IPCC Fourth Assessment Report

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The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) concludes that global warming is happening and that there is more than a 90% chance that it is caused by human emissions of greenhouse gases.

The Working Group I Summary for Policymakers report was published on 2 February, 2007; the full WGI report will be published in a few months. Working Group II and Working Group III reports are scheduled for early April and early May, respectively. The AR4 Synthesis Report (SYR) is expected to be finalised during the last quarter of 2007. Author lists and a chapter outline of the WGI report are available.[1]

The AR4 is expected to include, apart from updated science, a greater focus on uncertainty, risk concepts, and regional integration.

Working Group I reportEdit

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,[2] which took place in Paris, France, between 29 January and 1 February 2007.

The key conclusions were that[3]:

  • It is "unequivocal" that global warming is occurring
  • The probability that this is caused by natural climatic processes is less than 5%
  • The probability that this is caused by human emissions of greenhouse gases is over 90%

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

  • Surface air warming in the 21st century:
    • Best estimate for a "low scenario"[4] 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"[5] 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)
  • Based on a model that excludes ice sheet flow due to a lack of basis in published literature,[6] it is estimated that sea level rise will be:
    • in a low scenario[4] 18 to 38 cm (7 to 15 inches)
    • in a high scenario[5] 26 to 59 cm (10 to 23 inches)
  • It is more than 90% certain that there will be frequent warm spells, heat waves and heavy rainfall
  • It is more than 66% certain that there will be an increase in droughts, intensity of tropical cyclones (which include hurricanes and typhoons) and extreme high tides.

However, the report notes that conclusions as to the likelihood of many of these events (e.g. increasing heat waves, heavy precipitation, storm activity and sea level) were not drawn from formal attribution studies but rather based on informal conclusions drawn by some of the researchers.

Both past and future anthropogenic carbon dioxide emissions are predicted to 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.

The atmospheric concentration of carbon dioxide in 2005 exceeds the natural range over the last 650,000 years. The atmospheric concentration of methane in 2005 exceeds the natural range over the last 650,000 years.


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."

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." [7]

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."[8]

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.[9]


Dr. Chris Landsea withdrew his participation in the Fourth Assessment Report citing concerns 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 increased cyclone and hurricane activity, which Landsea described as a "misrepresentation of climate science while invoking the authority of the IPCC". He has stated that the Fourth Assessment Report is "motivated by pre-conceived agendas" and "scientifically unsound".[10]

See also Edit


  1. WG I Fourth Assessment Report (AR4)
  2. IPCC adopts major assessment of climate change science
  3. Climate Change 2007: The Physical Science Basis. Summary for Policymakers
  4. 4.0 4.1 "... 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."
  5. 5.0 5.1 "... 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"
  6. 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."
  7. "UK scientists' IPCC reaction", BBC NEWS, 2007-02-02.
  8. Duray, Dan. "Bush endorses climate study", Monterey County Herald, 2007-02-03.
  9. Doyle, Alister. "46 nations call for tougher U.N. environment role", Reuters, 2007-02-03.
  10. Chris Landsea's open letter on why he was leaving IPCC, accessed 2 Feb, 2007

External linksEdit

Part of a series on Global Warming
Scientific opinionAttribution of causesEffectsMitigationAdaptationControversyPoliticsEconomics
Related articles
Climate changeGlobal coolingGlobal dimmingGreenhouse effectGreenhouse gases
Intergovernmental Panel on Climate ChangeKyoto ProtocolPeak OilRenewable energyTemperature data

Adapted from the Wikipedia article "IPCC Fourth Assessment Report", updated February 5, 2007

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