The
2000 Antarctic Ozone Hole was largest ever observed.
Courtesy, NASA
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.
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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