|Comparison of observed continental- and global-scale changes in surface temperature (black line) with results simulated by climate models using natural and anthropogenic forcings. Click to enlarge.|
Warming of the world’s climate system is “unequivocal”, most of the observed increase in globally averaged temperatures since the mid-20th century is “very likely” due to the increase in greenhouse gas concentrations caused by anthropogenic emissions, and continued greenhouse gas emissions at or above current rates will cause further warming and induce many changes in the global climate that would “very likely” be larger than those observed during the 20th century, according to the just-released Summary of the first volume of “Climate Change 2007”—also known as the Fourth Assessment Report (AR4)—by Working Group I of the Intergovernmental Panel on Climate Change (IPCC).
”Very likely,” as used in the Summary, indicates a greater than 90% probability, using expert judgement, of an outcome or a result. This assessment reflects a greater confidence in the role of anthropogenic emissions than in the prior report in 2001, when their role was deemed “likely”—greater than 66% probability.
The entire Climate Change 2007 report will comprise three main volumes resulting from the efforts of three working groups:
- Working Group I: The Physical Science Basis (Release 2 February 2007)
- Working Group II: Impacts, Adaptation and Vulnerability (Acceptance and approval 2-5 April 2007)
- Working Group III: Mitigation of Climate Change (Acceptance and approval 30 April - 3 May 2007)
- A short 30-page synthesis report
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 document released today is a summary of that work.
The report was produced by some 600 authors from 40 countries. More than 620 expert reviewers and a large number of government reviewers also participated. Approximately 300 delegates from 113 countries reviewed and revised the Summary line-by-line during the course of this past week before adopting it and accepting the underlying report. Acceptance is through consensus; the implications is that whatever is accepted and approved has the acceptance of all the participating governments.
While you may probably feel that there is no direct connection between the physical science related to climate change and mitigation options which require actions, let’s say, in reducing or improving the efficiency of fossil fuel use, there is a direct connection because if you see the extent to which human activities are influencing the climate system, then the options for mitigation of the emissions of greenhouse gases appear in a totally different light...you are able to see then what the costs of inaction are.—Dr Rajendra K Pachauri, Chairman, IPCC
Key findings of the report include:
Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide 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 carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture.
There is a very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W m-2.
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 mean sea level.
At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones. Some aspects of climate have not been observed to change.
Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 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 meters of sea level rise.
Analysis of climate models together with constraints from observations enables an assessed likely range to be given for climate sensitivity for the first time and provides increased confidence in the understanding of the climate system response to radiative forcing.
For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES 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. Best estimates for the temperature increase by 2100 in the different scenarios range from 1.8°C to 4.0°C, within broader ranges.
There is now higher confidence in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and of ice.
Anthropogenic warming and sea level rise would continue for centuries due to the timescales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilized.
Climate Change 2007: The Physical Science Basis Summary for Policymakers