From http://www.sciencemag.org/cgi/content/full/312/5775/825
Fair use for educational purposes.
News of the Week
GLOBAL CHANGE:
No Doubt About It, the World Is Warming
Richard A. Kerr
Global warming contrarians can cross out one of their last talking points. A report released last week* settles the debate over how the atmosphere has been warming the past 35 years. The report, the first of 21 the Bush Administration has commissioned to study lingering problems of global climate change, finds that satellite-borne instruments and thermometers at the surface now agree: The world is warming throughout the lower atmosphere, not just at the surface, about the way greenhouse climate models predict.
"The evidence continues to support a substantial human impact on global temperature increases," added the report's chief editor Thomas Karl, director of the National Climatic Data Center in Asheville, North Carolina. The additional support for global warming will not change White House policy, however. Michele St. Martin, spokesperson for the White House Council on Environmental Quality, says President George W. Bush believes that greenhouse gas emissions can be brought down through better use of energy while the understanding of climate science continues to improve.
Critics who blasted research under the White House's Climate Change Science Program (CCSP) (Science, 27 February 2004, p. 1269) as mere obfuscation might not have expected such a forthright conclusion from the report. Karl attributes the clarity to the CCSP approach. "For the first time, we had people (who initially disagreed) sitting down across the table. That's a tremendous advantage," he says. "The process is great for improving understanding. It led to not just synthesis but to advancing the science." The CCSP synthesis and assessment process prompted new, independent analyses that helped eliminate some long-standing differences, Karl says.
The 21 authors of the report included researchers who for years had been battling in the literature over the proper way to analyze the satellite data. Meteorologists John Christy and Roy Spencer of the University of Alabama, Huntsville, were the first to construct a long record of lower-atmosphere temperature from temperature-dependent emissions observed by Microwave Sounding Units (MSUs) flown on satellites. By the early 1990s, Christy and Spencer could see little or no significant warming of the middle of the troposphere--the lowermost layer of the atmosphere--since the beginning of the satellite record in 1979, although surface temperature had risen.
Figure 1 A decent match. Warming of the lower atmosphere as measured from satellites (yellows and oranges, top) now resembles surface warming (bottom) measured by thermometers.
SOURCE: NOAA
In recent years, report authors Frank Wentz of Remote Sensing Systems in Santa Rosa, California, and Konstantin Vinnikov of the University of Maryland, College Park, led separate groups analyzing the MSU data. They and others found atmospheric warming more on a par with the observed surface warming (Science, 7 May 2004, p. 805). Hashing out those differences over the same table "was a pretty draining experience," says Christy.
In the end, the time and effort paid off, says Karl. The report authors eventually identified several errors in earlier analyses, such as not properly allowing for a satellite's orbital drift. They had additional years of data that lengthened a relatively short record. And they could compare observations with simulations from 20 different climate models, which researchers had prepared for an upcoming international climate change assessment. The report authors found that over the 25-year satellite record, the surface and the midtroposphere each warmed roughly 0.15°C per decade averaged over the globe, give or take 0.05°C or so per decade. The tropics proved to be an exception: The models called for more warming aloft than at the surface lately, whereas most observations showed the reverse. Reconciling that discrepancy will have to wait for the next round of synthesis and assessment.