Perilous Times
2 October 2011 Last updated at 13:05 ET
Arctic ozone loss at record level
Richard Black By Richard Black Environment correspondent, BBC News
Arctic ozone hole The Arctic ozone hole lay over over populated
regions for parts of winter and spring
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,
they say.
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.
It is currently impossible to predict if such losses will occur
again, the team writes in the journal Nature.
Early data on the scale of Arctic ozone destruction were released
in April, but the Nature paper is the first that has fully
analysed the data.
"Winter in the Arctic stratosphere is highly variable - some are
warm, some are cold," said Michelle Santee from Nasa's Jet
Propulsion Laboratory (JPL).
"But over the last few decades, the winters that are cold have
been getting colder.
“ Why [all this] occurred will take years of detailed study” -
Michelle Santee JPL
"So given that trend and the high variability, we'd anticipate
that we'll have other cold ones, and if that happens while
chlorine levels are high, we'd anticipate that we'd have severe
ozone loss."
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.
Longer, not colder
Winter temperatures in the Arctic stratosphere do not generally
fall as low as at the southern end of the world.
Polar stratospheric clouds Ozone destruction takes place within
polar stratospheric clouds, with chlorine the main culprit
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.
"Why [all this] occurred will take years of detailed study," said
Dr Santee.
"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.