
to understand the full impact of a feedback mechanism, its timescale needs to be considered. Melting of land
ice sheets can take days to millennia.
A spectrum of models is used to quantitatively project the climate response to forcings. The simplest energy
balance models use one box to represent the Earth system and solve the global energy balance to deduce
globally averaged surface air temperature. At the other extreme, full complexity 3-dimensional climate
models include the explicit solution of energy, momentum and mass conservation equations at millions of
points on the Earth in the atmosphere, land, ocean and cryosphere. More recently, capabilities for the explicit
simulation of the biosphere, the carbon cycle and atmospheric chemistry have been added to the full
complexity models, and these models are called Earth System Models (ESMs). Earth System Models of
Intermediate Complexity include the same processes as ESMs, but at reduced resolution, and thus can be
simulated for longer periods (see Annex III for Glossary and Section 9.1).
An equilibrium climate experiment is an experiment in which a climate model is allowed to fully adjust to a
specified change in RF. Such experiments provide information on the difference between the initial and final
states of the model simulated climate, but not on the time-dependent response. The equilibrium response in
global mean surface air temperature to a doubling of atmospheric concentration of CO2 above pre-industrial
levels (e.g., Arrhenius, 1896; see Le Treut et al., 2007 for a comprehensive list) has often been used as the
basis for the concept of equilibrium climate sensitivity (e.g., Hansen et al., 1981; see Meehl et al., 2007 for a
comprehensive list). For more realistic simulations of climate, changes in RF are applied gradually over
time, for example using historical reconstructions of the carbon dioxide, and these simulations are called
transient simulations. The temperature response in these transient simulations is different than in an
equilibrium simulation. The transient climate response is defined as the change in global surface temperature
at the time of atmospheric CO2 doubling in a global coupled ocean-atmosphere climate model simulation
where concentrations of CO2 were increased by 1% yr–1. The transient climate response is a measure of the
strength and rapidity of the surface temperature response to greenhouse gas forcing. It can be more
meaningful for some problems as well as easier to derive from observations (see Figure 10.19; Section 10.8;
Knutti et al., 2005; Chapter 12; Frame et al., 2006; Forest et al., 2008), but such experiments are not intended
to replace the more realistic scenario evaluations.
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Climate change commitment is defined as the future change to which the climate system is committed by
virtue of past or current forcings. The components of the climate system respond on a large range of
timescales, ranging from the essentially rapid responses that characterise some radiative feedbacks, to
millennial scale responses such as those associated with the behaviour of the carbon cycle (Section 6.1) and
ice sheets (see Figure 1.2 and Box 5.1). Even if anthropogenic emissions were immediately ceased
(Matthews and Weaver, 2010) or if climate forcings were fixed at current values (Wigley, 2005) the climate
system would continue to change until it came into equilibrium with those forcings (Section 12.5). Because
of the slow response time of some components of the climate system, equilibrium conditions will not be
reached for many centuries. Slow processes can sometimes only be constrained by data collected over long
periods, giving a particular salience to paleoclimate data for understanding equilibrium processes. Climate
change commitment is indicative of aspects of inertia in the climate system, since it captures the ongoing
nature of some aspects of change.
[INSERT FIGURE 1.2 HERE]
Figure 1.2: Climate feedbacks and timescales. The climate feedbacks related to increasing carbon dioxide and rising
temperature include negative feedbacks (–) such as longwave radiation, lapse rate (see Glossary in Annex III), and airsea carbon exchange and positive feedbacks (+) such as water vapour and snow/ice albedo feedbacks. Some feedbacks
may be positive or negative (±): clouds, ocean circulation changes, air-land carbon dioxide exchange, and emissions of
non-green house gases and aerosols from natural systems. In the smaller box, the large difference in timescales for the
various feedbacks is highlighted.
A summary of perturbations to the forcing of the climate system from changes in solar radiation, greenhouse
gases, surface albedo, and aerosols is presented in Box 13.1. The energy fluxes from these perturbations are
balanced by increased radiation to space from a warming Earth, reflection of solar radiation, and storage of
energy in the Earth system, principally the oceans (Box 3.1, Box 13.1).
The processes affecting climate can exhibit considerable natural variability. Even in the absence of external
forcing, periodic and chaotic variations on a vast range of spatial and temporal scales are observed. Much of
this variability can be represented by simple (e.g., unimodal or power law) distributions, but many
components of the climate system also exhibit multiple states – for instance, the glacial-interglacial cycles
and certain modes of internal variability such as El Niño-Southern Oscillation (ENSO) (see Box 2.5 for
details on patterns and indices of climate variability). Movement between states can occur as a result of
natural variability, or in response to external forcing. The relationship between variability, forcing and
response reveals the complexity of the dynamics of the climate system: the relationship between forcing and
response for some parts of the system seems reasonably linear; in other cases this relationship is much more
complex, characterised by hysteresis (the dependence on past states) and a non-additive combination of
feedbacks.
Related to multiple climate states, and hysteresis, is the concept of irreversibility in the climate system. In
some cases where multiple states and irreversibility combine, bifurcations or “tipping points” can been
reached (see Section 12.5). In these situations, it is difficult if not impossible for the climate system to revert
to its previous state, and the change is termed irreversible over some timescale and forcing range. A small
number of studies using simplified models find evidence for global-scale “tipping points” (e.g., Lenton et al.,
2008); however, there is no evidence for global-scale tipping points in any of the most comprehensive
models evaluated to date in studies climate evolution of the of the 21st century. There is evidence for
threshold behaviour in certain aspects of the climate system, such as ocean circulation (see Section 12.5) and
ice sheets (see Box 5.1), on multi-centennial-to-millennial timescales. There are also arguments for the
existence of regional tipping points, most notably in the Arctic (e.g., Lenton et al., 2008; Duarte et al., 2012;
Wadhams, 2012), although aspects of this are contested (Armour et al., 2011; Tietsche et al., 2011).
1.2.3
Multiple Lines of Evidence for Climate Change
While the first IPCC assessment depended primarily on observed changes in surface temperature and climate
model analyses, more recent assessments include multiple lines of evidence for climate change. The first line
of evidence in assessing climate change is based on careful analysis of observational records of the
atmosphere, land, ocean and cryosphere systems (Figure 1.3). There is incontrovertible evidence from in situ
observations and ice core records that the atmospheric concentrations of greenhouse gases such as CO2, CH4,
and N2O have increased substantially over the last 200 years (Section 6.3, Section 8.3). In addition,
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instrumental observations show that land and sea surface temperatures have increased over the last 100 years
(Chapter 2). Satellites allow a much broader spatial distribution of measurements, especially over the last 30
years. For the upper ocean temperature the observations indicate that the temperature has increased since at
least 1950 (Willis et al., 2010; Section 3.2). Observations from satellites and in situ measurements suggest
reductions in glaciers, Arctic sea ice and ice sheets (Sections 4.2, 4.3, 4.4). Additionally, analyses based on
measurements of the radiative budget and ocean heat content suggest a small imbalance (Section 2.3). These
observations, all published in peer-reviewed journals, made by diverse measurement groups in multiple
countries using different technologies, investigating various climate-relevant types of data, uncertainties and
processes, offer a wide range of evidence on the broad extent of the changing climate throughout our planet.
Conceptual and numerical models of the Earth’s climate system offer another line of evidence on climate
change (discussions in Chapters 5 and 9 provide relevant analyses of this evidence from paleoclimatic to
recent periods). These use our basic understanding of the climate system to provide self-consistent
methodologies for calculating impacts of processes and changes. Numerical models include the current
knowledge about the laws of physics, chemistry and biology, as well as hypotheses about how complicated
processes such as cloud formation can occur. Since these models can only represent the existing state of
knowledge and technology, they are not perfect; they are, however, important tools for analysing
uncertainties or unknowns, for testing different hypotheses for causation relative to observations, and for
making projections of possible future changes.
One of the most powerful methods for assessing changes occurring in climate involves the use of statistical
tools to test the analyses from models relative to observations. This methodology is generally called
detection and attribution in the climate change community (Section 10.2). For example, climate models
indicate that the temperature response to greenhouse gas increases is expected to be different than the effects
from aerosols or from solar variability. Radiosonde measurements and satellite retrievals of atmospheric
temperature show increases in tropospheric temperature and decreases in stratospheric temperatures,
consistent with the increases in greenhouse gas effects found in climate model simulations (e.g., increases in
CO2, changes in O3), but if the Sun was the main driver of current climate change, stratospheric and
tropospheric temperatures would respond with the same sign (Hegerl et al., 2007).
Prior to the instrumental period, historical sources, natural archives, and proxies for key climate variables
(e.g., tree rings, marine sediment cores, ice cores) can provide quantitative information on past regional to
global climate and atmospheric composition variability and these data provide another line of evidence.
Reconstructions of key climate variables based on these datasets have provided important information on the
responses of the Earth system to a variety of external forcings and its internal variability over a wide range of
timescales (Hansen et al., 2006; Mann et al., 2008). Paleoclimatic reconstructions thus provide a means for
placing the current changes in climate in the perspective of natural climate variability (Section 5.1). AR5
includes new information on external RFs caused by variations in volcanic and solar activity (e.g.,
Steinhilber et al., 2009; see Section 8.4). Extended data sets on past changes in atmospheric concentrations
and distributions of atmospheric greenhouse gas concentrations (e.g., Lüthi et al., 2008; Beerling and Royer,
2011) and mineral aerosols (Lambert et al., 2008) have also been used to attribute reconstructed paleoclimate
temperatures to past variations in external forcings (Section 5.2).
1.3
Indicators of Climate Change
There are many indicators of climate change. These include physical responses such as changes in surface
temperature, changes in atmospheric water vapour, changes in precipitation, changes in severe events,
changes in glaciers, changes in ocean and land ice, and changes in sea level. Some key examples of such
changes in important climate parameters are discussed in this section and all are assessed in much more
detail in other chapters.
As was done to a more limited extent in AR4 (Le Treut et al., 2007), this section provides a test of the
planetary-scale hypotheses of climate change against observations. In other words, how well do the
projections used in the past assessments compare with observations to date. Seven additional years of
observations are now available to evaluate earlier model projections. The projected range that was given in
each assessment is compared to observations. The largest possible range of scenarios available for a specific
variable for each of the previous assessment reports is shown in the figures.
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Based on the assessment of AR4, a number of the key climate and associated environmental parameters are
presented in Figure 1.3, which updates the similar figure in the Technical Summary (TS) of IPCC (2001).
This section discusses the recent changes in several indicators, while more thorough assessments for each of
these indicators are provided in other chapters. Also shown in parentheses in Figure 1.3 are the chapter and
section where those indicators of change are assessed in AR5.
Note that projections presented in the IPCC assessments are not predictions (see the Glossary in Annex III);
the analyses in the discussion below only examine the short-term plausibility of the projections up to AR4,
including the scenarios for future emissions and the models used to simulate these scenarios in the earlier
assessments. Model results from the Coupled Model Intercomparison Project Phase 5 (CMIP5) (Taylor et al.,
2012) used in AR5 are therefore not included in this section; Chapter 11 and Chapter 12 describe the
projections from the new modelling studies. Note that none of the scenarios examined in the IPCC
assessments were ever intended to be short-term predictors of change.
[INSERT FIGURE 1.3 HERE]
Figure 1.3: Overview of observed climate change indicators as listed in AR4. Chapter numbers indicate where detailed
discussions for these indicators are found in AR5 (temperature: red; hydrological: blue; others: black).
1.3.1
Global and Regional Surface Temperatures
Observed changes in global mean surface air temperature since 1950 (from three major databases, as
anomalies relative to 1961–1990) are shown in Figure 1.4. As in the prior assessments, global climate
models generally simulate global temperatures that compare well with observations over climate timescales
(Section 9.4). Even though the projections from the models were never intended to be predictions over such
a short time scale, the observations through 2012 generally fall within the projections made in all past
assessments. The 1990–2012 data have been shown to be consistent with the FAR projections (IPCC,
1990), and not consistent with zero trend from 1990, even in the presence of substantial natural
variability (Frame and Stone, 2013).
The scenarios were designed to span a broad range of plausible futures, but are not aimed at predicting the
most likely outcome. The scenarios considered for the projections from the earlier reports (FAR, SAR) had a
much simpler basis than the Special Report on Emission Scenarios (SRES) (IPCC, 2000) scenarios used in
the later assessments. For example, the FAR scenarios did not specify future aerosol distributions. In AR4 a
multiple-set of projections were presented that were simulated using comprehensive ocean-atmosphere
models provided by CMIP3 and these projections are continuations of transient simulations of the 20th
century climate. These projections of temperature provide additionally a measure of the natural variability.
This could not be obtained from the earlier projections based on models of intermediate complexity
(Cubasch et al., 2001).
Note that before TAR the climate models did not include natural forcing (such as volcanic activity and solar
variability). Even in AR4 not all models included natural forcing and some also did not include aerosols.
Those models that allowed for aerosol effects presented in the AR4 simulated for example the cooling effects
of the 1991 Mt. Pinatubo eruption and agree better with the observed temperatures than the previous
assessments that did not include those effects.
The bars on the side for FAR, SAR and TAR represent the range of results for the scenarios at the end of the
time period and are not error bars. By contrast to the previous reports the AR4 gave an assessment of the
individual scenarios with a mean estimate (cross-bar; ensemble mean of the CMIP3 simulations) and a likely
range (full bar; –40% to +60% of the mean estimate) (Meehl et al., 2007).
In summary, the trend in globally-averaged surface temperatures falls within the range of the previous IPCC
projections. During the last decade the trend in the observations is smaller than the mean of the projections
of AR4 (see Section 9.4.1, Box 9.2 for a detailed assessment of the hiatus in global mean surface warming in
the last 15 years). As shown by Hawkins and Sutton (2009), trends in the observations during short-time
scale periods (decades) can be dominated by natural variability in the Earth’s climate system. Similar
episodes are also seen in climate model experiments (Easterling and Wehner, 2009). Due to their
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experimental design these episodes cannot be duplicated with the same timing as the observed episodes in
most of the model simulations; this affects the interpretation of recent trends in the scenario evaluations
(Section 11.2). Notwithstanding these points, there is evidence that early forecasts that carried formal
estimates of uncertainty have proved highly consistent with subsequent observations (Allen et al., 2013). If
the contributions of solar variability, volcanic activity and ENSO are removed from the observations the
remaining trend of surface air temperature agree better with observations (Rahmstorf et al., 2012).
[INSERT FIGURE 1.4 HERE]
Figure 1.4: Estimated changes in the observed globally and annually averaged surface temperature anomaly relative to
1961–1990 (in C) since 1950 compared with the range of projections from the previous IPCC assessments. Values are
harmonized to start from the same value in 1990. Observed global annual mean surface air temperature anomaly,
relative to 1961–1990, is shown as squares and smoothed time series as solid lines (NASA (dark blue), NOAA (warm
mustard), and the UK Hadley Centre (bright green) reanalyses). The coloured shading shows the projected range of
global annual mean surface air temperature change from 1990 to 2035 for models used in FAR (Figure 6.11 in
Bretherton et al., 1990), SAR (Figure 19 in the TS of IPCC, 1996), TAR (full range of TAR Figure 9.13(b) in Cubasch
et al., 2001). TAR results are based on the simple climate model analyses presented and not on the individual full threedimensional climate model simulations. For the AR4 results are presented as single model runs of the CMIP3 ensemble
for the historical period from 1950 to 2000 (light grey lines) and for three scenarios (A2, A1B and B1) from 2001 to
2035. The bars at the right hand side of the graph show the full range given for 2035 for each assessment report. For the
three SRES scenarios the bars show the CMIP3 ensemble mean and the likely range given by –40% to +60% of the
mean as assessed in Meehl et al. (2007). The publication years of the assessment reports are shown. See Appendix 1.A
for details on the data and calculations used to create this figure.
1.3.2
Greenhouse Gas Concentrations
Key indicators of global climate change also include the changing concentrations of the radiatively important
greenhouse gases that are important drivers for this change (e.g., Denman et al., 2007; Forster et al., 2007).
Figures 1.5 through 1.7 show the recent globally and annually averaged observed concentrations for the
gases of most concern, CO2, CH4, and N2O (see Sections 2.2, 6.3 and 8.3 for more detailed discussion of
these and other key gases). As discussed in the later chapters, accurate measurements of these long-lived
gases come from a number of monitoring stations throughout the world. The observations in these figures are
compared with the projections from the previous IPCC assessments.
The model simulations begin with historical emissions up to 1990. The further evolution of these gases was
described by scenario projections. TAR and AR4 model concentrations after 1990 are based on the SRES
scenarios but those model results may also account for historical emissions analyses. The range of
projections from the FAR (IPCC, 1990) for CO2 is much larger than those from the scenarios used in more
recent assessments. The recent observed trends in CO2 concentrations tend to be in the middle of the
scenarios used for the projections (Figure 1.5).
As discussed in Dlugokencky et al. (2009), trends in CH4 showed a stabilization from 1999 to 2006, but CH4
concentrations have been increasing again starting in 2007 (see Sections 6.3 and 2.2 for more discussion on
the budget and changing concentration trends for CH4). Because at the time the scenarios were developed
(e.g., the SRES scenarios were developed in 2000), it was thought that past trends would continue, the
scenarios used and the resulting model projections assumed in FAR through AR4 all show larger increases
than those observed (Figure 1.6).
Concentrations of N2O have continued to increase at a nearly constant rate (Elkins and Dutton, 2010) since
about 1970 as shown in Figure 1.7. The observed trends tend to be in the lower part of the projections for the
previous assessments.
[INSERT FIGURE 1.5 HERE]
Figure 1.5: Observed globally and annually averaged CO2 concentrations in parts per million (ppm) since 1950
compared with projections from the previous IPCC assessments. Observed global annual CO2 concentrations are shown
in dark blue. The shading shows the largest model projected range of global annual CO2 concentrations from 1950 to
2035 from FAR (Figure A.3 in the Summary for Policymakers (SPM) of IPCC, 1990), SAR (Figure 5b in the TS of
IPCC, 1996), TAR (Appendix II of IPCC, 2001), and from the A2, A1B and B1 scenarios presented in the AR4 (Figure
10.26 in Meehl et al., 2007). The bars at the right hand side of the graph show the full range given for 2035 for each
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assessment report. The publication years of the assessment reports are shown. See Appendix 1.A for details on the data
and calculations used to create this figure.
[INSERT FIGURE 1.6 HERE]
Figure 1.6: Observed globally and annually averaged CH4 concentrations in parts per billion (ppb) since 1950
compared with projections from the previous IPCC assessments. Estimated observed global annual CH4 concentrations
are shown in dark blue. The shading shows the largest model projected range of global annual CH4 concentrations from
1950 to 2035 from FAR (Figure A.3 of the Annex of IPCC, 1990), SAR (Table 2.5a in Schimel et al., 1996), TAR
(Appendix II of IPCC, 2001), and from the A2, A1B and B1 scenarios presented in the AR4 (Figure 10.26 in Meehl et
al., 2007). The bars at the right hand side of the graph show the full range given for 2035 for each assessment report.
The publication years of the assessment reports are shown. See Appendix 1.A for details on the data and calculations
used to create this figure.
[INSERT FIGURE 1.7 HERE]
Figure 1.7: Observed globally and annually averaged N2O concentrations in parts per billion (ppb) since 1950
compared with projections from the previous IPCC assessments. Observed global annual N2O concentrations are shown
in dark blue. The shading shows the largest model projected range of global annual N2O concentrations from 1950 to
2035 from FAR (Figure A3 in the Annex of IPCC, 1990), SAR (Table 2.5b in Schimel et al., 1996), TAR (Appendix II
of IPCC, 2001), and from the A2, A1B and B1 scenarios presented in the AR4 (Figure 10.26 in Meehl et al., 2007). The
bars at the right hand side of the graph show the full range given for 2035 for each assessment report. The publication
years of the assessment reports are shown. See Appendix 1.A for details on the data and calculations used to create this
figure.
1.3.3
Extreme Events
Climate change, whether driven by natural or human forcings, can lead to changes in the likelihood of the
occurrence or strength of extreme weather and climate events such as extreme precipitation events or warm
spells (see Chapter 3 of the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to
Advance Climate Change Adaptation (SREX); Seneviratne et al., 2012). An extreme weather event is an
event that is rare at a particular place and time of year. Definitions of ‘rare’ vary, but an extreme weather
event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function
estimated from observations (see also Glossary in Annex III and FAQ 2.2). By definition, the characteristics
of what is called extreme weather may vary from place to place in an absolute sense. At present, single
extreme events cannot generally be directly attributed to anthropogenic influence, although the change in
likelihood for the event to occur has been determined for some events by accounting for observed changes in
climate (see Section 10.6). When a pattern of extreme weather persists for some time, such as a season, it
may be classified as an extreme climate event, especially if it yields an average or total that is itself extreme
(e.g., drought or heavy rainfall over a season). For some climate extremes such as drought, floods and heat
waves, several factors such as duration and intensity need to be combined to produce an extreme event
(Seneviratne et al., 2012).
The probability of occurrence of values of a climate or weather variable can be described by a probability
density function (PDF) that for some variables (e.g., temperature) is shaped similar to a ‘Gaussian’ curve. A
PDF is a function that indicates the relative chances of occurrence of different outcomes of a variable.
Simple statistical reasoning indicates that substantial changes in the frequency of extreme events (e.g., the
maximum possible 24-hour rainfall at a specific location) can result from a relatively small shift in the
distribution of a weather or climate variable. Figure 1.8a shows a schematic of such a PDF and illustrates the
effect of a small shift in the mean of a variable on the frequency of extremes at either end of the distribution.
An increase in the frequency of one extreme (e.g., the number of hot days) can be accompanied by a decline
in the opposite extreme (in this case the number of cold days such as frost days). Changes in the variability,
skewness or the shape of the distribution can complicate this simple picture (Figure 1.8b, c and d).
While the SAR found that data and analyses of extremes related to climate change were sparse, improved
monitoring and data for changes in extremes were available for the TAR, and climate models were being
analysed to provide projections of extremes. In AR4, the observational basis of analyses of extremes had
increased substantially, so that some extremes were now examined over most land areas (e.g., rainfall
extremes). More models with higher resolution, and more regional models have been used in the simulation
and projection of extremes, and ensemble integrations now provide information about PDFs and extremes.
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Since the TAR, climate change studies have especially focused on changes in the global statistics of
extremes, and observed and projected changes in extremes have been compiled in the so-called “Extremes”Table (Figure 1.9). This table has been further modified to account for the SREX assessment. For some
extremes (“higher maximum temperature”, “higher minimum temperature”, “precipitation extremes”,
“droughts or dryness”), all of these assessments found an increasing trend in the observations and in the
projections. In the observations for the “higher maximum temperature” the confidence level was raised from
likely in the TAR to very likely in SREX. While the diurnal temperature range was assessed in the ExtremesTable of the TAR, it was no longer included in the Extremes-Table of AR4, since it is not considered a
climate extreme in a narrow sense. Diurnal temperature range was, however, reported to decrease for 21st
century projections in AR4 (Meehl et al., 2007). In projections for precipitation extremes, the spatial
relevance has been improved from very likely "over many Northern Hemisphere mid-latitudes to high
latitudes land areas" from the TAR to very likely for all regions in AR4 (these “uncertainty labels” are
discussed in Section 1.4). However, confidence in trends in projected precipitation extremes was downscaled
to likely in the SREX as a result of a perception of biases and a fairly large spread in the precipitation
projections in some regions. SREX also had less confidence than TAR and AR4 in the trends for droughts
and dryness, “due to lack of direct observations, some geographical inconsistencies in the trends, and some
dependencies of inferred trends on the index choice” (IPCC, 2012b).
For some extremes (e.g., “changes in tropical cyclone activity”) the definition changed between the TAR and
the AR4. While the TAR only made a statement about the peak wind speed of tropical cyclones, the AR4
also stressed the overall increase in intense tropical cyclone activity. The "low confidence" for any long term
trend (>40 years) in the observed changes of the tropical cyclone activities is due to uncertainties in past
observational capabilities (IPCC, 2012b). The “increase in extreme sea level” has been added in the AR4.
Such an increase is likely according to the AR4 and the SREX for observed trends, and very likely for the
climate projections reported in the SREX.
The assessed likelihood of anthropogenic contributions to trends is lower for variables where the assessment
is based on indirect evidence. Especially for extremes that are the result of the combination of factors such as
droughts, linking a particular extreme event to specific causal relationships is difficult to determine (e.g.,
difficult to establish the clear role of climate change in the event) (see Section 10.6 and Peterson et al.,
2012). In some cases (e.g., precipitation extremes), however, it may be possible to estimate the humanrelated contribution to such changes in the probability of occurrence of extremes (Pall et al., 2011;
Seneviratne et al., 2012).
[INSERT FIGURE 1.8 HERE]
Figure 1.8: Schematic representations of the probability density function of daily temperature, which tends to be
approximately Gaussian, and daily precipitation, which has a skewed distribution. Dashed lines represent a previous
distribution and solid lines a changed distribution. The probability of occurrence, or frequency, of extremes is denoted
by the shaded areas. In the case of temperature, changes in the frequencies of extremes are affected by changes a) in the
mean, b) in the variance or shape, and c) in both the mean and the variance. d) In a skewed distribution such as that of
precipitation, a change in the mean of the distribution generally affects its variability or spread, and thus an increase in
mean precipitation would also likely imply an increase in heavy precipitation extremes, and vice-versa. In addition, the
shape of the right hand tail could also change, affecting extremes. Furthermore, climate change may alter the frequency
of precipitation and the duration of dry spells between precipitation events. a)-c) modified from Folland et al. (2001)
and d modified from Peterson et al. (2008) as in Zhang and Zwiers (2012).
[INSERT FIGURE 1.9 HERE]
Figure 1.9: Change in the confidence levels for extreme events based on prior IPCC assessments: TAR, AR4, and
SREX. Types of extreme events discussed in all three reports are highlighted in green. Confidence levels are defined in
Section 1.4. Similar analyses for AR5 are discussed in later chapters. Please note that the nomenclature for confidence
level changed from AR4 to SREX and AR5.
1.3.4
Climate Change Indicators
Climate change can lead to other effects on the Earth’s physical system that are also indicators of climate
change. Such integrative indicators include changes in sea level (ocean warming + land ice melt), in ocean
acidification (ocean uptake of CO2), and in the amount of ice on ocean and land (temperature and
hydrological changes). See Chapters 3, 4 and 13 for detailed assessment.
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1.3.4.1
Chapter 1
IPCC WGI Fifth Assessment Report
Sea Level
Global mean sea level is an important indicator of climate change (Section 3.7 and Chapter 13). The
previous assessments have all shown that observations indicate that the globally-averaged sea level is rising.
Direct observations of sea level change have been made for more than 150 years with tide gauges, and for
more than 20 years with satellite radar altimeters. Although there is regional variability from non-uniform
density change, circulation changes, and deformation of ocean basins, the evidence indicates that the global
mean sea level is rising, and that this is likely (according to AR4 and SREX) resulting from global climate
change (ocean warming plus land ice melt; see Chapter 13 for AR5 findings). The historical tide gauge
record shows that the average rate of global mean sea level rise over the 20th century was 1.7 ± 0.2 mm yr-1
(e.g., Church and White, 2011). This rate increased to 3.2 ± 0.4 mm yr-1 since 1990, mostly because of
increased thermal expansion and land ice contributions (Church and White, 2011; IPCC, 2012b). Although
the long-term sea level record shows decadal and multi-decadal oscillations, there is evidence that the rate of
global mean sea level rise during the 20th century was greater than during the 19th century.
All of the previous IPCC assessments have projected that global sea level will continue to rise throughout
this century for the scenarios examined. Figure 1.10 compares the observed sea level rise since 1950 with the
projections from the prior IPCC assessments. Earlier models had greater uncertainties in modelling the
contributions, because of limited observational evidence and deficiencies in theoretical understanding of
relevant processes. Also, projections for sea level change in the prior assessments are scenarios for the
response to anthropogenic forcing only; they do not include unforced or natural interannual variability.
Nonetheless, the results show that the actual change is in the middle of projected changes from the prior
assessments, and towards the higher end of the studies from TAR and AR4.
1.3.4.2
Ocean Acidification
The observed decrease in ocean pH resulting from increasing concentrations of carbon dioxide is another
indicator of global change. As discussed in AR4, the ocean’s uptake of carbon dioxide is having a significant
impact on the chemistry of sea water. The average pH of ocean surface waters has fallen by about 0.1 units,
from about 8.2 to 8.1 (total scale) since 1765 (Section 3.8). Long time series from several ocean sites show
ongoing declines in pH, consistent with results from repeated pH measurements on ship transects spanning
much of the globe (Section 3.8; Section 6.4; Byrne et al., 2010; Midorikawa et al., 2010). Ocean time-
series in the North Atlantic and North Pacific record a decrease in pH ranging between –0.0015 and
–0.0024 per year (Section 3.8). Due to the increased storage of carbon by the ocean, ocean acidification
will increase in the future (Chapter 6). In addition to other impacts of global climate change, ocean
acidification poses potentially serious threats to the health of the world’s oceans ecosystems (see AR5 WGII
assessment).
[INSERT FIGURE 1.10 HERE]
Figure 1.10: Estimated changes in the observed global annual mean sea level (GMSL) since 1950 relative to 1961–
1990. Estimated changes in global annual sea level anomalies are presented based on tide gauge data (Jevrejeva et al.,
2008 (warm mustard); Church and White, 2011 (dark blue); Ray and Douglas, 2011 (dark green)) and based on sea
surface altimetry (light blue). The altimetry data start in 1993 and are harmonized to start from the mean 1993 value of
the tide gauge data. Squares indicate annual mean values, solid lines smoothed values. The shading shows the largest
model projected range of global annual sea level rise from 1950 to 2035 for FAR (Figure 9.6 and Figure 9.7 in Warrick
and Oerlemans, 1990), SAR (Figure 21 in TS of IPCC, 1996), TAR (Appendix II of IPCC, 2001) and for Church et al.
(2011) based on the Coupled Model Intercomparison Project Phase 3 (CMIP3) model results not assessed at the time of
AR4 using the SRES B1, A1B, and A2 scenarios. Note that in the AR4 no full range was given for the sea level
projections for this period. Therefore, the figure shows results that have been published subsequent to the AR4. The
bars at the right hand side of the graph show the full range given for 2035 for each assessment report. For Church et al.
(2011) the mean sea level rise is indicated in addition to the full range. See Appendix 1.A for details on the data and
calculations used to create this figure.
1.3.4.3
Ice
Rapid sea ice loss is one of the most prominent indicators of Arctic climate change (Section 4.2). There has
been a trend of decreasing Northern Hemisphere sea ice extent since 1978, with the summer of 2012 being
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the lowest in recorded history (see Section 4.2 for details). The 2012 minimum sea ice extent was 49% below
the 1979 to 2000 average and 18% below the previous record from 2007. The amount of multiyear sea ice
has been reduced, i.e., the sea ice has been thinning and thus the ice volume is reduced (Haas et al., 2008;
Kwok et al., 2009). These changes make the sea ice less resistant to wind forcing. Sea ice extent has been
diminishing significantly faster than projected by most of the AR4 climate models (SWIPA, 2011). While
AR4 found no consistent trends in Antarctica sea ice, more recent studies indicate a small increase (Section
4.2). Various studies since AR4 suggest that this has resulted in a deepening of the low pressure systems in
West Antarctica that in turn caused stronger winds and enhanced ice production in the Ross Sea (Goosse et
al., 2009; Turner and Overland, 2009).
AR4 concluded that taken together, the ice sheets in Greenland and Antarctica have very likely been
contributing to sea level rise. The Greenland Ice Sheet has lost mass since the early 1990s and the rate of loss
has increased (see Section 4.4). The interior, high altitude areas are thickening due to increased snow
accumulation, but this is more than counterbalanced by the ice loss due to melt and ice discharge (AMAP,
2009; Ettema et al., 2009). Since 1979, the area experiencing surface melting has increased significantly
(Tedesco, 2007; Mernild et al., 2009), with 2010 breaking the record for surface melt area, runoff, and mass
loss, and the unprecedented areal extent of surface melt of the Greenland Ice Sheet in 2012 (Nghiem et al.,
2012). Overall, the Antarctic continent now experiences a net loss of ice (Section 4.4). Significant mass loss
has been occurring in the Amundsen Sea sector of West Antarctica and the northern Antarctic Peninsula. The
ice sheet on the rest of the continent is relatively stable or thickening slightly (Lemke et al., 2007; Scott et
al., 2009; Turner et al., 2009). Since AR4, there have been improvements in techniques of measurement,
such as gravity, altimetry and mass balance, and understanding of the change (Section 4.4).
As discussed in the earlier assessments, most glaciers around the globe have been shrinking since the end of
the Little Ice Age with increasing rates of ice loss since the early 1980s (Section 4.3). The vertical profiles of
temperature measured through the entire thickness of mountain glaciers, or through ice sheets, provide clear
evidence of a warming climate over recent decades (e.g., Lüthi and Funk, 2001; Hoelzle et al., 2011). As
noted in AR4, the greatest mass losses per unit area in the last four decades have been observed in the
Patagonia, Alaska, northwest USA, southwest Canada, the European Alps, and the Arctic. Alaska and the
Arctic are especially important regions as contributors to sea level rise (Zemp et al., 2008; Zemp et al.,
2009).
1.4
1.4.1
Treatment of Uncertainties
Uncertainty in Environmental Science
Science always involves uncertainties. These arise at each step of the scientific method: in measurements, in
the development of models or hypotheses, and in analyses and interpretation of scientific assumptions.
Climate science is not different in this regard from other areas of science. The complexity of the climate
system and the large range of processes involved bring particular challenges, since for example, gaps in
direct measurements of the past can only be filled by reconstructions using proxy-data.
Because the Earth’s climate system is characterized by multiple spatial and temporal scales, uncertainties do
not usually reduce at a single, predictable rate: for example, new observations may reduce the uncertainties
surrounding short timescale processes quite rapidly, while longer timescale processes may require very long
observational baselines before much progress can be made. Characterization of the interaction between
processes, as quantified by models, can be improved by model development, or can shed light on new areas
in which uncertainty is greater than previously thought. The fact that there is only a single realization of the
climate, rather than a range of different climates from which to draw, can matter significantly for certain
lines of enquiry, most notably for the detection and attribution of causes of climate change and for the
evaluation of projections of future states.
1.4.2
Characterizing Uncertainty
“Uncertainty” is a complex and multi-faceted property, sometimes originating in a lack of information, other
times from quite fundamental disagreements about what is known or even knowable (Moss and Schneider,
2000). Furthermore, scientists often disagree about the best or most appropriate way to characterize these
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uncertainties: some can be quantified easily while others cannot. Moreover, appropriate characterization is
dependent upon the intended use of the information and the particular needs of that user community.
Scientific uncertainty can be partitioned in various ways, in which the details of the partitioning usually
depend on the context. For instance, the process and classifications used for evaluating observational
uncertainty in climate science is not the same as that employed to evaluate projections of future change.
Uncertainty in measured quantities can arise from a range of sources, such as statistical variation, variability,
inherent randomness, inhomogeneity, approximation, subjective judgement, and linguistic imprecision
(Morgan et al., 1990), or from calibration methodologies, instrumental bias or instrumental limitations
(JCGM, 2008).
In the modelling studies that underpin projections of future climate change, it is common to partition
uncertainty into four main categories: scenario uncertainty, due to uncertainty of future emissions of
greenhouse gases and other forcing agents; “model uncertainty” associated with climate models; internal
variability and initial condition uncertainty; and forcing and boundary condition uncertainty for the
assessment of historical and paleoclimate simulations (e.g., Collins and Allen, 2002; Yip et al., 2011).
Model uncertainty is an important contributor to uncertainty in climate predictions and projections. It
includes, but is not restricted to, the uncertainties introduced by errors in the model's representation of
dynamical and physical and bio-geochemical aspects of the climate system as well as in the model's response
to external forcing. The phrase "model uncertainty" is a common term in the climate change literature, but
different studies use the phrase in different senses: some studies use the phrase to represent the range of
behaviour observed in ensembles of climate model (model spread), while other studies use it in more
comprehensive senses (see Sections 9.2, 11.2 and 12.2). Model spread is often used as a measure of climate
response uncertainty, but such a measure is crude as it takes no account of factors such as model quality
(Chapter 9) or model independence (e.g., Masson and Knutti, 2011; Pennell and Reichler, 2011), and not all
variables of interest are adequately simulated by global climate models.
To maintain a degree of terminological clarity this report distinguishes between “model spread” for this
narrower representation of climate model responses and “model uncertainty” which describes uncertainty
about the extent to which any particular climate model provides an accurate representation of the real climate
system. This uncertainty arises from approximations required in the development of models. Such
approximations affect the representation of all aspects of the climate including the response to external
forcings.
Model uncertainty is sometimes decomposed further into parametric and structural uncertainty, comprising,
respectively, uncertainty in the values of model parameters and uncertainty in the underlying model structure
(see Section 12.2). Some scientific research areas, such as detection and attribution and observationallyconstrained model projections of future climate, incorporate significant elements of both observational and
model-based science, and in these instances both sets of relevant uncertainties need to be incorporated.
Scenario uncertainty refers to the uncertainties that arise due to limitations in our understanding of future
emissions, concentration or forcing trajectories. Scenarios help in the assessment of future developments in
complex systems that are either inherently unpredictable, or that have high scientific uncertainties (IPCC,
2000). The societal choices defining future climate drivers are surrounded by considerable uncertainty, and
these are explored by examining the climate response to a wide range of possible futures. In past reports,
emissions scenarios from the SRES (IPCC, 2000) were used as the main way of exploring uncertainty in
future anthropogenic climate drivers. Recent research has made use of Representative Concentration
Pathways (RCP) (van Vuuren et al., 2011b; van Vuuren et al., 2011a).
Internal or natural variability, the natural fluctuations in climate, occur in the absence of any RF of the
Earth’s climate (Hawkins and Sutton, 2009). Climate varies naturally on nearly all time and space scales, and
quantifying precisely the nature of this variability is challenging, and is characterized by considerable
uncertainty. The analysis of internal and forced contributions to recent climate is discussed in Chapter 10.
The fractional contribution of internal variability compared with other forms of uncertainty varies in time
and in space, but usually diminishes with time as other sources of uncertainty become more significant
(Hawkins and Sutton, 2009; see also Chapter 11 and FAQ 1.1).
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In the WGI contribution to the AR5, uncertainty is quantified using 90% uncertainty intervals unless
otherwise stated. The 90% uncertainty interval, reported in square brackets, is expected to have a 90%
likelihood of covering the value that is being estimated. The value that is being estimated has a 5%
likelihood of exceeding the upper endpoint of the uncertainty interval, and the value has a 5% likelihood of
being less than that the lower endpoint of the uncertainty interval. A best estimate of that value is also given
where available. Uncertainty intervals are not necessarily symmetric about the corresponding best estimate.
In a subject as complex and diverse as climate change, the information available as well as the way it is
expressed, and often the interpretation of that material, varies considerably with the scientific context. In
some cases, two studies examining similar material may take different approaches even to the quantification
of uncertainty. The interpretation of similar numerical ranges for similar variables can differ from study to
study. Readers are advised to pay close attention to the caveats and conditions that surround the results
presented in peer-reviewed studies, as well as those presented in this assessment. To help readers in this
complex and subtle task, the IPCC draws on specific, calibrated language scales to express uncertainty
(Mastrandrea et al., 2010), as well as specific procedures for the expression of uncertainty (see Table 1.2).
The aim of these structures is to provide tools through which Chapter teams might consistently express
uncertainty in key results.
1.4.3
Treatment of Uncertainty in IPCC
In the course of the IPCC assessment procedure, Chapter teams review the published research literature,
document the findings (including uncertainties), assess the scientific merit of this information, identify the
key findings, and attempt to express an appropriate measure of the uncertainty that accompanies these
findings using a shared guidance procedure. This process has changed over time. The early Assessment
Reports (FAR and SAR) were largely qualitative. As the field has grown and matured, uncertainty is being
treated more explicitly, with a greater emphasis on the expression, where possible and appropriate, of
quantified measures of uncertainty.
Although IPCC’s treatment of uncertainty has become more sophisticated since the early reports, the rapid
growth and considerable diversity of climate research literature presents on-going challenges. In the wake of
the TAR the IPCC formed a Cross-Working Group team charged with identifying the issues and providing a
set of Uncertainty Guidance Notes that could provide a structure for consistent treatment of uncertainty
across the IPCC’s remit (Manning et al., 2004). These expanded on the procedural elements of Moss and
Schneider (2000) and introduced calibrated language scales designed to enable Chapter teams to use the
appropriate level of precision to describe findings. These notes were revised between the TAR and AR4 and
again between AR4 and AR5 (Mastrandrea et al., 2010).
Recently, increased engagement of social scientists (e.g., Patt and Schrag, 2003; Kandlikar et al., 2005;
Risbey and Kandlikar, 2007; Broomell and Budescu, 2009; Budescu et al., 2009; CCSP, 2009) and expert
advisory panels (CCSP, 2009; InterAcademy Council, 2010) in the area of uncertainty and climate change
has helped clarify issues and procedures to improve presentation of uncertainty. Many of the
recommendations of these groups are addressed in the revised Guidance Notes. One key revision relates to
clarification of the relationship between the “confidence” and “likelihood” language, and pertains to
demarcation between qualitative descriptions of “confidence” and the numerical representations of
uncertainty that are expressed by the likelihood scale. Additionally, a finding that includes a probabilistic
measure of uncertainty does not require explicit mention of the level of confidence associated with that
finding if the level of confidence is “high” or “very high”. This is a concession to stylistic clarity and
readability: if something is described as having a high likelihood, then in the absence of additional qualifiers
it should be inferred that it also has high or very high confidence.
1.4.4
Uncertainty Treatment in This Assessment
All three IPCC Working Groups in the AR5 have agreed to use two metrics for communicating the degree of
certainty in key findings (Mastrandrea et al., 2010):
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•
•
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confidence in the validity of a finding, based on the type, amount, quality, and consistency of
evidence (e.g., mechanistic understanding, theory, data, models, expert judgment) and the degree of
agreement. Confidence is expressed qualitatively;
quantified measures of uncertainty in a finding expressed probabilistically (based on statistical
analysis of observations or model results, or expert judgment).
A level of confidence synthesizes the Chapter teams’ judgments about the validity of findings as determined
through evaluation of the available evidence and the degree of scientific agreement. The evidence and
agreement scale underpins the assessment, since it is on the basis of evidence and agreement that statements
can be made with scientific confidence (in this sense, the evidence and agreement scale replaces the “level of
scientific understanding” scale used in previous WGI assessments). There is flexibility in this relationship;
for a given evidence and agreement statement, different confidence levels could be assigned, but increasing
levels of evidence and degrees of agreement are correlated with increasing confidence. Confidence cannot
necessarily be assigned for all combinations of evidence and agreement, but where key variables are highly
uncertain, the available evidence and scientific agreement regarding that variable is presented and discussed.
Confidence should not be interpreted probabilistically, and it is distinct from “statistical confidence”.
The confidence level is based on the evidence (robust, medium, and limited) and the agreement (high,
medium, and low). A combination of different methods, e.g., observations and modeling, is important for
evaluating the confidence level. Figure 1.11 shows how the combined evidence and agreement results in five
levels for the confidence level used in this assessment.
[INSERT FIGURE 1.11]
Figure 1.11: The basis for the confidence level is given as a combination of evidence (limited, medium, robust) and
agreement (low, medium, and high) (Mastrandrea et al., 2010).
The qualifier “likelihood” provides calibrated language for describing quantified uncertainty. It can be used
to express a probabilistic estimate of the occurrence of a single event or of an outcome, e.g., a climate
parameter, observed trend, or projected change lying in a given range. Statements made using the likelihood
scale may be based on statistical or modelling analyses, elicitation of expert views, or other quantitative
analyses. Where sufficient information is available it is preferable to eschew the likelihood qualifier in
favour of the full probability distribution or the appropriate probability range. See Table 1.2 for the list of
“likelihood” qualifiers to be used in AR5.
Table 1.2: Likelihood terms associated with outcomes used in the AR5.
Term
Likelihood of the Outcome
Virtually certain
99−100% probability
Very likely
90−100% probability
Likely
66−100% probability
About as likely as not
33−66% probability
Unlikely
0−33% probability
Very unlikely
0−10% probability
Exceptionally unlikely
0−1% probability
Notes:
Additional terms that were used in limited circumstances in the AR4 (extremely likely = 95−100% probability, more
likely than not = >50−100% probability, and extremely unlikely = 0−5% probability) may also be used in the AR5
when appropriate.
Many social sciences studies have found that the interpretation of uncertainty is contingent upon the
presentation of information, the context within which statements are placed, and the interpreter’s own lexical
preferences. Readers often adjust their interpretation of probabilistic language according to the magnitude of
perceived potential consequences (Patt and Schrag, 2003; Patt and Dessai, 2005). Furthermore, the framing
of a probabilistic statement impinges on how it is interpreted (Kahneman and Tversky, 1979): e.g., a 10%
chance of dying is interpreted more negatively than a 90% chance of surviving.
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In addition, work examining expert judgment and decision making shows that people—including scientific
experts—suffer from a range of heuristics and biases that affect their judgment (e.g., Kahneman et al., 1982).
For example, in the case of expert judgments there is a tendency towards overconfidence both at the
individual level (Morgan et al., 1990) and at the group level as people converge on a view and draw
confidence in its reliability from each other. However, in an assessment of the state of scientific knowledge
across a field such as climate change—characterized by complexity of process and heterogeneity of data
constraints—some degree of expert judgment is inevitable (Mastrandrea et al., 2010).
These issues were brought to the attention of Chapter teams so that contributors to the AR5 might be
sensitized to the ways presentation, framing, context and potential biases might affect their own assessments
and might contribute to readers’ understanding of the information presented in this assessment. There will
always be room for debate about how to summarize such a large and growing literature. The uncertainty
guidance is aimed at providing a consistent, calibrated set of words through which to communicate the
uncertainty, confidence and degree of consensus prevailing in the scientific literature. In this sense the
guidance notes and practices adopted by IPCC for the presentation of uncertainties should be regarded as an
interdisciplinary work in progress, rather than as a finalized, comprehensive approach. Moreover, one
precaution that should be considered is that translation of this assessment from English to other languages
may lead to a loss of precision.
1.5
Advances in Measurement and Modelling Capabilities
Since AR4, measurement capabilities have continued to advance. The models have been improved following
the progress in the understanding of physical processes within the climate system. This section illustrates
some of those developments.
1.5.1
Capabilities of Observations
Improved understanding and systematic monitoring of Earth’s climate requires observations of various
atmospheric, oceanic and terrestrial parameters and therefore has to rely on various technologies (ranging
from ground-based instruments to ships, buoys, ocean profilers, balloons, aircraft, satellite-borne sensors,
etc.). The Global Climate Observing System (GCOS, 2009) defined a list of so-called Essential Climate
Variables, that are technically and economically feasible to observe, but some of the associated observing
systems are not yet operated in a systematic manner. However, during recent years, new observational
systems have increased the number of observations by orders of magnitude and observations have been made
at places where there have been no data before (see Chapters 2, 3, and 4 for an assessment of changes in
observations). Parallel to this, tools to analyse and process the data have been developed and enhanced to
cope with the increase of information and to provide a more comprehensive picture of the Earth's climate. At
the same time, it should be kept in mind that there has been some limited progress in developing countries in
filling gaps in their in situ observing networks, but developed countries have made little progress in ensuring
long-term continuity for several important observing systems (GCOS, 2009). Additionally, more proxy (noninstrumental) data have been acquired to provide a more comprehensive picture of climate changes in the
past (see Chapter 5). Efforts are also occurring to digitize historic observations, mainly of ground-station
data from periods prior to the second half of the 20th century (Brunet and Jones, 2011).
Reanalysis is a systematic approach to produce gridded dynamically consistent data sets for climate
monitoring and research by assimilating all available observations with help of a climate model (Box 2.3).
Model-based reanalysis products play an important role in obtaining a consistent picture of the climate
system. However, their usefulness in detecting long-term climate trends is currently limited by changes over
time in observational coverage and biases, linked to the presence of biases in the assimilating model (see also
Box 2.3 in Chapter 2). Since AR4 both the quantity and quality of the observations that are assimilated
through reanalysis have increased (GCOS, 2009). As an example, there has been some overall increase in
mostly-atmospheric observations assimilated in European Centre for Medium-Range Weather Forecasts
Interim Reanalysis since 2007 (Dee et al., 2011). The overwhelming majority of the data, and most of the
increase over recent years, come from satellites (Figure 1.12) (GCOS, 2011). For example, information from
Global Positioning System radio occultation measurements has increased significantly since 2007. The
increases in data from fixed stations are often associated with an increased frequency of reporting, rather
than an increase in the number of stations. Increases in data quality come from improved instrument design,
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or more accurate correction in the ground-station processing that is applied before the data are transmitted to
users and data centres. As an example for in-situ data, temperature biases of radiosonde measurements from
radiation effects have been reduced over recent years. The new generation of satellite sensors such as the
high spectral resolution infrared sounders (such as the Atmospheric Infrared Sounder and the Infrared
Atmospheric Sounding Interferometer) are instrumental to achieving a better temporal stability for
recalibrating sensors like the High-Resolution Infrared Radiation Sounder. Few instruments (e.g., the
Advanced Very High Resolution Radiometer) have now been in orbit for about three decades, but these have
originally not been designed for climate applications and therefore require careful re-calibration.
A major achievement in ocean observation is due to the implementation of the Argo global array of profiling
floats system (GCOS, 2009). Deployment of Argo floats began in 2000, but it took until 2007 for numbers to
reach the design target of 3000 floats. Since 2000 the ice-free upper 2000 m of the ocean have been observed
systematically for temperature and salinity for the first time in history, because both the Argo profiling float
and surface drifting buoy arrays have reached global coverage at their target numbers (in January 2009, there
were 3291 floats operating). Biases in historical ocean data have been identified and reduced, and new
analytical approaches have been applied (e.g., Willis et al., 2009). One major consequence has been the
reduction of an artificial decadal variation in upper ocean temperature and heat content that was apparent in
the observational assessment for AR4 (see Section 3.2). The spatial and temporal coverage of
biogeochemical measurements in the ocean has also expanded. Satellite observations for sea level (Sections
3.7 and 13.2), sea surface salinity (Section 3.3), sea ice (Section 4.2), and ocean colour have also been
further developed over the past few years.
Progress has also been made with regard to observation of terrestrial Essential Climate Variables. Major
advances have been achieved in remote sensing of soil moisture due to the launch of the Soil Moisture and
Oceanic Salinity mission in 2009 but also due to new retrieval techniques that have been applied to data from
earlier and ongoing missions (see Seneviratne et al., 2010 for a detailed review). However, these
measurements have limitations. For example the methods fail under dense vegetation and they are restricted
to the surface soil. Updated Advanced Very High Resolution Radiometer-based Normalized Differenced
Vegetation Index data provide new information on the change in vegetation. During the International Polar
Year 2007–2009 the number of borehole sites was significantly increased and therefore allows a better
monitoring of the large-scale permafrost features (see Section 4.7).
[INSERT FIGURE 1.12 HERE]
Figure 1.12: Development of capabilities of observations. Top: Changes in the mix and increasing diversity of
observations over time create challenges for a consistent climate record (adapted from Brönnimann et al., 2008).
Bottom left: First year of temperature data in global historical climatology network daily database (available at
http://www.ncdc.noaa.gov/oa/climate/ghcn-daily/; Menne et al., 2012). Bottom right: Number of satellite instruments
from which data have been assimilated in the European Centre for Medium-Range Weather Forecasts production
streams for each year from 1996 to 2010. This figure is used as an example to demonstrate the fivefold increase in the
usage of satellite data over this time period.
1.5.2
Capabilities in Global Climate Modelling
Several developments have especially pushed the capabilities in modelling forward over recent years (see
Figure 1.13 and a more detailed discussion in Chapter 6, Chapter 7 and Chapter 9).
There has been a continuing increase in horizontal and vertical resolution. This is especially seen in how the
ocean grids have been refined, and sophisticated grids are now used in the ocean and atmosphere models
making optimal use of parallel computer architectures. More models with higher resolution are available for
more regions. Figure 1.14a and 1.14b show the large effect on surface representation from a horizontal grid
spacing of 87.5 km (similar to the current global models) to a grid spacing of 30.0 km (similar to the current
regional climate models).
Representations of Earth system processes are much more extensive and improved, particularly for the
radiation and the aerosol cloud interactions and for the treatment of the cryosphere. The representation of the
carbon cycle was added to a larger number of models and has been improved since AR4. A high resolution
stratosphere is now included in many models. Other ongoing process development in climate models
includes the enhanced representation of nitrogen effects on the carbon cycle. As new processes or treatments
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are added to the models, they are also evaluated and tested relative to available observations (see Chapter 9
for more detailed discussion).
Ensemble techniques (multiple calculations to increase the statistical sample, to account for natural
variability, and to account for uncertainty in model formulations) are being used more frequently, with larger
samples and with different methods to generate the samples (different models, different physics, different
initial conditions). Coordinated projects have been set up to generate and distribute large samples
(ENSEMBLES, climateprediction.net, Program for Climate Model Diagnosis and Intercomparison).
The model comparisons with observations have pushed the analysis and development of the models. CMIP5,
an important input to the AR5, has produced a multi-model dataset that is designed to advance our
understanding of climate variability and climate change. Building on previous CMIP efforts, such as the
CMIP3 model analysis reported in AR4, CMIP5 includes “long-term” simulations of 20th century climate
and projections for the 21st century and beyond. See Chapters 9, 10, 11 and 12 for more details on the results
derived from the CMIP5 archive.
Since AR4, the incorporation of "long-term" paleoclimate simulations in the CMIP5 framework has allowed
incorporation of information from paleoclimate data to inform projections. Within uncertainties associated
with reconstructions of past climate variables from proxy records and forcings, paleoclimate information
from the Mid Holocene, Last Glacial Maximum, and Last Millennium have been used to test the ability of
models to simulate realistically the magnitude and large-scale patterns of past changes (Section 5.3, Box 5.1
and 9.4).
The capabilities of Earth System Models continue to be enhanced. For example, there are currently extensive
efforts towards developing advanced treatments for the processes affecting ice sheet dynamics. Other
enhancements are being aimed at land surface hydrology, and the effects of agriculture and urban
environments.
As part of the process of getting model analyses for a range of alternative assumptions about how the future
may unfold, scenarios for future emissions of important gases and aerosols have been generated for the IPCC
assessments (e.g., see the SRES scenarios used in TAR and AR4). The emissions scenarios represent various
development pathways based on well-defined assumptions. The scenarios are used to calculate future
changes in climate, and are then archived in the Climate Model Intercomparison Project (e.g., CMIP3 for
AR4; CMIP5 for AR5). For CMIP5, four new scenarios, referred to as Representative Concentration
Pathways (RCPs) were developed (Section 12.3; Moss et al., 2010). See Box 1.1 for a more thorough
discussion of the RCP scenarios. Since results from both CMIP3 and CMIP5 will be presented in the later
chapters (e.g., Chapters 8, 9, 11 and 12), it is worthwhile considering the differences and similarities between
the SRES and the RCP scenarios. Figure 1.15, acting as a prelude to the discussion in Box 1.1, shows that
the RF for several of the SRES and RCP scenarios are similar over time and thus should provide results that
can be used to compare climate modelling studies.
[INSERT FIGURE 1.13 HERE]
Figure 1.13: The development of climate models over the last 35 years showing how the different components were
coupled into comprehensive climate models over time. In each aspect (e. g. the atmosphere, which comprises a wide
range of atmospheric processes) the complexity and range of processes has increased over time (illustrated by growing
cylinders). Note that during the same time the horizontal and vertical resolution has increased considerably e.g., for
spectral models from T21L9 (roughly 500 km horizontal resolution and 9 vertical levels) in the 1970s to T95L95
(roughly 100 km horizontal resolution and 95 vertical levels) at present, and that now ensembles with at least three
independent experiments can be considered as standard.
[INSERT FIGURE 1.14 HERE]
Figure 1.14: Horizontal resolutions considered in today’s higher resolution models and in the very high resolution
models now being tested: a) Illustration of the European topography at a resolution of 87.5 x 87.5 km; b) same as a) but
for a resolution of 30.0 x 30.0 km.
[INSERT FIGURE 1.15 HERE]
Figure 1.15: Historical and projected total anthropogenic RF (W m–2) relative to preindustrial (~1765) between 1950
and 2100. Previous IPCC assessments (SAR IS92a, TAR/AR4 SRES A1B, A2 and B1) are compared with
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representative concentration pathway (RCP) scenarios (see Chapter 12 and Box 1.1 for their extensions until 2300 and
Annex II for the values shown here). The total RF of the three families of scenarios, IS92, SRES and RCP, differ for
example for the year 2000, resulting from the knowledge about the emissions assumed having changed since the TAR
and AR4.
[START BOX 1.1 HERE]
Box 1.1: Description of Future Scenarios
Long term climate change projections require assumptions on human activities or natural effects that could
alter the climate over decades and centuries. Defined scenarios are useful for a variety of reasons, e.g.,
assuming specific time series of emissions, land-use, atmospheric concentrations or RF across multiple
models allows for coherent climate model intercomparisons and synthesis. Scenarios can be formed in a
range of ways, from simple, idealized structures to inform process understanding, through to comprehensive
scenarios produced by Integrated Assessment Models as internally consistent sets of assumptions on
emissions and socio-economic drivers (e.g., regarding population and socio-economic development).
Idealized Concentration Scenarios
As one example of an idealized concentration scenario, a 1%-yr-1 compound increase of atmospheric CO2
concentration until a doubling or a quadrupling of its initial value has been widely used in the past (Covey et
al., 2003). An exponential increase of CO2 concentrations induces an essentially linear increase in RF
(Myhre et al., 1998) due to a ‘saturation effect’ of the strong absorbing bands. Such a linear ramp function is
highly useful for comparative diagnostics of models’ climate feedbacks and inertia. The CMIP5
intercomparison project again includes such a stylized pathway up to a quadrupling of CO2 concentrations, in
addition to an instantaneous quadrupling case.
The Socio-Economic Driven SRES Scenarios
The SRES suite of scenarios were developed using Integrated Assessment Models and resulted from specific
socio-economic scenarios from storylines about future demographic and economic development,
regionalization, energy production and use, technology, agriculture, forestry, and land-use (IPCC, 2000). The
climate change projections undertaken as part of CMIP3 and discussed in AR4 were primarily based on the
SRES A2, A1B and B1 scenarios. However, given the diversity in models’ carbon cycle and chemistry
schemes, this approach implied differences in models’ long lived greenhouse gas and aerosol concentrations
for the same emissions scenario. As a result of this and other shortcomings, revised scenarios were
developed for AR5 to allow atmosphere-ocean general circulation model (AOGCM) (using concentrations)
simulations to be compared with those Earth system model (ESM) simulations that use emissions to
calculate concentrations.
Representative Concentration Pathway Scenarios and their Extensions
Representative Concentration Pathway (RCP) scenarios (see Section 12.3 for a detailed description of the
scenarios; Moss et al., 2008; Moss et al., 2010; van Vuuren et al., 2011b) are new scenarios that specify
concentrations and corresponding emissions, but are not directly based on socio-economic storylines like the
SRES scenarios. The RCP scenarios are based on a different approach and include more consistent shortlived gases and land use changes. They are not necessarily more capable of representing future developments
than the SRES scenarios. Four RCP scenarios were selected from the published literature (Fujino et al.,
2006; Smith and Wigley, 2006; Riahi et al., 2007; van Vuuren et al., 2007; Hijioka et al., 2008; Wise et al.,
2009) and updated for use within CMIP5 (Masui et al., 2011; Riahi et al., 2011; Thomson et al., 2011; van
Vuuren et al., 2011a). The four scenarios are identified by the 21st century peak or stabilization value of the
RF derived by the reference model (in W m-2) (Box 1.1, Figure 1): the lowest RCP, RCP2.6 (also referred to
as RCP3-PD) which peaks at 3 W m-2 and then declines to approximately 2.6 W m-2 by 2100; the mediumlow RCP4.5 and the medium-high RCP6 aiming for stabilization at 4.5 and 6 W m-2, respectively around
2100; and the highest one, RCP8.5, which implies a RF of 8.5 W m-2 by 2100, but implies rising RF beyond
that date (Moss et al., 2010). In addition there is a supplementary extension SCP6to4.5 with an adjustment of
emissions after 2100 to reach RCP 4.5 concentration levels in 2250 and thereafter. The RCPs span the full
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range of RF associated with emission scenarios published in the peer-reviewed literature at the time of the
development of the RCPs, and the two middle scenarios where chosen to be roughly equally spaced between
the two extremes (2.6 and 8.5). These forcing values should be understood as comparative labels
representative of the forcing associated with each scenario, which will vary somewhat from model to model.
This is because concentrations or emissions (rather than the RF) are prescribed in the CMIP5 climate model
runs.
[INSERT BOX 1.1, FIGURE 1 HERE]
Box 1.1, Figure 1: Total RF (anthropogenic plus natural) for RCPs and extended concentration pathways (ECP) − for
RCP2.6, RCP4.5, and RCP6, RCP8.5, as well as a supplementary extension SCP6to4.5 with an adjustment of emissions
after 2100 to reach RCP 4.5 concentration levels in 2250 and thereafter. Note that the stated RF levels refer to the
illustrative default median estimates only. There is substantial uncertainty in current and future RF levels for any given
scenario. Short-term variations in RF are due to both volcanic forcings in the past (1800–2000) and cyclical solar
forcing assuming a constant 11-year solar cycle (following the CMIP5 recommendation), except at times of
stabilization (reproduced from Figure 4 in Meinshausen et al., 2011).
Various steps were necessary to turn the selected ‘raw’ RCPs into emission scenarios from Integrated
Assessment Models and to turn these into datasets usable by the climate modelling community. , including
the extension with historical emissions (Granier et al., 2011; Meinshausen et al., 2011), the harmonization
(smoothly connected historical reconstruction) and gridding of land-use datasets (Hurtt et al., 2011), the
provision of atmospheric chemistry modelling studies, particularly for tropospheric ozone (Lamarque et al.,
2011), analyses of 2000–2005 GHG emission levels, and extension of GHG concentrations with historical
GHG concentrations and harmonization with analyses of 2000–2005 GHG concentrations levels
(Meinshausen et al., 2011). The final RCP datasets comprise land-use data, harmonized GHG emissions and
concentrations, gridded reactive gas and aerosol emissions, as well as ozone and aerosol abundance fields
(Box 1.1, Figure 2, Figure 3, and Figure 4).
[INSERT BOX 1.1, FIGURE 2 HERE]
Box 1.1, Figure 2: Concentrations of GHG following the 4 RCPs and their extensions (ECP) to 2300. (Reproduced
from Figure 5 in Meinshausen et al., 2011). Also see Annex II Table AII.4.1 for CO2, Table AII.4.2 for CH4, Table
AII.4.3 for N2O.
[INSERT BOX 1.1, FIGURE 3 HERE]
Box 1.1, Figure 3: (a) Equivalent CO2 concentration and (b) CO2 emissions (except land use emissions) for the four
RCPs and their ECPs as well as some SRES scenarios.
[INSERT BOX 1.1, FIGURE 4 HERE]
Box 1.1, Figure 4: (a) Anthropogenic BC emissions (Annex II Table AII.2.22), (b) Anthropogenic NOx emissions
(Annex II Table AII.2.18), and (c) Anthropogenic SOx emissions (Annex II Table II.2.20).
To aid model understanding of longer-term climate change implications, these RCPs were extended until
2300 (Meinshausen et al., 2011) under reasonably simple and somewhat arbitrary assumptions regarding
post-2100 GHG emissions and concentrations. In order to continue to investigate a broad range of possible
climate futures, the two outer RCPs, RCP2.6 and RCP8.5 assume constant emissions after 2100, while the
two middle RCPs aim for a smooth stabilization of concentrations by 2150. RCP8.5 stabilizes concentrations
only by 2250, with CO2 concentrations of approximately 2000 ppm, nearly 7 times the pre-industrial levels.
As the RCP2.6 implies net negative CO2 emissions after around 2070 and throughout the extension, CO2
concentrations are slowly reduced towards 360 ppm by 2300.
Comparison of SRES and RCP Scenarios
The four RCP scenarios used in CMIP5 lead to RF values that span a range larger than that of the three
SRES scenarios used in CMIP3 (Figure 12.3.3). RCP4.5 is close to SRES B1, RCP6 is close to SRES A1B
(more after 2100 than during the 21st century) and RCP8.5 is somewhat higher than A2 in 2100 and close to
the SRES A1FI scenario (Box 1.1, Figure 3). RCP2.6 is lower than any of the SRES scenarios (see also
Figure 1.15).
[END BOX 1.1 HERE]
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1.6
Chapter 1
IPCC WGI Fifth Assessment Report
Overview and Road Map to the Rest of the Report
As this chapter has shown, understanding of the climate system and the changes occurring in it continue to
advance. The notable scientific advances and associated peer-reviewed publications since AR4 provide the
basis for the assessment of the science as found in Chapters 2 to 14. Below a quick summary of these
chapters and their objectives is provided.
Observations and Paleoclimate Information (Chapters 2, 3, 4, and 5): These chapters assess information
from all climate system components on climate variability and change as obtained from instrumental records
and climate archives. This group of chapters covers all relevant aspects of the atmosphere including the
stratosphere, the land surface, the oceans, and the cryosphere. Information on the water cycle, including
evaporation, precipitation, runoff, soil moisture, floods, drought, etc. is assessed. Timescales from daily to
decades (Chapters 2, 3 and 4) and from centuries to many millennia (Chapter 5) are considered.
Process Understanding (Chapters 6 and 7): These chapters cover all relevant aspects from observations and
process understanding, to projections from global to regional scale. Chapter 6 covers the carbon cycle and its
interactions with other biogeochemical cycles, in particular the nitrogen cycle, as well as feedbacks on the
climate system. Chapter 7 treats in detail clouds and aerosols, their interactions and chemistry, the role of
water vapour, as well as their role in feedbacks on the climate system.
From Forcing to Attribution of Climate Change (Chapters 8, 9, 10): In these chapters, all the information on
the different drivers (natural and anthropogenic) of climate change is collected, expressed in terms of RF,
and assessed (Chapter 8). As part of this, the science of metrics commonly used in the literature to compare
radiative effects from a range of agents (Global Warming Potential, Global Temperature Change Potential
and others) is covered. In Chapter 9, the hierarchy of climate models used in simulating past and present
climate change is assessed. Information regarding detection and attribution of changes on global to regional
scales is assessed in Chapter 10.
Future Climate Change and Predictability (Chapters 11 and 12): These chapters assess projections of future
climate change derived from climate models on time scales from decades to centuries at both global and
regional scales, including mean changes, variability and extremes. Fundamental questions related to the
predictability of climate as well as long term climate change, climate change commitments, and inertia in the
climate system are addressed.
Integration (Chapters 13 and 14): These chapters integrate all relevant information for two key topics in
WGI AR5: sea level change (Chapter 13) and climate phenomena across the regions (Chapter 14). Chapter
13 assesses information on sea level change ranging from observations and process understanding to
projections from global to regional scales. Chapter 14 assesses the most important modes of variability in the
climate system and extreme events. Furthermore, this chapter deals with interconnections between the
climate phenomena, their regional expressions, and their relevance for future regional climate change. Maps
produced and assessed in Chapter 14, together with Chapters 11 and 12, form the basis of the Atlas of Global
and Regional Climate Projections in Annex I. RFs and estimates of future atmospheric concentrations from
Chapters 7, 8, 11, and 12 form the basis of the Climate System Scenario Tables in Annex II.
1.6.1
Topical Issues
A number of topical issues are discussed throughout the assessment. These issues include those of areas
where there is contention in the peer-reviewed literature and where questions have been raised that are being
addressed through ongoing research. Table 1.3 provides a non-comprehensive list of many of these and the
chapters where they are discussed.
Table 1.3: Key topical issues discussed in the assessment.
Topic
Section
Abrupt change and irreversibility
5.7, 12.5, 13.4
Aerosols
6.4, 7.3, 7.4, 7.5, 7.6, 8.3, 11.3, 14.1
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Antarctic climate change
Arctic sea ice change
Hydrological cycle changes
Carbon-climate feedbacks
Climate sensitivity
Climate stabilization
Cloud feedbacks
Cosmic ray effects on clouds
Decadal climate variability
Earth's Energy (trends, distribution and budget)
El Nino-Southern Oscillation
Geo-engineering
Glacier change
Ice sheet dynamics and mass balance assessment
Monsoons
Ocean acidification
Permafrost change
Solar effects on climate change
Sea level change, including regional effects
Temperature trends since 1998
Tropical cyclones
Upper troposphere temperature trends
Chapter 1
IPCC WGI Fifth Assessment Report
5.8, 9.4, 10.3, 13.3
4.2, 5.5, 9.4, 10.3, 11.3, 12.4
2.5, 2.6, 3.3, 3.4, 3.5, 7.6, 10.3, 12.4
6.4, 12.4
5.3, 9.7, 10.8, 12.5
6.3, 6.4, 12.5
5.3, 7.2, 9.7, 11.3, 12.4
7.4
5.3, 9.1, 10.3
2.3, 3.2, 13.3
2.7, 5.4, 9.4, 9.5, 14.4
6.4, 7.7
4.3, 5.5, 10.5, 13.3
4.4, 5.3, 5.6, 10.5, 13.3
2.7, 5.5, 9.5, 14.2
3.8, 6.4
4.7, 6.3, 10.5
5.2, 8.4
3.7, 5.6, 13.1
2.4, 3.2, 10.3
2.6, 10.6, 14.6
2.4, 9.4
[START FAQ 1.1 HERE]
FAQ 1.1: If Understanding of the Climate System Has Increased, Why Hasn’t the Range of
Temperature Projections Been Reduced?
The models used to calculate the IPCC’s temperature projections agree on the direction of future global
change, but the projected size of those changes cannot be precisely predicted. Future greenhouse gas
emission rates could take any one of many possible trajectories, and some underlying physical processes are
not yet completely understood, making them difficult to model. Those uncertainties, combined with natural
year-to-year climate variability, produce an ‘uncertainty range’ in temperature projections.
The uncertainty range around projected greenhouse gas and aerosol precursor emissions, (which depend on
predictions of future social and economic conditions) cannot be materially reduced. Nevertheless, improved
understanding and climate models—along with observational constraints—may reduce the uncertainty range
around some factors that influence the climate’s response to those emission changes. The complexity of the
climate system, however, makes this a slow process. (FAQ1.1, Figure 1).
Climate science has made many important advances since the last IPCC assessment report, thanks to
improvements in measurements and data analysis in the cryosphere, atmosphere, land, biosphere and ocean
systems. Scientists also have better understanding and tools to model the role of clouds, sea ice, aerosols,
small-scale ocean mixing, the carbon cycle and other processes. More observations mean that models can
now be evaluated more thoroughly, and projections can be better constrained. For example, as models and
observational analysis have improved, projections of sea level rise have become more accurate, balancing the
current sea level rise budget.
Despite these advances, there is still a range in plausible projections for future global and regional climate—
what scientists call an ‘uncertainty range’. These uncertainty ranges are specific to the variable being
considered (precipitation vs. temperature, for instance) and the spatial and temporal extent (such as regional
vs. global averages). Uncertainties in climate projections arise from natural variability and uncertainty
around the rate of future emissions and the climate's response to them. They can also occur because
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representations of some known processes are as yet unrefined, and because some processes are not included
in the models.
There are fundamental limits to just how precisely annual temperatures can be projected, because of the
chaotic nature of the climate system. Furthermore, decadal-scale projections are sensitive to prevailing
conditions—such as the temperature of the deep ocean—that are less well known. Some natural variability
over decades arises from interactions between the ocean, atmosphere, land, biosphere and cryosphere, and is
also linked to phenomena such as the El Niño-Southern Oscillation (ENSO) and the North Atlantic
Oscillation (see Box 2.5 for details on patterns and indices of climate variability).
Volcanic eruptions and variations in the sun's output also contribute to natural variability, although they are
externally-forced and explainable. This natural variability can be viewed as part of the ‘noise’ in the climate
record, which provides the backdrop against which the ‘signal’ of anthropogenic climate change is detected.
Natural variability has a greater influence on uncertainty at regional and local scales than it does over
continental or global scales. It is inherent in the Earth system, and more knowledge will not eliminate the
uncertainties it brings. However, some progress is possible—particularly for projections up to a few years
ahead—which exploit advances in knowledge of, for instance, the cryosphere or ocean state and processes.
This is an area of active research. When climate variables are averaged over decadal time scales or longer,
the relative importance of internal variability diminishes, making the long-term signals more evident
(FAQ1.1, Figure 1). This long-term perspective is consistent with a common definition of climate as an
average over 30 years.
A second source of uncertainty stems from the many possible trajectories that future emission rates of
greenhouse gases and aerosol precursors might take, and from future trends in land use. Nevertheless,
climate projections rely on input from these variables. So to obtain these estimates, scientists consider a
number of alternative scenarios for future human society, in terms of population, economic and technological
change, and political choices. They then estimate the likely emissions under each scenario. The IPCC
informs policy making, therefore climate projections for different emissions scenarios can be useful as they
show the possible climatic consequences of different policy choices. These scenarios are intended to be
compatible with the full range of emissions scenarios described in the current scientific literature, with or
without climate policy. As such, they are designed to sample uncertainty in future scenarios.
Projections for the next few years and decades are sensitive to emissions of short-lived compounds such as
aerosols and methane. More distant projections, however, are more sensitive to alternative scenarios around
long-lived greenhouse gas emissions. These scenario-dependent uncertainties will not be reduced by
improvements in climate science, and will become the dominant uncertainty in projections over longer time
scales (e.g., 2100) (FAQ 1.1, Figure 1).
The final contribution to the uncertainty range comes from our imperfect knowledge of how the climate will
respond to future anthropogenic emissions and land use change. Scientists principally use computer-based
global climate models to estimate this response. A few dozen global climate models have been developed by
different groups of scientists around the world. All models are built on the same physical principles, but
some approximations are needed because the climate system is so complex. Different groups choose slightly
different approximations to represent specific processes in the atmosphere, such as clouds. These choices
produce differences in climate projections from different models. This contribution to the uncertainty range
is described as ‘response uncertainty’ or ‘model uncertainty’.
The complexity of the Earth system means that future climate could follow many different scenarios, yet still
be consistent with current understanding and models. As observational records lengthen and models
improve, researchers will be able to narrow that range in probable temperature for the next few decades
(FAQ 1.1, Figure 1). It is also possible to use information about the current state of the oceans and
cryosphere to produce better projections up to a few years ahead.
As science improves, new geophysical processes can be added to climate models, and representations of
those already included can be improved. These developments can appear to increase model-derived estimates
of climate response uncertainty, but such increases merely reflect the quantification of previously
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unmeasured sources of uncertainty (FAQ1.1, Figure 1). As more and more important processes are added,
the influence of unquantified processes lessens, and there can be more confidence in the projections.
[INSERT FAQ1.1, FIGURE 1 HERE]
FAQ 1.1, Figure 1: Schematic diagram showing the relative importance of different uncertainties, and their evolution
in time. a) Decadal mean surface temperature change (C) from the historical record (black line), with climate model
estimates of uncertainty for historical period (grey), along with future climate projections and uncertainty. Values are
normalised by means from 1961 to 1980. Natural variability (orange) derives from model interannual variability, and is
assumed constant with time. Emission uncertainty (green) is estimated as the model mean difference in projections from
different scenarios. Climate response uncertainty (blue-solid) is based on climate model spread, along with added
uncertainties from the carbon cycle, as well as rough estimates of additional uncertainty from poorly-modelled
processes. Based on Hawkins and Sutton (2011) and Huntingford et al. (2009). b) Climate response uncertainty can
appear to increase when a new process is discovered to be relevant, but such increases reflect a quantification of
previously unmeasured uncertainty, or c) can decrease with additional model improvements and observational
constraints. The given uncertainty range of 90% means that the temperature is estimated to be in that range, with a
probability of 90%.
[END FAQ 1.1 HERE]
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Appendix 1.A: Notes and Technical Details on Figures Displayed in Chapter 1
Figure 1.4
Documentation of Data Sources
Observed Temperature
NASA GISS evaluation of the observations: Hansen et al. (2010) updated: The data were downloaded from
http://data.giss.nasa.gov/gistemp/tabledata_v3/GLB.Ts+dSST.txt. Annual means are used (January to
December) and anomalies are calculated relative to 1961–1990.
NOAA NCDC evaluation of the observations: Smith et al. (2008) updated: The data were downloaded from
ftp://ftp.ncdc.noaa.gov/pub/data/anomalies/annual.land_ocean.90S.90N.df_1901–2000mean.dat. Annual
mean anomalies are calculated relative to 1961–1990.
Hadley Centre evaluation of the observations: Morice et al. (2012): The data were downloaded from
http://www.metoffice.gov.uk/hadobs/hadcrut4/data/current/download.html#regional_series. Annual mean
anomalies are calculated relative to 1961–1990 based on the ensemble median.
IPCC Range of Projections
Table 1.A.1: FAR: The data have been digitized using a graphics tool from FAR Chapter 6, Figure 6.11 (Bretherton et
al., 1990) in 5-year increments as anomalies relative to 1990 (°C).
Lower Bound
Upper Bound
Year
(Scenario D)
(Business as Usual)
1990
0.00
0.00
1995
0.09
0.14
2000
0.15
0.30
2005
0.23
0.53
2010
0.28
0.72
2015
0.33
0.91
2020
0.39
1.11
2025
0.45
1.34
2030
0.52
1.58
2035
0.58
1.86
Table 1.A.2: SAR: The data have been digitized using a graphics tool from Figure 19 of the TS (IPCC, 1996) in 5-year
increments as anomalies relative to 1990. The scenarios include changes in aerosols beyond 1990 (°C).
Lower Bound
Upper Bound
Year
(IS92c/1.5)
(IS92e/4.5)
1990
0.00
0.00
1995
0.05
0.09
2000
0.11
0.17
2005
0.16
0.28
2010
0.19
0.38
2015
0.23
0.47
2020
0.27
0.57
2025
0.31
0.67
2030
0.36
0.79
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2035
Chapter 1
0.41
IPCC WGI Fifth Assessment Report
0.92
Table 1.A.3: TAR: The data have been digitized using a graphics tool from Figure 9.13(b) (Cubasch et al., 2001) in 5year increments based on the GFDL_R15_a and DOE PCM parameter settings (°C).
Year
Lower Bound
Upper Bound
1990
0.00
0.00
1995
0.05
0.09
2000
0.11
0.20
2005
0.14
0.34
2010
0.17
0.52
2015
0.22
0.70
2020
0.28
0.87
2025
0.37
1.08
2030
0.43
1.28
2035
0.52
1.50
AR4: The temperature projections of the AR4 are presented for three SRES scenarios: B1, A1B and A2.
Annual mean anomalies relative to 1961–1990 of the individual CMIP3 ensemble simulations (as used in
AR4 SPM Figure SPM5) are shown. One outlier has been eliminated based on the advice of the model
developers because of the model drift that leads to an unrealistic temperature evolution. As assessed by
Meehl et al. (2007), the likely-range for the temperature change is given by the ensemble mean temperature
change +60% and –40% of the ensemble mean temperature change. Note that in the AR4 the uncertainty
range was explicitly estimated for the end of the 21st century results. Here, it is shown for 2035. The time
dependence of this range has been assessed in Knutti et al. (2008). The relative uncertainty is approximately
constant over time in all estimates from different sources, except for the very early decades when natural
variability is being considered (see Figure 3 in Knutti et al., 2008).
Data Processing
Observations
The observations are shown from 1950 to 2012 as annual mean anomaly relative to 1961–1990 (squares).
For smoothing, first, the trend of each of the observational datasets was calculated by locally weighted
scatter plot smoothing (Cleveland, 1979; f=1/3). Then, the 11-year running means of the residuals were
determined with reflected ends for the last 5 years. Finally, the trend was added back to the 11-year running
means of the residuals.
Projections
For FAR, SAR and TAR, the projections have been harmonized to match the average of the three smoothed
observational datasets at 1990.
Figure 1.5
Documentation of Data Sources
Observed CO2 Concentrations
Global annual mean CO2 concentrations are presented as annual mean values from Annex II Table AII.1.1a.
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Chapter 1
IPCC WGI Fifth Assessment Report
IPCC Range of Projections
Table 1.A.4: FAR: The data have been digitized using a graphics tool from Figure A.3 (Annex, IPCC, 1990) as
anomalies compared to 1990 in 5-year increments (ppm) and the observed 1990 value (353.6) has been added.
Upper Bound
Lower Bound
Year
(Scenario D)
(Business as Usual)
1990
353.6
353.6
1995
362.8
363.7
2000
370.6
373.3
2005
376.5
386.5
2010
383.2
401.5
2015
390.2
414.3
2020
396.6
428.8
2025
401.5
442.0
2030
406.0
460.7
2035
410.0
480.3
Table 1.A.5: SAR: The data have been digitized using a graphics tool from Figure 5b in the TS (IPCC, 1996) in 5-year
increments (ppm) as anomalies compared to 1990 and the observed 1990 value (353.6) has been added.
Lower Bound
Upper Bound
Year
(IS92c)
(IS92e)
1990
353.6
353.6
1995
358.4
359.0
2000
366.8
369.2
2005
373.7
380.4
2010
382.3
392.9
2015
391.4
408.0
2020
400.7
423.0
2025
408.0
439.6
2030
416.9
457.7
2035
424.5
477.7
TAR: The data were taken in 10-year increments from table Appendix II (IPCC, 2001) SRES Data Tables
Table II.2.1 (ISAM model high and low setting). The scenarios that give the upper bound or lower bound
respectively vary over time.
AR4: The data used was obtained from Figure 10.26 in Chapter 10 of AR4 (Meehl et al., 2007, provided by
Malte Meinshausen). Annual means are used.
Data Processing
The projections have been harmonized to start from the observed value in 1990.
Figure 1.6
Documentation of Data Sources
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Chapter 1
IPCC WGI Fifth Assessment Report
Observed CH4 Concentrations
Global annual mean CH4 concentrations are presented as annual mean values from Annex II Table AII.1.1a.
IPCC Range of Projections
Table 1.A.6: FAR: The data have been digitized using a graphics tool from FAR SPM Figure 5 (IPCC, 1990) in 5-year
increments (ppb) as anomalies compared to 1990 the observed 1990 value (1714.4) has been added.
Upper Bound
Lower Bound
Year
(Scenario D)
(Business as Usual)
1990
1714.4
1714.4
1995
1775.7
1816.7
2000
1809.7
1938.7
2005
1819.0
2063.8
2010
1823.1
2191.1
2015
1832.3
2314.1
2020
1847.7
2441.3
2025
1857.9
2562.3
2030
1835.3
2691.6
2035
1819.0
2818.8
SAR: The data were taken in 5-year increments from Table 2.5a (Schimel et al., 1996). The scenarios that
give the upper bound or lower bound respectively vary over time.
TAR: The data were taken in 10-year increments from Appendix II SRES Data Tables Table II.2.2 (IPCC,
2001). The upper bound is given by the A1p scenario, the lower bound by the B1p scenario.
AR4: The data used was obtained from Figure 10.26 in Chapter 10 of AR4 (Meehl et al., 2007, provided by
Malte Meinshausen). Annual means are used.
Data Processing
The observations are shown as annual means. The projections have been harmonized to start from the same
value in 1990.
Figure 1.7
Documentation of Data Sources
Observed N2O Concentrations
Global annual mean N2O concentrations are presented as annual mean values from Annex II Table AII.1.1a.
IPCC Range of Projections
Table 1.A.7: FAR: The data have been digitized using a graphics tool from FAR A.3 (Annex, IPCC, 1990) in 5-year
increments (ppb) as anomalies compared to 1990 and the observed 1990 value (308.7) has been added.
Lower bound
Upper bound
Year
(Scenario D)
(Business as Usual)
1990
308.7
308.7
1995
311.7
313.2
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Chapter 1
IPCC WGI Fifth Assessment Report
2000
315.4
317.7
2005
318.8
322.9
2010
322.1
328.0
2015
325.2
333.0
2020
328.2
337.9
2025
331.7
343.0
2030
334.0
348.9
2035
336.1
354.1
SAR: The data were taken in 5-year increments from Table 2.5b (Schimel et al., 1996). The upper bound is
given by the IS92e and IS92f scenario, the lower bound by the IS92d scenario.
TAR: The data were taken in 10-year increments from Appendix II SRES Data Tables Table II.2.3 (IPCC,
2001). The upper bound is given by the A1FI scenario, the lower bound by the B2 and A1T scenario.
AR4: The data used was obtained from Figure 10.26 in Chapter 10 of AR4 (Meehl et al., 2007, provided by
Malte Meinshausen). Annual means are used.
Data Processing
The observations are shown as annual means. No smoothing is applied. The projections have been
harmonized to start from the same value in 1990.
Figure 1.10
Documentation of Data Sources
Observed Global Mean Sea Level Rise
Three datasets based on tide gauge measurements are presented: Church and White (2011), Jevrejeva et al.
(2008), and Ray and Douglas (2011). Annual mean anomalies are calculated relative to 1961–1990.
Estimates based on sea surface altimetry are presented as the ensemble mean of five different data sets
(Section 3.7, Figure 3.13, Section 13.2, Figure 13.3) from 1993 to 2012. Annual means have been calculated.
The data are harmonized to start from the mean of the three tide gauge based estimates (see above) at 1993.
IPCC Range of Projections
Table 1.A.8: FAR: The data have been digitized using a graphics tool from Chapter 9, Figure 9.6 for the upper bound
and Figure 9.7 for the lower bound (Warrick and Oerlemans, 1990) in 5-year increments as anomalies relative to 1990
(cm) and the observed anomaly relative to 1961–1990 (2.0 cm) has been added.
Lower Bound
Upper Bound
Year
(Scenario D)
(Business as Usual)
1990
2.0
2.0
1995
2.7
5.0
2000
3.7
7.9
2005
4.6
11.3
2010
5.5
15.0
2015
6.3
18.7
2020
6.9
22.8
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Chapter 1
IPCC WGI Fifth Assessment Report
2025
7.7
26.7
2030
8.4
30.9
2035
9.2
35.4
Table 1.A.9: SAR: The data have been digitized using a graphics tool from Figure 21 (TS, IPCC, 1996) in 5-year
increments as anomalies relative to 1990 (cm) and the observed anomaly relative to 1961–1990 (2.0 cm) has been
added.
Lower Bound
Upper Bound
Year
(IS92c/1.5)
(IS92e/4.5)
1990
2.0
2.0
1995
2.4
4.3
2000
2.7
6.5
2005
3.1
9.0
2010
3.4
11.7
2015
3.8
14.9
2020
4.4
18.3
2025
5.1
21.8
2030
5.7
25.4
2035
6.4
29.2
TAR: The data are given in Table II.5.1 in 10-year increments. They are harmonized to start from mean of
the observed anomaly relative to 1961–1990 at 1990 (2.0 cm).
AR4: The AR4 did not give a time-dependent estimate of sea level rise. These analyses have been conducted
post AR4 by Church et al. (2011) based on the CMIP3 model results that were available at the time of AR4.
Here, the SRES B1, A1B, and A2 scenarios are shown from Church et al. (2011). The data start in 2001 and
are given as anomalies with respect to 1990. They are displayed from 2001 to 2035, but the anomalies are
harmonized to start from mean of the observed anomaly relative to 1961–1990 at 1990 (2.0 cm).
Data Processing
The observations are shown from 1950 to 2012 as the annual mean anomaly relative to 1961–1990 (squares)
and smoothed (solid lines). For smoothing, first, the trend of each of the observational datasets was
calculated by locally weighted scatter plot smoothing (Cleveland, 1979; f=1/3). Then, the 11-year running
means of the residuals were determined with reflected ends for the last 5 years. Finally, the trend was added
back to the 11-year running means of the residuals.
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1
2
3
Chapter 1
IPCC WGI Fifth Assessment Report
Tables
Table 1.1: Historical Overview of Major Conclusions of Previous IPCC Assessment Reports.
Topic
FAR SPM Statement
SAR SPM Statement
Human and Natural Drivers of There is a natural greenhouse effect Greenhouse gas concentrations
have continued to increase. These
which already keeps the Earth
Climate Change
warmer than it would otherwise be. trends can be attributed largely to
human activities, mostly fossil
Emissions resulting from human
activities are substantially increasing fuel use, land use change and
the atmospheric concentrations of the agriculture.
greenhouse gases carbon dioxide,
Anthropogenic aerosols are shortmethane, chlorofluorocarbons and
lived and tend to produce
nitrous oxide. These increases will
negative radiative forcing.
enhance the greenhouse effect,
resulting on average in an additional
warming of the Earth's surface.
Continued emissions of these gases at
present rates would commit us to
increased concentrations for centuries
ahead.
Direct
Temperature
Observations of
Recent Climate
Change
Global mean surface air temperature
has increased by 0.3°C to 0.6°C over
the last 100 years, with the five
global-average warmest years being
in the 1980s.
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AR4 SPM Statement
Global atmospheric concentrations
of carbon dioxide, methane and
nitrous oxide have increased
markedly as a result of human
activities since 1750 and now far
exceed pre-industrial values
determined from ice cores spanning
many thousands of years. The global
increases in carbon dioxide
Anthropogenic aerosols are short- concentration are due primarily to
lived and mostly produce negative fossil fuel use and land use change,
while those of methane and nitrous
radiative forcing by their direct
effect. There is more evidence for oxide are primarily due to
agriculture.
their indirect effect, which is
negative, although of very
Very high confidence that the global
uncertain magnitude.
average net effect of human
activities since 1750 has been one of
Natural factors have made small
contributions to radiative forcing warming, with a radiative forcing of
+1.6 [+0.6 to +2.4] W m–2.
over the past century.
Climate has changed over the
An increasing body of observations Warming of the climate system is
unequivocal, as is now evident from
past century. Global mean surface gives a collective picture of a
temperature has increased by
warming world and other changes observations of increases in global
average air and ocean temperatures,
between about 0.3 and 0.6°C
in the climate system.
since the late 19th century.
The global average temperature has widespread melting of snow and ice,
Recent years have been among
increased since 1861. Over the 20th and rising global average sea level.
the warmest since 1860, despite century the increase has been
Eleven of the last twelve years
the cooling effect of the 1991 Mt. 0.6°C.
(1995–2006) rank among the 12
Pinatubo volcanic eruption.
Some important aspects of climate warmest years in the instrumental
record of global surface temperature
appear not to have changed.
(since 1850). The updated 100-year
linear trend (1906 to 2005) of 0.74°C
[0.56°C to 0.92°C] is therefore larger
than the corresponding trend for
1901 to 2000 given in the TAR of
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TAR SPM Statement
Emissions of greenhouse gases and
aerosols due to human activities
continue to alter the atmosphere in
ways that are expected to affect the
climate. The atmospheric
concentration of CO2 has increased
by 31% since 1750 and that of
methane by 151%.
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0.6°C [0.4°C to 0.8°C].
Sea Level
Over the same period global sea level
has increased by 10–20 cm These
increases have not been smooth with
time, nor uniform over the globe.
Climate varies naturally on all timescales from hundreds of millions of
years down to the year-to-year.
Prominent in the Earth's history have
been the 100,000 year glacialinterglacial cycles when climate was
mostly cooler than at present. Global
surface temperatures have typically
varied by 5°C–7°C through these
cycles, with large changes in ice
volume and sea level, and
temperature changes as great as
10°C–15°C in some middle and high
latitude regions of the Northern
Hemisphere. Since the end of the last
ice age, about 10,000 years ago,
global surface temperatures have
probably fluctuated by little more
than 1°C. Some fluctuations have
lasted several centuries, including the
Little Ice Age which ended in the
nineteenth century and which appears
to have been global in extent.
Understanding and Attributing The size of this warming is broadly
consistent with predictions of climate
Climate Change
models, but it is also of the same
magnitude as natural climate
variability. Thus the observed
increase could be largely due to this
A Palaeoclimatic Perspective
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Global sea level has risen by
Tide gauge data show that global
between 10 and 25 cm over the average sea level rose between 0.1
past 100 years and much of the
and 0.2 m during the 20th century.
rise may be related to the increase
in global mean temperature.
Some aspects of climate have not
been observed to change.
Global average sea level rose at an
average rate of 1.8 [1.3 to 2.3] mm
per year over 1961 to 2003. The rate
was faster over 1993 to 2003: about
3.1 [2.4 to 3.8] mm per year. The
total 20th century rise is estimated to
be 0.17 [0.12 to 0.22] m.
Palaeoclimatic information supports
the interpretation that the warmth of
the last half century is unusual in at
least the previous 1,300 years.
The limited available evidence
from proxy climate indicators
suggests that the 20th century
global mean temperature is at
least as warm as any other
century since at least 1400 AD.
Data prior to 1400 are too sparse
to allow the reliable estimation of
global mean temperature.
New analyses of proxy data for the
Northern Hemisphere indicate that
the increase in temperature in the
20th century is likely to have been
the largest of any century during
The last time the polar regions were
the past 1,000 years. It is also
significantly warmer than present for
likely that, in the Northern
an extended period (about 125,000
Hemisphere, the 1990s was the
years ago), reductions in polar ice
warmest decade and 1998 the
warmest year. Because less data are volume led to 4 to 6 m of sea level
rise.
available, less is known about
annual averages prior to 1,000
years before present and for
conditions prevailing in most of the
Southern Hemisphere prior to
1861.
The balance of evidence suggests
a discernible human influence on
global climate. Simulations with
coupled atmosphere-ocean
models have provided important
information about decade to
There is new and stronger evidence
that most of the warming observed
over the last 50 years is attributable
to human activities. There is a
longer and more scrutinized
temperature record and new model
1-40
Most of the observed increase in
global average temperatures since
the mid-20th century is likely due to
the observed increase in
anthropogenic greenhouse gas
concentrations. Discernible human
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Projections of Temperature
Future
Changes in
Climate
Sea Level
Chapter 1
natural variability; alternatively this century time-scale natural
variability and other human factors internal climate variability.
could have offset a still larger
human-induced greenhouse warming.
The unequivocal detection of the
enhanced greenhouse effect from
observations is not likely for a decade
or more.
Under the IPCC Business-as-Usual Climate is expected to continue to
emissions of greenhouse gases, a rate change in the future. For the midof increase of global mean
range IPCC emission scenario,
temperature during the next century IS92a, assuming the ‘best
of about 0.3°C per decade (with an estimate’ value of climate
uncertainty range of 0.2°C to 0.5°C sensitivity and including the
per decade); this is greater than that effects of future increases in
seen over the past 10,000 years.
aerosols, models project an
increase in global mean surface
air temperature relative to 1990
of about 2°C by 2100.
An average rate of global mean sea
level rise of about 6 cm per decade
over the next century (with an
uncertainty range of 3–10 cm per
decade) is projected.
IPCC WGI Fifth Assessment Report
estimates of variability.
Reconstructions of climate data for
the past 1,000 years indicate this
warming was unusual and is
unlikely to be entirely natural in
origin.
influences now extend to other
aspects of climate, including ocean
warming, continental-average
temperatures, temperature extremes
and wind patterns.
Global average temperature and sea For the next two decades, a warming
level are projected to rise under all of about 0.2°C per decade is
projected for a range of SRES
IPCC SRES scenarios. The
emission scenarios. Even if the
globally averaged surface
temperature is projected to increase concentrations of all greenhouse
by 1.4°C to 5.8°C over the period gases and aerosols had been kept
constant at year 2000 levels, a
1990 to 2100.
further warming of about 0.1°C per
Confidence in the ability of models decade would be expected.
to project future climate has
There is now higher confidence in
increased.
projected patterns of warming and
Anthropogenic climate change will other regional-scale features,
including changes in wind patterns,
persist for many centuries.
precipitation and some aspects of
extremes and of ice.
Anthropogenic warming and sea
level rise would continue for
centuries, even if greenhouse gas
concentrations were to be stabilised.
Models project a sea level rise of Global mean sea level is projected Global sea level rise for the range of
50 cm from the present to 2100. to rise by 0.09 to 0.88 m between scenarios is projected as 0.18 to 0.59
1990 and 2100.
m by the end of the 21st century.
1
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Chapter 1
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Chapter 1: Introduction
Coordinating Lead Authors: Ulrich Cubasch (Germany), Donald Wuebbles (USA)
Lead Authors: Deliang Chen (Sweden), Maria Cristina Facchini (Italy), David Frame (UK/New Zealand),
Natalie Mahowald (USA), Jan-Gunnar Winther (Norway)
Contributing Authors: Achim Brauer (Germany), Valérie Masson-Delmotte (France), Frank Kaspar
(Germany), Janina Körper (Germany), Malte Meinshausen (Australia/Germany), Matthew Menne (USA),
Carolin Richter (Switzerland), Michael Schulz (Germany), Bjorn Stevens (Germany/USA), Rowan Sutton
(UK), Kevin Trenberth (USA), Murat Türkeş (Turkey), Daniel S. Ward (USA)
Review Editors: Yihui Ding (China), Linda Mearns (USA), Peter Wadhams (UK)
Date of Draft: 7 June 2013
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Figures
Figure 1.1: Main drivers of climate change. The radiative balance between incoming solar shortwave radiation (SWR)
and outgoing longwave radiation (LWR) is influenced by global climate “drivers”. Natural fluctuations in solar output
(solar cycles) can cause changes in the energy balance (through fluctuations in the amount of incoming SWR) (Section
2.3). Human activity changes the emissions of gases and aerosols, which are involved in atmospheric chemical
reactions, resulting in modified O3 and aerosol amounts (Section 2.2). O3 and aerosol particles absorb, scatter and
reflect SWR, changing the energy balance. Some aerosols act as cloud condensation nuclei modifying the properties of
cloud droplets and possibly affecting precipitation (Section 7.4). Since cloud interactions with SWR and LWR are
large, small changes in the properties of clouds have important implications for the radiative budget (Section 7.4).
Anthropogenic changes in greenhouse gases (e.g., CO2, CH4, N2O, O3, CFCs), and large aerosols (>2.5 μm in size)
modify the amount of outgoing LWR by absorbing outgoing LWR and re-emitting less energy at a lower temperature
(Section 2.2). Surface albedo is changed by changes in vegetation or land surface properties, snow or ice cover and
ocean colour (Section 2.3). These changes are driven by natural seasonal and diurnal changes (e.g., snow cover), as well
as human influence (e.g., changes in vegetation types) (Forster et al., 2007).
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Figure 1.2: Climate feedbacks and timescales. The climate feedbacks related to increasing carbon dioxide and rising
temperature include negative feedbacks (–) such as longwave radiation, lapse rate (see Glossary in Annex III), and airsea carbon exchange and positive feedbacks (+) such as water vapour and snow/ice albedo feedbacks. Some feedbacks
may be positive or negative (±): clouds, ocean circulation changes, air-land carbon dioxide exchange, and emissions of
non-green house gases and aerosols from natural systems. In the smaller box, the large difference in timescales for the
various feedbacks is highlighted.
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Figure 1.3: Overview of observed climate change indicators as listed in AR4. Chapter numbers indicate where detailed
discussions for these indicators are found in AR5 (temperature: red; hydrological: blue; others: black).
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Figure 1.4: Estimated changes in the observed globally and annually averaged surface temperature anomaly relative to
1961–1990 (in C) since 1950 compared with the range of projections from the previous IPCC assessments. Values are
harmonized to start from the same value in 1990. Observed global annual mean surface air temperature anomaly,
relative to 1961–1990, is shown as squares and smoothed time series as solid lines (NASA (dark blue), NOAA (warm
mustard), and the UK Hadley Centre (bright green) reanalyses). The coloured shading shows the projected range of
global annual mean surface air temperature change from 1990 to 2035 for models used in FAR (Figure 6.11 in
Bretherton et al., 1990), SAR (Figure 19 in the TS of IPCC, 1996), TAR (full range of TAR Figure 9.13(b) in Cubasch
et al., 2001). TAR results are based on the simple climate model analyses presented and not on the individual full threedimensional climate model simulations. For the AR4 results are presented as single model runs of the CMIP3 ensemble
for the historical period from 1950 to 2000 (light grey lines) and for three scenarios (A2, A1B and B1) from 2001 to
2035. The bars at the right hand side of the graph show the full range given for 2035 for each assessment report. For the
three SRES scenarios the bars show the CMIP3 ensemble mean and the likely range given by –40% to +60% of the
mean as assessed in Meehl et al. (2007). The publication years of the assessment reports are shown. See Appendix 1.A
for details on the data and calculations used to create this figure.
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Figure 1.5: Observed globally and annually averaged CO2 concentrations in parts per million (ppm) since 1950
compared with projections from the previous IPCC assessments. Observed global annual CO2 concentrations are shown
in dark blue. The shading shows the largest model projected range of global annual CO2 concentrations from 1950 to
2035 from FAR (Figure A.3 in the Summary for Policymakers (SPM) of IPCC, 1990), SAR (Figure 5b in the TS of
IPCC, 1996), TAR (Appendix II of IPCC, 2001), and from the A2, A1B and B1 scenarios presented in the AR4 (Figure
10.26 in Meehl et al., 2007). The bars at the right hand side of the graph show the full range given for 2035 for each
assessment report. The publication years of the assessment reports are shown. See Appendix 1.A for details on the data
and calculations used to create this figure.
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Figure 1.6: Observed globally and annually averaged CH4 concentrations in parts per billion (ppb) since 1950
compared with projections from the previous IPCC assessments. Estimated observed global annual CH4 concentrations
are shown in dark blue. The shading shows the largest model projected range of global annual CH4 concentrations from
1950 to 2035 from FAR (Figure A.3 of the Annex of IPCC, 1990), SAR (Table 2.5a in Schimel et al., 1996), TAR
(Appendix II of IPCC, 2001), and from the A2, A1B and B1 scenarios presented in the AR4 (Figure 10.26 in Meehl et
al., 2007). The bars at the right hand side of the graph show the full range given for 2035 for each assessment report.
The publication years of the assessment reports are shown. See Appendix 1.A for details on the data and calculations
used to create this figure.
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Figure 1.7: Observed globally and annually averaged N2O concentrations in parts per billion (ppb) since 1950
compared with projections from the previous IPCC assessments. Observed global annual N2O concentrations are shown
in dark blue. The shading shows the largest model projected range of global annual N2O concentrations from 1950 to
2035 from FAR (Figure A3 in the Annex of IPCC, 1990), SAR (Table 2.5b in Schimel et al., 1996), TAR (Appendix II
of IPCC, 2001), and from the A2, A1B and B1 scenarios presented in the AR4 (Figure 10.26 in Meehl et al., 2007). The
bars at the right hand side of the graph show the full range given for 2035 for each assessment report. The publication
years of the assessment reports are shown. See Appendix 1.A for details on the data and calculations used to create this
figure.
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Figure 1.8: Schematic representations of the probability density function of daily temperature, which tends to be
approximately Gaussian, and daily precipitation, which has a skewed distribution. Dashed lines represent a previous
distribution and solid lines a changed distribution. The probability of occurrence, or frequency, of extremes is denoted
by the shaded areas. In the case of temperature, changes in the frequencies of extremes are affected by changes a) in the
mean, b) in the variance or shape, and c) in both the mean and the variance. d) In a skewed distribution such as that of
precipitation, a change in the mean of the distribution generally affects its variability or spread, and thus an increase in
mean precipitation would also likely imply an increase in heavy precipitation extremes, and vice-versa. In addition, the
shape of the right hand tail could also change, affecting extremes. Furthermore, climate change may alter the frequency
of precipitation and the duration of dry spells between precipitation events. a)-c) modified from Folland et al. (2001)
and d modified from Peterson et al. (2008) as in Zhang and Zwiers (2012).
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Figure 1.9: Change in the confidence levels for extreme events based on prior IPCC assessments: TAR, AR4, and
SREX. Types of extreme events discussed in all three reports are highlighted in green. Confidence levels are defined in
Section 1.4. Similar analyses for AR5 are discussed in later chapters. Please note that the nomenclature for confidence
level changed from AR4 to SREX and AR5.
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Figure 1.10: Estimated changes in the observed global annual mean sea level (GMSL) since 1950 relative to 1961–
1990. Estimated changes in global annual sea level anomalies are presented based on tide gauge data (Church and
White, 2011 (dark blue); Jevrejeva et al., 2008 (warm mustard) ; Ray and Douglas, 2011 (dark green)) and based on sea
surface altimetry (light blue). The altimetry data start in 1993 and are harmonized to start from the mean 1993 value of
the tide gauge data. Squares indicate annual mean values, solid lines smoothed values. The shading shows the largest
model projected range of global annual sea level rise from 1950 to 2035 for FAR (Figure 9.6 and Figure 9.7 in Warrick
and Oerlemans, 1990), SAR (Figure 21 in TS of IPCC, 1996), TAR (Appendix II of IPCC, 2001) and for Church et al.
(2011) based on the Coupled Model Intercomparison Project Phase 3 (CMIP3) model results not assessed at the time of
AR4 using the SRES B1, A1B, and A2 scenarios. Note that in the AR4 no full range was given for the sea level
projections for this period. Therefore, the figure shows results that have been published subsequent to the AR4. The
bars at the right hand side of the graph show the full range given for 2035 for each assessment report. For Church et al.
(2011) the mean sea level rise is indicated in addition to the full range. See Appendix 1.A for details on the data and
calculations used to create this figure.
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Figure 1.11: The basis for the confidence level is given as a combination of evidence (limited, medium, robust) and
agreement (low, medium, and high) (Mastrandrea et al., 2010).
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Figure 1.12: Development of capabilities of observations. Top: Changes in the mix and increasing diversity of
observations over time create challenges for a consistent climate record (adapted from Brönnimann et al., 2008).
Bottom left: First year of temperature data in global historical climatology network daily database (available at
http://www.ncdc.noaa.gov/oa/climate/ghcn-daily/; Menne et al., 2012). Bottom right: Number of satellite instruments
from which data have been assimilated in the European Centre for Medium-Range Weather Forecasts production
streams for each year from 1996 to 2010. This figure is used as an example to demonstrate the fivefold increase in the
usage of satellite data over this time period.
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Figure 1.13: The development of climate models over the last 35 years showing how the different components were
coupled into comprehensive climate models over time. In each aspect (e. g. the atmosphere, which comprises a wide
range of atmospheric processes) the complexity and range of processes has increased over time (illustrated by growing
cylinders). Note that during the same time the horizontal and vertical resolution has increased considerably e.g., for
spectral models from T21L9 (roughly 500 km horizontal resolution and 9 vertical levels) in the 1970s to T95L95
(roughly 100 km horizontal resolution and 95 vertical levels) at present, and that now ensembles with at least three
independent experiments can be considered as standard.
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Figure 1.14: Horizontal resolutions considered in today’s higher resolution models and in the very high resolution
models now being tested: a) Illustration of the European topography at a resolution of 87.5 x 87.5 km; b) same as a) but
for a resolution of 30.0 x 30.0 km.
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Figure 1.15: Historical and projected total anthropogenic RF (W m–2) relative to preindustrial (~1765) between 1950
and 2100. Previous IPCC assessments (SAR IS92a, TAR/AR4 SRES A1B, A2 and B1) are compared with
representative concentration pathway (RCP) scenarios (see Chapter 12 and Box 1.1 for their extensions until 2300 and
Annex II for the values shown here). The total RF of the three families of scenarios, IS92, SRES and RCP, differ for
example for the year 2000, resulting from the knowledge about the emissions assumed having changed since the TAR
and AR4.
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Box 1.1, Figure 1: Total RF (anthropogenic plus natural) for RCPs and extended concentration pathways (ECP) − for
RCP2.6, RCP4.5, and RCP6, RCP8.5, as well as a supplementary extension SCP6to4.5 with an adjustment of emissions
after 2100 to reach RCP 4.5 concentration levels in 2250 and thereafter. Note that the stated RF levels refer to the
illustrative default median estimates only. There is substantial uncertainty in current and future RF levels for any given
scenario. Short-term variations in RF are due to both volcanic forcings in the past (1800–2000) and cyclical solar
forcing assuming a constant 11-year solar cycle (following the CMIP5 recommendation), except at times of
stabilization (reproduced from Figure 4 in Meinshausen et al., 2011).
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Box 1.1, Figure 2: Concentrations of GHG following the 4 RCPs and their extensions (ECP) to 2300. (Reproduced
from Figure 5 in Meinshausen et al., 2011). Also see Annex II Table AII.4.1 for CO2, Table AII.4.2 for CH4, Table
AII.4.3 for N2O.
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Box 1.1, Figure 3: (a) Equivalent CO2 concentration and (b) CO2 emissions (except land use emissions) for the four
RCPs and their ECPs as well as some SRES scenarios.
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Box 1.1, Figure 4: (a) Anthropogenic BC emissions (Annex II Table AII.2.22), (b) Anthropogenic NOx emissions
(Annex II Table AII.2.18), and (c) Anthropogenic SOx emissions (Annex II Table II.2.20).
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Decadal mean temperature anomalies
Observations
4
Natural variability
3.5
Climate response uncertainty
Emission uncertainty
Historical GCM uncertainty
3
All 90% uncertainty ranges
2.5
IPCC WGI Fifth Assessment Report
Global average temperature change (°C)
Chapter 1
(a)
2
1.5
1
0.5
0
1960
1980
2000
2020
2040
Year
2060
2080
2100
Global average temperature change (°C)
Global average temperature change (°C)
Final Draft (7 June 2013)
4
3.5
(b)
3
2.5
2
1.5
1
0.5
0
1960 1980 2000 2020 2040 2060 2080 2100
Year
4
(c)
3.5
3
2.5
2
1.5
1
0.5
0
1960 1980 2000 2020 2040 2060 2080 2100
Year
FAQ 1.1, Figure 1: Schematic diagram showing the relative importance of different uncertainties, and their evolution
in time. a) Decadal mean surface temperature change (C) from the historical record (black line), with climate model
estimates of uncertainty for historical period (grey), along with future climate projections and uncertainty. Values are
normalised by means from 1961 to 1980. Natural variability (orange) derives from model interannual variability, and is
assumed constant with time. Emission uncertainty (green) is estimated as the model mean difference in projections from
different scenarios. Climate response uncertainty (blue-solid) is based on climate model spread, along with added
uncertainties from the carbon cycle, as well as rough estimates of additional uncertainty from poorly-modelled
processes. Based on Hawkins and Sutton (2011) and Huntingford et al. (2009). b) Climate response uncertainty can
appear to increase when a new process is discovered to be relevant, but such increases reflect a quantification of
previously unmeasured uncertainty, or c) can decrease with additional model improvements and observational
constraints. The given uncertainty range of 90% means that the temperature is estimated to be in that range, with a
probability of 90%.
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