View source for IPCC Fourth Assessment Report

{{short description|2007 IPCC report}}
{{use dmy dates|date=August 2021|cs1-dates=y}}
<noinclude>
{| class="wikitable" align="right" border="1" width="350"
| colspan="3" align="center" style="background:#B3B7FF" |The four ''[[Special Report on Emissions Scenarios|SRES]]'' scenario families<ref>according to: [http://www.cics.uvic.ca/scenarios/index.cgi?More_Info-Emissions Canadian Institute for Climate Studies, CCIS project: ''Frequently Asked Questions ''] {{webarchive |url=https://web.archive.org/web/20160118101108/http://www.cics.uvic.ca/scenarios/index.cgi?More_Info-Emissions |date=January 18, 2016 }}; [http://www.grida.no/climate/ipcc/emission/091.htm#4.2.1. Table 4- 2: Overview of SRES scenario quantifications] {{webarchive |url=https://web.archive.org/web/20160602233315/http://www.grida.no/climate/ipcc/emission/091.htm#4.2.1. |date=June 2, 2016 }}, in {{Harvnb|IPCC SRES|2000}}; [http://www.grida.no/climate/ipcc_tar/wg3/081.htm Figure 2.11: Schematic illustration of SRES scenarios] {{webarchive |url=https://web.archive.org/web/20160602233315/http://www.grida.no/climate/ipcc_tar/wg3/081.htm |date=June 2, 2016 }}, in the ''Third'' AR?? (That's the url: ipcc_''tar''.){{Citation needed|date=September 2011}}
</ref> of the ''Fourth Assessment Report'' vs. projected global average surface warming until 2100
|-
|{{center|<small>[http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf AR4 <br />(Summary; PDF)]{{Page needed|date=September 2011}}</small>}}
|<br/>{{center|<small>More&nbsp;economic&nbsp;focus</small>}}
|<br/>{{center|<small>More&nbsp;environmental&nbsp;focus</small>}}
|- valign="middle"
|[[Globalisation]]<br /><small>(homogeneous&nbsp;world)</small>
| align="center" |'''A1'''<br /> <small>rapid&nbsp;economic&nbsp;growth</small><br /><small>(groups:&nbsp;A1T;&nbsp;A1B;&nbsp;A1Fl)</small><br />'''1.4–6.4&nbsp;°C'''
| align="center" | '''B1'''<br /><small>global environmental sustainability</small><small>&nbsp; </small><br />'''1.1–2.9&nbsp;°C'''
|- valign="middle"
|[[Regionalisation]]<br /><small>(heterogeneous&nbsp;world)</small>
| align="center" | '''A2'''<br /><small>regionally&nbsp;oriented<br />economic&nbsp;development</small><br />'''2.0–5.4&nbsp;°C'''
| align="center" | '''B2'''<br /><small>local environmental sustainability</small><br />'''1.4–3.8&nbsp;°C'''
|}
</noinclude>
''Climate Change 2007'', the '''Fourth Assessment Report''' ('''AR4''') of the [[United Nations]] [[Intergovernmental Panel on Climate Change]] (IPCC), was published in 2007 and is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning [[climate change]], its potential effects, and options for [[climate change adaptation|adaptation]] and mitigation.<ref name="ar4 press flyer">[https://www.ipcc.ch/site/assets/uploads/2018/04/ipcc-flyer-low.pdf Press flyer announcing 2007 report] {{Webarchive|url=https://web.archive.org/web/20210625172322/https://www.ipcc.ch/site/assets/uploads/2018/04/ipcc-flyer-low.pdf |date=2021-06-25  }} IPCC</ref> The report is the largest and most detailed summary of the climate change situation ever undertaken, produced by thousands of authors, editors, and reviewers from dozens of countries, citing over 6,000 [[Peer-reviewed journal|peer-reviewed]] scientific studies. People from over 130 countries contributed to the IPCC Fourth Assessment Report, which took six years to produce.<ref name="ar4 press flyer" /> Contributors to AR4 included more than 2,500 scientific expert reviewers, more than 800 contributing authors, and more than 450 lead authors.<ref name="ar4 press flyer" /> 

<noinclude>"Robust findings" of the Synthesis report include:<ref name="ar4 main observations">{{cite book |title=Synthesis Report of the IPCC Fourth Assessment Report (Climate Change 2007): Summary for Policymakers |date=2007 |publisher=IPCC |isbn=978-92-9169-122-7 |url=https://www.ipcc.ch/report/ar4/syr/ |access-date=2021-08-12  |archive-date=2021-08-20  |archive-url=https://web.archive.org/web/20210820204443/https://www.ipcc.ch/report/ar4/syr/ |url-status=live }}</ref>
* "Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level".<ref name="ar4 main observations" />
* Most of the global average warming over the past 50 years is "very likely" (greater than 90% probability, based on expert judgement)<ref name="ar4 uncertainty">{{cite book |title=Synthesis Report of the IPCC Fourth Assessment Report (Climate Change 2007) |date=2007 |publisher=IPCC |location=Introduction |isbn=978-92-9169-122-7 |page=27 |url=https://www.ipcc.ch/report/ar4/syr/ |access-date=2021-08-12  |archive-date=2021-08-20  |archive-url=https://web.archive.org/web/20210820204443/https://www.ipcc.ch/report/ar4/syr/ |url-status=live }}</ref> due to human activities.<ref name="ar4 main observations" />
* "Impacts [of climate change] will very likely increase due to increased frequencies and intensities of some [[extreme weather]] events".<ref name="ar4 main observations" />
* "Anthropogenic warming and sea level rise would continue for centuries even if GHG emissions were to be reduced sufficiently for GHG concentrations to stabilise, due to the time scales associated with climate processes and feedbacks".<ref name="ar4 main observations" /> Stabilization of atmospheric greenhouse gas concentrations is discussed in [[climate change mitigation]].
* "Some planned adaptation (of human activities) is occurring now; more extensive adaptation is required to reduce vulnerability to climate change".<ref name="ar4 main observations" />
* "Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt".<ref name="ar4 main observations" />
* "Many impacts [of climate change] can be reduced, delayed or avoided by mitigation".<ref name="ar4 main observations" /></noinclude>

{{TOC limit|3}}

==Overview==
Like previous assessment reports, it consists of four reports:
* Working Group I: The Physical Science Basis
* Working Group II: Impacts, Adaptation and Vulnerability
* Working Group III: Mitigation
* Synthesis Report
Global warming projections from AR4<ref name="ar4 main observations"/> are shown below. The projections apply to the end of the 21st century (2090–2099), relative to temperatures at the end of the 20th century (1980–1999). Add 0.7&nbsp;°C to projections to make them relative to pre-industrial levels instead of 1980–1999. (UK Royal Society, 2010, p=10{{citation needed|date=August 2019}}).<ref>{{Cite web|url=https://www.ipcc.ch/sr15/chapter/chapter-3/|title=Impacts of 1.5 °C global warming on natural and human systems|last=Hoegh-Guldberg|date=5 February 2020|access-date=2021-08-12 |archive-date=2021-07-26 |archive-url=https://web.archive.org/web/20210726094953/https://www.ipcc.ch/sr15/chapter/chapter-3/|url-status=live}}</ref> Descriptions of the greenhouse gas emissions scenarios can be found in [[Special Report on Emissions Scenarios]].

{| class="wikitable"
|+ AR4 global warming projections<ref name="ar4 main observations"/>
! Emissions<br />scenario!! Best estimate<br />(°C) !! "Likely" range<br />(°C)
|-
| B1 || 1.8 || 1.1 – 2.9
|-
| A1T || 2.4 || 1.4 – 3.8
|-
| B2 || 2.4 || 1.4 – 3.8
|-
| A1B || 2.8 || 1.7 – 4.4
|-
| A2 || 3.4 || 2.0 – 5.4
|-
| A1FI || 4.0 || 2.4 – 6.4
|}
<!--|}-->

"Likely" means greater than 66% probability of being correct, based on expert judgement.<ref name="ar4 uncertainty"/>

==Sections==
{{IPCC}}
The report was released in four principal sections:

* Contribution of Working Group I (WGI): ''Climate Change 2007: The Physical Science Basis''.<ref name="IPCC AR4 WG1 2007">{{Harvnb|IPCC AR4 WG1|2007}}.</ref>
* Contribution of Working Group II (WGII): ''Climate Change 2007: Impacts, Adaptation and Vulnerability''.<ref>{{Harvnb|IPCC AR4 WG2|2007}}.</ref>
* Contribution of Working Group III (WGIII): ''Climate Change 2007: Mitigation of Climate Change''.<ref>{{Harvnb|IPCC AR4 WG3|2007}}.</ref>
* Contribution of Working Groups I, II, and III: ''The Synthesis Report'' (SYR).<ref>{{Harvnb|IPCC AR4 SYR|2007}}.</ref>

== Working Group I: The Physical Science Basis ==

The full WGI report<ref name="IPCC AR4 WG1 2007"/> was published in March 2007, and last updated in September of that year. It includes a ''Summary for Policymakers'' (SPM), which was published in February 2007, and a ''Frequently Asked Questions'' section.

This section of the report, ''Climate Change 2007: The Physical Science Basis'', assessed current scientific knowledge of "the natural and human drivers of climate change" as well as observed changes in climate. It looked at the ability of science to attribute changes to different causes, and made projections of future climate change.

It was produced by 676 authors (152 lead authors, 26 review editors, and 498 contributing authors) from 40 countries, then reviewed by over 625 expert reviewers. More than 6,000 peer-reviewed publications were cited.<ref>[https://www.ipcc-wg1.unibe.ch/statement/WGIsummary22122009.html IPCC website: Summary Description of the IPCC Process] {{webarchive |url=https://web.archive.org/web/20160622190918/https://www.ipcc-wg1.unibe.ch/statement/WGIsummary22122009.html |date=June 22, 2016 }}.</ref>

Before being approved, the summary was reviewed line by line by representatives of 113 governments during the 10th session of WGI, in January to February 2007.<ref>[http://www.ipcc.ch/press/prwg2feb07.htm IPCC adopts major assessment of climate change science] {{webarchive |url=https://web.archive.org/web/20071115201553/http://www.ipcc.ch/press/prwg2feb07.htm |date=November 15, 2007 }}</ref>

On the issue of global warming and its causes, the SPM states that:
*"Warming of the climate system is [[wikt:unequivocal|unequivocal]]."<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-direct-observations.html SPM: Direct Observations of Recent Climate Change] {{webarchive|url=https://web.archive.org/web/20160516191232/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-direct-observations.html |date=May 16, 2016 }}, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf p. 5] {{Webarchive|url=https://web.archive.org/web/20171220143410/http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf |date=2017-12-20  }}, and figure SPM.3, in {{Harvnb|IPCC AR4 WG1|2007}}.</ref>
*"Most of the observed increase in global average temperatures since the mid-20th century is'' very likely'' due to the observed increase in [[human impact on the environment|anthropogenic]] greenhouse gas concentrations."<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-understanding-and.html SPM: Understanding and Attributing Climate Change] {{webarchive|url=https://web.archive.org/web/20160524223539/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-understanding-and.html |date=May 24, 2016 }}, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf p. 10] {{Webarchive|url=https://web.archive.org/web/20171220143410/http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf |date=2017-12-20  }}, in {{Harvnb|IPCC AR4 WG1|2007}}.</ref>

''Very likely'' and ''likely'' mean "the assessed likelihood, using expert judgment" are over 90% and over 66%, respectively.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html SPM: footnote 6] {{Webarchive|url=https://web.archive.org/web/20181102212113/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html |date=2018-11-02  }}, in {{Harvnb|IPCC AR4 WG1|2007}}.</ref>

=== Observations ===

The report notes many observed changes in the Earth's climate including atmospheric composition, global average temperatures, ocean conditions, and other climate changes.

==== Changes in the atmosphere ====

[[Carbon dioxide]], [[methane]], and [[nitrous oxide]] are all long-lived [[greenhouse gases]].
* "Carbon dioxide, methane, and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values."<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html Summary for Policy Makers, WG1, AR4, pg 3.] {{webarchive |url=https://web.archive.org/web/20161021173115/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html |date=October 21, 2016 }}</ref>
* The amount of carbon dioxide in the atmosphere in 2005 (379&nbsp;[[Parts per notation|ppm]]) exceeds by far the natural range of the last 650,000 years (180 to 300&nbsp;ppm).{{Page needed|date=September 2011}}
* The amount of methane in the atmosphere in 2005 (1774&nbsp;[[Parts per notation|ppb]]) exceeds by far the natural range of the last 650,000 years (320 to 790&nbsp;ppb).{{Page needed|date=September 2011}}
* The primary source of the increase in carbon dioxide is [[fossil fuel]] use, but [[land-use change]]s also make a contribution.{{Page needed|date=September 2011}}
* The primary source of the increase in methane is very likely to be a combination of human [[agriculture|agricultural activities]] and fossil fuel use. How much each contributes is not well determined.{{Page needed|date=September 2011}}
* Nitrous oxide concentrations have risen from a pre-industrial value of 270&nbsp;ppb to a 2005 value of 319&nbsp;ppb.  More than a third of this rise is due to human activity, primarily agriculture.{{Page needed|date=September 2011}}

==== Warming of the planet ====

Cold days, cold nights, and frost events have become less frequent.  Hot days, hot nights, and heat waves have become more frequent.  Additionally:
* Eleven of the twelve years in the period (1995–2006) rank among the top 12 warmest years in the instrumental record (since 1880).
* Warming in the last 100 years has caused about a 0.74&nbsp;°C increase in global average temperature. This is up from the 0.6&nbsp;°C increase in the 100 years prior to the Third Assessment Report.
* [[Urban heat island]] effects were determined to have negligible influence (less than 0.0006&nbsp;°C per decade over land and zero over oceans) on these measurements.
* Observations since 1961 show that the ocean has been absorbing more than 80% of the heat added to the climate system, and that ocean temperatures have increased to depths of at least 3000&nbsp;m (9800&nbsp;ft).
* "Average Arctic temperatures increased at almost twice the global average rate in the past 100 years."
* It is likely that greenhouse gases would have caused more warming than we have observed if not for the cooling effects of volcanic and human-caused aerosols. See [[global dimming]].
* Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years (including both the [[Medieval Warm Period]] and the [[Little Ice Age]]).

==== Ice, snow, permafrost, rain, and the oceans ====

The SPM documents increases in wind intensity, decline of permafrost coverage, and increases of both drought and heavy precipitation events. Additionally:
* "Mountain [[glacier]]s and snow cover have declined on average in both hemispheres."
* Losses from the land-based [[ice sheets]] of Greenland and Antarctica have very likely (>90%) contributed to sea level rise between 1993 and 2003.
* [[Ocean warming]] causes seawater to expand, which contributes to sea level rising.
* Sea level rose at an average rate of about 1.8&nbsp;mm/year during the years 1961–2003. The rise in sea level during 1993–2003 was at an average rate of 3.1&nbsp;mm/year. It is not clear whether this is a long-term trend or just variability.
* Antarctic [[sea ice]] shows no significant overall trend, consistent with a lack of warming in that region.

==== Hurricanes ====

* There has been an increase in [[Tropical cyclone|hurricane]] intensity in the [[North Atlantic]] since the 1970s, and that increase correlates with increases in [[sea surface temperature]].
* The observed increase in hurricane intensity is larger than [[climate model]]s predict for the sea surface temperature changes we have experienced.
* There is no clear trend in the number of hurricanes.
* Other regions appear to have experienced increased hurricane intensity as well, but there are concerns about the quality of data in these other regions.
* It is more likely than not (>50%) that there has been some human contribution to the increases in hurricane intensity.
* It is likely (>66%) that we will see increases in hurricane intensity during the 21st century.

Table SPM-2 lists recent trends along with certainty levels for the trend having actually occurred, for a human contribution to the trend, and for the trend occurring in the future. In relation to changes (including increased hurricane intensity) where the certainty of a human contribution is stated as "more likely than not" footnote f to table SPM-2 notes "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies."

=== Factors that warm or cool the planet ===
[[Image:Radiative-forcings.svg|thumb|300px|Changes in [[radiative forcing]]s between 1750 and 2005 as estimated by the [[Intergovernmental Panel on Climate Change|IPCC]]]]
AR4 describes warming and cooling effects on the planet in terms of [[radiative forcing]]—the rate of change of energy in the system, measured as power per unit area (in SI units, W/m<sup>2</sup>). The report shows in detail the individual warming contributions (positive forcing) of carbon dioxide, methane, nitrous oxide, [[halocarbons]], other human warming factors, and the warming effects of changes in solar activity.  Also shown are the cooling effects (negative forcing) of [[Particulate|aerosols]], [[land cover|land-use]] changes, and other human activities. All values are shown as a change from pre-industrial conditions.
* Total radiative forcing from the sum of all human activities is about +1.6 watts/m<sup>2</sup>
* Radiative forcing from an increase of [[Solar radiation|solar intensity]] since 1750 is about +0.12 watts/m<sup>2</sup>
* Radiative forcing from carbon dioxide, methane, and nitrous oxide combined is very likely (>90%) increasing more quickly during the current era (1750–present) than at any other time in the last 10,000 years.

=== Climate sensitivity ===

[[Climate sensitivity]] is defined as the amount of global average surface warming following a doubling of carbon dioxide concentrations.<ref name="ClimSens">[http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains2-3.html SYR 2.3: Climate sensitivity and feedbacks] {{webarchive |url=https://web.archive.org/web/20160524223541/http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains2-3.html |date=May 24, 2016 }}</ref> It is likely to be in the range of 2 to 4.5&nbsp;°C, with a best estimate of about 3&nbsp;°C.<ref name ="ClimSens"/> This range of values is not a projection of the temperature rise we will see in the 21st century, since the future change in carbon dioxide concentrations is unknown, and factors besides carbon dioxide concentrations affect temperature.<ref name ="ClimSens"/>

=== Model-based projections for the future ===

Model projections are made based on an analysis of various computer climate models running within the different scenarios that were established in 2000 in the [[Special Report on Emissions Scenarios]] (the "SRES scenarios"). As a result, predictions for the 21st century are as shown below.
* Surface air warming in the 21st century:
**Best estimate for a "low scenario"<ref name="low">"''... a convergent world with the same global population, that peaks in mid-century and declines thereafter, ... but with rapid change in economic structures toward a service and information economy, with reductions in material intensity and the introduction of clean and resource efficient technologies. The emphasis is on global solutions to economic, social and environmental sustainability, including improved equity, but without additional climate initiatives.''" [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html SPM: Projections of Future Changes in Climate] {{webarchive |url=https://web.archive.org/web/20161126145250/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html |date=November 26, 2016 }}</ref> is 1.8&nbsp;°C with a likely range of 1.1 to 2.9&nbsp;°C (3.2&nbsp;°F with a likely range of 2.0 to 5.2&nbsp;°F)
**Best estimate for a "high scenario"<ref name="high">"''... a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. ... technological emphasis: fossil intensive''" [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html SPM: Projections of Future Changes in Climate] {{webarchive |url=https://web.archive.org/web/20161126145250/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html |date=November 26, 2016 }}</ref> is 4.0&nbsp;°C with a likely range of 2.4 to 6.4&nbsp;°C (7.2&nbsp;°F with a likely range of 4.3 to 11.5&nbsp;°F)
**A temperature rise of about 0.1&nbsp;°C per decade would be expected for the next two decades, even if greenhouse gas and aerosol concentrations were kept at year 2000 levels.
**A temperature rise of about 0.2&nbsp;°C per decade is projected for the next two decades for all SRES scenarios.
**Confidence in these near-term projections is strengthened because of the agreement between past model projections and actual observed temperature increases.
*Based on multiple models that all exclude [[ice sheet]] flow due to a lack of basis in published literature,<ref>This contrasts with the TAR, which included these [[ice dynamics]], and had a higher top end sea level rise estimate. The report states that recent observations suggest that ice flow dynamics could lead to additional rise: "''Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.''" [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html SPM: Projections of Future Changes in Climate] {{webarchive |url=https://web.archive.org/web/20161126145250/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html |date=November 26, 2016 }}</ref> it is estimated that  [[sea level rise]] will be:
**in a low scenario<ref name="low" /> 18 to 38&nbsp;cm (7 to 15&nbsp;inches)
**in a high scenario<ref name="high" /> 26 to 59&nbsp;cm (10 to 23&nbsp;inches)
* It is ''very likely'' that there will be an increase in frequency of warm spells, [[heat wave]]s and events of heavy rainfall.
* It is ''likely'' that there will be an increase in areas affected by [[drought]]s, intensity of [[tropical cyclone]]s (which include hurricanes and [[typhoon]]s) and the occurrence of extreme [[high tide]]s.
* "[[Sea ice]] is projected to shrink in both the Arctic and Antarctic&nbsp;... In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century."

Scenario-specific projections are based on analysis of multiple runs by multiple climate models, using the various SRES Scenarios. "Low scenario" refers to B1, the most optimistic scenario family. "High scenario" refers to A1FI, the most pessimistic scenario family.

==== Temperature and sea level rise in the various scenarios ====
There are six families of SRES scenarios, and AR4 provides projected temperature and sea level rises (excluding future rapid dynamical changes in ice flow) for each scenario family.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html Table SPM.3] {{Webarchive|url=https://web.archive.org/web/20161126145250/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html |date=2016-11-26  }}, p. 13, in {{Harvnb|IPCC AR4 WG1|2007}}.</ref>

*Scenario B1
**Best estimate temperature rise of 1.8&nbsp;°C with a likely range of 1.1 to 2.9&nbsp;°C (3.2&nbsp;°F with a likely range of 2.0 to 5.2&nbsp;°F)
**Sea level rise likely range [18 to 38&nbsp;cm] (7 to 15&nbsp;inches)
*Scenario A1T
**Best estimate temperature rise of 2.4&nbsp;°C with a likely range of 1.4 to 3.8&nbsp;°C (4.3&nbsp;°F with a likely range of 2.5 to 6.8&nbsp;°F)
**Sea level rise likely range [20 to 45&nbsp;cm] (8 to 18&nbsp;inches)
*Scenario B2
**Best estimate temperature rise of 2.4&nbsp;°C with a likely range of 1.4 to 3.8&nbsp;°C (4.3&nbsp;°F with a likely range of 2.5 to 6.8&nbsp;°F)
**Sea level rise likely range [20 to 43&nbsp;cm] (8 to 17&nbsp;inches)
*Scenario A1B
**Best estimate temperature rise of 2.8&nbsp;°C with a likely range of 1.7 to 4.4&nbsp;°C (5.0&nbsp;°F with a likely range of 3.1 to 7.9&nbsp;°F)
**Sea level rise likely range [21 to 48&nbsp;cm] (8 to 19&nbsp;inches)
*Scenario A2
**Best estimate temperature rise of 3.4&nbsp;°C with a likely range of 2.0 to 5.4&nbsp;°C (6.1&nbsp;°F with a likely range of 3.6 to 9.7&nbsp;°F)
**Sea level rise likely range [23 to 51&nbsp;cm] (9 to 20&nbsp;inches)
*Scenario A1FI
**Best estimate temperature rise of 4.0&nbsp;°C with a likely range of 2.4 to 6.4&nbsp;°C (7.2&nbsp;°F with a likely range of 4.3 to 11.5&nbsp;°F)
**Sea level rise likely range [26 to 59&nbsp;cm] (10 to 23&nbsp;inches)

=== Selected quotes from the Working Group I Summary for Policymakers ===

* "Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere."

=== Reaction to Working Group I ===

In the weeks before publication of the first report, controversy broke out about the report's projections of sea-level change, which in the new report was estimated at less than previous estimates. The now-published text gives a warning that the new estimation of sea-level could be too low: "Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise." The mid-points of the sea level rise estimates are within ±10% of those from the [[IPCC Third Assessment Report|TAR]]; but the range has narrowed.

[[Martin Rees, Baron Rees of Ludlow|Lord Rees]], the president of the [[Royal Society]], said, "This report makes it clear, more convincingly than ever before, that human actions are writ large on the changes we are seeing, and will see, to our climate. The IPCC strongly emphasises that substantial climate change is inevitable, and we will have to adapt to this. This should compel all of us—world leaders, businesses and individuals—towards action rather than the paralysis of fear. We need both to reduce our emissions of greenhouse gases and to prepare for the impacts of climate change. Those who would claim otherwise can no longer use science as a basis for their argument."<ref>{{cite news|url=http://news.bbc.co.uk/1/hi/sci/tech/6324093.stm|title=UK scientists' IPCC reaction|publisher=[[BBC News]]|date=2007-02-02|access-date=2007-02-03 |archive-date=2007-08-30 |archive-url=https://web.archive.org/web/20070830231116/http://news.bbc.co.uk/1/hi/sci/tech/6324093.stm|url-status=live}}</ref>

U.S. Energy Secretary [[Samuel Bodman]] told a news conference that the report was "sound science" and "As the president has said, and this report makes clear, human activity is contributing to changes in our earth's climate and that issue is no longer up for debate."<ref>{{cite news|url=http://www.montereyherald.com/mld/montereyherald/living/health/16614710.htm|title=Bush endorses climate study|publisher=[[Monterey County Herald]]|date=2007-02-03|first=Dan|last=Duray|url-status=dead|archive-url=https://web.archive.org/web/20070218181000/http://www.montereyherald.com/mld/montereyherald/living/health/16614710.htm|archive-date=2007-02-18}}</ref> [[Kurt Volker]], Principal Deputy Assistant Secretary for European and Eurasian Affairs, said, "We support the recent IPCC report, in which U.S. scientists played a leading role."<ref>[http://2001-2009.state.gov/p/eur/rls/rm/80465.htm Post-Kyoto Surprise: America's Quiet Efforts to Cut Greenhouse Gases Are Producing Results<!-- Bot generated title -->] {{webarchive |url=https://web.archive.org/web/20160823092041/http://2001-2009.state.gov/p/eur/rls/rm/80465.htm |date=August 23, 2016 }}</ref>

Based on the report, 46 countries in a "Paris Call for Action" read out by French President [[Chirac]], have called for the creation of a [[United Nations Environment Organization]] (UNEO), which is to have more power than the current [[United Nations Environment Programme]] (UNEP), and is to be modelled after the more powerful [[World Health Organization]]. The 46 countries included the [[European Union]] nations, but notably did not include the [[United States]], [[China]], [[Russia]], and [[India]], the top four emitters of greenhouse gases.<ref>{{cite news|url=http://today.reuters.com/news/articlenews.aspx?type=environment&storyID=2007-02-03T144510Z_01_L03357553_RTRIDST_0_DCBRIGHTS-GLOBALWARMING-APPEAL-DC.XML|title=46 nations call for tougher U.N. environment role|publisher=[[Reuters]]|date=2007-02-03|first=Alister|last=Doyle|url-status=dead|archive-url=http://arquivo.pt/wayback/20160515101534/http://today.reuters.com/news/articlenews.aspx?type=environment&storyID=2007-02-03T144510Z_01_L03357553_RTRIDST_0_DCBRIGHTS-GLOBALWARMING-APPEAL-DC.XML|archive-date=2016-05-15}}</ref>

==Working Group II: Impacts, Adaptation and Vulnerability==
Working Group II's ''Summary for Policymakers''<ref>See on-line: [http://www.ipcc.ch/publications_and_data/ar4/wg2/en/spm.html Impacts, Adaptation and Vulnerability, Summary for Policymakers] {{webarchive |url=https://web.archive.org/web/20160419000003/http://www.ipcc.ch/publications_and_data/ar4/wg2/en/spm.html |date=April 19, 2016 }}. Or [http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-spm.pdf pdf] {{webarchive |url=https://web.archive.org/web/20161125034403/http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-spm.pdf |date=November 25, 2016 }}.</ref>  was released on 6 April 2007.  The full report was released 18 September 2007.

WGII states that "evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases".

===Observations===
Some observed changes have been associated with climate change at varying levels of confidence.

With a high confidence (about an 8 in 10 chance to be correct) WGII asserts that climate change has resulted in:
*More and larger glacial lakes.
*Increasing ground instability in permafrost regions.
*Increasing rock avalanches in mountain regions.
*Changes in some Arctic and Antarctic ecosystems.
*Increased run-off and earlier spring peak discharge in many glacier and snow-fed rivers.
*Changes affecting algae, plankton, fish and zooplankton because rising water temperatures and changes in:
**ice cover
**salinity
**oxygen levels
**water circulation

With a very high confidence (about a 9 in 10 chance to be correct) WGII asserts that climate change is affecting terrestrial biological systems in that:
*Spring events such as the unfolding of leaves, laying of eggs, and migration are happening earlier.
*There are poleward and upward (to higher altitude) shifts in ranges of plant and animal species.

WGII also states that the ocean has become more acidic because it has absorbed human-caused carbon dioxide.  Ocean pH has dropped by 0.1, but how this affects marine life is not documented.

===Attribution of changes===
WGII acknowledges some of the difficulties of attributing specific changes to human-caused global warming, stating that "Limitations and gaps prevent more complete attribution of the causes of observed system responses to anthropogenic warming."  but found that the agreement between observed and projected changes was "Nevertheless&nbsp;... sufficient to conclude with high confidence that anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems."

===Projections===
WGII describes some of what might be expected in the coming century, based on studies and model projections.

====Fresh water====
It is projected with high confidence that:

*Dry regions are projected to get drier, and wet regions are projected to get wetter: "By mid-century, annual average river runoff and water availability are projected to increase by 10–40% at high latitudes and in some wet tropical areas, and decrease by 10–30% over some dry regions at mid-latitudes and in the dry tropics..."
*Drought-affected areas will become larger.
*Heavy precipitation events are very likely to become more common and will increase flood risk.
*Water supplies stored in glaciers and snow cover will be reduced over the course of the century.

====Ecosystems====
It is projected with high confidence that:
*The [[Resilience (ecology)|resilience]] of many ecosystems is likely to be exceeded this century by a combination of climate change and other stressors.
*[[Carbon removal]] by terrestrial ecosystems is likely to peak before mid-century and then weaken or reverse.  This would amplify climate change.

====Food====
It is projected with medium confidence (about 5 in 10 chance to be correct) that globally, potential food production will increase for temperature rises of 1–3&nbsp;°C, but decrease for higher temperature ranges.

====Coastal systems====
It is projected with very high confidence that:
*Coasts will be exposed to increasing risks such as coastal erosion due to climate change and sea-level rise.
* "Increases in sea-surface temperature of about 1–3&nbsp;°C are projected to result in more frequent coral bleaching events and widespread mortality unless there is thermal adaptation or acclimatisation by corals."
* "Many millions more people are projected to be flooded every year due to sea-level rise by the 2080s."

===Objections to original WGII language===
US negotiators managed to eliminate language calling for cuts in greenhouse gas emissions, according to Patricia Romero Lankao, a lead author from the [[National Center for Atmospheric Research]] (NCAR).  The original draft read: "However, adaptation alone is not expected to cope with all the projected effects of climate change, and especially not over the long run as most impacts increase in magnitude. ''Mitigation measures will therefore also be required.''" The second sentence does not appear in the final version of the report.<ref name="USChinaWarningsTonedDown">{{cite news | url=https://www.washingtonpost.com/wp-dyn/content/article/2007/04/06/AR2007040600291.html | title=U.S., China Got Climate Warnings Toned Down | access-date=2007-04-09 | newspaper=The [[Washington Post]] | first=Juliet | last=Eilperin | date=2007-04-07 | archive-date=2007-04-11  | archive-url=https://web.archive.org/web/20070411044149/http://www.washingtonpost.com/wp-dyn/content/article/2007/04/06/AR2007040600291.html | url-status=live }}</ref>

China objected to wording that said "based on observed evidence, there is very high confidence that many natural systems, on all continents and in most oceans, are being affected by regional climate changes, particularly temperature increases". When China asked that the word "very" be stricken, three scientific authors balked, and the deadlock was broken only by a compromise to delete any reference to confidence levels.<ref name="USChinaWarningsTonedDown" />

==Working Group III: Mitigation of Climate Change==
{{See also|Mitigation of global warming}}

Working Group III's ''Summary for Policymakers'' (SPM)<ref name="WG3SPM">See on-line: [http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html Mitigation of Climate Change. Summary for Policymakers] {{webarchive |url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}.</ref> was published on 4 May 2007 at the 26th session of the IPCC.<ref>{{cite web
|url=http://www.ipcc.ch/calendar2007.htm
|title=IPCC meetings in 2007
|publisher=[[Intergovernmental Panel on Climate Change|IPCC]]
|access-date=2007-05-01 |archive-url = https://web.archive.org/web/20070415113532/http://www.ipcc.ch/calendar2007.htm <!-- Bot retrieved archive --> |archive-date = 2007-04-15}}</ref> The full WG III report was published online in September 2007.<ref>IPCC, 2007: [https://www.ipcc.ch/report/ar4/wg3/ Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20181223030127/https://archive.ipcc.ch/publications_and_data/ar4/wg3/en/contents.html |date=2018-12-23  }} [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, US.</ref>

The IPCC convened in [[Bangkok]] on April 30 to start discussions on the draft Summary, with the participation of over 400 scientists and experts from about 120 countries.<ref>{{cite news
|url=http://news.bbc.co.uk/2/hi/science/nature/6606465.stm
|title=Bangkok hosts key climate summit
|publisher=[[BBC]]
|access-date=2007-05-01
|date=2007-04-30
|archive-date=2007-05-04 
|archive-url=https://web.archive.org/web/20070504171834/http://news.bbc.co.uk/2/hi/science/nature/6606465.stm
|url-status=live
}}</ref> At the full IPCC meeting on May 4, agreement was reached by the larger gathering of some 2,000 delegates. One of the key debates concerned a proposal to limit concentrations of greenhouse gases in the atmosphere to between 445 parts per million and 650 parts per million to [[Avoiding dangerous climate change|avoid dangerous climate change]], with pressure from developing countries to raise the lower limit. Despite this, the figures from the original proposal were incorporated into the Summary for Policymakers.<ref>{{cite news|url=http://edition.cnn.com/2007/TECH/science/05/03/climate.report.ap/index.html?eref=rss_world |title=Deal reached on climate change report |publisher=[[CNN]] |date=2007-05-04 |access-date=2007-05-04 }}{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}</ref> The Summary concludes that stabilization of greenhouse gas concentrations is possible at a reasonable cost, with stabilization between 445&nbsp;ppm and 535&nbsp;ppm costing less than 3% of global [[gross domestic product|GDP]].<ref>{{cite news
|url=http://news.bbc.co.uk/1/hi/sci/tech/6620909.stm
|title=Climate change 'can be tackled'
|publisher=[[BBC]]
|access-date=2007-05-04
|date=2007-05-04
|archive-date=2007-06-02 
|archive-url=https://web.archive.org/web/20070602180542/http://news.bbc.co.uk/1/hi/sci/tech/6620909.stm
|url-status=live
}}</ref>

The WG III report analyses mitigation options for the main sectors in the near-term, addressing also cross-sectorial matters such as synergies, co-benefits, and trade-offs. It also provides information on long-term mitigation strategies for various stabilization levels, paying special attention to implications of different short-term strategies for achieving long-term goals.<ref name="principles">IPCC. {{cite web |url= http://www.ipcc.ch/about/princ.pdf |title= Principles Governing IPCC Work |url-status= dead |archive-url= https://web.archive.org/web/20070712111916/http://www.ipcc.ch/about/princ.pdf |archive-date= 2007-07-12 }}&nbsp;{{small|(8.38&nbsp;KB)}}</ref>

===Mitigation in the short and medium term (until 2030)===
The Summary for Policymakers concludes that there was a high level of agreement and much evidence that "there is substantial economic potential for the mitigation of global [[greenhouse gas emissions]] over the coming decades, that could offset the projected growth of global emissions or reduce emissions below current levels",<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM (Summary for Policymakers)] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, in {{Harvnb|IPCC AR4 WG3|2007}}, item 5, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 11] {{webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=November 25, 2016 }}.</ref> taking into account financial and social costs and benefits.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, in {{Harvnb|IPCC AR4 WG3|2007}}, Box SPM 2, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 10] {{webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=November 25, 2016 }}.</ref>  The technologies with the largest economic potential within this timescale are considered to be:<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, Table SPM 3, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 14] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

{|class="wikitable"
|+'''Key mitigation technologies and practices by sector'''
|-valign="top"
|'''Sector'''
|'''Key mitigation technologies and practices currently commercially available'''
|'''Key mitigation technologies and practices projected to be commercialized before 2030'''
|-valign="top"
|[[Energy supply|Energy Supply]]
|Improved supply and distribution efficiency; fuel switching from [[coal]] to [[natural gas|gas]]; [[nuclear power]]; [[renewable energy|renewable heat and power]] ([[hydropower]], [[solar power|solar]], [[wind power|wind]], [[geothermal power|geothermal]] and [[biofuel|bioenergy]]); [[cogeneration|combined heat and power]]; early applications of [[carbon capture and storage|CCS]] (e.g. storage of removed {{CO2}} from natural gas)
|Carbon Capture and Storage (CCS) for gas, biomass and coal-fired electricity generating facilities; advanced nuclear power; advanced renewable energy, including [[tidal power|tidal]] and [[wave power|waves energy]], [[concentrating solar power|concentrating solar]], and [[photovoltaics|solar PV]].
|-valign="top"
|[[Transport]]
|More fuel efficient vehicles; [[electric vehicle]]; [[hybrid vehicle]]s; cleaner diesel vehicles; [[biofuel]]s; modal shifts from [[road transport]] to [[railway|rail]] and [[public transport]] systems; non-motorised transport ([[cycling]], [[walking]]); land-use and [[Transportation planning|transport planning]]
|Second generation biofuels; higher efficiency aircraft; advanced electric and hybrid vehicles with more powerful and reliable batteries
|-valign="top"
|[[low energy building|Buildings]]
|[[low-energy lighting|Efficient lighting]] and [[Daylighting (architecture)|daylighting]]; more efficient electrical appliances and heating and cooling devices; improved cook stoves, improved [[building insulation|insulation]]; [[passive solar building design|passive]] and [[active solar]] design for heating and cooling; alternative refrigeration fluids, recovery and recycle of fluorinated gases
|Integrated design of commercial buildings including technologies, such as [[Smart meter|intelligent meter]]s that provide feedback and control; [[building integrated photovoltaics|solar PV integrated in buildings]]
|-valign="top"
|Industry
|More efficient end-use electrical equipment; [[heat recovery|heat]] and [[power recovery]]; material [[recycling]] and substitution; control of non-{{CO2}} gas emissions; and a wide array of process-specific technologies
|Advanced energy efficiency; CCS for [[cement]], [[ammonia]], and [[iron]] manufacture; inert [[electrode]]s for aluminium manufacture
|-valign="top"
|[[Agriculture]]
|Improved crop and grazing land management to increase soil carbon storage; restoration of cultivated peaty soils and degraded lands; improved rice cultivation techniques and livestock and manure management to reduce {{chem|link=methane|CH|4}} emissions; improved [[nitrogen fertilizer]] application techniques to reduce {{chem|link=nitrous oxide|N|2|O}} emissions; dedicated [[energy crop]]s to replace [[fossil fuel]] use; improved energy efficiency
|Improvements of [[crop yield]]s
|-valign="top"
|[[Forestry]]/[[forest]]s
|[[Afforestation]]; [[reforestation]]; [[forest management]]; reduced [[deforestation]]; harvested wood product management; use of forestry products for bio-energy to replace fossil fuel use
|Tree species improvement to increase biomass productivity and [[Carbon sequestration|carbon biosequestration]]. Improved remote sensing technologies for analysis of vegetation/ soil carbon sequestration potential and mapping land use change
|-valign="top"
|[[Waste]]
|[[Biogas|Landfill methane recovery]]; waste [[Waste-to-energy|incineration with energy recovery]]; [[composting]] of organic waste; controlled [[waste water treatment]]; [[recycling]] and [[waste minimization]]
|[[Biocover]]s and [[biofilter]]s to optimize {{chem|CH|4}} oxidation
|}

The IPCC estimates that stabilizing atmospheric greenhouse gases at between 445–535&nbsp;ppm {{CO2}} [[Carbon dioxide equivalent|equivalent]] would result in a reduction of average annual GDP growth rates of less than 0.12%. Stabilizing at 535 to 590&nbsp;ppm would reduce average annual GDP growth rates by 0.1%, while stabilization at 590 to 710&nbsp;ppm would reduce rates by 0.06%.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, Table SPM4, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 16] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref> There was high agreement and much evidence that a substantial fraction of these mitigation costs may be offset by benefits to health as a result of reduced air pollution, and that there would be further cost savings from other benefits such as increased [[energy security]], increased agricultural production, and reduced pressure on natural ecosystems as well as, in certain countries, balance of trade improvements, provision of modern energy services to rural areas and employment.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, item 30, p. 17, and item 10, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 18] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

The IPCC considered that achieving these reductions would require a "large shift in the pattern of investment, although the net additional investment required ranges from negligible to 5–10%".They also concluded that it is often more cost effective to invest in end-use [[energy conservation|energy efficiency]] improvement than in increasing energy supply.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, items 15, 20, &amp; 25, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 18] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

In terms of electricity generation, the IPCC envisage that [[renewable energy]] can provide 30 to 35% of electricity by 2030 (up from 18% in 2005) at a [[carbon price]] of up to US$50/t, and that [[nuclear power]] can rise from 16% to 18%. They also warn that higher oil prices might lead to the exploitation of high-carbon alternatives such as [[oil sand]]s, [[oil shale]]s, [[Heavy crude oil|heavy oils]], and [[synthetic fuel]]s from coal and gas, leading to increasing emissions, unless carbon capture and storage technologies are employed.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, items 25, 30, &amp; 35, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 18] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

In the transport sector there was a medium level of agreement and evidence that the multiple mitigation options may be counteracted by increased use, and that there were many barriers and a lack of government policy frameworks.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, item 11, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 19] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

There was high agreement and much evidence that, despite many barriers (particularly in the developing countries), new and existing buildings could reduce emissions considerably, and that this would also provide other benefits in terms of improved air quality, social welfare and energy security.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=May 24, 2016 }}, item 12, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf p. 19] {{Webarchive|url=https://web.archive.org/web/20161125034350/http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-spm.pdf |date=2016-11-25  }}, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

===Mitigation in the long term (after 2030)===

The IPCC reported that the effectiveness of mitigation efforts over the next two or three decades would have a large impact on the ability to stabilize atmospheric greenhouse gases at lower levels, and that the lower the ultimate stabilization levels, the more quickly emissions would need to peak and decline.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, item 18, p. 22, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref> For example, to stabilize at between 445 and 490&nbsp;ppm (resulting in an estimate global temperature 2 to 2.4&nbsp;°C above the pre-industrial average) emissions would need to peak before 2015, with 50 to 85% reductions on 2000 levels by 2050.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, Table SPM.5, p. 23, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

There was high agreement and much evidence that stabilization could be achieved by 2050 using currently available technologies, provided appropriate and effective incentives were put in place for their development, acquisition, deployment and diffusion, and that barriers were removed.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, item 19, p. 25, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref> For stabilization at lower levels the IPCC agreed that improvements of [[carbon intensity]] need to be made much faster than has been the case in the past, and that there would be a greater need for efficient public and private [[research, development and demonstration]] efforts and investment in new technologies during the next few decades.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, item 25, p. 25, and item 5, p. 26, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref> The IPCC points out that government funding in real absolute terms for most energy research programmes has been flat or declining for nearly 20 years, and is now about half the 1980 level.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, item 10, p. 32, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref> Delays in cutting emissions would lead to higher stabilization levels and increase the risk of more severe climate change impacts, as more of the current high-emission technologies would have been deployed.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, item 10, p. 28, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

Among the measures that might be used, there was high agreement and much evidence that policies that put a price on the cost of carbon emissions could provide incentives for consumers and producers. Carbon prices of 5 to 65 US$/t{{CO2}} in 2030 and 15 to 130 US$/t{{CO2}} by 2050 are envisaged for stabilization at around 550&nbsp;ppm by 2100.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html SPM] {{Webarchive|url=https://web.archive.org/web/20160524223546/http://www.ipcc.ch/publications_and_data/ar4/wg3/en/spm.html |date=2016-05-24  }}, items 23 &amp; 25, p. 29, in {{Harvnb|IPCC AR4 WG3|2007}}.</ref>

==Synthesis Report==

A draft version of the Synthesis Report, said to be subject to final copyedit, was  published on 16 November 2007.<ref>{{cite web |url= http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf |title= Summary for Policymakers |access-date= 2007-11-17  |archive-date= 2011-02-28  |archive-url= https://web.archive.org/web/20110228135154/http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf |url-status= live }}&nbsp;{{small|(1.92&nbsp;MB)}}</ref>

{{blockquote|
The Synthesis Report goes one step further [than the first three Climate Change 2007 Working Group Reports]: it is the decisive effort to integrate and compact this wealth of information into a readable and concise document explicitly targeted to the policymakers.

The Synthesis Report also brings in relevant parts some material contained in the full Working Group Reports over and above what is included in the Summary for Policymakers in these three Reports. It is designed to be a powerful, scientifically authoritative document of high policy relevance, which will be a major contribution to the discussions at the 13th Conference of the Parties in Bali during December 2007. In fact, this Conference was postponed to December to allow the IPCC Synthesis Report to come out first.|[http://www.ipcc.ch/pdf/press-ar4/ar4-media-advisory.pdf Press conference for the release of the IPCC Synthesis Report]}}

The six topics<ref>[http://www.ipcc.ch/activity/ar4outline.htm AR4 Synthesis Report — Outline of Topics] {{webarchive |url=https://web.archive.org/web/20071113135054/http://www.ipcc.ch/activity/ar4outline.htm |date=November 13, 2007 }}</ref> addressed in the Synthesis Report are:
# Observed changes in climate and its effects (WGI and WGII).
# Causes of change (WGI and WGIII).
# Climate change and its impacts in the near and long term under different scenarios (WGI and WGIII).
# Adaptation and mitigation options and responses, and the inter-relationship with sustainable development, at global and regional levels (WGII and WGIII).
# The long term perspective: scientific and socio-economic aspects relevant to adaptation and mitigation, consistent with the objectives and provisions of the Convention [sic], and in the context of sustainable development (WGI and WGIII).
# Robust findings, key uncertainties (WGI, WGII and WGIII).

The "Convention" mentioned in Topic 5 is the [[UN Framework Convention on Climate Change]] ([[UNFCCC]]).

The key findings from the AR4 Synthesis Report will be discussed Wednesday 13 December 2007<ref>{{cite web |url=http://www.ipcc.ch/ |title=IPCC home page |access-date=2007-11-17 |archive-date=2017-09-15  |archive-url=https://web.archive.org/web/20170915050210/http://www.ipcc.ch/ |url-status=live }}</ref> at the United Nations Climate Change Conference ([[UNFCCC]] COP 13—CMP 3) in Bali, Indonesia, which takes place 3–14 December (see UNFCCC home page<ref>{{cite web |url=http://unfccc.int/2860.php |title=UNFCCC home page |access-date=2007-11-17 |archive-date=2018-02-01  |archive-url=https://web.archive.org/web/20180201152427/http://unfccc.int/2860.php |url-status=live }}</ref>).
<!-- OUT OF DATE!! This bit needs to be rewritten, or deleted. -->

===Anthropogenic warming could lead to some impacts that are abrupt or irreversible===
The SPM states that "Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."{{Citation needed|date=September 2011}}

*"There is medium confidence that approximately 20–30% of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5–2.5°C (relative to 1980–1999). As global average temperature increase exceeds about 3.5°C, model projections suggest significant extinctions (40–70% of species assessed) around the globe."{{Citation needed|date=September 2011}}
*"Partial loss of ice sheets on polar land could imply metres of sea level rise, major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands. Such changes are projected to occur over millennial time scales, but more rapid sea level rise on century time scales cannot be excluded."{{Citation needed|date=September 2011}}

=== Change in greenhouse gas concentrations and effect ===
The following table is based on Chapter 2 of the Working Group 1 Report.<ref name="ar4">{{cite book |url=http://www.ipcc.ch/ipccreports/ar4-wg1.htm |title=AR4 Climate Change 2007: The Physical Science Basis |page=141 |contribution=Chapter 2}}</ref>
{| class="wikitable"
!
! colspan="2" |Mole fractions and their changes
! colspan="2" |[[Radiative forcing]]
|-
|'''Species'''
|'''2005'''
|'''Change since 1998'''
|'''2005 (W m<sup>−2</sup>)'''
|'''1998 (%)'''
|-
|CO<sub>2</sub>
|379 ± 0.65 μmol/mol
| +13 μmol/mol
|1.66
| +13
|-
|CH<sub>4</sub>
|1,774 ± 1.8 nmol/mol
| +11 nmol/mol
|0.48
|–
|-
|N<sub>2</sub>O
|319 ± 0.12 nmol/mol
| +5 nmol/mol
|0.16
| +11
|-
|CFC-11
|251 ± 0.36 pmol/mol
|−13
|0.063
|−5
|-
|CFC-12
|538 ± 0.18 pmol/mol
| +4
|0.17
| +1
|-
|CFC-113
|79 ± 0.064 pmol/mol
|−4
|0.024
|−5
|-
|HCFC-22
|169 ± 1.0 pmol/mol
| +38
|0.033
| +29
|-
|HCFC-141b
|18 ± 0.068 pmol/mol
| +9
|0.0025
| +93
|-
|HCFC-142b
|15 ± 0.13 pmol/mol
| +6
|0.0031
| +57
|-
|CH<sub>3</sub>CCl<sub>3</sub>
|19 ± 0.47 pmol/mol
|−47
|0.0011
|−72
|-
|CCl<sub>4</sub>
|93 ± 0.17 pmol/mol
|−7
|0.012
|−7
|-
|HFC-125
|3.7 ± 0.10 pmol/mol
| +2.6
|0.0009
| +234
|-
|HFC-134a
|35 ± 0.73 pmol/mol
| +27
|0.0055
| +349
|-
|HFC-152a
|3.9 ± 0.11 pmol/mol
| +2.4
|0.0004
| +151
|-
|HFC-23
|18 ± 0.12 pmol/mol
| +4
|0.0033
| +29
|-
|SF<sub>6</sub>
|5.6 ± 0.038 pmol/mol
| +1.5
|0.0029
| +36
|-
|CF<sub>4</sub> (PFC-14)
|74 ± 1.6 pmol/mol
|–
|0.0034
|–
|-
|C<sub>2</sub>F<sub>6</sub> (PFC-116)
|2.9 ± 0.025 pmol/mol
| +0.5
|0.0008
| +22
|-
|}


==Reception==
{{Main|Criticism of the IPCC Fourth Assessment Report}}
''The New York Times'' reported that "the leading international network of climate scientists has concluded for the first time that global warming is 'unequivocal' and that human activity is the main driver, 'very likely' causing most of the rise in temperatures since 1950".<ref name="rosenthal">{{cite news |url=https://www.nytimes.com/2007/02/03/science/earth/03climate.html?ex=1328158800&en=61f42312221df544&ei=5090&partner=rssuserland&emc=rss%3Cbr%20/%3E |title=Science Panel Calls Global Warming 'Unequivocal' |newspaper=The New York Times |first1=Elisabeth |last1=Rosenthal |first2= Andrew C. |last2=Revkin |date=3 February 2007 |quote=the leading international network of climate scientists has concluded for the first time that global warming is 'unequivocal' and that human activity is the main driver, 'very likely' causing most of the rise in temperatures since 1950 |access-date=28 August 2010}}</ref>

The same newspaper wrote: "The Intergovernmental Panel on Climate Change said the likelihood was 90 percent to 99 percent that emissions of heat-trapping greenhouse gases like carbon dioxide, spewed from tailpipes and smokestacks, were the dominant cause of the observed warming of the last 50 years. In the panel's parlance, this level of certainty is labeled 'very likely'. Only rarely does scientific odds-making provide a more definite answer than that, at least in this branch of science, and it describes the endpoint, so far, of a progression."<ref name="On the Climate Change Beat, Doubt Gives Way to Certainty">{{cite news |url=https://www.nytimes.com/2007/02/06/science/earth/06clim.html?pagewanted=1&ei=5088&en=53862c0cdf77d1c0&ex=1328418000&partner=rssnyt&emc=rss |title=On the Climate Change Beat, Doubt Gives Way to Certainty |newspaper=The New York Times |access-date=28 August 2010 |first=William K. |last= Stevens |quote=The Intergovernmental Panel on Climate Change said the likelihood was 90 percent to 99 percent that emissions of heat-trapping greenhouse gases like carbon dioxide, spewed from tailpipes and smokestacks, were the dominant cause of the observed warming of the last 50 years. In the panel's parlance, this level of certainty is labeled "very likely." Only rarely does scientific odds-making provide a more definite answer than that, at least in this branch of science, and it describes the endpoint, so far, of a progression. | date=6 February 2007 }}</ref>

The [[Associated Press]] summarized the position on sea level rise:<ref name="foxnews">{{cite web |url=http://www.foxnews.com/story/0,2933,249659,00.html |title=U.N. Report: Global Warming Man-Made, Basically Unstoppable |publisher=Fox News |access-date=30 July 2012 |archive-url=https://web.archive.org/web/20120630012604/http://www.foxnews.com/story/0,2933,249659,00.html |archive-date=30 June 2012 |url-status=dead }}</ref>
{{blockquote|On sea levels, the report projects rises of 7 to 23 inches by the end of the century. An additional 3.9 to 7.8 inches are possible if recent, surprising melting of polar ice sheets continues.}}

The Fourth Assessment Report has been the subject of criticism. Skeptics of anthropogenic global warming contend that their claims are not sufficiently incorporated in the report. Others regard the IPCC as too conservative in its estimates of potential harm from climate change.  The report has also been criticized for inclusion of an erroneous date for the projected demise of the [[Criticism of the IPCC AR4#Projected date of melting of Himalayan glaciers|Himalayan glaciers]].

Related to the subject of global warming in general, the IPCC Fourth Assessment Report has been discussed by various bodies such as government officials, special interest groups and scientific organizations; see the article "[[Politics of global warming]]" for a thorough discussion of the politics surrounding the phenomenon, and the positions of the various parties involved.

The United Nations appointed an independent board of scientists to  "review the workings of the world's top climate science panel" which reported in September 2010;<ref name="NYT Review Board">{{cite news|url=https://www.nytimes.com/2010/02/27/science/earth/27climate.html?src=twt&twt=nytimesscience|title=Independent Board to Review Work of Top Climate Panel|date=27 February 2010|work=New York Times|access-date=28 February 2010|archive-date=2018-10-31 |archive-url=https://web.archive.org/web/20181031170228/https://www.nytimes.com/2010/02/27/science/earth/27climate.html?src=twt&twt=nytimesscience|url-status=live}}</ref> see [[Intergovernmental Panel on Climate Change#InterAcademy Council review in 2010]].

=== Response to AR4 ===
Several [[science academy|science academies]] have referred to and/or reiterated some of the conclusions of AR4. These include:
* Joint-statements made in 2007,<ref name="2007 science academies statement">
{{harvnb|Academia Brasileira de Ciéncias|others|2007}}
</ref> 2008<ref>
{{harvnb|Academia Brasileira de Ciéncias|others|2008}}
</ref> and 2009<ref>
{{harvnb|Academia Brasileira de Ciéncias|others|2009}}
</ref> by the science academies of Brazil, China, India, Mexico, South Africa and the [[G8]] nations (the "G8+5").
* Publications by the [[Australian Academy of Science]].<ref>{{harvnb|Lambeck|2007}}, {{harvnb|Australian Academy of Science|no date}}, {{harvnb|Australian Academy of Science|2010}}.</ref>
* A joint-statement made in 2007 by the [[Network of African Science Academies]].<ref>
{{harvnb|NASAC|2007}}
</ref>
* A statement made in 2010 by the Inter Academy Medical Panel<ref>
{{harvnb|IAMP|2010}}
</ref> This statement has been signed by 43 scientific academies.{{efn|
{{div col}}
* Academia Nacional de Medicina de Buenos Aires
* Academy of Medical Sciences of Armenia
* Austrian Academy of Sciences
* Bangladesh Academy of Sciences
* Academia Boliviana de Medicina
* Brazilian Academy of Sciences
* Cameroon Academy of Sciences
* Chinese Academy of Engineering
* Academia Nacional de Medicina de Colombia
* Croatian Academy of Medical Sciences
* Croatian Academy of Sciences and Arts
* Academy of Scientific Research and Technology, Egypt
* Académie Nationale de Médecine, France
* The Delegation of the Finnish Academies of Science and Letters
* Union of German Academies of Sciences and Humanities
* Deutsche Akademie der Naturforscher, Leopoldina
* Academia de Ciencias Medicas, Fisicas y Naturales de Guatemala
* Hungarian Academy of Sciences
* Indonesian Academy of Sciences
* Accademia Nazionale dei Lincei
* TWAS, academy of sciences for the developing world
* Islamic World Academy of Sciences
* Science Council of Japan
* African Academy of Sciences
* Kenya National Academy of Sciences
* The National Academy of Sciences, Rep. of Korea
* Akademi Sains Malaysia
* National Academy of Medicine of Mexico
* Nigerian Academy of Science
* National Academy of Science and Technology, Philippines
* Polish Academy of Sciences
* The Caribbean Academy of Sciences
* Russian Academy of Medical Sciences
* Slovenian Academy of Sciences and Arts
* Academy of Science of South Africa
* National Academy of Sciences of Sri Lanka
* Royal Swedish Academy of Sciences
* The Tanzania Academy of Sciences
* Thai Academy of Science and Technology
* Turkish Academy of Sciences
* Uganda National Academy Sciences
* Academy of Medical Sciences, UK
* Institute of Medicine, US NAS
{{Div col end}}
}}

The [[Netherlands Environmental Assessment Agency]] (PBL, ''et al.'', 2009;<ref>
{{harvnb|PBL|others|2009}}
</ref> 2010)<ref>
{{harvnb|PBL|2010}}
</ref> has carried out two reviews of AR4. These reviews are generally supportive of AR4's conclusions.<ref>
Summary, in {{harvnb|PBL|others|2009|p=7}}
</ref><ref name="pbl 2010 ipcc review">
Executive summary, in {{harvnb|PBL|2010|p=9}}
</ref> PBL (2010)<ref name="pbl 2010 ipcc review" /> make some recommendations to improve the IPCC process. A literature assessment by the US National Research Council (US NRC, 2010)<ref name="us nrc 2010 summary">
[http://www.nap.edu/openbook.php?record_id=12782&page=3 Summary, p. 3], in {{harvnb|US NRC|2010}}
</ref> concludes:<blockquote>'''Climate change is occurring, is caused largely by human activities, and poses significant risks for—and in many cases is already affecting—a broad range of human and natural systems''' [''emphasis in original text''].&nbsp;... This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of recent assessments by the U.S. Global Change Research Program&nbsp;..., the Intergovernmental Panel on Climate Change's Fourth Assessment Report ..., and other assessments of the state of scientific knowledge on climate change.</blockquote>

=== Projected date of melting of Himalayan glaciers ===
Some errors have been found in the IPCC AR4 Working Group II report. Two errors include the melting of [[Himalayas|Himalayan]] [[glacier]]s (see [[Intergovernmental Panel on Climate Change#Projected date of melting of Himalayan glaciers|later section]]), and Dutch land area that is below sea level.<ref>
Section 3.2: Errors, in: Chapter 3: Results and discussion, in {{harvnb|PBL|2010|pp=35–37}}
</ref>

A paragraph in the 2007 Working Group II report ("Impacts, Adaptation and Vulnerability"), chapter 10 included a projection that Himalayan glaciers could disappear by 2035:<blockquote>Glaciers in the Himalaya are receding faster than in any other part of the world (see Table 10.9) and, if the present rate continues, the likelihood of them disappearing by the year 2035 and perhaps sooner is very high if the Earth keeps warming at the current rate. Its total area will likely shrink from the present 500,000 to 100,000 km<sup>2</sup> by the year 2035 (WWF, 2005).</blockquote>This projection was not included in the final summary for policymakers. The IPCC has since acknowledged that the date is incorrect, while reaffirming that the conclusion in the final summary was robust. They expressed regret for "the poor application of well-established IPCC procedures in this instance". The date of 2035 has been correctly quoted by the IPCC from the WWF report, which has misquoted its own source, an [[International Commission on Snow and Ice Hydrology|ICSI]] report "Variations of Snow and Ice in the past and at present on a Global and Regional Scale".

[[Rajendra K. Pachauri]] responded in an interview with ''Science''.<ref>{{Cite journal|last1=Bagla|first1=P.|date=29 January 2010|title=Climate Science Leader Rajendra Pachauri Confronts the Critics|journal=Science|volume=327|issue=5965|pages=510–511|bibcode=2010Sci...327..510B|doi=10.1126/science.327.5965.510|pmid=20110473}}</ref>

===Overstatement of effects===
Former IPCC chairman [[Robert Watson (scientist)|Robert Watson]] said, regarding the Himalayan glaciers estimation, "The mistakes all appear to have gone in the direction of making it seem like climate change is more serious by overstating the impact. That is worrying. The IPCC needs to look at this trend in the errors and ask why it happened".<ref name="Times Online 2010-02-15">
{{cite news|author=Ben Webster|author2=Robin Pagnamenta|date=15 February 2010|title=UN must investigate warming 'bias', says former climate chief – Times Online|newspaper=The Times|location=London|url=http://www.timesonline.co.uk/tol/news/environment/article7026932.ece#cid=OTC-RSS&attr=797093|access-date=19 February 2010|archive-url=https://web.archive.org/web/20100529140554/http://www.timesonline.co.uk/tol/news/environment/article7026932.ece|archive-date=29 May 2010}}
</ref> Martin Parry, a climate expert<ref>{{cite web|title=Martin Parry|url=http://www.mlparry.com/|access-date=27 December 2015|archive-date=2021-07-28 |archive-url=https://web.archive.org/web/20210728180615/http://www.mlparry.com/|url-status=live}}</ref> who had been co-chair of the IPCC working group II, said that "What began with a single unfortunate error over Himalayan glaciers has become a clamour without substance" and the IPCC had investigated the other alleged mistakes, which were "generally unfounded and also marginal to the assessment".<ref name="guardian 2010-02-14">{{cite news|author=David Adam|date=14 February 2010|title=Climate scientist says Himalayan glacier report is 'robust and rigorous' {{pipe}} Environment|newspaper=The Guardian|location=London|url=https://www.theguardian.com/environment/2010/feb/14/climate-scientist-himalayan-glacier-report|access-date=19 February 2010|archive-date=2010-02-17 |archive-url=https://web.archive.org/web/20100217041407/http://www.guardian.co.uk/environment/2010/feb/14/climate-scientist-himalayan-glacier-report|url-status=live}}</ref>

=== Other ===
The IPCC Fourth Assessment Report featured a graph showing 12 proxy based temperature reconstructions, including the three highlighted in the 2001 Third Assessment Report (TAR); {{harvnb|Mann|Bradley|Hughes|1999}} as before, {{harvnb|Jones|Briffa|Barnett|Tett|1998}} and {{harvnb|Briffa|2000}} had both been calibrated by newer studies. In addition, analysis of the [[Medieval Warm Period]] cited reconstructions by {{harvnb|Crowley|Lowery|2000}} (as cited in the TAR) and {{harvnb|Osborn|Briffa|2006}}. Ten of these 14 reconstructions covered 1,000 years or longer. Most reconstructions shared some data series, particularly tree ring data, but newer reconstructions used additional data and covered a wider area, using a variety of statistical methods. The section discussed the [[divergence problem]] affecting certain tree ring data.<ref name="AR4 2k years">{{Harvnb|Jansen|Overpeck|2007}}</ref>

== See also ==
{{Portal|Energy}}
<!--
Please do not add links already featured in the article and/or templates such as [[Global warming]].
Refer to Wikipedia guidelines for details.
 -->
{|
|-
|
* [[Avoiding Dangerous Climate Change]]
* [[Individual and political action on climate change]]
* [[Business action on climate change]]
* [[Energy policy]]
* [[Energy conservation]]
* [[Global climate model]]
|
* [[Post–Kyoto Protocol negotiations on greenhouse gas emissions]]
* [[Precautionary principle]]
* [[Renewable Energy Sources and Climate Change Mitigation]]
* [[Special Report on Emissions Scenarios]]
* [[World energy resources and consumption]]
|}

== Notes ==
{{Notelist}}

==References==
{{Reflist|30em}}

===Sources===
The Fourth Assessment Report consists of the following reports from each of the three Working Groups, and a Synthesis Report.  Additional reports and documents can be found at the IPCC's [http://www.ipcc.ch/publications_and_data/publications_and_data_reports.shtml documents web page].
* {{cite book |title=IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)) |editor-last=Solomon |editor-first= S. |editor2-last=Qin|editor2-first=D.|editor3-last=Manning|editor3-first= M.|editor4-last=Chen|editor4-first=Z.|editor5-last=Marquis|editor5-first=M.|editor6-last=Averyt|editor6-first=K.B.|editor7-last=Tignor|editor7-first=M. |editor8-last=Miller|editor8-first=H.L.  |year=2007 |publisher=Cambridge University Press |location=Cambridge |url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf |access-date=7 July 2014}} (Abbreviated as SPM-WG1, AR4)
* {{Citation
|year   = 2007
|author = IPCC AR4 WG1
|author-link = Intergovernmental Panel on Climate Change
|title  = Climate Change 2007: The Physical Science Basis
|series = Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor = Solomon, S. |editor2=Qin, D. |editor3=Manning, M. |editor4=Chen, Z. |editor5=Marquis, M. |editor6=Averyt, K.B. |editor7=Tignor, M. |editor8=Miller, H.L.
|publisher = [[Cambridge University Press]]
|url  = http://www.ipcc.ch/publications_and_data/ar4/wg1/en/contents.html
|isbn = 978-0-521-88009-1
}} (pb: {{ISBNT|978-0-521-70596-7}}).
* {{Citation
 |year         = 2007
 |author       = IPCC AR4 WG2
 |author-link  = Intergovernmental Panel on Climate Change
 |title        = Climate Change 2007: Impacts, Adaptation and Vulnerability
 |series       = Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
 |editor       = Parry, M.L.
 |editor2      = Canziani, O.F.
 |editor3      = Palutikof, J.P.
 |editor4      = van der Linden, P.J.
 |editor5      = Hanson, C.E.
 |publisher    = Cambridge University Press
 |url          = http://www.ipcc.ch/publications_and_data/ar4/wg2/en/contents.html
 |isbn         = 978-0-521-88010-7
 |access-date  = 2010-03-04 
 |archive-date = 2018-11-10 
 |archive-url  = https://web.archive.org/web/20181110071159/http://www.ipcc.ch/publications_and_data/ar4/wg2/en/contents.html
 |url-status   = dead
}} (pb: {{ISBNT|978-0-521-70597-4}}).
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==External links==
* [https://www.ipcc.ch/ Intergovernmental Panel on Climate Change home page]
* Ten-webpage [http://www.greenfacts.org/en/climate-change-ar4/ summary of the Fourth Assessment Report SPMs] by [[GreenFacts]]; the webpages as a self-contained {{cite web|url= http://www.greenfacts.org/en/climate-change-ar4/climate-change-ar4-greenfacts-level2.pdf |title=39-page document }}&nbsp;{{small|(1.04&nbsp;MB)}}
* Commentary on the Working Group I Report:
** [https://www.theguardian.com/environment/2007/jan/27/usnews.frontpagenews The Guardian] Article on US political pressure on WGI Report, 27 January 2007
** [https://web.archive.org/web/20070207124053/http://www.unep.org/Documents.Multilingual/Default.asp?DocumentID=499&ArticleID=5506&l=en UNEP.org] Press release: ''Evidence of Human-caused Global Warming "Unequivocal", says IPCC'', 2 February 2007
** [[RealClimate]] blog &mdash; [https://www.realclimate.org/index.php/archives/2007/02/the-ipcc-fourth-assessment-summary-for-policy-makers Commentary] on Working Group I Summary for Policymakers, 2 February 2007
* Videos:
** [https://web.archive.org/web/20100621194329/http://uc.princeton.edu/main/index.php/component/content/article/3163 From Science to Assessment: Overview of the IPCC AR4 Working Group I Report]. A lecture given at [[Princeton University]] by Ronald Stouffer, Senior Research Meteorologist, [[Geophysical Fluid Dynamics Laboratory]] (GFDL), 11 March 2008
** [https://web.archive.org/web/20100718171134/http://uc.princeton.edu/main/index.php/component/content/article/28-all-videos/3162-lessons-from-the-climate-wars-the-future-of-the-ipcc Lessons from the Climate Wars: The Future of the IPCC]. A lecture given at Princeton University by [[Gary Yohe]], Woodhouse/Sysco Professor of economics at [[Wesleyan University]] and Director of the John E. Andrus Public Affairs Center at Wesleyan, 7 May 2008
** [https://web.archive.org/web/20100621185550/http://uc.princeton.edu/main/index.php/component/content/article/3161 Emissions Mitigation and Climate Stabilization]. A lecture given at Princeton University by Jae Edmonds, Chief Scientist, Joint Global Change Research Institute, [[Pacific Northwest National Laboratory]], 22 April 2008
** [https://web.archive.org/web/20090722044146/http://mitworld.mit.edu/video/550 How Would Climate Change Influence Society in the 21st Century?] A lecture given at [[MIT]] by [[Rajendra Pachauri]], 29 January 2008

{{Global warming}}
[[Category:2007 documents]]
[[Category:Climate change assessment and attribution]]
[[Category:Environmental reports]]
[[Category:Intergovernmental Panel on Climate Change reports|A04]]
[[Category:2007 in the environment]]