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By Stephanie Nebehay
GENEVA (Reuters)
- The destruction of the earth's protective layer over the Northern
Hemisphere has been less than in recent years, but is due to warmer
temperatures rather than a drop in ozone-depleting chemicals,
U.N. experts said on Tuesday.
In a statement,
the World Meteorological Organization said ozone values recorded
at middle and high latitudes during the Northern Hemisphere's
winter and spring appeared to be higher than in recent years.
The depletion
recorded since December was about "five per cent less than
the average pre-1980 levels" used as a norm, it said. The
largest deviations from the pre-ozone period were recorded in
the mid-1990s.
WMO, a United
Nations agency, attributed the lower level of destruction to natural
causes, including warmer temperatures in the stratosphere above
the Arctic and wind patterns.
"The
reduced loss is not related to the expected long-term recovery
of the stratospheric ozone layer in the coming decades,"
WMO said.
The seasonal
phenomenon, now winding up over the Northern Hemisphere, contrasts
with the record depth of the ozone hole over the Antarctic last
October, according to Michael Proffitt, WMO's senior scientific
officer.
"The
Antarctic ozone hole last year was the largest and deepest we've
seen. Now in the Arctic, we're seeing kind of the opposite,"
Proffitt told a news briefing.
"There
is not much depletion....The temperatures were just not cold enough
to produce a lot of ozone loss," he added.
But the American
scientist stressed ozone depletion was a seasonal occurrence which
varied. "Don't expect it to be the same next year,"
he added.
EARTH'S PROTECTIVE
LAYER
The earth's
protective layer shields the planet and humans from harmful ultraviolet
radiation which can cause skin cancer and destroy tiny plants
at the beginning of the food chain.
Chemicals
-- including chlorine compounds used in refrigerants, aerosol
sprays and solvents and bromine compounds used in firefighting
halogens -- are blamed for causing depletion. Extremely low temperatures
in the stratosphere set off the chemical processes.
Proffitt said:
"This is the shallowest we have seen in years. But that doesn't
mean the problem is going away. We know that there is still high
chlorine content in the stratosphere.
"We have
placed a lot of chlorine and bromine compounds in the atmosphere.
It is well-known that once these compounds get into the stratosphere
it is difficult to get them out, they stay there and catalytically
destroy ozone under certain conditions."
Proffitt said
"chlorine-loading" in the atmosphere had virtually ended
under international environmental protection treaties.
But he said
that although the concentration of compounds was beginning to
come down in the lower atmosphere it could take decades before
they disappeared from the stratosphere.
"In the
stratosphere is where the chlorine is released...so it just sits
there and keeps killing ozone. That is why people say it is going
to be decades, perhaps 50 years, maybe more, before the stratosphere
will cleanse itself of this chlorine."
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