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For
nearly a billion years, ozone molecules in the atmosphere have protected life on
Earth from the effects of ultraviolet rays.
Center
for Global Environmental Research, National Institute for Environmental Studies
Japan
In the past 60 years or so human
activity has contributed to the deterioration of the ozone layer.
NASA
Graphic
Only
10 or less of every million molecules of air are ozone. The majority of these
ozone molecules resides in a layer between 10 and 40 kilometers (6 and 25 miles)
above the Earth's surface in the stratosphere.
Each
spring in the stratosphere over Antarctica (Spring in the southern hemisphere is
from September through November.), atmospheric ozone is rapidly destroyed by
chemical processes.
As
winter arrives, a vortex of winds develops around the pole and isolates the
polar stratosphere. When temperatures drop below -78C (-109F), thin clouds
form of ice, nitric acid, and sulphuric acid mixtures. Chemical reactions on the
surfaces of ice crystals in the clouds release active forms of CFCs. Ozone
depletion begins, and the ozone hole appears.
Over the course of two to three months, approximately 50% of
the total column amount of ozone in the atmosphere disappears. At some levels,
the losses approach 90%. This has come to be called the Antarctic ozone hole.
In
spring, temperatures begin to rise, the ice evaporates, and the ozone layer
starts to recover.
The Antarctic ozone
hole was discovered in 1985 by British scientists Joesph Farman, Brian Gardiner,
and Jonathan Shanklin of the British Antarctic Survey.
The
ozone "hole" is really a reduction in concentrations of ozone high
above the earth in the stratosphere. The ozone hole is defined geographically as
the area wherein the total ozone amount is less than 220 Dobson Units. The ozone
hole has steadily grown in size (up to 27 million sq. km.) and length of
existence (from August through early December) over the past two decades.
After
a series of rigorous meetings and negotiations, the Montreal Protocol on
Substances that Deplete the Ozone Layer was finally agreed upon on 16 september
1987 at the Headquarters of the International Civil Aviation Organization in
Montreal.
NASA/NOAA
satellite data showing the rise in stratospheric chlorine and corresponding
decline in ozone layer thickness from 1979 to 1997. As stratospheric chlorine
declined in response to enactment of the Montreal Protocol, the first stage of
ozone recovery began.
The
Montreal Protocol stipulates that the production and consumption of compounds
that deplete ozone in the stratosphere--chlorofluorocarbons (CFCs), halons,
carbon tetrachloride, and methyl chloroform--are to be phased out by 2000 (2005
for methyl chloroform). Scientific theory and evidence suggest that, once
emitted to the atmosphere, these compounds could significantly deplete the
stratospheric ozone layer that shields the planet from damaging UV-B radiation.
Man-made
chlorines, primarily chloroflourobcarbons (CFCs), contribute to the thinning of
the ozone layer and allow larger quantities of harmful ultraviolet rays to reach
the earth.
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