A new study suggests that for every degree Celsius of climate warming, water flow from the Colorado River will decrease approximately 9.3%.
Using that metric, scientists believe that it is possible to see the Colorado River's flow reduced by an estimated 14% to 31% by 2050, according to researcher Chris Milly, a co-author of the study that was published in Science Magazine.
Milly said the 9.3% loss is enough water to service approximately 10 million people.
Brad Udall, a scientist from Colorado State University who has conducted studies on the Colorado River, called the study “eye-popping” because their conclusions landed in the upper end of what scientists thought was possible. Previously, studies had suggested the number was anywhere from 2% to 15% per degree of Celsius, or 1.8 degrees Fahrenheit, change.
“It’s certainly a great cause for concern,” Udall said.
The water is expected to dry up due to the warming climates and decreasing snow in the region. Since 2000, about 1.5 million acre-feet of water did not flow down the Colorado River due to climate change, Milly said. The Colorado River currently does not flow the entire way to the ocean.
The study comes as scientists are trying to better understand what southwestern states and Mexico can expect for the future of the Colorado River, which supports $1 trillion of revenue, in light of expected strains on water resources due to climate change.
Flagstaff does not directly rely upon the Colorado River for drinking water, but economic and cultural impacts of the river still run deep.
The new study combines observed precipitation trends and estimates about increasing temperatures as a result of climate change to form their conclusion, Milly said.
As snow builds up in the Southwest during the winter months, sunlight reflects off of snow that protects the ground from the sun’s heat. Milly explained that as temperatures increase, snow will stop collecting as often, causing the earth’s surface to warm.
“Energy is what drives evaporation. More energy leads to more evaporation,” Milly said.
Milly said their study utilized satellite data to better understand how much energy was normally reflected into the atmosphere from snowpack, which was a departure from other studies on the topic.
Their data is based upon one major unknown: how much precipitation will fall across the region.
“The wild card now, going forward, is we have to figure out better estimates of what the changes in precipitation are going to be,” Milly said, referring to how scientists are still unsure how climate change will impact trends of rain and snow.
The worst case scenario suggests water conditions could get as bad as a 40% reduction in river flows, but the best case scenario shows flows could actually increase by 3%.
“It’s not clear whether [precipitation is] going to increase or decrease over the southwest, in particular in the Colorado River Basin,” Milly said.
Weather and climate are related, but climate is defined as weather events measured over the course of 30 years. Over that time, there could be wide fluctuations between high and low flows to reach those total numbers.
Udall said in light of that possible fluctuation, decade to decade, the future rules for managing the river’s water will need to account for the wide variety. The current rules will expire in 2026; new ones will be put in place in 2027.
“Everybody thinks that their water use is critical. And yet what this science is telling us, by the mid-century, is we’re going to see very large declines in the flow,” Udall said. “It is the ultimate challenge for water management.”
Scott Buffon can be reached at firstname.lastname@example.org, on Twitter @scottbuffon or by phone at (928) 556-2250.
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