BOULDER--Three factors--the thinning of the ozone layer, emissions from the Mt. Pinatubo volcano, and the influx of sulfate aerosols and greenhouse gases into the atmosphere--may help explain why the lowest five miles of the earth's atmosphere has not warmed as quickly as the earth's surface, say a group of scientists in a paper appearing in the February 18 issue of the journal Science. The results follow extensive data analysis and modeling studies by the 13 scientists. The team includes second author Tom Wigley and Gerald Meehl, both scientists 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 lower troposphere has intensified the climate change debate. Some have pointed to the surface data as more reliable, while others have focused on the satellite measurements. In January the National Research Council (NRC) issued a report from a team of scientists across the spectrum of climate change positions that partly reconciles the differences in data sets and offers some explanation of why the temperature trends would be different. The Santer-Wigley paper, though not published at the time, was fully taken into account in the report, says Kevin Trenberth, head of NCAR's Climate Analysis Section and a coauthor of the NRC report.
For the Science paper, the team examined three observational data sets and recent model studies to reach their conclusions. The data sources are
--a century of thermometer readings of sea surface temperatures and air temperatures a few meters above land
--a half century of radiosonde measurements of troposphere and lower stratosphere temperatures
--two decades of global observations of tropospheric temperatures (up to eight kilometers) taken by a series of satellites that measure the upwelling 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 no warming or only a slight temperature rise (0.1 degree C) in the lower troposphere over the same period.
Neither complicated problems with the measurements nor the climate's inherent variability over decades explains fully the temperature trend difference, say the authors. In a comprehensive modeling study, they found that the loss of stratospheric ozone and, to a lesser extent, the influx of Mt. Pinatubo emissions in the stratosphere cooled the lower troposphere more than the surface. The model also took into account the buildup of greenhouse gases and sulfate aerosols.
Says Wigley, "This is a very complex problem with large uncertainties in the effects of human activities on the climate. However, we have reasonable confidence that ozone depletion and the Mt. Pinatubo emissions are likely candidates for explaining at least part of the cooler temperatures in the lower to middle troposphere compared to the more intense warming at the surface."
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.