Climate change is slowly drying up the Colorado River

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.

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.