WG1AR5_all_final.txt

CLIMATE CHANGE 2013
The Physical Science Basis
WORKING GROUP I CONTRIBUTION TO THE 
FIFTH ASSESSMENT REPORT OF THE 
INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE
WG IINTERGOVERNMENTAL PANEL ON  climate change

 

ForewordClimate Change 2013
The Physical Science Basis
Working Group I Contribution to the 
Fifth Assessment Report of the
Intergovernmental Panel on Climate Change
Edited by
 Thomas F. Stocker Dahe Qin 
 Working Group I Co-Chair Working Group I Co-Chair
 University of Bern China Meteorological Administration
 Gian-Kasper Plattner  Melinda M.B. Tignor Simon K. Allen Judith Boschung
 Director of Science Director of Operations Senior Science Officer Administrative Assistant 
  
 Alexander Nauels Yu Xia Vincent Bex Pauline M. Midgley
 Science Assistant Science Officer IT Officer Head   
  
Working Group I Technical Support Unit

ii
ForewordCAMBRIDGE UNIVERSITY PRESS
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Please use the following reference to the whole report:
IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovern -
mental Panel on Climate Change [Stocker, T.F ., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y . Xia, V. Bex and P .M. Midgley 
(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY , USA, 1535 pp.
Cover photo:
Folgefonna glacier on the high plateaus of Sørfjorden, Norway (60°03’ N - 6°20’ E) © Yann Arthus-Bertrand / Altitude.

Introduction Chapter 2iii
ForewordForeword, Preface 
and Dedication

v
ForewordForeword
“Climate Change 2013: The Physical Science Basis” presents clear and 
robust conclusions in a global assessment of climate change science—
not the least of which is that the science now shows with 95 percent 
certainty that human activity is the dominant cause of observed warm -
ing since the mid-20th century. The report confirms that warming in 
the climate system is unequivocal, with many of the observed changes 
unprecedented over decades to millennia: warming of the atmosphere 
and the ocean, diminishing snow and ice, rising sea levels and increas -
ing concentrations of greenhouse gases. Each of the last three decades 
has been successively warmer at the Earth’s surface than any preced -
ing decade since 1850. 
These and other findings confirm and enhance our scientific under -
standing of the climate system and the role of greenhouse gas emis -
sions; as such, the report demands the urgent attention of both policy -
makers and the general public.
As an intergovernmental body jointly established in 1988 by the World 
Meteorological Organization (WMO) and the United Nations Environ -
ment Programme (UNEP), the Intergovernmental Panel on Climate 
Change (IPCC) has provided policymakers with the most authorita -
tive and objective scientific and technical assessments. Beginning in 
1990, this series of IPCC Assessment Reports, Special Reports, Tech -
nical Papers, Methodology Reports and other products have become 
standard works of reference. 
This Working Group I contribution to the IPCC’s Fifth Assessment 
Report contains important new scientific knowledge that can be used 
to produce climate information and services for assisting society to act 
to address the challenges of climate change. The timing is particularly 
significant, as this information provides a new impetus, through clear 
and indisputable physical science, to those negotiators responsible for 
concluding a new agreement under the United Nations Framework 
Convention on Climate Change in 2015.
Climate change is a long-term challenge, but one that requires urgent 
action given the pace and the scale by which greenhouse gases are 
accumulating in the atmosphere and the risks of a more than 2 degree 
Celsius temperature rise. Today we need to focus on the fundamentals 
and on the actions otherwise the risks we run will get higher with 
every year. 
This Working Group I assessment was made possible thanks to the 
commitment and dedication of many hundreds of experts worldwide, 
representing a wide range of disciplines. WMO and UNEP are proud 
that so many of the experts belong to their communities and networks. 
We express our deep gratitude to all authors, review editors and expert 
reviewers for devoting their knowledge, expertise and time. We would 
like to thank the staff of the Working Group I Technical Support Unit 
and the IPCC Secretariat for their dedication. We are also grateful to the governments that supported their scien -
tists’ participation in developing this report and that contributed to 
the IPCC Trust Fund to provide for the essential participation of experts 
from developing countries and countries with economies in transition. 
We would like to express our appreciation to the government of Italy 
for hosting the scoping meeting for the IPCC’s Fifth Assessment Report, 
to the governments of China, France, Morocco and Australia for host -
ing drafting sessions of the Working Group I contribution and to the 
government of Sweden for hosting the Twelfth Session of Working 
Group I in Stockholm for approval of the Working Group I Report. The 
generous financial support by the government of Switzerland, and the 
logistical support by the University of Bern (Switzerland), enabled the 
smooth operation of the Working Group I Technical Support Unit. This 
is gratefully acknowledged.
We would particularly like to thank Dr. Rajendra Pachauri, Chairman of 
the IPCC, for his direction and guidance of the IPCC and we express our 
deep gratitude to Professor Qin Dahe and Professor Thomas Stocker, 
the Co-Chairs of Working Group I for their tireless leadership through -
out the development and production of this report. 
M. Jarraud
Secretary-General
World Meteorological Organization
A. Steiner
Executive Director
United Nations Environment Programme


vii
PrefacePreface
The Working Group I contribution to the Fifth Assessment Report of 
the Intergovernmental Panel on Climate Change (IPCC) provides a 
comprehensive assessment of the physical science basis of climate 
change. It builds upon the Working Group I contribution to the IPCC’s 
Fourth Assessment Report in 2007 and incorporates subsequent new 
findings from the Special Report on Managing the Risks of Extreme 
Events and Disasters to Advance Climate Change Adaptation, as well 
as from research published in the extensive scientific and technical 
literature. The assessment considers new evidence of past, present and 
projected future climate change based on many independent scien -
tific analyses from observations of the climate system, paleoclimate 
archives, theoretical studies of climate processes and simulations using 
climate models.
 
Scope of the Report
During the process of scoping and approving the outline of its Fifth 
Assessment Report, the IPCC focussed on those aspects of the current 
understanding of the science of climate change that were judged to be 
most relevant to policymakers.
In this report, Working Group I has extended coverage of future climate 
change compared to earlier reports by assessing near-term projections 
and predictability as well as long-term projections and irreversibility 
in two separate chapters. Following the decisions made by the Panel 
during the scoping and outline approval, a set of new scenarios, the 
Representative Concentration Pathways, are used across all three 
Working Groups for projections of climate change over the 21st cen -
tury. The coverage of regional information in the Working Group I 
report is expanded by specifically assessing climate phenomena such 
as monsoon systems and their relevance to future climate change in 
the regions. 
The Working Group I Report is an assessment, not a review or a text 
book of climate science, and is based on the published scientific and 
technical literature available up to 15 March 2013. Underlying all 
aspects of the report is a strong commitment to assessing the science 
comprehensively, without bias and in a way that is relevant to policy 
but not policy prescriptive. 
Structure of the Report
This report consists of a short Summary for Policymakers, a longer 
Technical Summary and fourteen thematic chapters plus annexes. An 
innovation in this Working Group I assessment is the Atlas of Global 
and Regional Climate Projections (Annex I) containing time series and 
maps of temperature and precipitation projections for 35 regions of 
the world, which enhances accessibility for stakeholders and users.The Summary for Policymakers and Technical Summary of this report 
follow a parallel structure and each includes cross-references to the 
chapter and section where the material being summarised can be 
found in the underlying report. In this way, these summary compo -
nents of the report provide a road-map to the contents of the entire 
report and a traceable account of every major finding.
In order to facilitate the accessibility of the findings of the Working 
Group I assessment for a wide readership and to enhance their usabil -
ity for stakeholders, each section of the Summary for Policymakers has 
a highlighted headline statement. Taken together, these 19 headline 
statements provide an overarching summary in simple and quotable 
language that is supported by the scientists and approved by the 
member governments of the IPCC. Another innovative feature of this 
report is the presentation of Thematic Focus Elements in the Techni -
cal Summary that provide end to end assessments of important cross-
cutting issues in the physical science basis of climate change. 
Introduction (Chapter 1):  This chapter provides information on the 
progress in climate change science since the First Assessment Report 
of the IPCC in 1990 and gives an overview of key concepts, indica -
tors of climate change, the treatment of uncertainties and advances in 
measurement and modelling capabilities. This includes a description of 
the future scenarios and in particular the Representative Concentration 
Pathway scenarios used across all Working Groups for the IPCC’s Fifth 
Assessment Report. 
Observations and Paleoclimate Information (Chapters 2, 3, 4, 5):  These 
chapters assess information from all climate system components on 
climate variability and change as obtained from instrumental records 
and climate archives. They cover all relevant aspects of the atmosphere 
including the stratosphere, the land surface, the oceans and the cryo -
sphere. Timescales from days 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 pro -
jections from global to regional scales for two key topics. 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. For the first time, there is a chapter dedicated to the 
assessment of the physical science basis of clouds and aerosols, their 
interactions and chemistry, and the role of water vapour, as well as 
their role in feedbacks on the climate system (Chapter 7).
From Forcing to Attribution of Climate Change (Chapters 8, 9, 10): All 
the information on the different drivers (natural and anthropogenic) 
of climate change is collected, expressed in terms of Radiative Forc -
ing and assessed in Chapter 8. In Chapter 9, the hierarchy of climate 
models used in simulating past and present climate change is assessed 
and evaluated against observations and paleoclimate reconstructions. 

Preface  viii
PrefaceInformation regarding detection of changes on global to regional 
scales and their attribution to the increase in anthropogenic green -
house gases is assessed in Chapter 10.
Future Climate Change, Predictability and Irreversibility (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, variabil -
ity and extremes. Fundamental questions related to the predictability 
of climate as well as long term climate change, climate change com -
mitments and inertia in the climate system are addressed. Knowledge 
on irreversible changes and surprises in the climate system is also 
assessed.
Integration (Chapters 13 and 14):  These chapters synthesise all relevant 
information for two key topics of this assessment: sea level change 
(Chapter 13) and climate phenomena across the regions (Chapter 14). 
Chapter 13 presents an end to end assessment of information on sea 
level change based on paleoclimate reconstructions, observations and 
process understanding, and provides projections from global to region -
al scales. Chapter 14 assesses the most important modes of variability 
in the climate system, such as El Niño-Southern Oscillation, monsoon 
and many others, as well as extreme events. Furthermore, this chapter 
deals with interconnections between the climate phenomena, their 
regional expressions and their relevance for future regional climate 
change. 
Maps 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, which is also available in digital format. Radiative forcings 
and estimates of future atmospheric concentrations from Chapters 7, 
8, 11 and 12 form the basis of the Climate System Scenario Tables 
presented in Annex II. All material including high-resolution versions of 
the figures, underlying data and Supplementary Material to the chap -
ters is also available online: www.climatechange2013.org.
The scientific community and the climate modelling centres around the 
world brought together their activities in the Coordinated Modelling 
Intercomparison Project Phase 5 (CMIP5), providing the basis for most 
of the assessment of future climate change in this report. Their efforts 
enable Working Group I to deliver comprehensive scientific informa -
tion for the policymakers and the users of this report, as well as for 
the specific assessments of impacts carried out by IPCC Working Group 
II, and of costs and mitigation strategies, carried out by IPCC Working 
Group III.
Following the successful introduction in the previous Working Group I 
assessment in 2007, all chapters contain Frequently Asked Questions. 
In these the authors provide scientific answers to a range of general 
questions in a form that will be accessible to a broad readership and 
serves as a resource for teaching purposes. Finally, the report is accom -
panied by extensive Supplementary Material which is made available in the online versions of the report to provide an additional level of 
detail, such as description of datasets, models, or methodologies used 
in chapter analyses, as well as material supporting the figures in the 
Summary for Policymakers.
The Process
This Working Group I Assessment Report represents the combined 
efforts of hundreds of leading experts in the field of climate science 
and has been prepared in accordance with rules and procedures estab -
lished by the IPCC. A scoping meeting for the Fifth Assessment Report 
was held in July 2009 and the outlines for the contributions of the 
three Working Groups were approved at the 31st Session of the Panel 
in November 2009. Governments and IPCC observer organisations 
nominated experts for the author team. The team of 209 Coordinat -
ing Lead Authors and Lead Authors plus 50 Review Editors selected 
by the Working Group I Bureau was accepted at the 41st Session of 
the IPCC Bureau in May 2010. In addition, more than 600 Contribut -
ing Authors provided draft text and information to the author teams 
at their request. Drafts prepared by the authors were subject to two 
rounds of formal review and revision followed by a final round of gov -
ernment comments on the Summary for Policymakers. A total of 54,677 
written review comments were submitted by 1089 individual expert 
reviewers and 38 governments. The Review Editors for each chapter 
monitored the review process to ensure that all substantive review 
comments received appropriate consideration. The Summary for Poli -
cymakers was approved line-by-line and the underlying chapters were 
then accepted at the 12th Session of IPCC Working Group I from 23–27 
September 2007.
Acknowledgements
We are very grateful for the expertise, hard work, commitment to 
excellence and integrity shown throughout by the Coordinating Lead 
Authors and Lead Authors with important help by the many Contribut -
ing Authors. The Review Editors have played a critical role in assist -
ing the author teams and ensuring the integrity of the review process. 
We express our sincere appreciation to all the expert and government 
reviewers. We would also like to thank the members of the Bureau of 
Working Group I: Jean Jouzel, Abdalah Mokssit, Fatemeh Rahimizadeh, 
Fredolin Tangang, David Wratt and Francis Zwiers, for their thoughtful 
advice and support throughout the preparation of the report. 
We gratefully acknowledge the long-term efforts of the scientific com -
munity, organized and facilitated through the World Climate Research 
Programme, in particular CMIP5. In this effort by climate modelling 
centres around the world, more than 2 million gigabytes of numerical 
data have been produced, which were archived and distributed under 
the stewardship of the Program for Climate Model Diagnosis and Inter -
comparison. This represents an unprecedented concerted effort by the 
scientific community and their funding institutions.

 Prefaceix
PrefaceOur sincere thanks go to the hosts and organizers of the four Working 
Group I Lead Author Meetings and the 12th Session of Working Group 
I. We gratefully acknowledge the support from the host countries: 
China, France, Morocco, Australia and Sweden. The support for their 
scientists provided by many governments as well as through the IPCC 
Trust Fund is much appreciated. The efficient operation of the Working 
Group I Technical Support Unit was made possible by the generous 
financial support provided by the government of Switzerland and logis -
tical support from the University of Bern (Switzerland). 
We would also like to thank Renate Christ, Secretary of the IPCC, and 
the staff of the IPCC Secretariat: Gaetano Leone, Jonathan Lynn, Mary 
Jean Burer, Sophie Schlingemann, Judith Ewa, Jesbin Baidya, Werani 
Zabula, Joelle Fernandez, Annie Courtin, Laura Biagioni and Amy 
Smith. Thanks are due to Francis Hayes who served as the conference 
officer for the Working Group I Approval Session.
Rajendra K. Pachauri Qin Dahe Thomas F. Stocker
IPCC Chair IPCC WGI Co-Chair IPCC WGI Co-Chair
Finally our particular appreciation goes to the Working Group I Techni -
cal Support Unit: Gian-Kasper Plattner, Melinda Tignor, Simon Allen, 
Judith Boschung, Alexander Nauels, Yu Xia, Vincent Bex and Pauline 
Midgley for their professionalism, creativity and dedication. Their tire -
less efforts to coordinate the Working Group I Report ensured a final 
product of high quality. They were assisted in this by Adrien Michel 
and Flavio Lehner with further support from Zhou Botao and Sun Ying. 
In addition, the following contributions are gratefully acknowledged: 
David Hansford (editorial assistance with the Frequently Asked Ques -
tions), UNEP/GRID-Geneva and University of Geneva (graphics assis -
tance with the Frequently Asked Questions), Theresa Kornak (copyedit), 
Marilyn Anderson (index) and Michael Shibao (design and layout).

xi
DedicationDedication
Bert Bolin  
(15 May 1925 – 30 December 2007)
The Working Group I contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) 
Climate Change 2013: The Physical Science Basis  is dedicated to the memory of Bert Bolin, the first Chair of the IPCC. 
As an accomplished scientist who published on both atmospheric dynamics and the carbon cycle, including processes in the 
atmosphere, oceans and biosphere, Bert Bolin realised the complexity of the climate system and its sensitivity to anthropogenic 
perturbation. He made a fundamental contribution to the organisation of international cooperation in climate research, being 
involved in the establishment of a number of global programmes.
Bert Bolin played a key role in the creation of the IPCC and its assessments, which are carried out in a unique and formalized 
process in order to provide a robust scientific basis for informed decisions regarding one of the greatest challenges of our time. 
His vision and leadership of the Panel as the founding Chair from 1988 to 1997 laid the basis for subsequent assessments includ -
ing this one and are remembered with deep appreciation.


ForewordContents
 Front Matter  Foreword    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
 Preface    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  vii
 Dedication    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  xi
 SPM  Summary for Policymakers     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
 TS  Technical Summary     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   33
 Chapters  Chapter 1   Introduction    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   119
 Chapter 2   Observations : Atmosphere and Surface     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   159
 Chapter 3   Observations: Ocean    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   255
 Chapter 4   Observations: Cryosphere     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   317
 Chapter 5   Information from Paleoclimate Archives    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   383
 Chapter 6   Carbon and Other Biogeochemical Cycles    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   465
 Chapter 7   Clouds and Aerosols    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   571
 Chapter 8   Anthropogenic and Natural Radiative Forcing    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   659
 Chapter 9   Evaluation of Climate Models    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   741
 Chapter 10   Detection and Attribution of Climate Change: from Global to Regional    . . . . . . . . . . . . . . . .   867
 Chapter 11   Near-term Climate Change: Projections and Predictability    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   953
 Chapter 12   Long-term Climate Change: Projections, Commitments and Irreversibility    . . . . . . . . . . . .   1029
 Chapter 13   Sea Level Change    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1137
 Chapter 14   Climate Phenomena and their Relevance for Future Regional Climate Change    . . . . . .   1217
 Annexes  Annex I   Atlas of Global and Regional Climate Projections    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1311
 Annex II   Climate System Scenario Tables    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1395
 Annex III   Glossary   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1447
 Annex IV   Acronyms   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1467
 Annex V   Contributors to the IPCC WGI Fifth Assessment Report   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1477
 Annex VI   Expert Reviewers of the IPCC WGI Fifth Assessment Report   . . . . . . . . . . . . . . . . . . . . . . . . . . .  1497
 Index      . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1523

Introduction Chapter 2
Chapter 1Summary for Policymakers

3
1This Summary for Policymakers should be cited as:
IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis.  Contribution of 
Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change  [Stocker, 
T.F ., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y . Xia, V. Bex and P .M. Midgley (eds.)]. 
Cambridge University Press, Cambridge, United Kingdom and New York, NY , USA.Summary  
for Policymakers SPM
Drafting Authors:
Lisa V. Alexander (Australia), Simon K. Allen (Switzerland/New Zealand), Nathaniel L. Bindoff 
(Australia), François-Marie Bréon (France), John A. Church (Australia), Ulrich Cubasch 
(Germany), Seita Emori (Japan), Piers Forster (UK), Pierre Friedlingstein (UK/Belgium), Nathan 
Gillett (Canada), Jonathan M. Gregory (UK), Dennis L. Hartmann (USA), Eystein Jansen 
(Norway), Ben Kirtman (USA), Reto Knutti (Switzerland), Krishna Kumar Kanikicharla (India), 
Peter Lemke (Germany), Jochem Marotzke (Germany), Valérie Masson-Delmotte (France), 
Gerald A. Meehl (USA), Igor I. Mokhov (Russian Federation), Shilong Piao (China), Gian-Kasper 
Plattner (Switzerland), Qin Dahe (China), Venkatachalam Ramaswamy (USA), David Randall 
(USA), Monika Rhein (Germany), Maisa Rojas (Chile), Christopher Sabine (USA), Drew Shindell 
(USA), Thomas F . Stocker (Switzerland), Lynne  D. Talley (USA), David G. Vaughan (UK), Shang-
Ping Xie (USA)
Draft Contributing Authors:
Myles R. Allen (UK), Olivier Boucher (France), Don Chambers (USA), Jens Hesselbjerg Christensen 
(Denmark), Philippe Ciais (France), Peter U. Clark (USA), Matthew Collins (UK), Josefino C. 
Comiso (USA), Viviane Vasconcellos de Menezes (Australia/Brazil), Richard A. Feely (USA), 
Thierry Fichefet (Belgium), Arlene M. Fiore (USA), Gregory Flato (Canada), Jan Fuglestvedt 
(Norway), Gabriele Hegerl (UK/Germany), Paul J. Hezel (Belgium/USA), Gregory C. Johnson 
(USA), Georg Kaser (Austria/Italy), Vladimir Kattsov (Russian Federation), John Kennedy (UK), 
Albert M. G. Klein Tank (Netherlands), Corinne Le Quéré (UK), Gunnar Myhre (Norway), Timothy 
Osborn (UK), Antony J. Payne (UK), Judith Perlwitz (USA), Scott Power (Australia), Michael 
Prather (USA), Stephen R. Rintoul (Australia), Joeri Rogelj (Switzerland/Belgium), Matilde 
Rusticucci (Argentina), Michael Schulz (Germany), Jan Sedláček (Switzerland), Peter A. Stott 
(UK), Rowan Sutton (UK), Peter W. Thorne (USA/Norway/UK), Donald Wuebbles (USA)

SPMSummary for Policymakers41 In this Summary for Policymakers, 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 Chapter 1 and Box TS.1 for more details).
2 In this Summary for Policymakers, 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 
Chapter 1 and Box TS.1 for more details).Warming of the climate system is unequivocal, and since the 1950s, many of the observed 
changes are unprecedented over decades to millennia. The atmosphere and ocean have 
warmed, the amounts of snow and ice have diminished, sea level has risen, and the 
concentrations of greenhouse gases have increased (see Figures SPM.1, SPM.2, SPM.3 and 
SPM.4). {2.2, 2.4, 3.2, 3.7, 4.2–4.7, 5.2, 5.3, 5.5–5.6, 6.2, 13.2}A. Introduction
The Working Group I contribution to the IPCC’s Fifth Assessment Report (AR5) considers new evidence of climate change 
based on many independent scientific analyses from observations of the climate system, paleoclimate archives, theoretical 
studies of climate processes and simulations using climate models. It builds upon the Working Group I contribution to the 
IPCC’s Fourth Assessment Report (AR4), and incorporates subsequent new findings of research. As a component of the 
fifth assessment cycle, the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate 
Change Adaptation (SREX) is an important basis for information on changing weather and climate extremes.
This Summary for Policymakers (SPM) follows the structure of the Working Group I report. The narrative is supported by a 
series of overarching highlighted conclusions which, taken together, provide a concise summary. Main sections are introduced 
with a brief paragraph in italics which outlines the methodological basis of the assessment.
The degree of certainty in key findings in this assessment is based on the author teams’ evaluations of underlying scientific 
understanding and is expressed as a qualitative level of confidence (from very low  to very high ) and, when possible, 
probabilistically with a quantified likelihood (from exceptionally unlikely  to virtually certain ). Confidence in the validity of 
a finding is based on the type, amount, quality, and consistency of evidence (e.g., data, mechanistic understanding, theory, 
models, expert judgment) and the degree of agreement1. Probabilistic estimates of quantified measures of uncertainty in a 
finding are based on statistical analysis of observations or model results, or both, and expert judgment2. Where appropriate, 
findings are also formulated as statements of fact without using uncertainty qualifiers. (See Chapter 1 and Box TS.1 for more 
details about the specific language the IPCC uses to communicate uncertainty).
The basis for substantive paragraphs in this Summary for Policymakers can be found in the chapter sections of the underlying 
report and in the Technical Summary. These references are given in curly brackets. 
B. Observed Changes in the Climate System
Observations of the climate system are based on direct measurements and remote sensing from satellites and other platforms. 
Global-scale observations from the instrumental era began in the mid-19th century for temperature and other variables, with 
more comprehensive and diverse sets of observations available for the period 1950 onwards. Paleoclimate reconstructions 
extend some records back hundreds to millions of years. Together, they provide a comprehensive view of the variability and 
long-term changes in the atmosphere, the ocean, the cryosphere, and the land surface.

SPM Summary for Policymakers5Each of the last three decades has been successively warmer at the Earth’s surface than any 
preceding decade since 1850 (see Figure SPM.1). In the Northern Hemisphere, 1983–2012 
was likely  the warmest 30-year period of the last 1400 years ( medium confidence ). {2.4, 5.3}B.1 Atmosphere
• The globally averaged combined land and ocean surface temperature data as calculated by a linear trend, show a 
warming of 0.85 [0.65 to 1.06] °C3, over the period 1880 to 2012, when multiple independently produced datasets exist.  
The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, 
based on the single longest dataset available4 (see Figure SPM.1). {2.4}
• For the longest period when calculation of regional trends is sufficiently complete (1901 to 2012), almost the entire globe 
has experienced surface warming (see Figure SPM.1). {2.4}
• In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and 
interannual variability (see Figure SPM.1). Due to natural variability, trends based on short records are very sensitive to 
the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming 
over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller 
than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade)5. {2.4}
• Continental-scale surface temperature reconstructions show, with high confidence , multi-decadal periods during 
the Medieval Climate Anomaly (year 950 to 1250) that were in some regions as warm as in the late 20th century. 
These regional warm periods did not occur as coherently across regions as the warming in the late 20th century  (high 
confidence ). {5.5}
• It is virtually certain that globally the troposphere has warmed since the mid-20th century. More complete observations 
allow greater confidence in estimates of tropospheric temperature changes in the extratropical Northern Hemisphere 
than elsewhere. There is medium confidence  in the rate of warming and its vertical structure in the Northern Hemisphere 
extra-tropical troposphere and low confidence  elsewhere. {2.4}
• Confidence  in precipitation change averaged over global land areas since 1901 is low prior to 1951 and medium  
afterwards.  Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has increased since 
1901 ( medium confidence  before and high confidence  after 1951). For other latitudes area-averaged long-term positive 
or negative trends have low confidence  (see Figure SPM.2). {TS TFE.1, Figure 2; 2.5}
• Changes in many extreme weather and climate events have been observed since   about 1950 (see Table SPM.1 for 
details). It is very likely  that the number of cold days and nights has decreased and the number of warm days and nights 
has increased on the global scale6. It is likely  that the frequency of heat waves has increased in large parts of Europe, 
Asia and Australia. There are likely  more land regions where the number of heavy precipitation events has increased than 
where it has decreased. The frequency or intensity of heavy precipitation events has likely  increased in North America and 
Europe. In other continents, confidence  in changes in heavy precipitation events is at most medium . {2.6}
3 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. Uncertainty intervals are not necessarily symmetric about the corresponding 
best estimate. A best estimate of that value is also given where available. 
4 Both methods presented in this bullet were also used in AR4. The first calculates the difference using a best fit linear trend of all points between 1880 and 2012. The second 
calculates the difference between averages for the two periods 1850–1900 and 2003–2012. Therefore, the resulting values and their 90% uncertainty intervals are not 
directly comparable. {2.4}
5 Trends for 15-year periods starting in 1995, 1996, and 1997 are 0.13 [0.02 to 0.24] °C per decade, 0.14 [0.03 to 0.24] °C per decade, and, 0.07 [–0.02 to 0.18] °C per 
decade, respectively.
6 See the Glossary for the definition of these terms: cold days/cold nights, warm days/warm nights, heat waves.

SPMSummary for Policymakers6Figure SPM.1 |  (a) Observed global mean combined land and ocean surface temperature anomalies, from 1850 to 2012 from three data sets. Top panel: 
annual mean values. Bottom panel: decadal mean values including the estimate of uncertainty for one dataset (black). Anomalies are relative to the mean 
of 1961−1990. (b) Map of the observed surface temperature change from 1901 to 2012 derived from temperature trends determined by linear regression 
from one dataset (orange line in panel a). Trends have been calculated where data availability permits a robust estimate (i.e., only for grid boxes with 
greater than 70% complete records and more than 20% data availability in the first and last 10% of the time period). Other areas are white. Grid boxes 
where the trend is significant at the 10% level are indicated by a + sign. For a listing of the datasets and further technical details see the Technical Summary 
Supplementary Material. {Figures 2.19–2.21; Figure TS.2}
Temperature anomaly (°C) relative to 1961–1990(a)
(b) Observed change in surface temperature 1901–2012    −0.6−0.4−0.20.00.20.40.6
Annual average
    −0.6−0.4−0.20.00.20.40.6
1850 1900 1950 2000Decadal average
(°C) Observed globally averaged combined land and ocean 
surface temperature anomaly 1850–2012
−0.6 −0.4 −0.2 00 .2 0.40 .6 0.81 .0 1.25 1.51 .752 .5Year

SPM Summary for Policymakers7Phenomenon and 
direction of trendAssessment that changes occurred (typically 
since 1950 unless otherwise indicated)Assessment of a human  
contribution to observed changes Early 21st century Late 21st century
Warmer and/or fewer 
cold days and nights 
over most land areasVery likely  {2.6}
Very likely
Very likely  Very likely  {10.6}
Likely
Likely   Likely  {11.3} Virtually certain  {12.4}
Virtually certain
Virtually certain  
Warmer and/or more 
frequent hot days and  
nights over most land areasVery likely {2.6}
Very likely
Very likelyVery likely  {10.6}
Likely
Likely (nights only)Likely  {11.3} Virtually certain  {12.4}
Virtually certain
Virtually certain
Warm spells/heat waves.  
Frequency and/or duration  
increases over most 
land areasMedium confidence  on a global scale  
Likely in large parts of Europe, Asia and Australia {2.6}
Medium confidence in many (but not all) regions
LikelyLikelya
 {10.6}
Not formally assessed
More likely than notNot formally assessedb
 {11.3}Very likely 
 {12.4}
Very likely
Very likely
Heavy precipitation events.
Increase in the frequency, 
intensity, and/or amount 
of heavy precipitationLikely more land areas with increases than decreasesc 
 {2.6}
Likely more  land areas with increases than decreases
Likely  over most land areasMedium confidence  
 {7.6, 10.6}
Medium confidence
More likely than notLikely over many land areas  
 {11.3}Very likely  over most of the mid-latitude land  
masses and over wet tropical regions  {12.4}
Likely over many areas
Very likely over most land areas
Increases in intensity  
and/or duration of droughtLow confidence  on a global scale  
Likely changes in some regionsd {2.6}
Medium confidence in some regions
Likely  in many regions, since 1970e  Low confidence  {10.6}
Medium confidencef
More likely than notLow confidenceg {11.3} Likely  (medium confidence) on a regional to  
global scaleh  {12.4}
Medium confidence in some regions
Likelye
Increases in intense 
tropical cyclone activityLow confidence  in long term (centennial) changes  
Virtually certain in North Atlantic since 1970 {2.6}
Low confidence
Likely  in some regions, since 1970  Low confidencei  
 {10.6}
Low confidence
More likely than notLow confidence   
 {11.3}More likely than not  in the Western North Pacific  
and North Atlanticj {14.6}
More likely than not in some basins
Likely
Increased incidence and/or  
magnitude of extreme 
high sea level Likely (since 1970) {3.7}
Likely (late 20th century)
Likely  Likelyk {3.7}
Likelyk
More likely than notkLikelyl {13.7} Very likelyl {13.7}
Very likelym
LikelyLikelihood of further changesTable SPM.1 |  Extreme weather and climate events: Global-scale assessment of recent observed changes, human contribution to the changes, and projected further changes for the early (2016–2035) and late (2081–2100) 21st century. 
Bold indicates where the AR5 (black) provides a revised* global-scale assessment from the SREX (blue) or AR4 (red). Projections for early 21st century were not provided in previous assessment reports. Projections in the AR5 are relative to 
the reference period of 1986–2005, and use the new Representative Concentration Pathway (RCP) scenarios (see Box SPM.1) unless otherwise specified. See the Glossary for definitions of extreme weather and climate events.
* The direct comparison of assessment findings between reports is difficult. For some climate variables, different aspects have been assessed, and the revised guidance note on uncertainties has been used for the SREX and AR5. The availability of new information, improved scientific understanding, continued 
analyses of data and models, and specific differences in methodologies applied in the assessed studies, all contribute to revised assessment findings.
Notes:
a Attribution is based on available case studies. It is likely  that human influence has more than doubled the probability of occurrence of some observed heat waves in some locations.
b Models project near-term increases in the duration, intensity and spatial extent of heat waves and warm spells.
c In most continents, confidence  in trends is not higher than medium  except in North America and Europe where there have been likely  increases in either the frequency or intensity of heavy precipitation with some seasonal and/or regional variation. It is very likely  that there have been increases in central 
North America.
d The frequency and intensity of drought has  likely  increased in the Mediterranean and West Africa, and likely  decreased in central North America and north-west Australia.
e AR4 assessed the area affected by drought. 
f SREX assessed medium confidence  that anthropogenic influence had contributed to some changes in the drought patterns observed in the second half of the 20th century, based on its attributed impact on precipitation and temperature changes. SREX assessed low confidence  in the attribution of changes 
in droughts at the level of single regions.
g There is  low confidence  in projected changes in soil moisture.
h Regional to global-scale projected decreases in soil moisture and increased agricultural drought are likely (medium confidence)  in presently dry regions by the end of this century under the RCP8.5 scenario. Soil moisture drying in the Mediterranean, Southwest US and southern African regions is consistent 
with projected changes in Hadley circulation and increased surface temperatures, so there is high confidence  in likely  surface drying in these regions by the end of this century under the RCP8.5 scenario.
i There is medium confidence  that a reduction in aerosol forcing over the North Atlantic has contributed at least in part to the observed increase in tropical cyclone activity since the 1970s in this region.
j Based on expert judgment and assessment of projections which use an SRES A1B (or similar) scenario.
k Attribution is based on the close relationship between observed changes in extreme and mean sea level.
l There is high confidence  that this increase in extreme high sea level will primarily be the result of an increase in mean sea level. There is low confidence  in region-specific projections of storminess and associated storm surges.
m SREX assessed it to be very likely  that mean sea level rise will contribute to future upward trends in extreme coastal high water levels.

SPMSummary for Policymakers8B.2 Ocean
Ocean warming dominates the increase in energy stored in the climate system, accounting 
for more than 90% of the energy accumulated between 1971 and 2010 ( high confidence ). 
It is virtually certain  that the upper ocean (0−700 m) warmed from 1971 to 2010 (see Figure 
SPM.3), and it likely  warmed between the 1870s and 1971. {3.2, Box 3.1}
• On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C 
per decade over the period 1971 to 2010. Since AR4, instrumental biases in upper-ocean temperature records have been 
identified and reduced, enhancing  confidence in the assessment of change. {3.2}
• It is likely  that the ocean warmed between 700 and 2000 m from 1957 to 2009. Sufficient observations are available for 
the period 1992 to 2005 for a global assessment of temperature change below 2000 m. There were likely  no significant 
observed temperature trends between 2000 and 3000 m for this period. It is likely that the ocean warmed from 3000 m 
to the bottom for this period, with the largest warming observed in the Southern Ocean. {3.2}
• More than 60% of the net energy increase in the climate system is stored in the upper ocean (0–700 m) during the 
relatively well-sampled 40-year period from 1971 to 2010, and about 30% is stored in the ocean below 700 m. The 
increase in upper ocean heat content during this time period estimated from a linear trend is likely  17 [15 to 19] ×  
1022 J 7 (see Figure SPM.3). {3.2, Box 3.1} 
• It is about as  likely as not  that ocean heat content from 0–700 m increased more slowly during 2003 to 2010 than during 
1993 to 2002 (see Figure SPM.3). Ocean heat uptake from 700–2000 m, where interannual variability is smaller, likely  
continued unabated from 1993 to 2009. {3.2, Box 9.2}
• It is very likely that regions of high salinity where evaporation dominates have become more saline, while regions of 
low salinity where precipitation dominates have become fresher since the 1950s. These regional trends in ocean salinity 
provide indirect evidence that evaporation and precipitation over the oceans have changed ( medium confidence ). {2.5, 
3.3, 3.5}
• There is no observational evidence of a trend in the Atlantic Meridional Overturning Circulation (AMOC), based on the 
decade-long record of the complete AMOC and longer records of individual AMOC components. {3.6} Figure SPM.2 |  Maps of observed precipitation change from 1901 to 2010 and from 1951 to 2010 (trends in annual accumulation calculated using the 
same criteria as in Figure SPM.1) from one data set. For further technical details see the Technical Summary Supplementary Material. {TS TFE.1, Figure 2; 
Figure 2.29} −100 −50 −25 −10 −5 −2.5 0 2.5 51 02 55 0 100
(mm yr-1 per decade)1901– 2010 1951– 2010Observed change in annual precipitation over land
7 A constant supply of heat through the ocean surface at the rate of 1 W m–2 for 1 year would increase the ocean heat content by 1.1 × 1022 J.

SPM Summary for Policymakers9B.3 Cryosphere
Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, 
glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern 
Hemisphere spring snow cover have continued to decrease in extent ( high confidence ) (see 
Figure SPM.3). {4.2–4.7}
• The average rate of ice loss8 from glaciers around the world, excluding glaciers on the periphery of the ice sheets9, was 
very likely  226 [91 to 361] Gt yr−1 over the period 1971 to 2009, and very likely  275 [140 to 410] Gt yr−1 over the period 
1993 to 200910. {4.3}
• The average rate of ice loss from the Greenland ice sheet has very likely  substantially increased from 34 [–6 to 74] Gt yr–1 
over the period 1992 to 2001 to 215 [157 to 274] Gt yr–1 over the period 2002 to 2011. {4.4}
• The average rate of ice loss from the Antarctic ice sheet has likely increased from 30 [–37 to 97] Gt yr–1 over the period 
1992–2001 to 147 [72 to 221] Gt yr–1 over the period 2002 to 2011. There is very high confidence  that these losses are 
mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica. {4.4}
• The annual mean Arctic sea ice extent decreased over the period 1979 to 2012 with a rate that was very likely  in the 
range 3.5 to 4.1% per decade (range of 0.45 to 0.51 million km2 per decade), and  very likely  in the range 9.4 to 13.6% 
per decade (range of 0.73 to 1.07 million km2 per decade) for the summer sea ice minimum (perennial sea ice). The 
average decrease in decadal mean extent of Arctic sea ice has been most rapid in summer ( high confidence ); the spatial 
extent has decreased in every season, and in every  successive decade since 1979 ( high confidence ) (see Figure SPM.3). 
There is medium confidence  from reconstructions that over the past three decades, Arctic summer sea ice retreat was 
unprecedented and sea surface temperatures were anomalously high in at least the last 1,450 years. {4.2, 5.5}
• It is very likely  that the annual mean Antarctic sea ice extent increased at a rate in the range of 1.2 to 1.8% per decade 
(range of 0.13 to 0.20 million km2 per decade) between 1979 and 2012. There is high confidence  that there are strong 
regional differences in this annual rate, with extent increasing in some regions and decreasing in others. {4.2}
• There is very high confidence  that the extent of Northern Hemisphere snow cover has decreased since the mid-20th 
century (see Figure SPM.3). Northern Hemisphere snow cover extent decreased 1.6 [0.8 to 2.4] % per decade for March 
and April, and 11.7 [8.8 to 14.6] % per decade for June, over the 1967 to 2012 period. During this period, snow cover 
extent in the Northern Hemisphere did not show a statistically significant increase in any month. {4.5}
• There is high confidence  that permafrost temperatures have increased in most regions since the early 1980s. Observed 
warming was up to 3°C in parts of Northern Alaska (early 1980s to mid-2000s) and up to 2°C in parts of the Russian 
European North (1971 to 2010). In the latter region, a considerable reduction in permafrost thickness and areal extent 
has been observed over the period 1975 to 2005 ( medium confidence ). {4.7}
• Multiple lines of evidence support very substantial Arctic warming since the mid-20th century. {Box 5.1, 10.3}
8 All references to ‘ice loss’ or ‘mass loss’ refer to net ice loss, i.e., accumulation minus melt and iceberg calving. 
9 For methodological reasons, this assessment of ice loss from the Antarctic and Greenland ice sheets includes change in the glaciers on the periphery. These peripheral glaciers 
are thus excluded from the values given for glaciers.
10 100 Gt yr−1 of ice loss is equivalent to about 0.28 mm yr−1 of global mean sea level rise.

SPMSummary for Policymakers101900 1920 1940 1960 1980 2000−20−1001020
Year (1022 J)Change in global average upper ocean heat content (c)
Global average sea level change
1900 1920 1940 1960 1980 2000−50050100150200
Year(mm)(d)Arctic summer sea ice extent
1900 1920 1940 1960 1980 2000468101214
Year(million km2)(b)Northern Hemisphere spring snow cover
1900 1920 1940 1960 1980 200030354045
Year(million km2)(a)
Figure SPM.3 |  Multiple observed indicators of a changing global climate: (a) Extent of Northern Hemisphere March-April (spring) average snow cover; (b) 
extent of Arctic July-August-September (summer) average sea ice; (c) change in global mean upper ocean (0–700 m) heat content aligned to 2006−2010, 
and relative to the mean of all datasets for 1970; (d) global mean sea level relative to the 1900–1905 mean of the longest running dataset, and with all 
datasets aligned to have the same value in 1993, the first year of satellite altimetry data. All time-series (coloured lines indicating different data sets) show 
annual values, and where assessed, uncertainties are indicated by coloured shading. See Technical Summary Supplementary Material for a listing of the 
datasets. {Figures 3.2, 3.13, 4.19, and 4.3; FAQ 2.1, Figure 2; Figure TS.1}

SPM Summary for Policymakers11B.4 Sea Level
The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have 
increased to levels unprecedented in at least the last 800,000 years. Carbon dioxide 
concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel 
emissions and secondarily from net land use change emissions. The ocean has absorbed 
about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification (see 
Figure SPM.4). {2.2, 3.8, 5.2, 6.2, 6.3}
11 ppm (parts per million) or ppb (parts per billion, 1 billion = 1,000 million) is the ratio of the number of gas molecules to the total number of molecules of dry air. For example, 
300 ppm means 300 molecules of a gas per million molecules of dry air.The rate of sea level rise since the mid-19th century has been larger than the mean rate 
during the previous two millennia ( high confidence ). Over the period 1901 to 2010, global 
mean sea level rose by 0.19 [0.17 to 0.21] m (see Figure SPM.3). {3.7, 5.6, 13.2}
• Proxy and instrumental sea level data indicate a transition in the late 19th to the early 20th century from relatively low 
mean rates of rise over the previous two millennia to higher rates of rise ( high confidence ). It is likely  that the rate of 
global mean sea level rise has continued to increase since the early 20th century. {3.7, 5.6, 13.2}
• It is very likely  that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm yr–1 between 1901 and 2010, 
2.0 [1.7 to 2.3] mm yr–1 between 1971 and 2010, and 3.2 [2.8 to 3.6] mm yr–1 between 1993 and 2010. Tide-gauge and 
satellite altimeter data are consistent regarding the higher rate of the latter period. It is likely  that similarly high rates 
occurred between 1920 and 1950. {3.7}
• Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the 
observed global mean sea level rise ( high confidence ). Over the period 1993 to 2010, global mean sea level rise is, with 
high confidence , consistent with the sum of the observed contributions from ocean thermal expansion due to warming 
(1.1 [0.8 to 1.4] mm yr–1), from changes in glaciers (0.76 [0.39 to 1.13] mm yr–1), Greenland ice sheet (0.33 [0.25 to 0.41] 
mm yr–1), Antarctic ice sheet (0.27 [0.16 to 0.38] mm yr–1), and land water storage (0.38 [0.26 to 0.49] mm yr–1). The sum 
of these contributions is 2.8 [2.3 to 3.4] mm yr–1. {13.3}
• There is very high confidence  that maximum global mean sea level during the last interglacial period (129,000 to 116,000 
years ago) was, for several thousand years, at least 5 m higher than present, and high confidence  that it did not exceed 
10 m above present. During the last interglacial period, the Greenland ice sheet very likely  contributed between 1.4 and 
4.3 m to the higher global mean sea level, implying with medium confidence  an additional contribution from the Antarctic 
ice sheet. This change in sea level occurred in the context of different orbital forcing and with high-latitude surface 
temperature, averaged over several thousand years, at least 2°C warmer than present ( high confidence ). {5.3, 5.6}
B.5 Carbon and Other Biogeochemical Cycles
• The atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) 
have all increased since 1750 due to human activity. In 2011 the concentrations of these greenhouse gases were 391 
ppm11, 1803 ppb, and 324 ppb, and exceeded the pre-industrial levels by about 40%, 150%, and 20%, respectively. {2.2, 
5.2, 6.1, 6.2}
• Concentrations of CO2, CH4, and N2O now substantially exceed the highest concentrations recorded in ice cores during 
the past 800,000 years. The mean rates of increase in atmospheric concentrations over the past century are, with very 
high confidence , unprecedented in the last 22,000 years. {5.2, 6.1, 6.2}

SPMSummary for Policymakers12• Annual CO2 emissions from fossil fuel combustion and cement  production were 8.3 [7.6 to 9.0] GtC12 yr–1 averaged over 
2002–2011  (high confidence ) and were 9.5 [8.7 to 10.3] GtC yr–1 in 2011, 54% above the 1990 level. Annual net CO2 
emissions from  anthropogenic land use change were 0.9 [0.1 to 1.7] GtC yr–1 on average during 2002 to 2011 ( medium 
confidence ). {6.3}
• From 1750 to 2011, CO2 emissions from fossil fuel combustion and cement production have released 375 [345 to 405] 
GtC to the atmosphere, while deforestation and other land use change are estimated to have released 180 [100 to 260] 
GtC. This results in cumulative anthropogenic emissions of 555 [470 to 640] GtC. {6.3}
• Of these cumulative anthropogenic CO2 emissions, 240 [230 to 250] GtC have accumulated in the atmosphere, 155 [125 
to 185] GtC have been taken up by the ocean and 160 [70 to 250] GtC have accumulated in natural terrestrial ecosystems 
(i.e., the cumulative residual land sink). {Figure TS.4, 3.8, 6.3}
• Ocean acidification is quantified by decreases in pH13. The pH of ocean surface water has decreased by 0.1 since the 
beginning of the industrial era ( high confidence ), corresponding to a 26% increase in hydrogen ion concentration (see 
Figure SPM.4). {3.8, Box 3.2}
Figure SPM.4 |  Multiple observed indicators of a changing global carbon cycle: (a) atmospheric concentrations of carbon dioxide (CO2) from Mauna Loa 
(19°32’N, 155°34’W – red) and South Pole (89°59’S, 24°48’W – black) since 1958; (b) partial pressure of dissolved CO2 at the ocean surface (blue curves) 
and in situ pH (green curves), a measure of the acidity of ocean water. Measurements are from three stations from the Atlantic (29°10’N, 15°30’W – dark 
blue/dark green; 31°40’N, 64°10’W – blue/green) and the Pacific Oceans (22°45’N, 158°00’W − light blue/light green). Full details of the datasets shown 
here are provided in the underlying report and the Technical Summary Supplementary Material. {Figures 2.1 and 3.18; Figure TS.5}(a)
(b)1950 1960 1970 1980 1990 2000 2010300320340360380400
YearCO 2 (ppm) 
1950 1960 1970 1980 1990 2000 2010320340360380400
YearpCO 2 (μatm)
8.068.098.12
in situ pH unitSurface ocean CO 2 and pH  Atmospheric CO 2 
12 1 Gigatonne of carbon = 1 GtC = 1015 grams of carbon. This corresponds to 3.667 GtCO2.
13 pH is a measure of acidity using a logarithmic scale: a pH decrease of 1 unit corresponds to a 10-fold increase in hydrogen ion concentration, or acidity. 

SPM Summary for Policymakers1314 The strength of drivers is quantified as Radiative Forcing (RF) in units watts per square metre (W m–2) as in previous IPCC assessments. RF is the change in energy flux 
caused by a driver, and is calculated at the tropopause or at the top of the atmosphere. In the traditional RF concept employed in previous IPCC reports all surface and 
tropospheric conditions are kept fixed. In calculations of RF for well-mixed greenhouse gases and aerosols in this report, physical variables, except for the ocean and sea 
ice, are allowed to respond to perturbations with rapid adjustments. The resulting forcing is called Effective Radiative Forcing (ERF) in the underlying report. This change 
reflects the scientific progress from previous assessments and results in a better indication of the eventual temperature response for these drivers. For all drivers other than 
well-mixed greenhouse gases and aerosols, rapid adjustments are less well characterized and assumed to be small, and thus the traditional RF is used. {8.1}
15 This approach was used to report RF in the AR4 Summary for Policymakers.Total radiative forcing is positive, and has led to an uptake of energy by the climate system. 
The largest contribution to total radiative forcing is caused by the increase in the atmospheric 
concentration of CO2 since 1750 (see Figure SPM.5). {3.2, Box 3.1, 8.3, 8.5}C. Drivers of Climate Change
Natural and anthropogenic substances and processes that alter the Earth’s energy budget are drivers of climate change. 
Radiative forcing14 (RF) quantifies the change in energy fluxes caused by changes in these drivers for 2011 relative to 1750, 
unless otherwise indicated. Positive RF leads to surface warming, negative RF leads to surface cooling. RF is estimated based 
on in-situ and remote observations, properties of greenhouse gases and aerosols, and calculations using numerical models 
representing observed processes. Some emitted compounds affect the atmospheric concentration of other substances. The RF 
can be reported based on the concentration changes of each substance15. Alternatively, the emission-based RF of a compound 
can be reported, which provides a more direct link to human activities. It includes contributions from all substances affected 
by that emission. The total anthropogenic RF of the two approaches are identical when considering all drivers. Though both 
approaches are used in this Summary for Policymakers, emission-based RFs are emphasized.
• The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W m−2 (see Figure SPM.5), and it has increased 
more rapidly since 1970 than during prior decades. The total anthropogenic RF best estimate for 2011 is 43% higher than 
that reported in AR4 for the year 2005. This is caused by a combination of continued growth in most greenhouse gas 
concentrations and improved estimates of RF by aerosols indicating a weaker net cooling effect (negative RF). {8.5}
• The RF from emissions of well-mixed greenhouse gases (CO2, CH4, N2O, and Halocarbons) for 2011 relative to 1750 is 
3.00 [2.22 to 3.78] W m–2 (see Figure SPM.5). The RF from changes in concentrations in these gases is 2.83 [2.26 to 3.40] 
W m–2. {8.5}
• Emissions of CO2 alone have caused an RF of 1.68 [1.33 to 2.03] W m–2 (see Figure SPM.5). Including emissions of other 
carbon-containing gases, which also contributed to the increase in CO2 concentrations, the RF of CO2 is 1.82 [1.46 to 
2.18] W m–2. {8.3, 8.5}
• Emissions of CH4 alone have caused an RF of 0.97 [0.74 to 1.20] W  m−2 (see Figure SPM.5). This is much larger than the 
concentration-based estimate of 0.48 [0.38 to 0.58] W m−2 (unchanged from AR4). This difference in estimates is caused 
by concentration changes in ozone and stratospheric water vapour due to CH4 emissions and other emissions indirectly 
affecting CH4. {8.3, 8.5}
• Emissions of stratospheric ozone-depleting halocarbons have caused a net positive RF of 0.18 [0.01 to 0.35] W m−2 (see 
Figure SPM.5). Their own positive RF has outweighed the negative RF from the ozone depletion that they have induced. 
The positive RF from all halocarbons is similar to the value in AR4, with a reduced RF from CFCs but increases from many 
of their substitutes. {8.3, 8.5}
• Emissions of short-lived gases contribute to the total anthropogenic RF . Emissions of carbon monoxide (CO) are virtually 
certain  to have induced a positive RF , while emissions of nitrogen oxides (NOx) are likely  to have induced a net negative 
RF (see Figure SPM.5). {8.3, 8.5}
• The RF of the total aerosol effect in the atmosphere, which includes cloud adjustments due to aerosols, is –0.9 [–1.9 to 
−0.1] W m−2 (medium confidence ), and results from a negative forcing from most aerosols and a positive contribution 

SPMSummary for Policymakers14from black carbon absorption of solar radiation. There is high confidence  that  aerosols and their interactions with clouds 
have offset a substantial portion of global mean forcing from well-mixed greenhouse gases. They continue to contribute 
the largest uncertainty to the total RF estimate. {7.5, 8.3, 8.5}
• The forcing from stratospheric volcanic aerosols can have a large impact on the climate for some years after volcanic 
eruptions. Several small eruptions have caused an RF of –0.11 [–0.15 to –0.08] W m–2 for the years 2008 to 2011, which 
is approximately twice as strong as during the years 1999 to 2002. {8.4}
• The RF due to changes in solar irradiance is estimated as 0.05 [0.00 to 0.10] W m−2 (see Figure SPM.5). Satellite obser -
vations of total solar irradiance changes from 1978 to 2011 indicate that the last solar minimum was lower than the 
previous two. This results in an RF of –0.04 [–0.08 to 0.00] W m–2 between the most recent minimum in 2008 and the 
1986 minimum. {8.4}
• The total natural RF from solar irradiance changes and stratospheric volcanic aerosols made only a small contribution to 
the net radiative forcing throughout the last century, except for brief periods after large volcanic eruptions. {8.5}
Figure SPM.5 |  Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climate change. Values are 
global average radiative forcing (RF14), partitioned according to the emitted compounds or processes that result in a combination of drivers. The best esti -
mates of the net radiative forcing are shown as black diamonds with corresponding uncertainty intervals; the numerical values are provided on the right 
of the figure, together with the confidence level in the net forcing (VH – very high , H – high, M – medium , L – low, VL – very low ). Albedo forcing due to 
black carbon on snow and ice is included in the black carbon aerosol bar. Small forcings due to contrails (0.05 W m–2, including contrail induced cirrus), 
and HFCs, PFCs and SF6 (total 0.03 W m–2) are not shown. Concentration-based RFs for gases can be obtained by summing the like-coloured bars. Volcanic 
forcing is not included as its episodic nature makes is difficult to compare to other forcing mechanisms. Total anthropogenic radiative forcing is provided 
for three different years relative to 1750. For further technical details, including uncertainty ranges associated with individual components and processes, 
see the Technical Summary Supplementary Material. {8.5; Figures 8.14–8.18; Figures TS.6 and TS.7}
Anthropogeni c Natural
−1 0 1 2 3 
 
 
Radiative forcing relative to 1750 (W m−2)Level of
confidenceRadiative forcing by emissions and drivers
1.68 [1.33 to 2.03] 
0.97 [0.74 to 1.20]
0.18 [0.01 to 0.35]
0.17 [0.13 to 0.21]
0.23 [0.16 to 0.30]
0.10 [0.05 to 0.15]
-0.15 [-0.34 to 0.03]
-0.27 [-0.77 to 0.23]
-0.55 [-1.33 to -0.06]
-0.15 [-0.25 to -0.05]
0.05 [0.00 to 0.10]
2.29 [1.13 to 3.33]
1.25 [0.64 to 1.86]
0.57 [0.29 to 0.85]VH
H
H
VH
M
M
M
H
L
M
M
H
H
MCO2
CH4
Halo-
carbons
N2O
CO
NMVOC
NOxEmitted
compound
Aerosols and
precursors
(Mineral dust , 
SO2, NH3,
Organic carbon
and Black carbon )Well-mixed greenhouse gases Short lived gases and aerosolsResulting atmospheric
drivers
CO2
CO2H2OstrO3CH4
O3CFCs HCFCs
CO2CH4O3N2O
CO2CH4O3
Nitrate CH4O3
Black carbonMineral dust
Organic carbonNitrate Sulphate
Cloud adjustments
due to aerosols
Albedo change
due to land use
Changes in
solar irradiance
Total anthropogenic
RF relative to 1750
195019802011