Global temperatures are likely to warm by at least 1.8 degrees Celsius this century, the U.N.'s Pachauri said. That would add to the existing gain of 0.76 degree since industrialization began and overshoot the threshold beyond which European leaders have said climate change will become dangerous.

Such a temperature increase would trigger a 38 percent chance of shortages in states such as California and Arizona, said McCabe, whose study was completed late last year.

"It turns out in the Colorado, just modest warming can have significant impacts," McCabe said. "I'm just trying to make people aware that there's possibilities, both from the natural variables as well as from this continued warming, of having some big problems."

Strong is more optimistic.

"We're fairly confident the water supply is not going to crash," he said. "It's not going to stop flowing immediately. ... We feel like we're doing a good job managing water supplies.

"I don't see where the information is consistent with the actual facts," he said about some opinions people are forming in recent months about climate change and its impact on the river.

However, Strong said global warming is "real," and he is concerned about its potential impact on the Colorado, but he wants more science coming in the areas of modeling temperature change and anticipated precipitation for the future.

The Colorado River is a primary water supply for residents in Utah, Colorado, Wyoming, New Mexico, Arizona, Nevada and California. The U.S. Bureau of Reclamation, which manages the two main reservoirs on the river at Lake Powell and Lake Mead, last year developed guidelines on how to cut supplies to users in the event of a shortage.

The Colorado River is allotted to users under terms of the 1922 Colorado River Pact. Allocations were set during an "unusually wet" period compared with the rest of the 20th century, according to the report by Reston, Virginia-based USGS.

Demand for Colorado River water has "increased substantially," the USGS said. As a result, even without global warming, allotments that were set "at high levels that may be difficult to maintain," according to the report.

While developing supply guidelines last year, the bureau projected a 5 percent or less chance of a water shortage by 2010, Terry Fulp, a bureau regional director, said in a March interview. That jumps to a 25 to 30 percent chance by 2020.

Until recently, "the concept of a shortage was contemplated but there were no rules in place on how to deal with it," Fulp said. Climate change "could potentially decrease the mean average flow. We don't know by how much."

Carbon dioxide, the main pollutant blamed for global warming, is produced primarily from burning fossil fuels such as oil and coal. Rising global temperatures driven by human emissions of CO2 and other heat-trapping gases is causing Arctic ice to melt and sea levels to rise, a U.N. panel of climate scientists said in 2007.

European countries and most other developed nations have agreed to limit emissions under the Kyoto treaty.

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