April 17 (Bloomberg) -- The Colorado River may shrink in this century to its lowest level in at least 500 years because of global warming, threatening water supplies to California and six other states, researchers at the U.S. Geological Survey said.

A ``modest'' 0.86 degree Celsius (1.5 degree Fahrenheit) increase in the 21st century could trim the average flow of the river -- the primary water supply for residents in much of the U.S. Southwest -- to the low end of a range marked between 1490 and 1998, USGS scientist Gregory McCabe said yesterday.

The Earth is likely to warm by more than twice that amount in the period, Rajendra Pachauri, chairman of the United Nations Intergovernmental Panel on Climate Change, said last month. McCabe will brief Congress on the findings in June, when legislators expect to debate plans for the world's largest producer of greenhouse gases to begin capping its emissions.

``A 2-degree Celsius warming pushes the risk so high that it's beyond anything that has happened in the last 500 years,'' McCabe said on a conference call yesterday. ``The average flow in the Colorado drops to lower than anything we've seen.''

The U.S.'s seventh-biggest river by drainage area is fed by melting snow. Less precipitation from periodic droughts or climate change leaves reduced snow to feed the 1,450-mile waterway.

In a report presented yesterday in Boston and co-written by USGS research hydrologist David Wolock, McCabe used data from an earlier study that reconstructed annual stream flows from measurements of tree rings. The technique provides a way to show how temperatures will affect flows in the future, Wolock said.

Southern California

``It allows us to place the 20th century conditions that were used in developing plans for managing water resources in the basin in the context of a much longer record of flow,'' Wolock said in an interview. ``We can estimate flow during periods when we were never able to measure.''

About 40 percent of southern California's water supply is likely to be vulnerable within the next two decades as rising temperatures lead to reductions in snow pack in the Sierra Nevada and the Colorado River basin.

Global temperatures are likely to warm by at least 1.8 degrees Celsius this century, the UN'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.

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

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.

`Big Problems'

``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.''

The Colorado River is the primary water supply for residents in Colorado, Utah, 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.

Water Shortages

The bureau last year developed guidelines on how to cut supplies in the event of a shortage. It 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 UN panel of climate scientists said in 2007.

To contact the reporter on this story: Jim Efstathiou Jr. in Washington at jefstathiou@bloomberg.net

Last Updated: April 17, 2008 11:16 EDT