Climate forecasts likely incorrect.

• Position: Global Warming

A NASA-funded study found some climate models might be overestimating the amount of water vapor entering the atmosphere as the Earth warms. Since it is the most important heat-trapping greenhouse gas, climate forecasts may be incorrect concerning future temperature increases.

In response to human emissions of greenhouse gases such as carbon dioxide, the Earth warms, causing more water to evaporate from the ocean. Thus, the amount of water vapor in the atmosphere swells, leading to a further spike in the surface temperature. This effect is known as "positive water vapor feedback." Its existence and size have been argued contentiously for several years.

The work of Ken Minschwaner, a physicist at the New Mexico Institute of Mining and Technology, Socorro; Andrew Dessler, a researcher with the University of Maryland, College Park; and scientists at NASA's Goddard Space Flight Center, Greenbelt, Md., verified water vapor is rising in the atmosphere as the surface warms. However, the researchers found the jump in water vapor was not as high as many climate-forecasting computer models have assumed.

"Our study confirms the existence of a positive water vapor feedback in the atmosphere, but it may be weaker than we expected," Minschwaner maintains.

"One of the responsibilities of science is making good predictions of the future climate, because that's what policymakers use to make their decisions," adds Dessler. "This study is another incremental step toward improving those climate predictions."

The size of the positive water vapor feedback is a key debate within climate science circles. Some scientists have claimed atmospheric water vapor will not magnify in response to global warming, and even may go down. General circulation models, the primary tool scientists use to predict...