While not a hole, it is unprecedented

By Janet Raloff

Web edition: April 5, 2011

In mid-March, our online story about the thinning of stratospheric ozone over the Arctic noted that conditions appeared primed for regional ozone losses to post an all-time record. On April 5, World Meteorological Organization Secretary-General Michel Jarraud announced that Arctic ozone had indeed suffered an unprecedented thinning.

Ozone losses this year “still don’t compare to what occurs in the Antarctic,” says Bryan Johnson of the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo. It would be really big news, he says, if the Arctic polar vortex stayed stable long enough to permit a near disappearance — a proverbial hole — in ozone at certain altitudes. 

Because stratospheric ozone protects Earth’s inhabitants from the sun’s harmful ultraviolet rays, regions impacted by the thinned ozone can face exaggerated sunburn risks. At the April 5 WMO press briefing, Markus Rex of the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany, reported that ozone-depleted Arctic air masses are on the move — and recently drifted down over southern Finland.

In coming days, Rex said, the ozone-shy stratospheric air masses could cover parts of Russia and perhaps extend to the Russian-Chinese border. Portions of Central Europe might also be affected, he said, including regions as far south as the Mediterranean.

A low-pressure ring of winds known as a vortex forms over the poles each winter, isolating air masses in these regions from mixing with mid-latitude air. The destruction of ozone, which occurs in these isolated air masses, can worsen until the vortex breaks up.

The spring thinning of Arctic ozone commences in February and has been far less severe than the hole that begins developing over the Antarctic each October, owing to a host of factors that affect ground-level weather in and around the poles. This year, however, conditions aligned to make the Arctic stratosphere especially cold — below -78 °Celsius — a key requirement for heavy ozone losses. In some parts of the polar stratosphere, temperatures plummeted to below -85 °C, Rex told me.

He suspects that global warming played a role in why the Arctic’s high altitudes were so cold in 2011. When greenhouse gases trap heat near Earth’s surface, that energy doesn’t rise to warm the stratosphere. Additional factors can conspire to keep that heat from rising, he adds, such as a paucity of winds and active atmospheric waves that might breach into the stratosphere and destabilize the polar vortex.

There’s no question that the Arctic ozone thinning was “caused by human chemicals,” says Ross Salawitch of the University of Maryland in College Park. The ozone's destruction is driven by chlorofluorocarbons and related pollutants, production of which was banned by the Montreal Protocol. What causes ozone depletion to differ from one year to the next is temperature. And this year, he notes, the polar vortex was very stable and its temperature especially frigid.

“When this happens,” Salawitch explains, “chlorine is converted from a benign form (known as reservoir species), to a very reactive form, which we call radicals.” And 2011 saw very high levels of radicals in the polar vortex, he says, triggering lots of ozone depletion.

When the vortex is cold and stable, polar stratospheric clouds of ice crystals can form. These cloud particles serve as the platform on which those radicals unleash unusual reactions that break apart ozone. This year proved a good year for cloud formation. And even after the vortex breaks apart, Rex says that it could take weeks for the radicals to dissipate, eventually shutting down ozone’s destruction.

Concludes the WMO’s Jarraud: “The 2011 ozone loss shows that we have to remain vigilant and keep a close eye on the situation in the Arctic in the coming years.”