By Deborah Zabarenko
WASHINGTON
(Reuters) - Ozone-eating clouds that erode Earth's protection
against ultraviolet radiation are born in thin rings of supercold
air over the North and South Poles, scientists reported on Thursday.
The Sun's
ultraviolet rays could cause skin cancer in humans and biological
damage to other living things if Earth were not shielded by the
ozone layer high in the atmosphere. But polar stratospheric clouds
made of nitric acid and water deplete this protective layer.
Scientists
have known about the clouds for years, but U.S. researchers have
just discovered the bands of frigid air in the stratosphere that
help to create them, according to an article in the current edition
of the journal Science.
And as the
Earth's surface gets warmer, due to heat trapped by so-called
greenhouse gases, the stratosphere gets colder, making it an even
better place to create the ozone-depleting clouds, NASA (news
- web sites) researcher Azadeh Tabazadeh said.
The more these
high polar clouds proliferate, the slower Earth's recovery from
ozone depletion, Tabazadeh said in a telephone interview from
the National Aeronautics and Space Administration's Ames Research
Center in California.
The polar
stratospheric clouds do their work by sucking nitrogen out of
the cold air. Because they are made up of large particles, each
the size of a bit of road dust, the clouds are heavy and pull
out the nitrogen as they fall toward Earth, Tabazadeh said.
Nitrogen is
important because it reacts with the chlorine in human-made chlorofluorocarbons
(CFCs). Now banned under international agreements, CFCs have long
been identified as a prime cause of ozone depletion.
The polar
stratospheric clouds pack a double punch, Tabazadeh said: they
take away nitrogen, which can mitigate the effects of ozone depletion,
and they also activate chlorine, which spurs ozone depletion.
Still, if
Earth's climate stayed constant, the ozone layer should start
recovering because CFCs are being limited. But Earth's surface
climate is warming, which means the stratosphere is cooling.
``The surface
warming causes a cooling in the stratosphere and the cooling promotes
more ozone depletion,'' Tabazadeh said. ''Global warming (news
- web sites) is actually affecting the ozone depletion.''
``I think
the best thing to do is try to control the global warming issue,''
she said. ``And that could be controlled by less emissions of
greenhouse gases and also less emissions of soot. It's very hard
to regulate.''
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