Satellite measurements suggested that during the winter–spring of 2010–11, ozone was severely depleted at a height of 18–20 kilometres above the Earth’s surface. The maximum area in which ozone levels were classed as being low (below 250 Dobson Units) was about two million square kilometres – roughly five times the size of Germany and similar in size to the hole over the Antarctic during the mid-1980s. This was the first time that a hole of this size has been observed.

The hole was triggered by the polar vortex, a cyclonic weather system that forms in the Arctic stratosphere each winter. Last winter, the vortex formed during a period of particularly cold weather. At some altitudes, the scientists found, the cold period in the Arctic lasted more than 30 days longer than in any previously studied Arctic winter. This, in turn, led to the ozone loss described in the report.

‘In the 2010–11 Arctic winter, we did not have temperatures that were lower than in the previous cold Arctic winters,’ said University of Toronto physicist Kaley Walker, a member of the international team that identified the hole. ‘What was different about this year was that [they] were low enough to generate ozone-depleting forms of chlorine for a much longer period of time. Arctic ozone loss events such as those observed this year could become more frequent if winter Arctic stratospheric temperatures decrease in future as Earth’s climate changes.’

November 2011