The conclusions by the Intergovernmental Panel on Climate Change, the most authoritative scientific voice on global warming, is expected to unleash new controversy as scientists and governments debate the earth's climate in the coming decade.
The report's summary, a copy of which was obtained by The Associated Press, was distributed to government officials worldwide this week and will be fine-tuned at a meeting of world government representatives early next year.
While some wording will change, the panel's scientific findings cannot be altered, several participants in crafting the summary report said.
It is the first full-scale review and update of the state of climate science by the IPCC panel since 1995 when the same group concluded there is "a discernible human influence"on the earth's climate, the so-called "greenhouse" effect caused by the buildup of heat-trapping chemicals in the atmosphere.
While there remain uncertainties, studies of the last five years and more sophisticated computer modeling shows "there is now stronger evidence for a human influence" on the climate and more certainty that man-made greenhouse gases "have contributed substantially to the observed warming over the last 50 years."
"What this report is clearly saying is that global warming is a real problem and it is with us and we are going to have to take this into account in our future planning," said Kevin Trenberth, head of climate analysis section at the National Center for Atmospheric Research in Boulder, Colo.
Equally significant, is the conclusion in the new assessment that if greenhouse emissions are not curtailed the earth's average surface temperatures could be expected to increase substantially more than previously estimated.
The panel concluded that average global temperature increases ranging from 2.7 to as much as 11 degrees Fahrenheit can be expected by the end of this century if current trends of concentration of heat-trapping gases continues unabated in the atmosphere.
Five years ago, the panel put the projected increases at a range 1.8 to 6.3 degrees Fahrenheit. The panel said the higher temperatures stem mainly from more sophisticated computer modeling and expected decline in sulfate releases into the atmosphere, especially from power plants for other environmental reasons. These sulfates tend to act as a cooling agent by reflecting the sun's radiation.
Michael Oppenheimer, an atmospheric physicist at Environmental Defense, said the new warming estimates pose "a risk of devastating consequences within this century."
A co-author of the full report, he said he could only discuss it in general terms since it has yet to be released.
The findings were the product of sevral hundred climate scientists including, like Oppenheimer, longtime proponents of global warming as well as skeptics who say there remain significant uncertainties.
Michael Schlesinger, a climatologist at the University of Illinois at Champaign-Urbana, who also contributed to the report, emphasized in an interview that there is still insufficient knowledge about natural climate variables such as solar radiation that could influence any assessment on human impact on climate.
Three years ago industrial nations tentatively agreed to curtail the release of greenhouse gases mainly carbon dioxide from burning fossil fuels to below 1990 levels as a first step to address global warming. None of the major industrial emitters has yet to ratify the treaty.
The IPCC panel's 1995 summary represented the key scientific underpinning for the Kyoto accord. Likewise the new assessment, reflecting the most current scientific understanding of the climate system and potential for future warming, will be key to the coming decade's climate debate.
"An increasing body of observations gives a collective picture of warming world," the scientists determined.
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