An ozone hole five times the size of California opened over the Arctic this spring, matching ozone loss over Antarctica for the first time on record, scientists said on Sunday.
Formed by a deep chill over the North Pole, the unprecedented hole at one point shifted over eastern Europe, Russia and Mongolia, exposing populations to higher, but unsustained, levels of ultra-violet light.
Ozone, a molecule of oxygen, forms in the stratosphere, filtering out ultraviolet rays that damage vegetation and can cause skin cancer and cataracts.
The shield comes under seasonal attack in both polar regions in the local winter-spring.
Part of the source comes from man-made chlorine-based compounds, once widely used in refrigerants and consumer aerosols, that are being phased out under the UN's Montreal Protocol.
But the loss itself is driven by deep cold, which causes water vapour and molecules of nitric acid to condense into clouds in the lower stratosphere.
These clouds in turn become a "bed" where atmospheric chlorine molecules convert into reactive compounds that gobble up ozone.
Ozone loss over the Antarctic is traditionally much bigger than over the Arctic because of the far colder temperatures there.
In the Arctic, records have -- until now -- suggested that the loss, while variable, is far more limited.
Satellite measurements conducted in the 2010-2011 Arctic winter-spring found ozone badly depleted at a height of between 15 and 23 kilometres (9.3 and 14.3 miles).
The biggest loss -- of more than 80 percent -- occurred between 18 and 20 kms (11.25 and 12.5 miles).
"For the first time, sufficient loss occurred to be reasonably be described as an Arctic ozone hole," says the study, appearing in the British science journal Nature.
The trigger was the polar vortex, a large-scale cyclone that forms every winter in the Arctic stratosphere but which last winter was born in extremely cold conditions, Gloria Manney, of the Jet Propulsion Laboratory in California, told AFP in email.
"The ozone destruction began in January, then accelerated in late February and March, so that ozone values in the polar vortex region were much lower than usual from early March through late April, after which the polar vortex dissipated.
"Especially low total column ozone values (below 250 Dobson Units) were observed for about 27 days in March and early April.
"The maximum area with values below 250 Dobson Units was about two million square kilometres (772,000 square miles), roughly five times the area of Germany or California."
This was similar in size to ozone loss in Antarctica in the mid-1980s.
In April, the vortex shifted over more densely populated parts of Russia, Mongolia and eastern Europe for about two weeks.
Measurements on the ground showed "unusually high values" of ultra-violet, although human exposure was not constant as the vortex shifted location daily before eventually fading, said Manney.
The study, published by the journal Nature, challenges conventional thinking about the Arctic's susceptibility to ozone holes. This thinking is based on only a few decades of satellite observations.
Stratospheric temperatures in the Arctic have been extraordinarily varied in the past decade, the paper notes. Four out of the last 10 years have been amongst the warmest in the past 32 years, and two are the coldest.
In the stratosphere, ozone is protective. At ground level, where it is produced in a reaction between traffic exhaust and sunlight, it is a dangerous irritant for the airways.