6 February 2007
Most of the observed increase in globally averaged temperatures is very likely due to the observed increase in anthropogenic greenhouse gas concentrations, concludes the UN’s Intergovernmental Panel on Climate Change (IPCC) in his newest assessment of climate change science. The IPCC is nearing the completion of its “Climate Change 2007”, also known as the Fourth Assessment Report (AR4). The major conclusions of the first volume of the report, Climate Change 2007: The Physical Science Basis, have been released as the “Summary for Policymakers” document.
The “very likely” designation is an advance from the previous, 2001 report, which concluded that global warming was “likely” caused by increasing greenhouse gas concentrations. The IPCC report uses the term very likely to indicate the assessed likelihood, using expert judgement, of an outcome of over 90%, while the term likely denotes the likelihood of 66%.
Climate Change 2007: The Physical Science Basis, 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 some 600 authors from 40 countries, with the participation of over 620 expert reviewers and a large number of government reviewers from 113 countries.
Selected findings of the report include:
- Global atmospheric concentrations of CO2, CH4 and N2O have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in CO2 concentration are due primarily to fossil fuel use and land-use change, while those of CH4 and N2O are primarily due to agriculture.
- The understanding of anthropogenic warming and cooling influences on climate has improved since the previous report, leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming.
- 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.
- Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 metres of sea level rise.
- 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. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
- For the next two decades a warming of about 0.2°C per decade is projected for a range of emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected.
- Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
The report analyzed six emission scenarios. The predicted warming by the end of the 21st century is 1.8°C (a range from 1.1 to 2.9) for the most optimistic scenario, and 4.0°C (2.4 - 6.4) for the most pessimistic one. The corresponding ranges of rise in sea levels are 0.18-0.38 m and 0.26-0.59 m, respectively.
For the 20th century, the average temperature increase is estimated to be 0.74°C (0.56 - 0.92), and the sea level rise 0.17 m (0.12 - 0.22).
When completed later this year, the Climate Change 2007 report will include three main volumes:
- The Physical Science Basis (Working Group I),
- Impacts, Adaptation and Vulnerability (Working Group II),
- Mitigation of Climate Change (Working Group III),
and a short 30-page synthesis report.