Study Forecasts Ill Effects of Possible Warming Trend : Climate: Southwest could face crop damage, droughts and flooding if ‘greenhouse effect’ predictions are accurate. But some scientists are skeptical about the assumptions researchers used in the report.

A new federal government study on the possible effects of global warming in the Southwest warns of dramatic reductions in hydroelectric power generation, severe water shortages for Arizona cities, widespread crop damage in Southern California and northern Mexico, and the degradation of fish and wildlife habitat from Colorado to the Mexican border.

Released today, the study, sponsored by the U.S. Environmental Protection Agency and conducted by Oakland’s Pacific Institute, focuses on the effects of temperature increases of 2 to 4 degrees Celsius in the Colorado River Basin, an area covering parts of seven states and northern Mexico.

Global warming, itself a controversial theory, occurs as man-made “greenhouse gases” such as carbon dioxide intensify in the atmosphere, primarily from the burning of fossil fuels, and trap more of the sun’s heat. The EPA study of the Colorado River Basin assumes a doubling of such gases over the next half-century.

The study’s anticipated temperature rise is well within the range of increases predicted by most global warming studies. However, many scientists are skeptical about the accuracy of those forecasts. Scientists also disagree about the effects of global warming, some believing that it could produce beneficial effects such as balmier nights and milder winters that make for longer growing seasons.

The authors of the study acknowledge some of the uncertainties surrounding global warming research and caution that an extended drought is just one possible result of a rise in temperature. They point out that a large increase in rainfall also is a possible consequence and could offset shortages in runoff caused by a reduced snowpack.

The greatest threat posed by global warming, the study warns, is the prospect of extreme fluctuations in weather patterns, “increasing the frequency of both sustained drought events and high-flow events (floods)” and underscoring the need for much more flexible management of the basin’s water supply.

“One of the greatest fears is that climate change will make floods and droughts both more common. We don’t know for sure that is what is going to happen. But there are indications from the modeling that it will,” said Peter Gleick, a co-author of the study and director of the global environment program at the nonprofit Pacific Institute for Studies in Development, Environment and Security.

Whatever happens as a result of rising temperatures, the new study states, the system of apportioning water in the basin--with its fixed allocations and priorities--would not respond well in a crisis, especially if runoff is depleted.

Gleick recommends management changes that make it easier to divert surplus water from one part of the basin to another in case an ecosystem is imperiled by drought or to head off a municipal water shortage. “Under the present management system,” the study says, “even modest decreases in runoff would lead to drastic declines in the levels of major reservoirs, big drops in the generation of hydroelectricity and cutbacks in water deliveries to some users.”

An increase in temperature of 2 to 4 degrees Celsius could cause annual runoff in the Colorado River Basin to decline up to 21%, according to the study. Such a reduction would leave Lake Mead in Southern Nevada “essentially empty 25% of the time” and without minimum water levels needed to generate hydroelectric power at Hoover Dam. A 20% decline in runoff also would have a crippling effect on power generation at the Glen Canyon Dam in northern Arizona.

Agriculture could face the biggest threat if river flow is severely curtailed. The report contends that a 20% reduction in river flow would cause an increase in salinity in the river of 15% to 20%, a potentially devastating problem for farmers in California’s Imperial Valley and in Mexico’s Mexicali Valley.

But there would be an altogether different scenario in the region if a long-term warming trend triggered unusually high levels of rainfall.

“Should the region experience only a moderate increase in temperature (2 degrees Celsius) and a large increase in precipitation (20%), one would see roughly a 20% increase in mean annual runoff, a 30% to 60% increase in (reservoir) storage and a 40% increase in power production,” the study says.

Global warming may well lead to more precipitation, but predicting where in the world the rain would fall is not easy.

“A major uncertainty is how a change in global climate affects regional precipitation,” Gleick said. “While we expect to have higher precipitation globally, some models show less precipitation for the midcontinent U.S. But the predictions are not considered reliable.”