The Planet Earth
The 2000 Antarctic Ozone Hole Was Largest Ever
The 2000 Antarctic Ozone Hole was the largest ever observed. Earth's wetter
upper atmosphere may delay global ozone recovery.
NASA researchers have found that an increase in water vapor in the
stratosphere, stemming partially from greenhouse gases, may delay ozone
recovery and increase the rate of climate change.
To check on the long-term stratospheric cooling and ozone depletion, NASA put
data from satellites and other remote sensors into its GISS global climate
model. It was the first study to link greenhouse gases to increased ozone
depletion over populated areas.
Water and ozone. Climate models show cooler stratospheric temperatures happen
when there is more water vapor present. Water vapor also leads to the
breakdown of ozone molecules.
The stratosphere is the dry layer of the atmosphere above the troposphere,
where temperatures increase with height.
According to satellite data, upper atmospheric temperatures around the world -
at altitudes of 20 to 35 miles high -- have cooled between 5.4 and 10.8
degrees Fahrenheit over recent decades.
Driving forces. NASA found two driving forces behind the change in
stratospheric moisture:
Increased emissions of the greenhouse gas methane are transformed into water
in the stratosphere, accounting for about a third of the observed increase
in moisture there.
More water is transported from the lower atmosphere. 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. Greenhouse
gases, such as carbon dioxide and methane, may enhance the transport of
water into the stratosphere.
The increased transport of water vapor to the stratosphere seems likely to
have been induced by human activities.
Ozone destruction. Rising greenhouse gas emissions account for all or part of
the water vapor increase, 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. 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.
Warmer ground. 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.
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