A Second Perilous Breach in the Ozone Hole?

Greenhouse gases will pry open a springtime ozone hole over the Arctic during the next few decades--much like the one now over the Antarctic--according to predictions from a computer model described in tomorrow's issue of Nature. An annual Arctic hole would likely mean more harmful ultraviolet radiation reaching people and ecosystems at high northern latitudes.

Although carbon dioxide and other greenhouse gases heat the atmosphere, it's not the warmth that's posing the threat. Ozone-destroying reactions in polar regions are catalyzed by icy cloud crystals that form only in extreme cold. The colder it gets and the longer that cold persists into the spring--when the other key ingredient, sunshine, appears--the more ozone will be destroyed. Greenhouse gases warm the lower atmosphere, but they also cool the polar stratosphere by radiating heat to space and possibly by changing winds that carry heat from lower latitudes.

When Drew Shindell, David Rind, and Patrick Lonergan--all computer modelers at NASA's Goddard Institute for Space Studies in New York City--ran their model, they found that projected increases in greenhouse gases would progressively chill the wintertime stratosphere over the poles by 8 to 10 degrees Celsius. Arctic ozone losses, rather than declining as international controls on ozone-depleting chlorine compounds take hold, worsened until stabilizing in the 2010s. By then, relative annual Arctic losses would be worse than in the current Antarctic ozone hole.

Atmospheric researchers are taking these results seriously. "It's a bold calculation and an important result," says atmospheric physicist Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Maryland. But he cautions that the result is "really soft" because the model is relatively simple. Still, the progressive cooling and accelerating ozone losses seen over the springtime Arctic during the 1990s--with the exception of this year--tends to support the model. Says ozone researcher Ross Salawitch of the Jet Propulsion Laboratory in Pasadena, California: "I think they may be on to something."