Ozone loss over the Arctic this year was so severe that for
the first time it could be called an "ozone hole" like the Antarctic
one, scientists report. About 20km (13 miles) above the ground, 80% of the
ozone was lost. The cause was an unusually long spell of cold weather at
altitude. In cold conditions, the chlorine chemicals that destroy ozone are at
their most active."Winter in the Arctic stratosphere is highly variable -
some are warm, some are cold."But over the last few decades, the winters
that are cold have been getting colder.Ozone-destroying chemicals originate in
substances such as chlorofluorocarbons (CFCs) that came into use late last
century in appliances including refrigerators and fire extinguishers.Their
destructive effects were first documented in the Antarctic, which now sees
severe ozone depletion in each of its winters.Their use was progressively
restricted and then eliminated by the 1987 Montreal Protocol and its
successors.
The ozone layer blocks ultraviolet-B rays from the Sun,
which can cause skin cancer and other medical conditions.Winter temperatures in
the Arctic stratosphere do not generally fall as low as at the southern end of
the world.No records for low temperature were set this year, but the air
remained at its coldest for an unusually long period of time, and covered an
unusually large area.In addition, the polar vortex was stronger than usual.
Here, winds circulate around the edge of the Arctic region, somewhat isolating
it from the main world weather systems."It was continuously cold from
December through April, and that has never happened before in the Arctic in the
instrumental record."The size and position of the ozone hole changed over
time, as the vortex moved northwards or southwards over different regions.
Some monitoring stations in northern Europe and Russia
recorded enhanced levels of ultraviolet-B penetration, though it is not clear
that this posed any risk to human health. While the Arctic was setting records,
the Antarctic ozone hole is relatively stable from year to year. This year has
seen ozone-depleting conditions extending a little later into the southern
hemisphere spring than usual - again, as a result of unusual weather
conditions. Chlorine compounds persist for decades in the upper atmosphere,
meaning that it will probably be mid-century before the ozone layer is restored
to its pre-industrial health.
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