Most-serious greenhouse gas is increasing, international study finds

April 23, 2001

Scientists know that atmospheric concentrations of greenhouse gases such as carbon dioxide have risen sharply in recent years, but a study released today in Paris reports a surprising and dramatic increase in the most important greenhouse gas - water vapor - during the last half-century.

The buildup of other greenhouse gases (those usually linked with climate change) is directly attributable to human activity, and the study indicates the water vapor increase also can be traced in part to human influences, such as the buildup of atmospheric methane. However, other causes not directly related to humans must also be at work, said Philip Mote, a University of Washington research scientist who is one of seven lead authors on the report.

"Half the increase in the stratosphere can be traced to human-induced increases in methane, which turns into water vapor at high altitudes, but the other half is a mystery," said Mote. "Part of the increase must have occurred as a result of changes in the tropical tropopause, a region about 10 miles above the equator, that acts as a valve that allows air into the stratosphere."

Readings of water vapor increases 3 to 10 miles up are more ambiguous, Mote said.

The international study, produced by 68 scientists in seven countries as part of the World Climate Research Programme, examined only the changes at higher altitudes, 3 to 30 miles above sea level.

Although carbon dioxide has been relatively easy to monitor and increases have been observed since the 1950s, water vapor has proven much more difficult to monitor. The new effort for the first time was able to draw conclusions about the behavior of water vapor based on a large number of measurements during a long period of time. The report covered both the upper troposphere (3 to 10 miles high), where trends are harder to detect, and the stratosphere (10 to 30 miles high).

"A wetter and colder stratosphere means more polar stratospheric clouds, which contribute to the seasonal appearance of the ozone hole," said James Holton, UW atmospheric sciences chairman and expert on stratospheric water vapor. "These trends, if they continue, would extend the period when we have to be concerned about rapid ozone depletion."

Atmospheric heating happens when the Earth's atmosphere and surface absorb solar radiation, while cooling occurs when thermal infrared radiation escapes the atmosphere and goes into space. If certain key gases that absorb and emit infrared radiation, the most important being water vapor and carbon dioxide, were not present in the atmosphere, Earth's temperature would cool to minus 19 degrees celsius, or minus 2 degrees Fahrenheit. The global annual mean temperature is 14 degrees celsius.

Key findings of the water vapor assessment are:
* Ground-based, balloon, aircraft and satellite measurements show a global stratospheric water vapor increase of as much as 2 parts per million by volume in the last 45 years, a 75 percent jump.
* Modelling studies by the University of Reading in England show that since 1980 the stratospheric water vapor increase has produced a surface temperature rise about half of that attributable to increased carbon dioxide alone.
* Methane, which has been increasing in the atmosphere since the 1950s, could be contributing to the water vapor increase. Chemical conversion of methane to water vapor occurs in the stratosphere but can only account for at most half of the water vapor increase.

A satellite record of relative humidity data for the upper troposphere shows a 2 percent increase during the last 20 years in the equatorial region. However, the uncertainty in this determination is too large to allow a clear conclusion as to whether this is part of a long-term trend.

Among other things, the report recommends continuing to launch balloons monthly from Boulder, Colo., as a means to measure water vapor, a low-cost effort that nevertheless faces possible discontinuation. The balloon measurements, dating from 1981, are the only continuous record of water vapor.

Holton said the report is significant because, by careful comparison, it largely has resolved contradictions in measurements among a number of instruments.
-end-
For more information, contact Mote at (206) 616-5346 or philip@atmos.washington.edu, or Holton at (206) 543-4010 or holton@atmos.washington.edu

University of Washington

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