Climate forecasts likely incorrect

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 the future of climate, forecast the atmosphere will experience a significant spike in water vapor. NASA's Upper Atmosphere Research Satellite was used to measure water vapor on a global scale in the upper troposphere. Humidity levels in this part of the atmosphere, especially in the tropics, are important for global climate because this is where the water vapor has the strongest impact as a greenhouse gas.

UARS recorded both specific and relative humidity. Specific humidity refers to the actual amount of water vapor in the air. Relative humidity relates to the saturation point, the amount of water vapor in the air divided by the maximum amount of water the air is capable of holding at a given temperature. As air temperatures rise, warm air can hold more water, and the saturation point of the air expands.

In most computer models, relative humidity tends to remain fixed at current levels. Models that include water vapor feedback with constant relative humidity predict the Earth's surface will warm nearly twice as much over the next 100 years as models that contain no feedback.

Using the UARS data to quantify specific and relative humidity, the researchers found that, while water vapor does rise with temperature in the upper troposphere, the feedback effect is not as strong as models have predicted.

"The increases in water vapor with warmer temperatures are not large enough to maintain a constant relative humidity," Minschwaner reveals.

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