Unintended consequences

Healing Antarctica’s ozone hole has a possible downside

Via a complicated cascade of effects, a full recovery of the ozone hole over Antarctica in the coming years could significantly boost warming of the atmosphere over and around the icy continent.

After years of decline, the springtime concentrations of ozone in the atmosphere high over Antarctica have begun to increase — a sign that the ozone hole is recovering (SN: 12/24&31/05, p. 418).

Stratospheric ozone blocks much of the sun’s ultraviolet radiation that would otherwise reach Earth’s surface and boost rates of skin cancer.

In one sense, however, the ozone hole is somewhat beneficial: It has kept Antarctica cooler than it otherwise would have been, says Seok-Woo Son, an atmospheric scientist at ColumbiaUniversity.

Because the lower stratosphere over Antarctica lacks ozone in the springtime, it doesn’t absorb much ultraviolet radiation and therefore is much cooler than normal. This, in turn, increases the temperature difference between air over the mid-latitude regions of the Southern Hemisphere and the air over Antarctica, Son notes.

That temperature gradient is the driving force for strong, steady winds that blow from the west over the seas off Antarctica’s coast at latitudes of about 50°S — the circumpolar westerlies.

The ozone hole has strengthened this wall of winds in recent decades, preventing many storm systems that head south from temperate latitudes — as well as the large quantities of warm air they contain — from reaching central Antarctica.

Now, Son and his colleagues have investigated how ozone recovery might affect Antarctic climate. Seven of the simulations they used account for changes in atmospheric chemistry, and five of those suggest that the increases in ozone concentrations would cause significant warming in the lower stratosphere.

The climate changes resulting from full ozone recovery, expected sometime later this century, could be substantial, the researchers speculate in the June 13 Science.

Warming of the lower stratosphere would tend to slow the circumpolar westerlies but strengthen winds at lower latitudes, a combination that would significantly shift weather patterns. Much of Australia would become drier, and portions of South America would, on average, receive more precipitation, the models suggest.

Results of a climate model run by Judith Perlwitz, an atmospheric scientist at the University of Colorado, Boulder, and her colleagues confirm these notions. In that simulation, atmospheric temperatures at altitudes between 10 and 20 kilometers are as much as 9 degrees Celsius warmer after the ozone hole has recovered than they are today, the team reported in the April 28 Geophysical Research Letters.

The model that Perlwitz’s team used suggests that ozone-related heating of the stratosphere triggers the same cascade of effects that Son’s team now reports: the circumpolar westerlies weaken, enabling more storm systems to breach that barrier and bring warm winds to Antarctica.

“If the successful control of ozone-depleting substances allows for a full recovery of the ozone hole over Antarctica, we may finally see the interior of Antarctica begin to warm with the rest of the world,” Perlwitz says.

David Karoly, a climate scientist at the University of Melbourne in Australia, notes that the amount of carbon dioxide being absorbed by oceans around Antarctica has subsided in recent decades (SN: 5/26/07, p. 333). Changes in the strength and pattern of winds over the region could boost carbon uptake in the future, he speculates.

Karoly says that it’s not clear which source of warming — the gradual recovery of ozone, or the ever-increasing concentrations of carbon dioxide and other greenhouse gases — will more strongly influence Antarctic climate in the near future. Regardless, he notes, the coming changes in weather patterns “are of major importance for a number of countries” in the Southern Hemisphere.

“Climate change and ozone depletion are connected, but not in simple ways,” Karoly says.

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