United States Geological Society research hydrologists Chris Milly and Krista Dunne created a computer model that accurately mimicked the river's flow from 1913 to 2017, and then used it to predict how the river will fare decades in the future. The model factors in detailed data about a variety of impacts—including snowfall, rainfall, snow melt, temperature and evaporation in the landscape that drains into the river—that could change how the that could alter the 1,450-mile-long river.
As Science News’ Maria Temming reports, the researchers found that the river flow fell by 11 percent in the last century. And for each 1.8 degree Fahrenheit of warming, they predict that the flow of water will fall by an additional 9.3 percent.
Colorado State University climate scientist Brad Udall, who was not involved in the study, tells Scientific American’s Shannon Hall that the findings are “eye-popping.” In 2017, Udall co-authored a study that found that the Colorado River’s flow could shrink by 3 to 10 percent at the same interval of warming.
Regarding the new analysis, “I would argue that they did it more elegantly and more rigorously” than previous studies, Udall tells Scientific American. “And you have to take this result pretty seriously.”
The Colorado River is fed by snowpack that builds up throughout the winter and melts slowly until mid-spring. Historically, the snow’s blindingly reflective surface moderates the thawing effects of springtime sunlight. But as temperatures rose by about 2.5 degrees Fahrenheit since 1913, less snow has covered the landscape. That, in turn, leaves more ground exposed to the sun, which causes water to evaporate before it reaches the river.
To Scientific American, Milly describes the snow cover as a “protective shield” that keeps the ground cool enough for the water to reach the river. So as the snow recedes, less water will enter the river. Rising air temperatures also mean that water is evaporating out of the river at a faster rate than it used to.
The researchers predict that if greenhouse gas emissions continue at their current pace, the river could see a 19 to 31 percent drop in water flow by the middle of the 21st century. If emissions are curbed, that figure falls to a 14 to 26 percent loss of flow. For the 40 million people who rely on the Colorado River for water, that would mean severe water shortages, the study suggests.
“Every drop in that river is being used. And any reduction like that is going to cause serious pain,” Udall tells Scientific American. But he is optimistic that there are still options, emphasizing the need to reduce greenhouse gas emissions, Udall explains in an interview with the Guardian’s Oliver Milman. And last year, the effects of a 19-year drought in the American Southwest prompted seven states around the river to voluntarily reduce water usage.
“I like to say, ‘Hey, if we’ve got 20 percent less, that still means the glass is 80 percent full,’” Udall tells Scientific American. “Let’s get smart and savvy and figure out how to use what we’ve got.”