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Water vapor will amplify global warming
mongabay.com
November 17, 2008


The heat-amplifying effect of water vapor in the atmosphere could more than double the climate warming caused by increased carbon dioxide levels, report researchers using NASA data.

While is has long been known that water vapor is an important greenhouse gas, its contribution to global warming has been hotly debated. Now, according to NASA, Andrew Dessler and colleagues from Texas A&M University in College Station have calculated to heat-trapping capacity of water vapor.

"Everyone agrees that if you add carbon dioxide to the atmosphere, then warming will result," Dessler said. "So the real question is, how much warming?"

NASA says the impact can be determined by estimating the magnitude of water vapor feedback:

Increasing water vapor leads to warmer temperatures, which causes more water vapor to be absorbed into the air. Warming and water absorption increase in a spiraling cycle.

Water vapor feedback can also amplify the warming effect of other greenhouse gases, such that the warming brought about by increased carbon dioxide allows more water vapor to enter the atmosphere.

"The difference in an atmosphere with a strong water vapor feedback and one with a weak feedback is enormous," Dessler said. "This new data set shows that as surface temperature increases, so does atmospheric humidity. Dumping greenhouse gases into the atmosphere makes the atmosphere more humid. And since water vapor is itself a greenhouse gas, the increase in humidity amplifies the warming from carbon dioxide."

"We now think the water vapor feedback is extraordinarily strong, capable of doubling the warming due to carbon dioxide alone."

The researchers say the results support existing models that show rising CO2 levels will boost global temperatures by the end of the century.

"This study confirms that what was predicted by the models is really happening in the atmosphere," said Eric Fetzer, an atmospheric scientist at NASA's Jet Propulsion Laboratory who works with Atmospheric Infrared Sounder (AIRS) data used in the study. "Water vapor is the big player in the atmosphere as far as climate is concerned."