Updated Feb. 1 with comments from U.S. Bureau of Land Reclamation commissioner.

The Colorado River watershed may be reaching a climate tipping point, drying under the influence of global warming to the point that states and tribes in the basin can no longer put off a day of reckoning about the water allocations that have been their lifeblood for the past century.

On Thursday night, Arizona joined other states that share the river basin in agreeing to voluntary water conservation plans. Its legislature approved a plan that helps balance the state's competing water rights with of those of California, Colorado, Nevada, New Mexico, Utah and Wyoming, along with Native American tribes and Mexico. The states faced a Jan. 31 deadline for completing interstate contingency plans on water rights; without them, federal officials could order mandatory cuts later this year. Only a California water district had yet to agree.

U.S. Bureau of Land Reclamation Commissioner Brenda Burman said Friday that with details in the complex web of agreements still being finalized in California and Arizona, her office would start the federal review process but halt it if all states had formal agreements in place by March 4.

A crisis point in the region has been approaching for years. When the regional water arrangements were first devised in 1922, assumptions about the river's bounty were way off because they were based on data from wet years. Even when the mistake was recognized decades later, managers continued to permit new withdrawals.

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And as dry heat has enwrapped the Southwest, the great river's flow has been going down while cities and farms slurp up ever more water. The gap between supply and demand can no longer be papered over by shunting billions of gallons of the river's waters back and forth among reservoirs in what has been one of the nation's most significant attempts to adapt to climate change.

To the contrary, recent scientific research shows that the Colorado's flow is very likely to drop even more in the years ahead.

Since 2000, temperatures have persistently run well above average. The heat sucks water out of the ground, as do thirsty plants. And the high country snowpack has dwindled, too.

From 1916 to 2014, flows in the Colorado dropped 16.5 percent, even though total precipitation in the upper basin increased slightly during that period, said Jonathan Overpeck, a climate researcher at the University of Michigan, who has written several studies showing the lasting impacts of warming.

In a recent study, Brad Udall and other researchers found that rising temperatures and changing precipitation patterns reduced Colorado River flows between 2000 and 2014 by 19 percent compared to the 1906–1999 average.

"This drought is not going to end until we stop global warming," Overpeck said. "It's not just precipitation, it's temperatures. We need to understand how what's happening on the land and to plants affects flows. It would be crazy to bet on increased precipitation."

What's Reducing the Basin's Runoff?

If recent research shows the fingerprints of global warming all across the basin, advanced modeling helps explain why.

A 2018 study used hydrology models to tease out what was causing the reduced runoff. It blamed a little more than half of the decline on unprecedented regional warming, which melted the snowpack and increased water use by plants. The rest was due to lower snowfall in four key pockets of Colorado where most of the water originates.

Model simulations run by Keith Musselman of the University of Colorado for a 2017 study indicated that some Western mountains could be expected to lose 10 percent of their mountain snowpack for every 1 degree Celsius of warming. (The models simulated flows in the Southern Sierra Nevada.)

A third application of advanced models across six mountainous regions of the West saw global warming driving the snowline — the altitude where snow falls above, but rain below — significantly higher up the slopes. Rain runs off immediately, while snow is stored until spring or summer.

The results "overwhelmingly indicate" the vulnerability of snowpack to a warmer climate," wrote the authors, from the University of Utah.

More Warming and Drying Ahead

Alarmingly, climate scientists expect another 2.5 to 4.5 degrees Fahrenheit of warming in the region by 2070 even if global greenhouse gas emissions are substantially reduced.

By mid-century, flows could drop another 20 percent, and there is a significant risk of long droughts in the coming century that will cut river flows even if there is an increase in precipitation. Some climate models do suggest that precipitation may increase — warmer air holds more water — but there is a lot of uncertainty around the projections.

The term "drought" may not be useful anymore because it implies a short-term condition with an end in sight, said Udall, who works for the Colorado River Research Group. He calls it aridification instead, and says the negotiating over sharing the water is a dry run for the future of water use in the Southwest under climate change.

Ominously, some studies have already suggested the region is at the beginning of a megadrought, based in part on reliable projections that global warming will drive an expansion of subtropical dry areas, which means the deserts of the Southwest could encroach on what are now the water producing-areas of the Colorado River Basin, he added.

The lower end of the river basin, in Southern California and Southern Arizona, is already one of the hottest parts of the country. The 2018 National Climate Assessment shows places like Phoenix and Las Vegas will have more frequent heat waves with extreme life-threatening temperatures in the decades ahead. Those extremes will also affect agriculture in the lower basin, but these types of impacts haven't even been considered in the current Colorado River talks, Udall said.

State Contingency Plans

The Drought Contingency Plans are designed to keep Lake Mead's water level above a threshold that would trigger disruptive mandatory cutbacks in water use. Upper Basin states have agreed to keep enough water flowing to the desert lowlands. And the lower basin states, now including Arizona, have agreed to divert less for farms and cities in order to bolster Lake Mead.

The Southwest is going to have to live with less water, said Doug Kenney, director of the Western Water Policy Program at the University of Colorado, Boulder. "We can do it with people kicking and screaming, or we can try to do it in a way that's fair to everybody."

The process still has a long way to go, but Gary Wockner, director of the conservation advocacy group Save the Colorado, said getting past the Jan. 31 deadline was a small sign that water managers recognize the elephant in the room.

"It acknowledges that the agencies involved know that the collapse of the Colorado River management system is impending. Now the collapse is happening faster than they thought it would. But because of global warming time is running out. Incrementalism not adequate any longer. We have a global climate emergency, everyone needs to first and foremost approach every issue based on that."

Water engineers, planners and scientists understand how global warming threatens water supplies and are generally averse to risk, Overpeck said. That's not always the case in politics.

"The polarization that exists on climate policy in the U.S. has prevented a lot of conservative politicians from doing something," he said. "And the decisions that are being made now, or the lack of decisions being made now, are going to condemn the Colorado to additional flow reductions."

Overpeck, who recently moved from Arizona to Michigan, said global warming was part of his family's decision to leave. Among other things, they were worried about the value of their property in the coming years.

"If we don't deal with climate change, the Southwest will become a place of exodus. It was getting depressing to see politicians writing off the future of the Southwest," he said.