WETTER
UPPER ATMOSPHERE MAY DELAY GLOBAL OZONE RECOVERY
NASA
research has shown that increasing water-vapor in the stratosphere,
which results partially from greenhouse gases, may delay ozone
recovery and increase the rate of climate change.
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The
2000 Antarctic Ozone Hole was largest ever observed.
Courtesy, NASA
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Drew
Shindell, an atmospheric scientist from NASA's Goddard Institute
for Space Studies (GISS) and Columbia University, NY, used
the NASA/GISS global climate model with satellite and other
remote sensing data to investigate long-term stratospheric
cooling and ozone depletion. This study is the first to link
greenhouse gases to increased ozone depletion over populated
areas.
Shindell
found that he was able to best simulate the behavior of temperature
and ozone in the upper atmosphere when he added water vapor
data into the climate model.
"Climate
models show cooler stratospheric temperatures happen when
there is more water vapor present, and water vapor also leads
to the breakdown of ozone molecules," Shindell said.
According to satellite data, upper atmospheric temperatures
around the world (20-35 miles high) have cooled between 5.4-10.8
degrees Fahrenheit over recent decades. The stratosphere is
the typically dry layer of the atmosphere above the troposphere,
where temperatures increase with height.
According
to Shindell there are two driving forces behind the change
in stratospheric moisture. "Increased emissions of the
greenhouse gas, methane, are transformed into water in the
stratosphere," Shindell said, "accounting for about
a third of the observed increase in moisture there."
The
second cause of change in the upper atmosphere is a greater
transport of water from the lower atmosphere, which happens
for several reasons. Warmer air holds more water vapor than
colder air, so the amount of water vapor in the lower atmosphere
increases as it is warmed by the greenhouse effect. Climate
models also indicate that greenhouse gases such as carbon
dioxide and methane may enhance the transport of water into
the stratosphere. Though not fully understood, the increased
transport of water vapor to the stratosphere seems likely
to have been induced by human activities.
"Rising
greenhouse gas emissions account for all or part of the water
vapor increase," said Shindell, "which causes stratospheric
ozone destruction."
When
more water vapor works its way into the stratosphere, the
water molecules can be broken down, releasing reactive molecules
that can destroy ozone. Shindell noted that his global climate
model agrees with satellite observations of the world's stratospheric
ozone levels when the water vapor factor is increased in the
stratosphere over time. Shindell said, "If the trend
of increasing stratospheric water vapor continues, it could
increase future global warming and impede ozone stratospheric
recovery."
The
impact on global warming comes about because both water vapor
and ozone are greenhouse gases, which trap heat leaving the
Earth. "When they change, the Earth's energy balance
changes too, altering the surface climate," said Shindell.
Increased water vapor in the stratosphere makes it warmer
on the ground by trapping heat, while the ozone loss makes
it colder on the ground. Water vapor has a much larger effect,
so that overall the changes increase global warming. Shindell
stressed that although ozone depletion cools the Earth's surface,
repairing stratospheric ozone is very important to block harmful
ultraviolet radiation, and other greenhouse gas emissions
need to be reduced.
Shindell
used seven years of data from the Upper Atmosphere Research
Satellite's (UARS) Halogen Occultation Experiment (HALOE)
with ground based data to paint a complete picture of the
upper atmosphere. He also used 14 years of lower stratospheric
measurements that show large increases in water vapor. Though
some studies conflict with lower stratospheric observations
of water vapor trends, studies released since Shindell's paper
was written, agree with the increases he used, and indicate
that they have been taking place for more than four decades
already.
Shindell's
paper, "Climate and Ozone Response to Increased Stratospheric
Water Vapor," appears in the April 15th issue of Geophysical
Research Letters.
NASA's
HALOE was launched on the UARS spacecraft September 12, 1991
as part of the Earth Science Enterprise Program. Its mission
includes improvement of understanding stratospheric ozone
depletion by analyzing vertical profiles of ozone, hydrogen
chloride, hydrogen fluoride, methane, water vapor, nitric
oxide, nitrogen dioxide, and aerosols.
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