New climate simulations from NASA show that under the warmer global temperatures of the 20th century, water vapor in the atmosphere took longer than normal to fall out of the sky fall as rain, snow and other precipitation. With a few exceptions, the amount decreased over land but increased over oceans.

The simulations are the first to take into consideration a part of Earths water cycle that until now has been overlookedthe storage of water vapor in the atmosphere. These findings could play an important role in climate models used to provide short-term weather forecasts critical to water resource managers as well as models used to predict long-term climate trends.

"Going back and looking at a time where we know what happened helps give us confidence that the models can do a good job of predicting the future," says meteorologist Michael Bosilovich, lead author of the study, published online in the American Meteorological Society's Journal of Climate.

When modeling the exchange of moisture that occurs among the land, poles, oceans and atmosphere, scientists typically use sea surface temperature data to predict how much water vapor will evaporate from the earth and how much of it will turn into precipitation. And although researchers have cautioned that warmer global temperatures could increase the atmosphere's ability to hold moisture, no one has modeled the extent to which this could occur.

Bosilovich and his team, from NASA's Goddard Space Flight Center in Greenbelt, Md., decided that rather than create climate models that predicted the future, they would create models of the past using historic information. Simulations were necessary because satellite observations are nonexistent for most of South America, Africa and huge gaps in Eurasia. Using available data, the team produced two climate simulations. One was based on two 20-year sets of sea surface temperature data gathered by satellites from 1902 to 1921 and 1979 to 1998. The other model was created from sea surface temperature measured from 1948 to 1998.

Both simulations showed that along with a .25 to .5 percent increase in global temperatures during these periods came a rise in evaporation and precipitation. The higher temperatures also improved the atmosphere's ability to retain water vapor and although precipitation was significant, it was not as great as the potential amount held in the clouds. "Water passing through the atmosphere was taking longer to evaporate, and then find its way back out as precipitation," Bosilovich says.

The study also found that, in general, the water cycle slowed over tropical land masses and sped up over oceans. But the trend was not equal across all regions. For example, precipitation increased over land in the central U.S., while decreasing over the Gulf of Mexico. According to the team, although global averages show significant trends, further studies that take a regional focus must be conducted in order to refine the models.