In December 2010 NIWA announced the Antarctic ozone hole was the smallest it had been in the past five years. Now comes news from the World Meteorological Organisation (WMO) that the depletion of the ozone layer has reached what the WMO describes as an “unprecedented level” between December 2010 to March 2011, thanks to the ongoing presence of ozone-depleting substances in the atmosphere.
Observations from the ground and from balloons over the Arctic region as well as from satellites show that the Arctic region has suffered an ozone column loss of about 40 percent from the beginning of the winter to late March. The highest ozone loss previously recorded was about 30 percent over the entire winter.
“The Arctic stratosphere continues to be vulnerable to ozone destruction caused by ozone-depleting substances linked to human activities,” says WMO Secretary-General Michel Jarraud. “The degree of ozone loss experienced in any particular winter depends on the meteorological conditions. The 2011 ozone loss shows that we have to remain vigilant and keep a close eye on the situation in the Arctic in the coming years.”
Chlorofluorocarbons (CFCs) and other man-made halogen compounds used in refrigeration, aerosol sprays and numerous other applications throughout the world, had previously had a massive negative impact on the ozone layer, until the the Montreal Protocol was signed in 1987. And although the signing of the Protocol fostered an agreement to phase out the use of chemicals—quite successfully—the problem lies in the fact that these compounds have long atmospheric lifetimes, meaning it will take several decades before concentrations of these compounds are back down to pre-1980 levels.
The WMO says the level of destruction of the ozone is unprecedented, but not entirely unexpected. It says ozone scientists have foreseen that significant Arctic ozone loss is possible in the case of a cold and stable Arctic stratospheric winter. Here’s the WMO’s technical explanation.
In Antarctica the so-called ozone hole is an annually recurring winter/spring phenomenon due to the existence of extremely low temperatures in the stratosphere. In the Arctic the meteorological conditions vary much more from one year to the next and the temperatures are always warmer than over Antarctica. Hence, some Arctic winters experience almost no ozone loss, whereas cold stratospheric temperatures in the Arctic lasting beyond the polar night can occasionally lead to substantial ozone loss.
Stratospheric ozone depletion occurs over the polar regions when temperatures drop below -78°C. At such low temperatures clouds form in the stratosphere. Chemical reactions that convert innocuous reservoir gases (e.g. hydrochloric acid) into active ozone depleting gases take place on the clouds particles. The result is rapid destruction of ozone if sunlight is present.
Seasonal variations in size is common and very much influenced from one year to the next and the temperatures are always warmer than over Antarctica.
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