Inside the Ozone Layer

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Researchers used a model to observe the stratosphere, the layer above the troposphere, and better understand what has contributed to its cooling over the past approximately 25 years. The stratosphere contains the ozone layer, which absorbs sunlight and heats the stratosphere. This long-term cooling trend is generally accepted to result from the loss of the ozone layer as a result of man-made influences. However, the cooling trend is not uniform like ozone loss, but rather broken into a series of jumps or discontinuities. These jumps are associated with major volcanic (El Chichon in 1982 and Mt. Pinatubo in 1991) eruptions that inject aerosols into the stratosphere. The aerosols also absorb sunlight and heat the stratosphere, thus temporarily offsetting the cooling trend from ozone loss. The volcanic eruptions are considered to be a "natural" forcing.

The model that researchers used to evaluate the influences, both man-made and naturally occurring, included not only the ozone loss and volcanic eruptions, but also solar activity, and a few other variables that also affect stratospheric temperature. What they found was that when all these factors are included, the model does an excellent job of reproducing the observed cooling of the stratosphere over the record of satellite observation from 1980 to 2005 - both the long-term cooling trend and the jumps.

"Since we can make this determination, it gives us confidence in the model's ability also to predict future cooling of the stratosphere over the next century from increasing greenhouse gases, such as carbon dioxide," said University of Miami Rosenstiel School's Dr. Brian Soden, who is among the paper's six authors.

Venkatchalam Ramaswamy, from NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton NJ, is the lead author on this paper, which also includes the work of Soden, Dan Schwarzkopf from NOAA's GFDL, Bill Randel from the National Center for Atmospheric Research, Ben Santer from the Program for Climate Model Diagnosis and Intercomparison at Lawrence Livermore National Laboratory, and Georgiy Stenchikov at the Department of Environmental Sciences at Rutgers University.

"While most people equate a warmer atmosphere with global warming, the stratosphere is a section of the atmosphere which actually cools in response to increasing greenhouse gases," Soden said.

Increasing carbon dioxide warms the earth's surface and troposphere, but eventually it actually cools the stratosphere. In the next century, carbon dioxide will likely become the dominant contributor to stratospheric cooling. And, future stratospheric cooling, in turn, may have important implications for the rate of recovery of the ozone hole in the southern hemisphere and possibly even the loss of ozone over the polar regions of the northern hemisphere.

University of Miami