What's eating the snowpack?

 

The water gods created haves and have-nots this year, and nowhere more dramatically than in Colorado. In March, after another dry winter, the whole state was biting its nails. Then: Snowpacalypse! An unusually stormy April built up the snowpack in most of northern Colorado to just about average. In the southern part of the state, however, snowpack in the Rio Grande, Dolores, Animas, San Miguel and San Juan basins sat just above 40 percent of average at the start of May.

Water inequities aren’t new to the West. Between the Northern and Southern Rockies, disparities even used to be easy to predict. If northwest Wyoming, western Montana and northern Idaho had a banner snow year, southern Wyoming, western Colorado and northern New Mexico usually suffered, and vice versa. But since the 1980s, the pattern has shifted. Snowpacks have simultaneously declined everywhere. That’s extremely unusual, says Greg Pederson, a U.S. Geological Survey scientist who has studied the phenomenon. “Tree ring records suggest that it’s only happened a few times in 800 years.”

Snowy mountains in Rocky Mountain National Park

So what changed? Was less snow falling from the sky? Or was the mercury rising too high to keep it on the ground? Pederson and two colleagues aimed to find out. Just recently, in the journal Geophysical Research Letters, they fingered their culprit: warmer temperatures.

You may be thinking, "Well, duh." And it’s true: This isn’t the most shocking conclusion science has ever reached. But it is still an important one. Before this study, Pederson says, whether temperature or snowfall was driving the declines was still a matter of some contention. During the Dust Bowl, another anomalous time when the Rockies were significantly water-short top to bottom, temperatures were cool enough to build snowpacks, but the snow never materialized. But almost every other time, it’s happened during periods of unusual warmth.

The impact in the last 30 years has been particularly acute at low to mid elevations, where the study found snowcover has declined by about 20 percent since the 1980s. “In February and March, especially, when you warm it up, it really melts out the snowpack,” Pederson says. “Even if you get substantial precipitation, it’s an undercutting effect. Lots of regions remain cold enough to snow, but after the storm passes, we’re getting much warmer air. So you get three feet dumped, and then it gets reduced to six inches. Our model shows that to be one of the major processes – that snowpack gets undercut and doesn’t accumulate or persist as long.”

Remember that while there’s still uncertainty about how climate change will effect precipitation in the West, scientists are extremely confident that temperatures will continue their upward trajectory. I blogged last fall about a climate researcher named Park Williams in New Mexico, who has been studying tree mortality, and similarly trying to tease apart the influence of temperature versus precipitation. Williams, too, found that higher temperatures would be enough to stress some Western forests to death in the coming century, whether or not the region becomes more arid.

That, more or less, is the conclusion of Pederson's work on snowpack, too. So if our water supplies are already being reduced by warmer temps, what does that bode for the future? I asked Pederson if any of the conclusions of his recent research surprised him. “Probably the recent magnitude of warming and of snowcover declines in low to mid elevations,” he said. “Warming in February to March across the West has had a pretty profound impact. We’re ostensibly at the start of this long-term trend. So if this is just the tip of the iceberg, we’re standing at the edge of some very large changes to our water supplies and mountain ecosystems.”

Cally Carswell is HCN's assistant editor. She tweets @callycarswell.

Photo courtesy Flickr user Nature's Images, licensed under Creative Commons.

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