For the first time ever, federal water managers took emergency measures in July to prevent the water level from dropping to levels too low to produce power at Glen Canyon dam, which generates electricity for 5 million people across seven western states. Lower water levels had already reduced the dam’s output by 16 percent when water managers made emergency releases from upstream reservoirs in Wyoming, Utah, Colorado and New Mexico to keep production from cutting out altogether — for now.

The swift and dramatic changes within the Colorado River basin — home to 40 million people, 15 percent of America’s crop output and 11 national parks — exemplify the accelerating, snowballing effects from climate change that the world’s leading scientists warned last week are on track to dramatically remake the planet unless governments can rapidly reduce carbon dioxide emissions.

“We are seeing the effects of climate change in the Colorado River basin through extended drought, extreme temperatures, expansive wildfires, and, in some places, flooding and landslides, and now is the time to take action to respond to them,” Tanya Trujillo, the top Interior Department official overseeing water science, said Monday as she announced the new water delivery cuts.

Despite the worsening conditions, the sprawling Colorado River watershed has thus far been a success story for climate change adaptation. The states and big water users there accepted years ago that their long-term future would be a drier one, and that they couldn’t count on a wet year or two to make up for the drought years. For the past decade and a half, they have opted for an approach that set aside fighting over the dwindling water resources and got down to the hard work of planning how to share the pain to head off disaster.

But now, as the workhorse waterway enters a new era of water scarcity, the limits of political buy-in for that approach are being tested.

“Climate change is changing the face of the river. It’s changing how much water will be there, it’s adding deep uncertainty to a system that was designed for a stationary river,” said Eric Kuhn, the former general manager of the Colorado River District, which represents Western Colorado’s water interests.

Academics, environmentalists and climate researchers say that elected officials must face politically unpalatable possibilities head on: There simply isn’t enough water for new pipeline and reservoir projects in Colorado, Utah, Wyoming and New Mexico, despite the promise made to those upper basin states a century ago under the foundational 1922 Colorado River Compact. At the same time, the steep water cuts that Arizona, Nevada and California painstakingly crafted two years ago almost certainly won’t be deep enough.

The situation is coming to a head now as the current rules governing water sharing among the seven states, the federal government and Mexico are set to expire in 2026. The parties must soon begin negotiations on the framework that will take their place.

But close river watchers say they see a dearth of political will for confronting the tough future possibilities among the key players, particularly in the upper basin.

“I am concerned that we are not seeing leadership in the upper Colorado River basin that grasps the depth of the challenge that we face in terms of the need, as a broad basin community, to use less water for all our water users,” said John Fleck, director of the University of New Mexico’s water resources program and a former science journalist.

“That’s politically difficult because you can get voted out of office for staying stuff like that, but that’s, I think, what leadership requires,” he said.

Now, one of the most crucial questions facing the states and water managers is how dire of a future the new rules should plan for.

“You need a mutually accepted definition of the problem and the magnitude of the risk,” said John Entsminger, who will be at the negotiating table as general manager of the Southern Nevada Water Authority.

Science doesn’t offer a clear answer about how much water will be left in the river in the coming years. Over the past two decades, it has seen a 20 percent decline in average annual flows, and researchers roughly expect another 9 percent decline for each additional 1 degree Celsius of warming.

But, while climate models have become quite accurate at predicting temperature changes, precipitation — arguably the most crucial lever in the Colorado River basin’s future hydrology — is still a problem for modelers.

“They’re great on what’s the trajectory on average warming, that’s not the problem. They’re not so great on the water cycle and that’s a problem and that blinds us to the potential really harsh outcomes,” said Brad Udall, a leading water and climate scientist at Colorado State University.

Udall argues that water leaders should prepare for a “reasonable worst-case scenario,” that is, one within the worse range of what climate models based on the past 20 years of drought predict, but that is not so dire that it is politically impossible to plan for.

But that could push some extremely delicate political conversations to the fore.

State leaders are loath to acknowledge scenarios in which cuts in water use in the upper basin become necessary, or where urban water supplies in the Southwest could be threatened.

Meanwhile, a major legal question looms over the river regarding who bears the brunt of reduced flows due to climate change. It has been avoided so far because, even during the past two decades of drought, the amount of unused water in the upper basin has just barely matched the amount of water that that needs to be sent downstream to Nevada, Arizona, California and Mexico. But any changes to that balance, whether through increased water use in the upper basin or through more dire drought, will force a reckoning among the states.

All of this has some water experts calling for a fundamental change to how the Colorado River is managed. Kuhn, the retired Colorado River District leader who participated in multiple earlier rounds of negotiations on the river, argues that the level of uncertainty that climate change has introduced means water managers must shift away from a system based on delivering certain fixed amounts of water to users each year, as current water contracts are written, and instead portion out supplies based on what’s available in any given year.

“How the river responds to climate change doesn’t particularly care about how we crafted water contracts in the 1930’s,” he said.

But, by and large, water managers prefer to work within the system that already exists — even as they recognize the assumptions on which it is based will no longer hold true in the drier future.

The level of cooperation and sense of shared destiny among the region’s water leaders is their most important tool, they say. Shaking the framework on which that is based is simply not worth it, even when individual states believe strongly that parts of it are unjust.

“It doesn’t make sense to me to go back and redo the Colorado River Compact, even though we know today that the Colorado River can’t produce 16.5 million acre feet to satisfy the upper basin, the lower basin and the treaty with Mexico,” said Ted Cooke, general manager of the Central Arizona Project, the Grand Canyon State’s major Colorado River water purveyor.

“We know that, this is a fact, we have to deal with it. But it doesn’t mean knock over everything we’ve built and start over with something else, because, fundamentally that involves a lot of taking. That will take things from people,” he said.

Regardless of the framework for governing the river, the solution to the fundamental problem of supply remains the same: Use less water. And the tools for doing so are well proven.

Over the past two decades, the arid Southwestern states have lined up technically and legally complex frameworks for reducing water use and transferring water from lower-value agricultural uses to cities during dry years. Major urban water utilities in southern California and Nevada are also preparing to invest more than $3 billion in a massive new water reuse facility. But in the upper basin, where elected officials have not acknowledged the need to rein in future water use, let alone to shrink current usages, innovative approaches to easing the pain of cuts remain in the study phase.

The bipartisan infrastructure bill approved by the Senate last week includes hefty funding for such measures, including $1 billion in water recycling, $250 million in desalination, and nearly $1 billion for programs such as environmental restoration and water use efficiency projects.

“It’s a major investment in moving down the path toward greater water security and water system resilience,” said Kevin Moran, director of the Environmental Defense Fund’s Colorado River program.

But, he acknowledged, getting it out the door of federal agencies and on the ground to projects that can make a difference at a speed that matches the deteriorating hydrology will be a major challenge.

“It’s going to test us,” he said.