The 40 million people who rely on Colorado River water need to prepare for a drier future.

Global warming is shrinking the Rocky Mountain snowpack that feeds the river and flows are declining at a rate of about 9.3 percent for every 1.8 degrees Fahrenheit increase in temperature, according to a new study that "identifies a growing potential for severe water shortages in this major basin."

The decline is "mainly driven by snow loss and consequent decrease of reflection of solar radiation," a pair of scientists with the U.S. Geological Survey wrote in a new paper published Thursday in the journal Science. The study helps resolve a "longstanding disagreement in previous estimates of the river's sensitivity to rising temperatures."

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The study links dwindling flow of water with the loss of albedo, a measure of the snowpack's reflective quality. Like ice in the Arctic, white snow reflects solar radiation back to space. But as the snowpack in the Colorado River declines, the ground and, crucially, the air directly above the ground, warm up. Water from the melting snow or from rain evaporates from the soil, rather than trickling into the streams that feed the Colorado River.

The scientists found the link by measuring the relationship between the amount of water in the snow, the amount of the sun's incoming radiation and how much of that was reflected back by the snowpack's albedo, showing that, as the snowpack dwindled, the river's flow declined.

Brad Udall, a climate scientist with the Colorado River Research Group, said the study "adds another brick in the wall of evidence that it's very likely we're going to see significant declines in Colorado River flows.

"Scientists have been trying to figure out how sensitive the river is to global warming," he said, "and these numbers put the sensitivity at the upper end of what's possible."

The research divided the Colorado River Basin into 960 sub-areas and broke down the data, including satellite measurements of albedo, month by month. That enabled the scientists to see that the effect was dominant in the late spring and early summer, when the snowpack was being depleted, said Chris Milly, the senior U.S. Geological Survey researcher who led the new research. Previous studies on the Colorado River's climate sensitivity focused primarily on precipitation and temperatures, without considering the radiation balance, he added.

"Before our study there was a huge range of estimates of how sensitive Colorado River flows are to warming, from 2 percent to 15 percent for every 1 degree Celsius of warming. We really wanted to try and understand and narrow that uncertainty," Milly said.

It's not just a Colorado problem. "Many water-stressed regions around the world depend on runoff from seasonally snow-covered mountains," the authors wrote in the journal report, "and more than one sixth of the global population relies on seasonal snow and glaciers for water supply."

The findings suggest that the snow cover offers a "protective shield" that limits evaporation from this natural reservoir, the scientists wrote in the study. As the shield shrinks, it will crimp water availability in snow-fed regions that are already stressed, including the Colorado River Basin.

In all, Colorado River water is shared by seven states under a complex set of agreements dating back to the Colorado River Compact of 1922. Wyoming, Colorado, Utah and New Mexico were designated Upper Basin states and California, Arizona and Nevada as Lower Basin states. Last year the states agreed on drought plans to avert a short-term crisis that would result in mandatory water cut-offs for some users.

Unending Stream Flow Decline

University of Michigan climate researcher Jonathan Overpeck said the new study is valuable because it details the mechanism "by which regional human-caused warming is reducing flows in the Colorado River."

Continued warming, he said, "will lead to significant and unending reductions in river flows. Until global warming is stopped, the Colorado and other key rivers of the Southwest will continue to provide less and less water to the region."

The Colorado River gets most of its water from tributaries filled by melting snow in the Rocky Mountains of Colorado, Utah and Wyoming. It then flows more than 1,000 miles via Arizona, Utah, Nevada and California, supplying water to all major regional cities and vast tracts of farmland in the Southwest. Most of its water is diverted for agriculture and municipal use before the river trickles to an end in dried up wetlands and mudflats near the Sea of Cortez.

Water levels in two major reservoirs built to store and distribute the river's water have been steadily dropping since 2000, a period that has been identified as one of the five driest periods on record in the past 1,200 years. Most regional water experts say the drying trend goes beyond drought (which implies eventual recovery), to long-term aridification caused by global warming.

By 2018, water storage in Lake Mead (the largest reservoir in the U.S.) has dropped 60 percent since 2000, to its lowest level since its creation in 1935, according to the 2018 National Climate Assessment. Upstream, at the border of Utah and Arizona, water storage in Lake Powell has been close to record-low levels since 2005. Lake Powell is currently less than half-full.

Colorado River states have been seriously grappling with expected shortages ever since a 2012 U.S. Bureau of Reclamation's report with an iconic graph showed the rising red water demand line crossing above the blue supply line that appeared during Colorado's hottest year on record.

Historical supply and use and projected future supply and demand in the Colorado River Basin. Credit: U.S. Department of the Interior Bureau of Reclamation

Research since then has confirmed that global warming is affecting water supplies in the West in several different ways. As early as 2013, U.S. Geological Survey research showed that warmer spring temperatures since 1980 have cut the Rocky Mountain snowpack by 20 percent.

A 2016 study in California's Sierra Nevada Mountains showed how the snowfall line is speeding uphill. At lower elevations where the mountains aren't so steep, tens of thousands of square miles that used to be white all winter now stay brown and heat up, and the moisture in the soil evaporates.

In 2017, Overpeck, along with Udall, showed a clear relationship between warming temperatures and less water in the Colorado River Basin, as they studied the Colorado River's 21st century "hot drought."

"Water that used to run down our rivers is now evaporating out. We have this big natural lever that is working against us and it's a nonlinear response," Udall said. "With global warming, the snowpack shrinks and there's more dark-colored ground that heats up. That melts more snow, which leads to even more ground being exposed."

Extremes

The new study doesn't take into account extreme events like the crippling 2012 drought that sent Colorado River flows to record lows while reservoir storage plummeted.

By the end of May that year, 100 percent of Colorado was in some stage of drought, including the mountains that supply more than three-quarters of the Colorado's total flow. It would end up being Colorado's hottest year on record, as well as one of the state's worst wildfire seasons, burning a quarter million acres and causing temporary evacuations of 35,000 people.

But so-called Black Swan climate events like megadroughts lasting several decades have happened regularly in the last few thousand years, and are increasingly likely in a world that's cooking in a thickening stew of greenhouse gases.

In May 2019, the Colorado River Research Group published a warning about "unexpected shocks from Black Swan events." That includes megadroughts or extreme floods, as well as "socioeconomic events that might stress the existing legal/management framework beyond any known circumstance," the report said.

Because of global warming, the chances of such events are increasing at the same time that reservoir storage and groundwater reserves are being depleted, a disconcerting situation "given the role of multiple megadroughts in undermining past civilizations in the region," the river researchers wrote.

They said planning scenarios should be based on water records that stretch back longer than the last century, and should take into account that "the abnormally wet period of the early 20th century ... might be better viewed as a highly unlikely hydrologic event that cannot be assumed to be part of the future."

The paleoclimate record clearly shows that the first 100 years of the European settlement era in the Colorado River Basin was an unusually stable period of abundant water, and that there were sudden extreme swings between drought and floods during past geologic eras of rapid climate change.

One of most severe drought periods on record in the Colorado River Basin was between the years 900 to 1300, when regional temperatures close to today's triggered "a period of extensive and persistent aridity over western North America," according to a 2010 study in the Proceedings of the National Academy of Sciences.

The Colorado River Research Group warned that the current long-term drying trend, driven by warming and precipitation deficits, could worsen for years or decades into the future.

"The risk of such aridification certainly increases with time unless greenhouse gas emissions are curbed, and could yield a more or less permanent (on human time scales) megadrought worse than any of the last 1,000 years," the scientists wrote.

Overpeck said, "The good news is that we understand what is happening to the Colorado River and why. This means we can have confidence on the solution, which is putting a rapid stop to climate change, mainly by ending the burning of fossil fuels."

He added, "Simply put, the more oil and gas we burn, the less water will be available to the American Southwest."