IPCC Fourth Assessment Report

The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) is the fourth in a series of reports on climate change. So far only the Working Group 1 - Summary for Policy Makers (SPM) has been published.

SPM contains the following statements:

• "Warming of the climate system is unequivocal"
• "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

Extensive studies of future evolution of climate based on different social and economic projections are modeled and compared.

Report Overview

The Fourth Assessment Report (Climate Change 2007) is released in four distinct sections:

• Working Group I Report (WGI) Climate Change 2007: The Physical Science Basis.
• Working Group II Report (WGII) Climate Change 2007: Impacts, Adaptation and Vulnerability
• Working Group III Report (WGIII) Climate Change 2007: Mitigation of Climate Change
• The Synthesis Report (SYR)

For each section, the IPCC will release the main report and a summary version, known as the Summary for Policy Makers. To date, only the Summary for Policy Makers of the WGI report has been completed and released. Author lists and a chapter outline of the reports are available.

Working Group I (WGI) The Physical Science Basis

The Working Group I Summary for Policymakers (SPM) [2] was published on 2 February, 2007 and revised on 5 February, 2007; the full WGI report will be published a few months later.

Climate Change 2007: The Physical Science Basis, the report of Working Group I, "assesses the current scientific knowledge of the natural and human drivers of climate change, observed changes in climate, the ability of science to attribute changes to different causes, and projections for future climate change".

The report was produced by around 600 authors from 40 countries, and reviewed by over 620 experts and governments. Before being accepted, the summary was reviewed line-by-line by representatives from 113 governments during the 10th Session of Working Group I, which took place in Paris, France, between 29 January and 1 February 2007.

The report concluded that:

• "Warming of the climate system is unequivocal"
• "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

Footnotes on page 4 indicate very likely means "the assessed likelihood, using expert judgement", is over 90%

Observations

The report noted 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."
• The amount of carbon dioxide in the atmosphere in 2005 (379 ppm) exceeds by far the natural range of the last 650,000 years (180 to 300 ppm).
• The amount of methane in the atmosphere in 2005 (1774 ppb) exceeds by far the natural range of the last 650,000 years (320 to 790 ppb).
• The primary source of the increase in carbon dioxide is fossil fuel use, but land-use changes also make a contribution.
• The primary source of the increase in methane is very likely to be a combination of human agricultural activities and fossil fuel use. How much each contributes is not well determined.
• Nitrous oxide concentrations have risen from a pre-industrial value of 270 ppb to a 2005 value of 319 ppb. More than a third of this rise is due to human activity, primarily agriculture.

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 amont the top 12 warmest years in the instrumental record (since 1850).
• Warming in the last 100 years has caused about a 0.74 °C increase in global average temperature. This is up from the 0.6 °C increase in the 100 years prior to the Third Assessment Report.
• Urban heat island effects were determined to have neglible influence (0.0006 °C per decade over land and zero over oceans) effect 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 3000m (9800 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.

Ice, snow, permafrost, and the oceans

The SPM documents increases in wind intensity, decline of permafrost coverage, and increases of both drough and heavy precipitation events. Additionally:

• "Mountain glaciers 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 over 1993 to 2003.
• Ocean warming causes seawater to expand, which contributes to sea level rise.
• Sea level rose at an average rate of about 1.8mm/year over 1961-2003. Sea level rise over 1993-2003 was at an average rate of 3.1mm/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 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 models 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 ocurred, 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 certainity 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 judgement rather than formal attribution studies."

Factors that warm or cool the planet

AR4 describes warming and cooling effects on the planet in terms of radiative forcing. The report show in detail the individual warming contributions 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 of aerosols, 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 a warming force of about +1.6 watts/m²
• Radiative forcing from an increase of solar intensity since 1750 is about +0.12 watts/m²
• 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. It is likely to be in the range of 2 to 4.5 °C, with a best estimate of about 3 °C. This range of values is not a projection of the temperature rise we will see in the 21st century.

Model-based projections for the future

Model projections are made based on an analysis of various computer climate models running within different SRES scenarios

As a result it is predicted that, during the 21st century:

• Surface air warming in the 21st century:
  • Best estimate for a "low scenario" is 1.8 °C with a likely range of 1.1 to 2.9 °C (3.2 °F with a likely range of 2.0 to 5.2 °F)
  • Best estimate for a "high scenario" is 4.0 °C with a likely range of 2.4 to 6.4 °C (7.2 °F with a likely range of 4.3 to 11.5 °F)
  • A temperature rise of about 0.1C 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 0.2C 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, it is estimated that sea level rise will be:
  • in a low scenario 18 to 38 cm (7 to 15 inches)
  • in a high scenario 26 to 59 cm (10 to 23 inches)
• It is very likely that there will be an increase in frequency of warm spells, heat waves and events of heavy rainfall
• It is likely that there will be an increase in areas affected by droughts, intensity of tropical cyclones (which include hurricanes and typhoons) and the occurrence of extreme high tides.
• "Sea ice is projected to shrink in both the Arctic and Antarctic … 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 the most optimistic scenario family, B1. "High scenario" refers to the most pessimistic scenario family, A1FI.

Footnotes on page 4 of the summary indicate very likely means "the assessed likelihood, using expert judgement", is over 90%, likely means >66%.

Temperature and sea level rise for each SRES scenario family

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

Selected quotes from the WGI Summary for Policy Makers

• "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 WGI

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 TAR; but the range has narrowed.

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."

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 attributing to changes in our earth's climate and that issue is no longer up for debate."

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 gasses.

After the report's release, The Guardian reported that the American Enterprise Institute, who the newspaper calls "an ExxonMobil-funded think tank with close links to the Bush administration", had offered several scientists $10,000 USD each to write articles disputing the IPCC's conclusions.

Working Group II (WGII) The Physical Science Basis

The Working Group II report is scheduled to be released in early April, 2007.

Working Group III (WGIII) Mitigation of Climate Change

The Working Group II report is scheduled to be released in early May, 2007.

Criticism

• In January 2005, more than 2 years before the publication, Dr. Chris Landsea withdrew his participation in the Fourth Assessment Report expressing his opinion that the IPCC had become "politicized" and that the IPCC leadership simply dismissed his concerns. The conflict centers around Dr. Kevin Trenberth's public contention that global warming was contributing to "recent hurricane activity", which Landsea described as a "misrepresentation of climate science while invoking the authority of the IPCC". Landsea does not believe that global warming has a strong influence on hurricanes: "global warming might be enhancing hurricane winds, but only by 1 percent or 2 percent". He has stated that the process of producing the Fourth Assessment Report is "motivated by pre-conceived agendas" and "scientifically unsound" [3]. Landsea writes that "the IPCC leadership said that Dr. Trenberth was speaking as an individual even though he was introduced in the press conference as an IPCC lead author"

Roger A. Pielke, who originally published Landsea's letter, expressed his opinion that the actual report "maintain[s] consistency with the actual balance of opinion(s) in the community of relevant experts."

• Scientists like former US department of energy member Joseph Romm have claimed that the report underestimates positive feedbacks that could lead to a runaway greenhouse effect, thus greatly underestimating the future warming and its effects. The report is also said to be out of date because it omits recent observations such as the release of greenhouse gases, including methane, from thawing tundra.

The actual report gives a warning that positive feedbacks could release more carbon dioxide in a yet uncertain magnitude, but it does not mention gases with an even greater global warming potential like methane: "Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. This increases the uncertainty in the trajectory of carbon dioxide emissions required to achieve a particular stabilisation level of atmospheric carbon dioxide concentration."

• Shortly after publication of the AR4 Summary for Policymakers, The libertarian Fraser Institute issued an alternative "Independent Summary for Policymakers" (ISPM) drawing sceptic conclusions. An analysis by climate scientists writing for noted climate science blog RealClimate describes it as "profoundly ignorant" of IPCC processes, and stated that it is better to think of the ISPM as the "Incorrect Summary for Policymakers".

See also

• Individual and political action on climate change
• Business action on climate change
• Energy policy
• Energy conservation
• Global climate model
• Precautionary principle
• World energy resources and consumption

External links

• IPCC website AR4 pages
• IPCC website Outline of topics in the AR4 synthesis report
• UNEP.org Press release: Evidence of Human-caused Global Warming “Unequivocal”, says IPCC
• The Guardian Article on US political pressure on the report
• RealClimate — The IPCC Fourth Assessment SPM
• Grains of Sand: A 21st century greenhouse gas budget? Lessons from AR4, 4 February 2007
• Spiked online magazine: A man-made morality tale — James Woudhuysen and Joe Kaplinsky, 5 February 2007
• Grist: Bush administration put on defensive over climate change, 9 February 2007