Affirmation on Global Warming
May 5, 2000
The International Panel on Climate Change (IPCC), an international collaboration of several hundred scientists sponsored by the United Nations and the World Meteorological rganization, will soon be releasing their periodic report on climate change.
According to preliminary analysisof their new report, humans have "discernibly" influenced the planet's climate. The temperature of the Earth's surface is likely to warm by at least 2 degrees and as much as 9 degrees Fahrenheit over the next 100 years.
This is no a far departure from their previous assessment, issued in 1995, which 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 more definitive, indicating that since 1860 (the start of large-scale fossil fuel combustion of the industrial revolution) average gobal temperatures have risen somewhere between 0.7 and 1.5 degrees F, which is about 0.2 degree F higher than the group's 1995 estimates.
While most scientists that originally questioned global warming now agree there is a warming trend, there still is a question concerning the contribution human activity has to increasing temperatures. The new report addresses this topic by calling the warming trend "exceptional and unlikely to be solely natural in origin."
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, CO. Trenberth is also a lead author on some of the draft report's sections.
Most of the new report mirrors estimates and conclusions from the 1995 report: 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).
Trenberth said the trend of increasing temperatures of the record-setting 1990s (including the two hottest years on record in 1998 and '97) combined with enhanced computer models, better statistical analysis of data, 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 pointing out several areas of uncertainty. One is the role of sulfate aerosols, typically released by fossil fuel combustion, that could possibly help reduce the influx of solar radiation by reflecting the suns rays.
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. 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 [warming trends since 1860] 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