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 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

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.