BOULDER, Colo., Sept. 27 (UPI) -- Climate change is a global phenomenon but it does not affect all regions equally, and trying to parse out the local impacts of warming remains a great challenge for scientists.
A paper by research scientist Shaleen Jain and colleagues at the CIRES Climate Diagnostics Center in Boulder, has found the pattern of precipitation in major river basins across the American West has changed over the 20th century, a event that could have serious implications for the water supply in that relatively dry region.
CIRES is the Cooperative Institute for Research in Environmental Sciences, part of the U.S. National Oceanic and Atmospheric Administration.
Researchers at the Regional Climate Modeling Laboratory at Iowa State University in Ames think global warming might not be as severe in the U.S. Midwest as in other regions. They found summertime daily maximum temperatures will warm less in an area centered on eastern Kansas than anywhere else in the United States.
The Jain paper -- soon to be published in the Journal of Climate -- looked at four western river basins -- the Fraser, Columbia, Sacramento-San Joaquin, and Upper Colorado. The researchers found although average annual runoff does not appear to have changed significantly, the year-to-year figure has taken some wide swings. In particular since 1970, there have been more "extreme events" -- drought or heavy rainfall -- in any given year. In addition, these extremes are occurring more often, and at the same time, in all four basins.
"The key aspect we've tried to highlight in the paper is the kind of changes we are seeing -- first, in terms of increased variability and, second, in terms of synchronicity," Jain told United Press International. The variation has been unusual over the past 30-or-so years, compared with the overall 20th century record, he said.
"That in itself is an important consideration for how we manage our water resources," he added.
In the West, much of the water available to agriculture, industry and cities falls in the winter and spring and must be stored in reservoirs for use in the dry seasons. Changes in the pattern of that availability can affect important endeavors, such as agriculture and skiing, as well as municipal water supplies.
In addition, if water levels drop in all basins at the same time -- a trend Jain's research has documented -- it reduces the region's flexibility to deal with water scarcity.
Jain's paper does not try to pin the rainfall changes definitively on global warming, but there are some indications climate change is involved.
"In terms of trying to attribute it to larger-scale climate in this paper, I have only gone as far trying to show that, indeed, these changes have some consistent analogs in the atmospheric circulation regime," he said.
The paper finds the changes are connected to shifts in atmospheric circulation in the cold season -- October to March -- and they also seem to be related to ocean temperature variations in the north and tropical Pacific, where El Niño weather patterns originate.
"The basic analysis is undeniable," Aiguo Dai, a scientist at the National Center for Atmospheric Research's division of climate and global dynamics in Boulder, told UPI. "But some of the conclusions -- like the pattern may be a potential global-warming effect in the western United States -- may be more speculative. On a national basis, many climate models do not agree on precipitation changes."
Dai added, "I don't think there is very close consensus among many models about precipitation changes on river-basin scales, even on big river basins like the Amazon or the Mississippi. This is more speculative."
Jain's conclusions do, however, echo work done in 1999 by Dennis Lettenmaier, a professor of civil and environmental engineering at the University of Washington in Seattle. Lettenmaier's model suggested that, under a rising global-temperature regime, the snowline tended to move to higher elevations. More precipitation would fall as rain in lower elevations, making it harder to store in reservoirs for the summer, when it is needed.
The Iowa State contingent found an apparently opposite effect in the U.S. central region, however. They found daytime temperatures in that aforementioned area centered on eastern Kansas will warm less than other places in the United States, but would experience more rainfall and wetter soil.
"The modeling showed that warming in the United States will be stronger in winter than summer and stronger at night than during the day," said lead researcher Zaitao Pan, currently on the faculty of St. Louis University. "But we found what looked to us like a 'hole' in the daytime warming in summer, which was a surprise."
When the group discovered this result, Pan said, they looked at the observed temperatures from 1975 to 2000 in the central Great Plains and found "this hole already has started to develop."
"Our model tells us the future climate will have more rainfall and wetter soil, so more of the sun's energy goes into evaporating water than heating the air," said team member Ray Arritt, an agronomy professor. "Rainfall in the northern Great Plains already has increased by about 10 percent over the past few decades, which is consistent with our predictions."
The research is published in the current issue of the journal Geophysical Research Letters.
Most atmospheric scientists now agree the warming of the globe over the next century is virtually assured. One remaining major challenge is to determine the effects of climate change locally, because the positive and negative effects probably will not be distributed equally.
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Climate is series by UPI examining the potential impact of global climate change by Dan Whipple, who covers the environment for UPI Science News. E-mail [email protected]
