By Liz Kalaugher, at the EGU General Assembly in Vienna
Spring 2011 has seen the largest-ever degree of ozone loss over the northern hemisphere, journalists at the EGU General Assembly in Vienna heard this morning.
This year about 40% of the ozone column above the Arctic has disappeared, breaking the previous record of 30%. The cause? An unusual persistence of cold temperatures in the stratosphere into March, allowing longer lifetimes for the polar stratospheric clouds that enable conversion of pollutant gases into ozone-destroying chlorine.
Dubbed “mother-of-pearl” clouds because of their attractive appearance, polar stratospheric clouds form at temperatures below -78 ° C. The chlorine they help create, meanwhile, can only destroy ozone in the presence of sunlight, which reappears in the polar spring.
The ozone layer acts, according to Geir Braather of the World Meteorological Organization “like a suncream with factor 70” it cuts by 70% the amount of short-wave ultraviolet rays reaching the Earth’s surface. So any disruption of this protection could have implications for humans.
As weather systems cause the polar vortex to shift, ozone-depleted air masses can move above Europe, Russia and North America. Indeed in 2005, when the second-largest ozone decrease took place, the ultraviolet index in March in one European country was five, bringing a sunburn time of 20-30 minutes for the fair-skinned. While this is not above summertime levels, it is unusual for spring and the researchers feel that people should be informed.
To date, air affected by the record-breaking ozone loss has hovered over Canada, eastern Russia and Scandinavia but has not extended down to the heavily-populated regions of Germany and central Europe, although this situation could change. The polar vortex is currently over central Russia and is forecast to be stable until April 9th.
At the south pole, where stratospheric temperatures are typically colder, springtime ozone loss of around 50% occurs each year. Fortunately, while the resulting ozone-depleted air sometimes reaches the southern tip of Chile, it generally does not extend above heavily-populated areas.
The more variable temperatures in the Arctic mean that some winters see ozone loss of just 5 or 10% whereas a “normal” winter could see 30% loss. Although this year’s ozone loss has been unprecedented, it was not unexpected – scientists had predicted that such cold conditions in the stratosphere would lead to increased ozone loss.
While the Arctic was warmer than average at ground level this winter, temperatures in the stratosphere were colder. And when it comes to the stratosphere, the cold winters have been getting colder. “We don’t know what’s driving this long-term change,” said Markus Rex of the AWI, Germany, who will be publishing his analysis of ozonesonde data in a Nature paper. Greenhouse gases could be a factor, but that’s by no means certain.
Speaking on behalf of the World Meteorological Organization, Braather was keen to stress that this year’s Arctic ozone loss record was not because the Montreal Protocol isn’t working. Set up in 1987 this agreement has seen levels of ozone-depleting gases such as chlorofluorcarbons and halons above the Arctic fall by 10% of the amount that would bring them back to the 1980 benchmark level. Outside the poles the ozone layer is projected to recover by around 2030-2040. In the Antarctic recovery is expected by 2045-2060 and the picture is one or two decades rosier for the Arctic.