Good news on the Colorado River is rare. Its reservoirs, the two largest in the country, have shrunk to record lows . The policymakers who will decide its future are stuck at an impasse . Climate change has driven more than two decades of megadrought and strained the water supply for 40 million people across the Southwest.

But a new study is delivering a potential dose of optimism for the next 25 years of the Colorado River. The findings, published in the Journal of Climate , forecast a 70% chance the next quarter century will be wetter than the last.

Projections for Colorado River water supply have largely focused on the impact of temperature . Climate change means the region is getting hotter, which in turn drives a raft of environmental factors that mean less water ends up in rivers and reservoirs. For example, snow melts quicker and is more likely to evaporate . Dry, thirsty soil soaks up snow melt before it has a chance to flow into the nearest stream.

This new study , though, takes a closer look at the impact of precipitation.

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Eighty five percent of the Colorado River starts as snow in the region’s headwaters – the high-altitude mountains of Colorado and Wyoming. The scientists behind the new paper predict an increase in precipitation over the next 25 years that could be big enough to offset the drying caused by rising temperatures, at least in the short term.

Researchers with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder used data from the United Nations Intergovernmental Panel on Climate Change, or IPCC, to run forecasting models and form their conclusions.

Those scientists stressed the importance of variability in their findings. While the high end of their forecasts paint a positive picture, their models also showed a small chance that precipitation could go down in the next two decades. There’s a 4% chance that river flows could drop by 20% in the next 25 years.

“All of our thinking, our acting, our management should be humble and recognize the nature in which we live, which is, yeah, you have water, but it is very highly variable,” Balaji Rajagopalan, a water engineering professor who co-authored the study, said.

Good science about the region’s climate future is particularly important right now, as Colorado River policy makers renegotiate the rules for sharing its water. The region’s water crisis is driven by two big themes – climate change is shrinking supply, and the people in charge have struggled to rein in demand in response.

Right now, they’re hashing out a new set of rules for managing the river to replace the guidelines that expire in 2026. Rajagopalan said the findings from the new study underscore the need to build flexible rules that can adapt along with climate conditions.

“We want to emphasize that it's not like, ‘Oh, there's going to be water around, so let’s go party – we don't have to do the hard work that needs to be done in terms of conservation and thoughtful management,’” he said. “If anything, it speaks to even more reason that you have to.”

Another climate scientist, Brad Udall, who was not involved in the study, cast a bit of skepticism on its findings and message. Udall, a climate researcher at Colorado State University's Colorado Water Institute, said he holds the paper’s authors in high regard, but some aspects of the study’s approach gave him some “unease.”

“We just can’t rely on these models for precipitation,” he said. “We can rely on them for temperature, but we can’t rely on them for precipitation. There are just too many issues with them.”

He said climate models can’t always dependably predict precipitation because they are based on statistics, as opposed to the physics-based methods used to build long-term temperature forecasts.

Udall, who has referred to himself as “the skunk in the room” after years of sharing tough-to-stomach forecasts about the dire future of Western water, pointed to this year’s runoff as an example of temperature’s ability to chip away at the benefits of a wet winter.

While snow totals in the Colorado River headwaters region peaked at around 100% of normal, warm temperatures mean flows in the Colorado River are expected to reach about 80% of normal levels.

This story is part of ongoing coverage of the Colorado River, produced by KUNC in Colorado and supported by the Walton Family Foundation. KUNC is solely responsible for its editorial coverage.