Global warming could mean that mountain snow melts at a slower pace, researchers said Monday, a peculiar finding that might be bad news for the West and other regions that depend on snow for water.
Scientists have long known the annual snowmelt is starting sooner as the climate warms. New research by the National Center for Atmospheric Research in Boulder, Colorado, found that when the snow begins to melt earlier in the season, it dissipates more slowly than it does in late spring or summer, in part because the angle of the early year sun is lower so its rays are less intense.
The sun's intensity is particularly important because the energy in direct sunlight is the biggest driver of snowmelt, said Keith Musselman, the lead researcher.
Another factor that slows the snowmelt in the early season is the fact that nights are still cooler.
The conclusions, published in the journal Nature Climate Change, could help explain why computer models show rivers fed by mountain snow are expected to run lower in a warming climate, even if rainfall increases as snowfall decreases, said Musselman, a post-doctoral fellow at the Boulder research centre.
The new study didn't examine the broader consequences of the findings, but Musselman said some slow-melting snow that lingers in the mountain soil could be taken up by trees and plants or evaporate into the atmosphere instead of flowing into rivers and reservoirs, where people can use it.
Changing the timing and pace of snowmelt could also have implications for the annual surge of spring runoff that helps keep rivers healthy, and for floods and flood control.
"We think the impacts could be far-reaching," Musselman said.
Mountain snow is a vital water source around the globe. NASA, which is conducting an unrelated study on how to measure snow from space, says one-sixth of the world's population gets most of its fresh water from snow.
The snow that falls in Colorado's mountains alone feeds four rivers that provide water to a dozen states and Mexico. One of those rivers, the Colorado, supplies water to about 40 million people and 6,300 square miles of farmland.
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