Satellites find no global warming in 1980s.

Satellites find no global warming in 1980s

Satellite measurements indicate Earth's lower atmosphere has not warmed over the last decade. But because these data span such a short period, they cannot settle the question of whether increasing levels of greenhouse gases have started to raise global temperatures.

"I certainly wouldn't say we've proven there isn't a global warming [underway]," says Roy W. Spencer of NASA's Marshall Space Flight Center in Huntsville, Ala., who analyzed the atmospheric measurements with John R. Christy of the University of Alabama in Huntsville.

The satellite instruments measure the temperature of the atmosphere's lower 10 kilometers by absorbing microwave radiation emitted by oxygen molecules. Between 1979 and 1988, the instruments recorded strong upswings and downswings in average global temperature, each lasting several months, but showed no general warming trend, the researchers report in the March 30 SCIENCE.

Concern over global warming stems in part from measurements taken at land stations and on ships, which indicate Earth's average surface temperature has risen by 0.5[degrees] since the late 1800s. The surface record differs slightly from the satellite data, showing a subtle rise since 1979. However, most of the surface warming occurred before 1979.

The surface record has come under fire in recent years from critics who contend that it fails to represent true global temperatures for two important reasons: The network does not sufficiently cover vast regions of the remote oceans, and urban heat from developing cities may artificially boost temperatures at land stations.

In contrast, the satellite sensors survey the entire lower atmosphere, and city warmth does not skew their readings. The sensors thus provide a powerful tool for monitoring future global temperatures, and could detect a global warming sooner than surface measurements, Spencer and Christy say. They plan to use satellite measurements to test the reliability of computer climate models that predict a 1.5[degrees]C to 4.5[degrees]C warming by the middle of the next century.

Spencer says the disagreement between satellite and surface data for the 1980s does not necessarily mean one source is wrong. Satellite measurements encompass a thick region of the lower atmosphere, he explains, and over a time span as brief as a decade, Earth's surface and its lower atmosphere might follow different temperature trends.