BOULDER --- Three factors--the thinning of the ozone layer, emissionsfrom the Mt. Pinatubo volcano, and the influx of sulfate aerosols andgreenhouse gases into the atmosphere--may help explain why the lowestfive miles of the earth's atmosphere has not warmed as quickly as theearth's surface, say a group of scientists in a paper appearing inthe February 18 issue of the journal Science. The results followextensive data analysis and modeling studies by the 13 scientists.The team includes second author Tom Wigley and Gerald Meehl, bothscientists at the National Center for Atmospheric Research (NCAR).Lead author Ben Santer is at Lawrence Livermore National Laboratory.NCAR's primary sponsor is the National Science Foundation.
The difference in temperature trends at the surface and in the lowertroposphere has intensified the climate change debate. Some havepointed to the surface data as more reliable, while others havefocused on the satellite measurements. In January the NationalResearch Council (NRC) issued a report from a team of scientistsacross the spectrum of climate change positions that partlyreconciles the differences in data sets and offers some explanationof why the temperature trends would be different. The Santer-Wigleypaper, though not published at the time, was fully taken into accountin the report, says Kevin Trenberth, head of NCAR's Climate AnalysisSection and a coauthor of the NRC report.
For the Science paper, the team examined three observational datasets and recent model studies to reach their conclusions. The datasources are
--a century of thermometer readings of sea surface temperatures andair temperatures a few meters above land
--a half century of radiosonde measurements of troposphere and lowerstratosphere temperatures
--two decades of global observations of tropospheric temperatures (upto eight kilometers) taken by a series of satellites that measure theupwelling microwave radiation from oxygen molecules
Over the period 1979 to 1998, the surface data show a warming of 0.2-0.4 degree Celsius, while the radiosonde and satellite data show nowarming or only a slight temperature rise (0.1 degree C) in the lowertroposphere over the same period.
Neither complicated problems with the measurements nor the climate'sinherent variability over decades explains fully the temperaturetrend difference, say the authors. In a comprehensive modeling study,they found that the loss of stratospheric ozone and, to a lesserextent, the influx of Mt. Pinatubo emissions in the stratospherecooled the lower troposphere more than the surface. The model alsotook into account the buildup of greenhouse gases and sulfateaerosols.
Says Wigley, "This is a very complex problem with large uncertaintiesin the effects of human activities on the climate. However, we havereasonable confidence that ozone depletion and the Mt. Pinatuboemissions are likely candidates for explaining at least part of thecooler temperatures in the lower to middle troposphere compared tothe more intense warming at the surface."
NCAR is managed by the University Corporation for AtmosphericResearch, a consortium of more than 60 universities offering Ph.D.sin atmospheric and related sciences.
Materials provided by National Center For Atmospheric Research (NCAR). Note: Content may be edited for style and length.
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