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Summary
for Policymakers
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)
This 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.
3
SPM
1
Summary
for Policymakers
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)
This 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.
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Summary for Policymakers
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
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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 agreement
. Probabilistic estimates of quantified measures of uncertainty in a
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finding are based on statistical analysis of observations or model results, or both, and expert judgment
. Where appropriate,
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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.
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}
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:
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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).
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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).
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Summary for Policymakers
B.1 Atmosphere
Each 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}
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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] °C
, over the period 1880 to 2012, when multiple independently produced datasets exist.
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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 available
(see Figure SPM.1). {2.4}
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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)
. {2.4}
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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 scale
. It is likely that the frequency of heat waves has increased in large parts of Europe,
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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}
In the WGI contribution to the AR5, uncertainty is quantified using 90% uncertainty intervals unless otherwise stated. The 90% uncertainty interval, reported in square
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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.
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
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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}
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
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decade, respectively.
See the Glossary for the definition of these terms: cold days/cold nights, warm days/warm nights, heat waves.
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Summary for Policymakers
Observed globally averaged combined land and ocean
(a)
surface temperature anomaly
1850–2012
0.6
Annual average
0.4
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0.2
0.0
−0.2
−0.4
−0.6
0.6
Decadal average
0.4
0.2
0.0
−0.2
−0.4
−0.6
1850
1900
1950
2000
Year
(b)
Observed change in surface temperature 1901–2012
−0.6 −0.4 −0.2
0
0.2
0.4
0.6
0.8
1.0
1.25 1.5
1.75
2.5
(°C)
Figure 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}
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Summary for Policymakers
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