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February 2000

A reconciliation on global warming

A long-simmering debate over the nature of tropospheric warming has been at least partially resolved. The news broke on 13 January at the 80th annual meeting of the American Meteorological Society in Long Beach, California. That day, the National Research Council (NRC) issued the report Reconciling Observations of Global Temperature Change. Eight of the 11 scientists behind the document were on hand for an NRC-AMS presentation in Long Beach that afternoon. Among them was Kevin Trenberth (CGD), a key figure in the debate.

Through the 1990s, scientists puzzled and sometimes openly argued over a widening gap between global temperature trends at the surface and in the lower troposphere. Surface readings have risen more rapidly in the past 20 years (0.25-0.4°C) than have temperatures in the lowest few kilometers of the atmosphere (0.0-0.2°C). The latter readings have been inferred since 1979 through microwave emissions detected by satellite. In a series of papers, Kevin and colleagues argued that--in addition to the fact that the surface and satellite records measured different things--inconsistencies in the satellite record and its sampling techniques were to blame for the difference. Others stressed problems with the surface record. (More recently, improvements have been made to all the data sets.)

The NRC panel, convened at the suggestion of former NWS director Elbert "Joe" Friday, was headed by John Wallace (University of Washington) and included John Christy (University of Alabama in Huntsville), the lead scientist behind the temperature analyses derived from microwave sounding units aboard NOAA satellites.

The panelists concluded that, although instrument and sampling biases are a significant concern, different layers of the atmosphere do appear to be warming at different rates. "The troposphere actually may have warmed much less rapidly than the surface . . . due both to natural causes [e.g., volcanic eruptions] and human activities [e.g., cooling of the upper troposphere due to nearby ozone depletion in the stratosphere]." The report stresses that the dramatic surface warming trend since 1979 is "undoubtedly real," and then adds that there is no basis for assuming that the surface will continue to warm more quickly than the troposphere in future decades.

Kevin sums up, "All [of the observation systems] still have some problems and have potential for further improvements, but the biggest source of discrepancy between the surface and satellite records is that they measure different things and are influenced differently by things such as ozone depletion and changes in cloud cover."

Bob Henson

On the Web:
A summary of the NRC report and video from a press briefing are available at http://nationalacademies.org/topnews under the heading "Climate Conclusion."

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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall