Above-average snowpack in the Rocky Mountains this year may bring some relief to the Colorado River Basin, which has been in a drought since 2000. But the long-term picture for the region is less rosy after a newly published study found just how much higher temperatures are impacting river flow.

The paper, published in the American Geophysical Union’s peer-reviewed journal Water Resources Research, found that river flows fell 19 percent between 2000 and 2014 and reduced precipitation could not be blamed for all of the decline. On average, one-third of those reductions in river flow were likely due to rising temperatures.

The study’s authors, Brad Udall, senior water and climate research scientist at Colorado State University, and Jonathan Overpeck, director of the Institute of the Environment at the University of Arizona, found that if we continue on our current path and do little to address greenhouse gas emissions lower river flows from warming alone may reduce the Colorado by 35 percent or more.

The Colorado is the lifeblood of most major southwestern cities, including Las Vegas, Phoenix, Los Angeles, Salt Lake City and others, and 40 million people, industry and agriculture depend on it.

“In the 20th century, droughts were associated almost exclusively with a lack of precipitation,” Udall and Overpeck wrote. “In this century, however, high temperatures alone can lead to anomalously dry conditions.”

Water Deeply recently spoke with Udall about the research, how it differs from other research on the impacts of climate change in the region and what a warmer future means for the Colorado.

Water Deeply: How did this research come about?

Brad Udall: A couple of years ago, I was at a Colorado Water Congress meeting and one of the water managers there pointed out to me that the Colorado River drought looked abnormal – that it looked like precipitation couldn’t completely explain the decline in river flows. And it got me thinking about [whether] one could really see the impacts of temperature on flow.

I previously had done work on this, on a theoretical basis with other scientists, running hydrology models and tweaking them by slight changes in temperature and slight changes in precipitation to see what would happen. So we knew, theoretically, that you should see something, but it then occurred to me that this was a great opportunity to go look and see if you can actually find it in the data.

Water Deeply: So what did you find when you were looking at the data?

Udall: The first finding is that you can explain one-third or more of the decline [in Colorado River flows] due to warming. What you find is that the 2000 to 2014 period [that] the study covered, the Upper Basin, where most of these flows originate is about 1.6F (0.9C) warmer than the 20th-century average. There’s a big range on that, but the midpoint is one-third – that’s 500,000 acre-feet (617 million cubic meters), which is a lot of water.

The other part, we really want to make clear to people, and oftentimes scientific studies sort of tiptoe around this issue, but if you really want to save the flow of the Colorado River, we need to address greenhouse gas emissions. We need, at some point in this 21st century, to cease all emissions of greenhouse gases.

It’s that clear.

Water Deeply: How does your most recent study compare to other research about how climate change may impact the region?

Udall: You can take past work that I’ve done and a bunch of other people have done, and look at future temperature projections and figure out what kind of a decline [in flow] temperature increases will get us in the river. You got these eye-opening numbers – 35 percent or more due to temperature declines by the end of century, if we continue to emit greenhouse gases unabated. We call those temperature induced declines.

Water Deeply: In the study you mention that your findings differ from those done by the Bureau of Reclamation. Can you explain what the differences were and why?

Udall: Previous studies on the Colorado River have blended the inputs or outputs from climate models of both temperature and precipitation change. The most recent one of these is one that Reclamation has done. It sort of presents a no-big deal future for the Colorado River, where you don’t really see much of a decline [in river flows]. That’s because the certain temperature-induced declines get washed out by very uncertain precipitation futures.

What we point out, and I think this is a really important part of the study, is that the climate models are all over the map with regard [to] what happens to future precipitation. There’s no certainty with them, the way there is certainty with [what happens when] temperatures increase. What we’ve tried to say here is it’s a mistake to combine certain temperature increases with uncertain precipitation increases and present what amounts to a pretty rosy future for the river, in what is arguably a not very rosy future under climate change worldwide, and even throughout the American West.

Water Deeply: If we’re following research done by the bureau or others using these kinds of models and we drastically underestimate the impact that temperature increases will have on the river, what would be the ramifications?

Udall: So we talk kind of midcentury, the potential for 20 percent declines in flow. End of century, 35 percent decline in flow, just due to temperature. Again, precipitation could either make that worse or make it better. At 20 percent average annual decline [in river flows], it is pretty clear that you end up breaking the rules that the river operates under.

Those rules go back to 1922 and the Colorado River Compact. Those rules include Supreme Court decrees, an Upper Basin compact and a variety of agreements among the states. They’re painstakingly negotiated and put in place. They take years to change, even decades to change, in some cases. One of our messages is, we need to think now about how we would operate this river, if you had serious flow declines, because the current rules won’t work.

Water Deeply: The recent emergency at Oroville Dam in California has increased the spotlight on whether we are using the best climate science and the understanding of climate extremes when making water infrastructure and policy decisions. Are you seeing more of that in Colorado and do you think that this research might help guide some thinking around infrastructure in future planning?

Udall: Well, I hope so.

We focus, obviously, on the downside here, the loss [of flow] side. The flood side is interesting, too. So one of the things that has been known, well, since the 1970s or even before, is just how connected the hydrologic cycle, the water cycle, is to the temperature of the Earth. If you increase the energy that the Earth is receiving, and that’s exactly what we’re doing by emitting CO2 and other greenhouse gases, you change the hydrologic cycle in fundamental ways.

What does that mean? It means more evaporation globally, more precipitation globally, but regional winners and losers.

It means more extremes of all kinds – more floods, more droughts.

The Colorado River Basin is mid-continent. So, it obviously has access to the moisture in the Pacific Ocean, but it’s far more indirect. So we will see extremes, just like California, but they’ll be muted, somewhat.

Water Deeply: In your study you mention that another factor that can influence streamflow is dust on snow. Can you explain?

Udall: We get dust out of southern Utah and northern Arizona in springtime that comes and gets picked up by these strong spring winds, then deposited on our snowpack. It’s dark. It absorbs more solar energy; it makes the snow melt faster.

One study by Tom Painter indicated, on average, about a 5 percent loss in Colorado River flow due to dust in these heavy dust years. That’s a worry as climate change in the 21st century unfolds. The dust clearly seems related to drought, so when you have drier winters in northern Arizona and southern Utah, it’s much easier for the winds to come along in the spring and pick this dust up and carry it.

The reason it actually decreases runoff is interesting. It’s sort of related to climate change. The dust causes the snow to melt earlier, so you then have the ground bare for a longer period of time, it means plants can transpire water for that longer period of time. It also increases the heat for a longer period of time.

There’s a concern, obviously, that this might get worse as the century unfolds.

Water Deeply: You mentioned earlier that climate scientists need to be a bit bolder, and I’m wondering right now, given that we have an administration that seems hostile to climate science, what does that mean for scientists?

Udall: It means we continue to need to do our job. We need to speak bluntly, backed by facts. I think one of the great frustrations of the era we find ourselves in is the concept of alternative facts. Democracy runs on truths. Societies run on known truths.

If we truly are to be a great nation, we have to make science-based decisions. The public has no idea how unified the scientific community is about the nature of this threat. The potential for catastrophe, the need to do something in the very short-term timeframe, most of the scientists I know do not sleep well at night knowing just how dangerous a territory we are entering.

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