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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