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NATURE

Global-warming warnings are more than hot air

Sun Set
Scientists say the surface temperature of Earth rose about 0.7 to 1.4 degrees Fahrenheit in the past century.  
ENN



February 1, 2000
Web posted at: 10:37 AM EST (1537 GMT)

By Environmental News Network staff

Despite contradictory temperature readings taken from the air, surface global warming is a fact of 21st century life, a panel of atmospheric scientists concludes in a National Research Council report.

"The difference between surface and upper-air trends in no way invalidates the conclusion that the Earth's temperature is rising," said John Wallace, chairman of the panel and a professor of atmospheric sciences at the University of Washington.

  MESSAGE BOARD
Global climate
 

The panel did not say whether human activities such as fossil-fuel burning contribute to the warming. "The task of establishing causality is much more difficult than reaffirming the reality of the warming," said Wallace.

At issue is the apparent conflict between surface temperatures and upper-air temperatures, which has fueled a controversy over whether global warming is truly occurring.

Earth's surface temperature has risen about 0.7 to 1.4 degrees Fahrenheit in the past century. Data collected by satellites and balloon-borne instruments since 1979, however, indicates little, if any, warming of the atmospheric layer extending about five miles above Earth's surface.

Mount Pinatubo
An aerial view of Mount Pinatubo after it erupted on June 15, 1991. Disparity between surface temperatures and temperatures of the atmosphere can be attributed to natural causes such volcanic eruptions.  

Climate models generally predict an increase in upper-air as well as surface temperature if greater concentrations of greenhouse gases are causing the warming.

The panel concluded that at least part of the disparity between surface temperatures and temperatures of the atmosphere can be attributed to natural causes such volcanic eruptions and human causes such as depletion of the ozone layer.

For example, natural events such as the eruption of Mount Pinatubo in 1991 tended to decrease atmospheric temperature for several years. The burning of fossil fuels produces tiny aerosol particles that have a cooling effect, and chloroflourocarbons and other chemicals also reduce upper-air temperatures.

"In the presence of the uncertainties in the measurements, we don't know how to assign weights to these two factors (natural and human). It might be all one, all the other or a mix of the two," said Wallace.

Nevertheless, when these variables are accounted for in atmospheric models, satellite and balloon data more closely align with surface-temperature observations, the scientists report.

The difference between surface and upper-air temperatures can also be attributed to uncertainties in temperature measurements. Satellite measurements go back only 20 years, and validation of the data is limited.

To resolve all the uncertainty, the panel recommends an improved temperature monitoring system, including more accurately calibrated satellites, a refurbished weather balloon system and consistent surface observations.

"Our reports represents only a small step toward alleviating the public's confusion about global warming, but at least it's a positive one," said Wallace. "Our most important achievement is reaching a consensus among a diverse group of experts on an issue that has been hotly contested."

Copyright 2000, Environmental News Network, All Rights Reserved



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