Deep Chill Triggers Record-Breaking Ozone Hole

Dick writes about Earth and planetary science for Science magazine.

WASHINGTON, D.C.--This year's thinning of the stratospheric ozone layer over Antarctica is as severe as almost any seen before, and it stretches over an area larger than North America, a new record. Unprecedented stratospheric cold is driving the extreme ozone destruction, researchers said in an announcement made here today, which could be a preview of bad ozone holes to come before the ozone layer recovers toward the middle of the next century.

This year's Antarctic ozone hole is a whopper. Seen from a National Oceanic and Atmospheric Administration (NOAA) satellite, the area of depleted ozone extends over about 26 million square kilometers, the largest observed since annual holes first appeared in the late 1970s. Measured by balloon-borne NOAA instruments ascending from the South Pole, the layer of total ozone destruction extends from an altitude of 15 to 21 kilometers. That's higher than ever seen before, says ozone researcher David Hofmann of NOAA in Boulder, Colorado. And by yesterday the total amount of ozone over the South Pole had dropped from its usual 280 Dobson units to 92 Dobson units, Hofmann says; that nadir is second only to 1993's, when the catalytic effect of the haze from the eruption of Mt. Pinatubo helped drive ozone down to 88 Dobson units.

Scientists are attributing the great depth and breadth of this year's ozone hole to the deep chill that gripped the Antarctic stratosphere this past austral winter. Every winter it gets cold enough there--colder than -78oC--to form icy stratospheric clouds that abet ozone destruction by the chlorine from chlorofluorocarbons (CFCs). But as the hole bottoms out this year, the area cold enough to form polar stratospheric clouds "is larger than anything we've seen to date" for the same time of year, says meteorologist Melvyn Gelman of NOAA's Climate Prediction Center in Camp Springs, Maryland. "There's much less heat being pumped up into the stratosphere than usual," he says.

An underlying cooling trend in the stratosphere induced by, of all things, accumulating greenhouse gases is probably aggravating the situation, researchers say. While greenhouse gases warm the lower atmosphere, they cool the stratosphere by radiating heat to space.

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