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Study: Colorado River’s Flow Decreases 9 Percent With Every Degree Of Warming

Melissa Sevigny

A new paper published today in Science shows a rising risk of water shortages in the Colorado River Basin. Scientists say diminishing snowpack from climate change plays a critical role—not just because snow supplies the river with water, but because it acts as a protective shield against evaporation. KNAU’s Melissa Sevigny spoke with lead author Chris Milly of the U.S. Geological Survey.

Melissa Sevigny: Your study was about what’s going to happen to the Colorado River as the world continues to get warmer. What did you find out?

Chris Milly: We wanted to know what to expect about the future for the Colorado River since it is the major source of water supply for the U.S. Southwest….We analyzed historical data for streamflow, climate, that is, precipitation and temperature, we even looked at satellite observations from the last couple of decades which allow us to see how white the basin looks, that is, how much snow can be seen from space. After all that analysis we were able to determine how sensitive the flow of the river is to rising temperatures.

I have the number here with me, so you found the river’s flow decreases about 9 percent per every degree Celsius of warming.

That number is not the highest number that’s been reported in the past… The range goes from 2 to 15. We were able, we think, to nail down what this number really is, and why previous studies have had such a wide dispersion of their estimates.

So one of the unique things about your approach was looking at the reflection of sunlight off the snow affects the environment, can you talk about that?

That’s right. That’s the thing I found most fascinating about this study. As the snow cover dwindles due to warming, it’s reflecting less sunlight back to the atmosphere. So the basin’s absorbing more sunlight. That sunlight is powering evaporation out of the basin. Now when that evaporation—the water is taken out of the basin due to the evaporation, there’s less water flowing down the river to the 40 million people that are waiting for it.

So on our current track of warming, what do you think the Colorado River is going to look like a couple of decades from now?

Going into the future, there’s a large degree of uncertainty of how the climate is going to be changing… If we take the changes in temperature alone, something we’re pretty clear about what’s going on, the effect of those warming temperatures by the middle of the century, the year 2050, are expected to decrease the flow of the river from its historical levels by anywhere from 14-31 percent. If precipitation is brought in, that range becomes bigger on both ends. We might see in the very, very best case, a couple percent increase in the flow, but if you look at the range of projections for precipitation changes they take you down to as much as a 40 percent decrease in flow when combined with the warming that’s going on.

Chris Milly, thank you so much for joining me today.

Pleasure to be here.


Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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