Global Warming Causes Stratospheric Cooling
By Jeffrey Masters, Ph.D. — Director of Meteorology, Weather Underground, Inc.
Global temperatures during January – August 2011 were the third coldest on record in the lower stratosphere, according to the National Climatic Data Center, and have been generally declining in recent decades (Figure 1). Why is this important? Well, the stratosphere is that layer of the upper atmosphere approximately 14-22 km (9-14 miles) above the surface that contains our protective ozone layer. The main reason for the recent stratospheric cooling is due to the destruction of ozone by human-emitted CFC gases. Ozone absorbs solar UV radiation, which heats the surrounding air in the stratosphere. Loss of ozone means that less UV light gets absorbed, resulting in cooling of the stratosphere. Cooling of the stratosphere results in the formation of more polar stratospheric clouds, which require very cold temperatures to form. The presence of these clouds allows even more ozone destruction to occur, since the reactions responsible for ozone destruction occur much faster in clouds than in dry air. Thus, the recent cooling of the stratosphere allows high levels of harmful UV light to reach the surface. As CFC gases begin to decline in coming years thanks to banning of these substances in 1987, the stratosphere should start to warm, and ozone levels will recover.
Annual Lower Stratospheric Temperature Anomalies
Figure 1. Global lower stratospheric departure of temperature from average since 1979, as measured by satellites. The large spikes in 1982 and 1991 are due to the eruptions of El Chicon and Mt. Pinatubo, respectively. These volcanos ejected huge quantities of sulphuric acid dust into the stratosphere. This dust absorbed large quantities of solar radiation, heating the stratosphere. Image credit: National Climatic Data Center.
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