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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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