San Francisco Chronicle
January 7, 2003
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
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
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
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
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
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
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.