Water vapor may warm Earth less than some models predict

March 19, 2004 — Some climate forecasts may be overestimating future increases in global temperature because they are based on an overestimate of the amount of water vapor that will enter the atmosphere as the Earth warms, suggests a new study by researchers.

Newswise – Some climate forecasts may be overestimating future increases in global temperature because they are based on an overestimate of the amount of water vapor the will enter the atmosphere as the Earth warms, suggests a new study by researchers at the University of Maryland, NASA, and the New Mexico Institute of Mining and Technology.

Greenhouse gases, of which water vapor is the most significant, trap heat and warm the Earth. This warming increases evaporation from the oceans, thus putting more water vapor into the atmosphere. The rise in water vapor in turn leads to a further increase in the surface temperature. The existence and size of this “positive water vapor feedback” have been contentiously argued for several years.

The NASA-supported study by Andrew Dessler, a researcher with the University of Maryland’s Earth System Science Interdisciplinary Center and NASA’s Goddard Space Flight Center, and Ken Minschwaner, a physicist at the New Mexico Institute of Mining and Technology, Socorro, N.M., verified that water vapor is increasing in the atmosphere as the surface warms. However, they found the increases in water vapor were not as high as many climate-forecasting computer models have assumed.

The researchers used data on water vapor in the upper troposphere (10-14 km or 6-9 miles altitude) from NASA’s Upper Atmosphere Research Satellite (UARS). The study is published in the March 15 issue of the American Meteorological Society’s Journal of Climate.

According to Dessler, the size of the positive water vapor feedback is a key debate within climate science circles. Some climate scientists have claimed atmospheric water vapor will not increase in response to global warming, and may even decrease. General circulation models, the primary tool scientists use to forecast the future of our climate, predict a significant increase in the amount of water vapor in Earth’s atmosphere.

“One of the responsibilities of science is making good predictions of the future climate, because that’s what policy makers use to make their decisions,” Dessler said. “This study is another incremental step toward improving those climate predictions.”

NASA’s UARS satellite was used to measure water vapor on a global scale and with unprecedented accuracy 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.

The UARS recorded both specific and relative humidity in the upper troposphere. 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 also increases.

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 water vapor feedback.

Using the UARS data to actually quantify both specific humidity and relative humidity, the researchers found, while water vapor does increase 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 said. These new findings will be useful for testing and improving global climate models.

The University of Maryland is a recognized leader in global climate and land cover change research, led by the Joint Global Change Research Institute, the Earth System Science Interdisciplinary Center, the Global Land Cover Facility, and the departments of meteorology and geography.

Source: University of Maryland, College Park

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