A huge hole that appeared in the Earth's protective ozone layer above the Arctic in 2011 was the largest recorded in the Northern Hemisphere, triggering worries the event could occur again and be even worse, scientists said in a report on Monday.
The ozone layer high in the stratosphere acts like a giant shield against the Sun's ultraviolet (UV) radiation, which can cause skin cancers and cataracts.
Since the 1980s, scientists have recorded an ozone hole every summer above the Antarctic at the bottom of the globe.
Some years, the holes have been so large they covered the entire continent and stretched to parts of South America, leading to worries about a surge in skin cancers.
During extreme events, up to 70 percent of the ozone layer can be destroyed, before it recovers months later.
A matching hole above the Arctic was always much smaller, until March this year, when a combination of powerful wind patterns and intense cold high in the atmosphere created the right conditions for ozone-eating chlorine chemicals to damage the layer.
The findings, reported on Monday in the journal Nature, show the hole opened over northern Russia, parts of Greenland, and Norway, meaning people in these areas were likely to have been exposed to high levels of UV radiation.
The chemical ozone destruction over the Arctic in early 2011 was, for the first time in the observational record, comparable to that in the Antarctic ozone hole, say the scientists, led by Gloria Manney of the Jet Propulsion Laboratory in Pasadena, California.
Scientists say man-made chemicals such as chlorofluorocarbons destroy ozone in the stratosphere. Sunlight breaks up the complex chemicals into simpler forms that react with ozone. While some of the chemicals are covered by a U.N. treaty that aims to stop their use, it will be decades before they are fully phased out of production.
Normally, atmospheric conditions high above the Arctic do not trigger a large-scale plunge in ozone levels. But in the 2010/11 winter, a high-altitude wind pattern called the polar vortex was unusually strong, leading to very cold conditions in the stratosphere that also lasted for several months.
This created the right conditions for the ozone-destroying forms of chlorine to slash ozone levels over a long period.
Chemical ozone destruction in the 2011 Arctic polar vortex attained, for the first time, a level clearly identifiable as an Arctic ozone hole, said the authors.
The researchers pointed to the risk if the Arctic hole becomes an annual event and spreads.
More acute Arctic ozone destruction could exacerbate biological risks from increased ultraviolet radiation exposure, especially if the vortex shifted over densely populated mid-latitudes, as it did in April 2011, they wrote.