A team of Harvard scientists led by Weld professor of atmospheric chemistry James G. Anderson announced today the discovery of a serious and wholly unexpected risk of ozone loss over the United States in summer.
The finding, published in advance online on July 26 at Science’s Science Express website, is startling because the complex atmospheric chemistry that destroys ozone has previously been thought to occur only at very cold temperatures over polar regions where there is very little threat to humans. (A large hole in the ozone layer persists over Antarctica.) The discovery also links—for the first time—ozone loss (an issue around which world leaders successfully organized to ban chlorofluorocarbons, or CFCs) to climate change (a global problem that has so far proven politically intractable).
Courtesy of James G. Anderson/Art by Rob Stanhope
Harvard scientists have discovered that intense summer storms can force water vapor into the dry and cold stratosphere through a process called convective injection.
The presence of such water vapor, which normally stops at the tropopause (the boundary between the troposphere and the stratosphere), changes the threshold temperature at which ozone is destroyed by chemistry dependent on manmade chlorofluorocarbons (CFCs), which are still present in the atmosphere despite an international ban on their use.
The chemical reactions that destroy ozone typically occur only at very cold temperatures. The presence of water vapor raises the temperature at which ozone loss takes place, to the point that threshold conditions for ozone destruction are routinely crossed during the summer above the United States and possibly elsewhere. The frequency and intensity of these summer storms is expected to increase with climate forcing due to increasing levels of heat-trapping atmospheric carbon dioxide and methane. Reductions in stratospheric ozone would allow more DNA-damaging ultraviolet radiation to reach Earth, with potential biological effects on human beings, animals, and plants.
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