The annual thinning of ozone over the Arctic is shaping up to be especially severe this spring, measurements by European scientists indicate. During the past six weeks, a large portion of the region’s stratosphere has lost at least half of the layer that normally filters out much of the sun’s harmful ultraviolet rays.
The new data “are kind of a warning that we might be getting into an ozone hole situation,” says Francis Schmidlin of NASA’s Wallops Flight Facility in Virginia, whose team is not affiliated with the European polar-ozone monitoring group.
Data radioed back from sensors aboard high-altitude weather balloons show the ozone-depleted region spans about 15 million square kilometers — an area about 22 times the size of Texas. Although the affected parcel of air 20 kilometers above Earth’s surface tends to remain roughly centered over the North Pole, it can wander as far south as Italy or Greece for a few days at a time. So people throughout Europe, Canada and much of the northern United States could briefly face exaggerated exposures to ultraviolet radiation this spring.
Ozone destruction occurs in low-pressure rings of winds, known as polar vortices, that form over the poles each winter and isolate air masses in these regions from midlatitude air. The destruction of ozone in the Arctic vortex could worsen for another month, observes Markus Rex, an atmospheric scientist with the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany, which coordinated the measurements and announced the results March 14.
Though stratospheric ozone thins annually in the Arctic, the loss has been so rapid and severe this year that it appears headed to chalk up a record, Rex says.
The current state of Arctic ozone loss constitutes a severe thinning, Rex emphasizes — not a hole like the one that opens in the Antarctic stratosphere every year. But if the vortex doesn’t break up in late March as it usually does and instead persists into May (as it occasionally has), “we could get conditions very close to an ozone hole in the Arctic,” Rex said.
Ground-level ultraviolet radiation would still be lower under the thin patch than normal levels in the tropics, Rex says, but might be high enough to cause a sunburn within 20 minutes in places where people don’t expect sunburns in March.
Bryan Johnson of the National Oceanic and Atmospheric Administration in Boulder, Colo., says his group’s balloon measurements of stratospheric ozone over Greenland confirm that this year is exceptional. Total atmospheric ozone ranged from 430 to 480 dobson units (a measure of ozone levels in a column of air) in mid-March during each of the last three years. On March 15 this year, the team measured only 306 dobson units.
A cold stratosphere is the key to Arctic ozone depletion. And the Arctic stratosphere was especially cold this winter — in some parts below –85° Celsius, Rex reports. He suspects that global warming played a role in the high-altitude cooling, because when greenhouse gases trap heat near Earth’s surface, that energy doesn’t rise to warm the stratosphere.
When the vortex is cold and stable, polar stratospheric clouds of ice crystals can form. These particles serve as the platform on which pollutant-induced reactions can break apart ozone. This year proved a good year for cloud formation, and even after the vortex starts to break apart, Rex says that it could take a few more weeks to shut down ozone’s destruction.
(Originally posted online March 17)
