From Inside Climate News (Bob Berwyn):

The loss of the reflective snowpack drives evaporation and reduces the flow of water, the study found.

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.”

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…

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.”

[…]

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.”

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…

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.”

From KUNC (Luke Runyon):

Using hydrologic models, researchers with the U.S. Geological Survey, found that the Colorado River basin is extremely sensitive to slight changes in temperature. In their new paper in the journal Science, they show for each degree Celsius temperatures rise, flows in the river are likely to decline more than 9%.

That decline is likely to cause severe water shortages in the Colorado River basin, where more water exists on paper in the form of water rights than in the river itself. Warmer temperatures diminish snowpack, lessening the amount of water available…

The reductions might sound small, Milly said, but they will be felt throughout the basin.

“There’s not a lot of slack in the system,” Milly said. “In the long-term communities, states will be making adjustments to how they allocate water.”

[…]

The finding comes as water managers throughout the watershed are gearing up for negotiations over a long-term plan for the river’s management. The Colorado River’s current operating guidelines expire at the end of 2026, and the states that make up the watershed are required to start negotiating new ones by the end of this year.

“The new rules must consider how to manage the river with unprecedented low flows in the 21st century,” Udall said. “The science is crystal clear — we must reduce greenhouse gas emissions immediately. We now have the technologies, the policies and favorable economics to accomplish greenhouse gas reductions. What we lack is the will.”

Click here to get access to the paper.

From The Washington Post (Juliet Eilperin, Chris Mooney):

Up to half of the drop in the Colorado’s average annual flow since 2000 has been driven by warmer temperatures, four recent studies found. Now, two U.S. Geological Survey researchers have concluded that much of this climate-induced decline — amounting to 1.5 billion tons of missing water, equal to the annual water consumption of 10 million Americans — comes from the fact that the region’s snowpack is shrinking and melting earlier. Less snow means less heat is reflected from the sun, creating a feedback loop known as the albedo effect, they say.

“The Colorado River Basin loses progressively more water to evaporation, as its sunlight-reflecting snow mantle disappears,” write the authors, USGS senior resource scientist Chris Milly and physical scientist Krista A. Dunne…

Milly and Dunne, who analyzed 960 different areas in the Upper Colorado River Basin to determine how disappearing snowpack influenced the river’s average annual flow, determined that the flow has dipped 9.3 percent for each temperature rise of 1 degree Celsius (1.8 degrees Fahrenheit). The average annual temperature for the area they surveyed has risen 1.4 degrees C (2.5 degrees F) in the past century, Milly said in a phone interview.

The region is poised to warm even more in the years ahead, Milly said, and it isn’t “likely” that precipitation can compensate for these hotter and drier conditions. Comparing the Colorado River’s historic flow between 1913 and 2017 to future conditions, he added: “That flow, we estimate, due to the warming alone would be reduced anywhere from 14 to 31 percent by 2050.”

Colorado State University senior scientist Brad Udall, who has written two papers attributing half of the Colorado River’s lower flows to warming temperatures, said in a phone interview that researchers now “have multiple lines of evidence pointing to a very similar number.”
“And this number is worrying,” Udall said of the new study. “I would say eye-popping.”

[…]

Andrew Mueller, general manager for the Colorado River District, said in an email that the new findings provide “confirmation of significantly grim indicators about future flow in the Colorado River.”

The amount of water that would disappear with another 1 degree C temperature rise, he added, is nearly five times what Las Vegas uses each year. “A decline in flows of this magnitude will present a significant challenge to all inhabitants in the Colorado River Basin.”

The current operating rules for the river expire at the end of 2026, and negotiations over how to share the water going forward start this year.

Udall said that in light of current projections, policymakers need to consider crafting an agreement where all the major players in the West will use less water than they do now.
“These projections are dire, but we’re looking at a glass that’s 70 percent full, not half full,” he said. “It could be grimmer.”

Officials at the U.S. Bureau of Reclamation, who brokered a drought contingency plan among seven states and Mexico last year, said that they are continuing to monitor the way climate change is affecting the river.

“Reclamation works closely with ​leading scientists at the state and federal level, as well as universities to understand the potential impacts of climate change on the Colorado River,” said bureau spokesman Marlon Duke. “We will continue to use the best available science to manage the river to sustain reliable water far into the future.”