Weather balloons deflate climate blow-up.

Measurements made with weather balloons may cool a debate raging among climate scientists over the degree of global warming. A new study suggests that the past 20 years have been a climatologically curious time, when Earth's surface and the lower atmosphere have warmed at different rates.

Researchers first caught wind of the difference several years ago while analyzing atmospheric temperatures taken by a series of satellite instruments called the Microwave Sounding Unit (MSU). The MSU data seemed to indicate that temperatures several thousand meters above the surface were falling even as weather station readings showed a significant warming at ground level. In recent years, researchers have questioned the reliability of both data sets, especially the satellite record, which reaches back only to 1979 (SN: 8/15/98, p. 100).

"This caused quite a controversy because the people who don't believe in global warming use any data which doesn't show it, such as the MSU data," says James K. Angell of the National Oceanic and Atmospheric Administration in Silver Spring, Md. "The other people, who are in the majority, feel there is evidence for a warming based on the surface data," he adds.

"I seem to be bridging the gap here and saying that they may both be basically correct," says Angell, who reports his results in the Sept. 1 GEOPHYSICAL RESEARCH LETTERS.

Angell analyzed twice-daily temperature measurements made at 63 sites around the globe since 1958. He compared readings taken at the surface with balloon data giving an average tropospheric temperature, representing conditions between about 1,500 and 9,000 meters above the ground.

Over the entire 41-year period, the surface temperatures climbed at a rate of 0.14 [degrees] C per decade, while the atmosphere warmed at 0.10 [degrees] C per decade. The rates are roughly equal, given the wide range of uncertainty, says Angell.

Since 1979--the period during which satellites have collected data--the surface and atmosphere have behaved quite differently, according to Angell. The surface has warmed at a rate of 0.15 [degrees] C per decade, while the lower atmosphere temperatures have increased only 0.04 [degrees] C per decade.

"This is extremely consistent with what we have seen," says John R. Christy of the University of Alabama in Huntsville, who analyzes the MSU data. Christy attributes the discrepancy between surface and troposphere to a series of climate-disrupting episodes. "We're talking about a 20-year period that had very unusual events: two of the largest El Ninos of this century and two volcanic eruptions that are the largest of this century," he says. "The surface and the troposphere respond to these things differently."

Over several decades, Christy predicts, the two trends will realign themselves.

Even as it supports the satellite data, the new study also bolsters the surface data that have come under attack from greenhouse skeptics. "One of the things that bothers me is that people were using the MSU data to say that the surface isn't warming," says Frank J. Wentz of Remote Sensing Systems in Santa Rosa, Calif. "Clearly, the surface is warming," says Wentz, a member of a National Research Council committee examining the difference between surface and satellite data.