From Grist (John Upton):
The new research, published in Geophysical Research Letters by academics and a federal scientist, focused on the upper stretches of the river. It attempted to parse out the different roles of temperature, precipitation, and soil moisture on the variability of yearly water flows since reliable record-keeping began in 1906.
Annual Colorado River flows have naturally swung up and down over time, but the natural trends have been bucked in recent years and decades.
“What we’re seeing now is that, consistent with more of the global observations in terms of warming, that it’s not just a fluctuation that’s within that historical back and forth,” Prairie said. “That oscillation is starting to break from that range.”
Temperatures appear to have been playing a larger role in reducing the flows of water down the Colorado River since the late 1980s, the findings from the new study suggested.
“If you look at the trend in temperature over this period, we see a warming trend,” said Connie Woodhouse, a University of Arizona professor who led the new research. “We’re finding in those years temperatures explaining a lot more of the variability.”
Warmer temperatures cause more snow to fall instead as rain, and they cause snowpacks to melt earlier. Both of those effects lengthen growing seasons of riverside vegetation, which allows it to suck up more water as it grows. Higher temperatures also increase evaporation.
The likely effects of climate change on rainfall, snow, and streamflow in the West remain difficult to assess, though they’re expected to lead to more rain and less snow, reducing the water volume of the snowpack that melts slowly to fill up the rivers. Storms may also shift southward.
Recent assessments suggest that even without changes to precipitation, the flow through “most of the Colorado River” would “shift to moderately lower” levels as the planet continues to warm, said Andy Wood, a National Center for Atmospheric Research scientist who was not involved with the study.