The ozone layer high in the stratosphere acts like a giant shield against the sun's ultraviolet (UV) radiation, which can cause skin cancer and cataracts. Since the 1980s, scientists have charted the size of the ozone hole every summer above the Antarctic.
Some years, the holes have been so large that they covered the entire continent and stretched to parts of South America.
During extreme events, up to 70% of the ozone layer can be destroyed, before it recovers months later. The hole above the Arctic was always much smaller – until March this year, when a combination of powerful wind patterns and intense cold temperatures high up in the atmosphere created the right conditions for already-present, ozone-eating chlorine chemicals to damage the layer.
The findings, reported in the journal Nature, show that the hole had 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.
Adapted from an article by the Guardian Newspaper, Monday 3 October.