Climate change is threatening to dry up the Colorado River — jeopardizing a water supply that serves some 40 million people from Denver to Phoenix to Las Vegas and irrigates farmlands across the U.S. Southwest.

Computer simulations of the Colorado River Basin indicate that, on average, a regional temperature increase of 1.4 degrees Celsius over the last century reduced the annual amount of water flowing through the river by more than 11 percent. Researchers from the U.S. Geological Survey in Princeton, N.J., report these results online February 20 in Science.

These findings “should be a cause for serious concern,” says climate scientist Brad Udall of Colorado State University in Fort Collins. As the world continues to warm, significant changes to the Colorado River’s flow — like other snow-fed waterways around the globe — could leave many communities with severe water shortages (SN: 5/29/19).

For the study, research hydrologist Paul “Chris” Milly and physical scientist Krista Dunne simulated snow accumulation and water runoff in the Colorado River Basin from 1912 to 2017, based on factors including historical data on temperatures, precipitation and snowpack. Those simulations allowed the researchers to tease out how specific variables, like air temperature, affected the river.

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The team found that over the 20th century, warmer weather allowed for less snow cover, exposing darker ground that absorbed more sunlight. That caused more water on the ground to evaporate before it could feed into the Colorado River, diminishing river flow.

To forecast the river’s future, Milly and Dunne combined their simulations with climate models that predict temperature increases under hypothetical emissions scenarios. If fossil fuel emissions are curbed so that atmospheric carbon dioxide concentrations level off by midcentury, the simulations predict that annual river flow would drop 14 to 26 percent compared with the average annual flow during the last century.

In a “business-as-usual” scenario where carbon emissions continue apace, simulated river flow dropped 19 to 31 percent by midcentury compared with 20th century flow.