Tugging Satellite Data Into Step With Global Warming

A newly unmasked error in satellite probes of temperatures in Earth's atmosphere suggests that over the past 20 years the lower atmosphere has warmed slightly--not cooled, as the data had suggested. The correction brings air temperatures into line with measurements of steady warming at the ground, according to a report in tomorrow's issue of Nature. Some climate experts argue that the finding undermines the main criticism of scientists who contend that greenhouse gases are not warming the planet. But not all agree.

Climatologists looking for signs of global warming have been vexed by a contradiction between ground-based and airborne temperature measurements. At the surface, average temperatures have climbed by about 0.13° Celsius per decade since 1979. But satellite measurements of radiation shed by oxygen molecules suggested that the bottom 10 kilometers of Earth's atmosphere had cooled 0.05°C per decade in the same period.

The discrepancy prompted atmospheric physicists Frank Wentz and Matthias Schabel of Remote Sensing Systems in Santa Rosa, California, to scrutinize the satellite data. To their surprise, they found that no one had accounted for changes in the satellites' altitudes caused by slight drags in the upper atmosphere. The gradual drop alters the angle at which each satellite looks at part of the lower atmosphere. That, in turn, introduces a slight but key error into temperature calculations. When they corrected the error, Wentz and Schabel derived a warming trend of about 0.07°C per decade, more in line with surface thermometers and climate models. "It's a simple and undeniable correction," Wentz says.

The adjustment seems reasonable, says meteorologist Dian Gaffen of the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Maryland. "I'm surprised it had gone undetected until now." In a commentary in Friday's issue of Science, climatologist James Hansen of NASA's Goddard Institute for Space Studies in New York City and his colleagues argue that the new work has important repercussions for the global warming debate. "Until now, the [satellite] data have been the principal refuge for those who deny the reality of global warming," Hansen's team writes. "The issue should no longer be whether global warming is occurring, but what is the rate of warming, what is its practical significance, and what should be done about it."

But global warming skeptics aren't ready to concede yet. Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says that temperature measurements aboard balloons show essentially constant temperature or perhaps even slight cooling. "This is far from conciliation with computer simulations that project a warming trend," says Baliunas.