By Curt Suplee
April 18, 2000
An early draft of an intently awaited report from the Intergovernmental Panel on Climate Change (IPCC) contains no surprises about the prospect of continued global warming, and comes to approximately the same major conclusions as its celebrated predecessor five years ago.
According to the new preliminary analysis by the IPCC, an international collaboration of several hundred scientists sponsored by the United Nations and the World Meteorological Organization, human beings have "discernibly" influenced the planet's climate and the Earth's surface is likely to warm at least 2 degrees and as much as 9 degrees Fahrenheit by the end of the 21st century. The group's previous assessment, issued in 1995, stated that "the balance of evidence suggests a discernible human influence on global climate."
The preliminary draft of the new assessment, released for expert comment yesterday, is somewhat more definitive, indicating that global warming since 1860--somewhere between 0.7 and 1.5 degrees F, about 0.2 degree F higher than the 1995 estimates--is "exceptional and unlikely to be solely natural in origin." But the range of possible temperature increases, the extent of potential sea-level increase (4 inches to 3 feet, depending on how much ice melts and how much the ocean expands from rising temperatures) and the estimates of additional carbon dioxide in the atmosphere by 2100 (two to three times the pre-industrial level of about 280 parts per million) are all extremely similar to the 1995 findings.
In general, improved understanding of world climate during the past five years has "made for a sharper statement" of the human contribution this time around, said Kevin E. Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colo., and a lead author on some of the draft report's sections. Trenberth said the remarkable warming of the record-setting 1990s, including the two hottest years on record in 1998 and '97--combined with better statistical analysis of data, enhanced computer models and greatly improved ability to reconstruct ancient climates--have convinced him and many others that man-made "climate change has emerged from the noise of natural variability."
The new draft, however, echoes the 1995 report in emphasizing several areas of uncertainty. One is the role of sulfate aerosols--typically released by fossil fuel combustion--that may serve to discourage global warming by making clouds shinier. Other factors include variations in the intensity of sunlight from decade to decade, and the so-far inexplicable changes in the rate at which methane, a potent greenhouse gas, enters the atmosphere. The rate of methane increase has slowed during the past two decades, the draft report notes, "for reasons that are not clear." "We probably emphasized a lot more of the negative" elements (that is, those that tend to mitigate warming) than the 1995 report did, Trenberth said. And the authors were even more careful to note major uncertainties, such as the likely amount and distribution of precipitation in a warmer world. The authors also determined that potential melting of the West Antarctic Ice Sheet is now regarded as much less of a threat than gradual decline of the Greenland Ice Sheet.
Not all the authors share Trenberth's view of the new draft report. "I think, if anything, it is a little bit more uncertain than it was last time," said Richard S. Lindzen of the Massachusetts Institute of Technology, a lead author of one section of the 14-chapter report, which totals nearly 1,000 pages. "We're really no closer to attributing [global warming since the 19th century] to anything in particular." In large measure, that is because of extreme uncertainties about the role of aerosols and "the assumption that [computer climate] models are good surrogates for the data," Lindzen said.
D. James Baker, administrator of the National Oceanic and Atmospheric Administration, said yesterday that his agency would have no comment on the draft assessment "until we have completed our review of all the chapters." The draft material released for comment yesterday is the first of three parts and focuses specifically on the quality of scientific research and data underlying the assessment of climate change.
During May, two other IPCC groups are expected to issue drafts of their reports on coping with impacts of warming and mitigating global warming. If all goes as planned, the final version of all three will be approved in January of 2001 and the entire report published shortly thereafter. The long process, Trenberth said, "is part of the price you pay for truly building a consensus."
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