The IPCC reports do not publish new science. More than 2,500 scientists from some 130 countries compile and analyze previously published peer-reviewed research. They spend years reconciling the many differences and putting the information together in a summary fashion.
The latest assessment, for example, says temperatures will rise 1.8 to 4.0 degrees Celsius by 2100. In contrast, the Third Assessment Report in 2001 predicted a range of 1.4 to 5.8 degrees -- a wider range and a lower minimum increase.
"The main difference (between the two reports) is that the conclusions are now so well-supported by observed data that nobody can reasonably doubt that we are in the midst of global warming," Stefan Rahmstorf, an IPCC contributor, told Tierramerica.
Beyond a few exceptions, the panel's reports say essentially the same things, said Rahmstorf, a professor of ocean physics at Germany's Potsdam Institute for Climate Impact Research.
But there are exceptions, and perhaps the most controversial has been the impact of melting glaciers and ice sheets on how much the sea level will rise.
Versions of the Fourth Assessment Report leaked to the media prior to Friday's official release predicted a sea-level rise by between 28 and 43 centimetres by 2100, substantially less than the 2001 prediction of between nine and 88 cm.
Because of a mandatory cut-off date for submissions, the newest IPCC report does not include assessments of the latest published studies showing that ice sheets in Greenland and elsewhere are melting much faster than previously thought.
Rahmstorf's own research published last December in Science magazine states that the sea level rise is more likely to be between 50 and 140 cm by 2100.
"The sea level numbers (in the IPCC report) are extremely conservative and the climate models don't reflect the most recent observations of what is happening in Greenland," says Andrew Weaver, a climatologist at the earth and ocean sciences school of Canada's University of Victoria.
Climate models do include findings by the World Glacier Monitoring Service (WGMS) in Zurich, Switzerland, that mountain glaciers are retreating three times faster than they were in the 1980s.
Since 1850, half of the world's mountain glaciers have melted and global temperatures only rose by about 0.8 degrees Celsius. A future temperature increase of 2.0 degrees means that "only biggest and highest glaciers would survive the 21st century," according to WGMS expert Michael Zemp.
Despite the report's conservatism and the virtually unanimity amongst climate scientists, there will be organizations and individuals who will deny humans are responsible for climate change. They blame the increased temperatures on variations in the sun or criticize the climate models' projections as inaccurate predictors of a future "hot house" planet.
University of Chicago scientist Archer says the alternative explanations have all been scientifically disproved, but they resurface regularly in new forms. "There are still many people, especially in North America, who are looking for any excuse to bury their heads in the sand," he said.
The new IPCC report used 19 different climate models, more than twice the last IPCC report, says John Fyfe, of the Canadian Center for Climate Modelling and Analysis, based in the western province of British Columbia. "A huge effort was made over the last six years to standardise and improve the accuracy of these models."
Extremely complex, climate models can only be run on the world's biggest supercomputers. Fyfe says they do a very good job of accurately simulating the climate over the past 150 years: "They are also very good at calculating future temperatures."
Models have also become much better at predicting what future conditions will be like over large areas such as the Canadian prairies, where more frequent droughts are expected.
The text released in Paris is the first of three parts making up the 4th Assessment Report. The next two, dealing with the impacts of climate change and how to mitigate these impacts, will be released in April and May.
Compared to the 2001 report, the first part, which deals with the physical science related to climate change, incorporates many more direct observations, which has greatly increased scientists' confidence in the results, says David Fahey of the U.S. National Oceanic and Atmospheric Administration's Earth Systems Research Laboratory.
"This report represents a tremendous leap forward in our understanding of climate," Fahey, who wrote a part of the IPCC report, told Tierramerica.
However, other than some details, this report says what past IPCC reports have said: climate change is happening now and it will get worse. And, adds Fahey, it could be much worse, if action isn't taken now to reduce emissions of greenhouses gases.
"To me climate change is a slow-motion train wreck."
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Albion Monitor February
5, 2007 (http://www.albionmonitor.com)
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