http://www.commondreams.org/headlines03/0107-06.htm
Published on Tuesday,
January 7, 2003 by the San Francisco Chronicle
The Sky is Slowly Rising, Scientists Say
Upward movement of atmospheric layer points to
global warming
by Keay Davidson
Contrary to Chicken Little's warning, the sky isn't falling
-- it's rising.
An important part of
it, anyway -- the "tropopause," the roof
of Earth's lower atmosphere. Its rise -- by an average of about
650 feet globally over the last 22 years -- is new evidence for
the reality of global warming, scientists say.
New computer models
show that humans are largely to blame for
the rising of the tropopause, says physicist-atmospheric scientist
Ben Santer of Lawrence Livermore National Laboratory. He and
11 colleagues around the world announced the finding in the
latest issue of the Journal of Geophysical Research.
The atmosphere is layered,
sort of like a building with multiple
floors. The lowest major layer is the troposphere, within which
humans dwell. The moist, turbulent troposphere is home to the
most exciting weather, from rainstorms and blizzards to hurricanes
and tornadoes. The troposphere varies from 5 to 10 miles deep
and is deepest at the equator.
Just above the troposphere
is the stratosphere, the comparatively
calm region through which commercial jets fly. The barrier between
the troposphere and the stratosphere is the tropopause.
In recent years, observers
using weather balloons and other
instruments began to suspect the tropopause was slowly rising.
No one, though, knew whether to blame the alleged rise on
Mother Nature or on humans. Some speculated about purely
natural causes, including that volcanic eruptions might
have triggered intermittent tropospheric warming.
Now the Santer team
has largely acquitted Mother Nature of
blame. The main villain, they say, is humanity.
Greenhouse gases, including
carbon dioxide from the burning of
fossil fuels, trap infrared radiation, warming the atmosphere.
Santer and his associates believe that as the warming accelerates,
the troposphere expands, just as a balloon warms and expands
when it drifts from a cool room into a warmer one. Tropospheric
expansion nudges the tropopause upward.
Another reason for
the tropopause rise, Santer said in an
interview, is the disintegration of stratospheric ozone gas
by commercially generated pollutants called chlorofluorocarbons
(CFCs). It's too early to say which of the two deserves the
larger blame, he adds.
Ozone gas is a natural
constituent of the stratosphere.
Not only does it shield Earth from cancer-causing solar
radiation, it also absorbs much incoming sunlight, warming
the stratosphere. Since the 1980s, scientists have generally
agreed that CFCs have chemically attacked and destroyed part
of the ozone layer.
Result: the stratosphere
cools and contracts, thus pulling
its lowest part, the tropopause, even further upward.
"Our work illustrates
that changes in tropopause height
may be a useful 'fingerprint' of human effects on climate
and are deserving of further attention," Santer and his
colleagues conclude in their article.
The tropopause has
risen the most around the poles, by more
than 1,000 feet in some places, Santer said.
It's too early to say
how a higher tropopause will affect
terrestrial weather, scientists acknowledge. The tropopause
typically limits the height of severe storms. The famous
wispy "anvil" atop thunderclouds marks where the cloud
usually
stops rising as it bumps up against the tropopause and the
high-speed winds of the stratosphere.
Pending further research,
it's anyone's guess whether a higher
tropopause would lead to taller thunderclouds, with possible
consequences such as more violent downdrafts as rain-cooled
air plunges from greater heights.
©2003 San Francisco
Chronicle
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