Tropical myth

Published: Wednesday 31 December 2008

They're cooler than predicted

THOSE studying global warming are often baffled by one observation: in the past three decades, the atmosphere over the tropics has not warmed as much as predicted. Climate change skeptics use this discrepancy in observed and predicted temperatures to argue against human contribution to global warming. A study has now identified the reason for this discrepancy and shown the way to resolve it.

Climate models used since the late 1960s have consistently predicted a rise in temperature both near the surface of the earth and in the atmosphere due to increased greenhouse gas levels. While the temperature near the earth's surface was in sync with what the models predicted, atmospheric data was not. Another prediction that could not be proved in the region was that the rise in temperature in the atmosphere would be higher than at the earth's surface. Data from satellites and weather balloons in the past 30 years showed the tropical atmosphere to be cooler than predicted. It was also cooler than on the surface. Last year, this led David Douglass, a professor of physics at the University of Rochester in the us, and other researchers to suggest in a paper that increase in greenhouse gases was not responsible for global warming.

A paper published in the November 15, 2008, issue of the International Journal of Climatology said Douglass and his collaborators' analysis was flawed and the data they used, old. Global temperature is dependent on natural and man-made factors. Researchers from Lawrence Livermore National Laboratory in the us pointed out that Douglass' paper failed to deduct the effects of natural factors from the temperature data while analyzing temperature rise due to greenhouse gases. The statistical method did not factor in El Nino and La Nina weather phenomena that primarily affect the temperature in the Tropics.

When the Livermore team modified the statistical test, they found the temperature of the tropical atmosphere to be consistent with climate models that track effects of greenhouse gases. The group used new data, which led to a better consistency between observed and predicted temperatures. The authors concluded that results of these studies should be applied towards improving existing climate monitoring systems.

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