The stratospheric ozone layer, which shields the Earth from the Sun's harmful ultraviolet rays, has been damaged to its greatest-ever extent over the Arctic this winter.
The protective layer of gas, which can be destroyed by reactions with industrial chemicals, has suffered a loss of about 40 per cent from the start of winter until late March, exceeding the previous seasonal loss of about 30 per cent, according to the World Meteorological Organisation (WMO).
The phenomenon is annual in the Antarctic, where after its discovery in the 1980s it came to be known as the "ozone hole". Although CFC levels are now dropping, they remain in the atmosphere for so long that they will still be causing ozone depletion for decades in certain conditions, particularly the intense cold of the stratosphere.
Arctic ozone conditions vary more and the temperatures are always warmer than over Antarctica, where the ozone hole forms high in the stratosphere near the South Pole each winter and spring. Because of changing weather and temperatures, some Arctic winters experience almost no ozone loss – but others with exceptionally cold stratospheric conditions can occasionally lead to substantial ozone depletion.
This is what has happened over the Arctic this winter; for while at ground level the Arctic region was unusually warm, temperatures 15-20km above the Earth's surface plummeted. WMO officials say the latest losses, which are unprecedented, were detected in observations from the ground and from balloons and satellites over the Arctic.
"The Arctic stratosphere continues to be vulnerable to ozone destruction caused by ozone-depleting substances linked to human activities," said the WMO's secretary general, Michel Jarraud.
Loss of ozone allows more of the Sun's harmful ultraviolet-B rays to penetrate the atmosphere. They have been linked to increased rates of skin cancer, cataracts and immune system damage.
In late March, winds blew the ozone-depleted region over Greenland and Scandinavia, and the WMO is warning people there to heed national alerts and forecasts of ozone levels. The development is unlikely to be risky for humans as the depleted ozone will soon merge into the background atmosphere, according to one of the world's leading ozone experts, Professor John Pyle, professor of chemistry at Cambridge University and co-director of the National Centre for Atmospheric Science.
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