The Intergovernmental Panel on Climate Change (IPCC) is a United Nations organization established by the World Meterological Organization and the United Nations Environment Programme in 1988. Its reports are considered authoritative by governments and other entities worldwide. It has published a series of assessment reports (in 1990, 1995, and 2001) and is currently working on the Fourth Assessment Report. These reports are massive, with enormous amounts of data and analyses. Four sections are planned for the Fourth Assessment Report.
A couple weeks ago, the IPCC released a summary of the first section (Working Group 1 — “Climate Change 2007: The Physical Science Basis”). This summary, the WG1 Summary for Policymakers, “was produced by some 600 authors from 40 countries. Over 620 expert reviewers and a large number of government reviewers also participated. Representatives from 113 governments reviewed and revised the Summary line-by-line during the course of this week before adopting it and accepting the underlying report,” according to the IPCC (emphasis mine). The list of governments approving the summary does include the United States; U. S. Energy Secretary Samuel Bodman said the report was based on “sound science” and added, “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” (source).
I have omitted references to figures, footnotes, and supporting paragraphs in an attempt to distill this summary. It is not my intention to distort the meaning of it; please see the full summary for further information. In the summary, the authors use the following definitions: very high confidence is a greater than 90% chance of being correct; for outcomes or results, very likely is a probability greater than 90%, while likely is greater than 66%.
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.
The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to 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 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 [10 to 20 feet] 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. This is an advance since the [Third Assessment Report’s] conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
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 [0.4° F] per decade is projected for a range of SRES [the IPCC Special Report on Emission Scenarios] 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 [0.2° F] 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.
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.