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CLIMATE
CHANGE MAY BECOME MAJOR PLAYER IN OZONE LOSS
While
industrial products like chlorofluorocarbons are largely responsible
for current ozone depletion, a NASA study finds that by the
2030s climate change may surpass chlorofluorocarbons (CFCs)
as the main driver of overall ozone loss.
Drew
Shindell, an atmospheric scientist from NASA's Goddard Institute
for Space Studies (GISS) and Columbia University, N.Y., finds
that greenhouse gases like methane and carbon dioxide are
changing the climate in many ways. Some of those effects include
water vapor increases and temperature changes in the upper
atmosphere, which may delay future ozone recovery over heavily
populated areas.
Scientists
have expected the ozone layer to recover as a result of international
agreements to ban CFCs that destroy ozone. CFCs, once used
in cooling systems and aerosols, can last for decades in the
upper atmosphere, where they break down, react with ozone,
and destroy it. They remain the major cause of present-day
ozone depletion.
"It's
hard to tell if those great international agreements [to ban
CFCs] work if we don't understand the other big things that
are going on in the stratosphere, such as increases in greenhouse
gases and water vapor," Shindell said. The stratosphere
is a dry atmospheric layer between 6 and 30 miles (9.7 and
48.3 kilometers) up where most ozone exists.
Ozone
shields the planet's surface from the Sun's harmful ultraviolet
radiation and makes life on Earth possible. The study examined
the ozone layer over heavily populated areas around the equator
and mid-latitudes where ozone thinning occurs, excluding the
Polar regions, where 'ozone holes' form.
Ozone
thinning can occur when increased emissions of methane get
transformed into water in the stratosphere. At high altitudes,
water vapor can be broken down into molecules that destroy
ozone.
Also,
methane and carbon dioxide change our climate by trapping
heat in the atmosphere before it can escape out to space.
This greenhouse effect, much like the inside of a car with
all the windows closed, heats the air within the lowest layer
of the atmosphere, called the troposphere. Warming in the
troposphere can alter atmospheric circulation and make the
air wetter, since warmer air holds more water. Though complex
and not well understood, there is evidence that water vapor
can get wafted from the troposphere into the stratosphere
by shifting air currents caused by climate change.
Climate
change from greenhouse gases can also affect ozone by heating
the lower stratosphere where most of the ozone exists. When
the lower stratosphere heats, chemical reactions speed up,
and ozone gets depleted.
The
chemical and atmospheric processes in the lower stratosphere
are complex, quite variable, and not well understood. Shindell
focused his study largely on the upper stratosphere where
processes are simpler and better understood, and then used
those findings to make inferences about ozone in the lower
stratosphere.
Computer
model simulations were used to separate the different factors
that contribute to ozone changes. According to the models,
which contain some uncertainty, ozone levels are expected
to reach their lowest point in recorded history by around
2006. Scientists hope that by banning CFCs, ozone will eventually
return to healthier levels, like those that existed prior
to 1979.
One
simulation isolated the impacts of CFCs on ozone, and showed
that as CFCs decline, by the year 2040 overall ozone makes
close to a full recovery from current low levels. When CFCs,
water vapor and temperature changes were all combined in a
computer model, by 2040, overall ozone levels recovered only
slightly from their current low point.
These
computer simulations suggest that climate change from greenhouse
gases may greatly slow any anticipated ozone recovery. Shindell
said the effects of climate change need to be better accounted
for as scientists and others try to track the success of international
agreements, like the 1987 Montreal Protocol that banned CFCs.
The
paper appears in the latest issue of the Journal of Geophysical
Research - Atmospheres.
The
study was supported by NASA's Atmospheric Chemistry Modeling
and Analysis Program, and a NASA Earth Observing System postdoctoral
Fellowship. Some of the data used was obtained from the NASA
Langley Research Center's EOSDIS Distributed Active Archive
Center.
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