Toronto: Scientists have stumbled on an unprecedented depletion of the earth's protective ozone layer above the Arctic last winter and spring. The depletion was caused by an unusually prolonged period of extremely low temperatures in the stratosphere, says a NASA study.

A NASA team, which included University of Toronto physicist Kaley Walker, found the amount of ozone destroyed in the Arctic in 2011 was comparable to that seen in some years in the Antarctic, where an ozone 'hole' has formed each spring since the mid 1980s.

The stratospheric ozone layer, extending from about 15-35 km above the surface, protects life on earth from the sun's harmful ultraviolet radiation, the journal Nature reports.

The scientists found that at some altitudes, the cold period in the Arctic lasted more than 30 days longer in 2011 than in any previously studied Arctic winter, leading to the unprecedented ozone loss, according to a NASA statement.

The Antarctic ozone hole forms when extremely cold conditions, common in the winter Antarctic stratosphere, trigger reactions that convert atmospheric chlorine from human-produced chemicals into forms that destroy ozone.

Walker and 18 other scientists from the US, Germany, Holland, Russia, Finland, Denmark, Japan and Spain investigated the 2011 Arctic ozone loss.

“Arctic ozone loss events such as those observed this year could become more frequent if winter Arctic stratospheric temperatures decrease in future as the earth's climate changes,” Walker said.