UCAR Communications

staff notes monthly

May 2003

Study finds lower atmosphere warming

The reanalysis of satellite data could refute skeptics of global warming.

A new analysis of satellite data collected since the late 1970s from the lowest few miles of the atmosphere indicates a global temperature rise of about one-third of a degree Fahrenheit between 1979 and 1999. The results are at odds with previous analyses that show virtually no warming in the satellite record over the 20-year period—and they provide more evidence that global warming is actually occurring.

A team that includes Climate and Global Dynamics scientists Tom Wigley, Gerald Meehl, Caspar Ammann, Julie Arblaster, Thomas Bettge, and Warren Washington reported its findings in the 2 May online issue of Science. The lead author is Ben Santer of Lawrence Livermore National Laboratory.

“There is a lot of statistical uncertainty when you’re trying to estimate a trend from very noisy data,” Tom says. “But it’s undeniable that the agreement with both global climate models and surface data is better for the new analysis than for the old one.”

Over the past 25 years, a series of instruments aboard 12 U.S. satellites has provided a unique temperature record extending as high as the lower stratosphere. Each sensor intercepts microwaves emitted by various parts of the atmosphere, with the emissions increasing as temperatures rise. These data are used to infer the temperature at key atmospheric layers.

Since the 1990s, skeptics have pointed to the absence of a warming signal in the satellite-derived temperatures, which stood in contrast to a distinct warming trend in average air temperature at Earth’s surface. A 2000 report from the National Research Council concluded that both trends might be correct—in other words, the global atmosphere might be warming more quickly near the ground than higher up.

Although Tom agreed, he felt there was more to be explained.

“The real issue is the trend in the satellite data from 1979 onward,” says Tom. “If the original analysis of the satellite data were right, then something must be missing in the models. With the new data set, the agreement with the models is improved, and the agreement with the surface data is quite good.”

In order to glean temperatures from the raw satellite data, several adjustments and corrections had to be made. Until now, only one group, based at the University of Alabama in Huntsville, had produced a complete set of global temperatures from the raw data. It found minimal warming.

For the new study, a group based at Remote Sensing Systems in Santa Rosa, California, applied a revised set of corrections to the satellite data. This accounted for the effects of heating on the radiation sensor itself—the first time this source of error had been addressed fully, according to the authors—and made adjustments for the drifting orbit of each satellite and other factors.

The group found a warming trend of 0.16°F (0.10°C) per decade in the layer between about 1.5 and 7.5 miles (2.4–12.1 kilometers) high, compared to a trend of 0.02°F (0.01°C) in the previously published University of Alabama in Huntsville analysis. Both estimates have a margin of error of plus-or-minus nearly 0.2°F (up to 0.12°C)

According to the authors, the new results are a closer match with surface warming, as well with four simulations of 20th-century climate produced by global-scale models of the ocean and atmosphere. These simulations were produced by CGD scientists and their colleagues using the Parallel Climate Model, a global-scale model of the ocean and atmosphere that was built by NCAR and Los Alamos National Laboratory. The simulations included solar variations, volcanoes, greenhouse gases, and sulfate aerosols, all of which affect climate.

As a further check on the new satellite data set, the team examined regional patterns. Using a statistical technique, the group analyzed the 20th-century simulations and searched for an underlying “fingerprint” of climate change. For instance, the rates of warming in the satellite-monitored data vary by latitude from north to south. The authors found that the overall fingerprint of climate change in the models resembled this and other regional patterns found in the new satellite data set.

•Bob Henson


Also in this issue:

In the midnight hour: BAMEX takes aim at dangerous night storms

The long riders: How some staffers cope with epic commutes

Study finds lower atmosphere warming

An information divide

Building bridges for Latina students

Short takes

Delphi Question: Publications on the Web


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