Warm spring temps shrank Rockies snowpack over last 30 years

Posted: May 21, 2013

Written by

Stephanie Paige Ogburn, ClimateWire
Rocky Mountains

When a Denver resident turns on the sprinkler in midsummer, it applies last winter's Rocky Mountain snowfall to the lawn. In the northern Rockies, that snowmelt fills the Columbia River, whose hydroelectric dams provide power to Seattle residents. Meanwhile, the meltwater flowing into the Colorado River irrigates farms across the West.

But since 1980, snowpack in the Rocky Mountains has steady declined in spring, right when the winter's bounty of snow is needed to flow into the reservoirs that provide Westerners with the precious water that sustains their society.

Scientists have been working to tease out why this is. And in a paper published online Sunday in the journal Geophysical Research Letters, three U.S. Geological Survey scientists definitively state that the culprit is spring warmth.

There were four main options for what was causing the dearth. It could have been a decline in winter precipitation, a decline in spring precipitation, a warming of winter temperatures or a warming of spring temperatures, said Julio Betancourt, a senior scientist with USGS who co-authored the study.

This is interesting because typically precipitation in the Rockies operates in a sort of seesaw pattern, because of El Niño and La Niña.

If it's a high precipitation year in the northern Rockies, the southern Rockies have a low year, and vice versa, said Gregory Pederson, a USGS research ecologist and lead author on the study.

In an earlier study published in Science, Pederson and Betancourt used tree ring data to show this was the pattern for essentially the last 800 years, until the 1980s, when both the northern and southern Rockies showed a consistent downturn in their spring snowpack.

'Breakdown' in 800-year-old pattern

"So we've been really interested in the causes of the recent breakdown," Pederson said.

Together with another USGS scientist, Gregory McCabe, they created a model to "disentangle," as Betancourt put it, the causes behind the decrease of the last 30 years.

The model shows that warmer temperatures are overriding the typical precipitation patterns of the Rockies, so the usual differences of the northern mountains being wetter when the southern ranges are drier, or the other way around, are not showing up in recent decades.

"The increase in temperature seems to be kind of dominating what we see as a Westwide decline in snow," McCabe said.

While a warming planet is likely part of the reason for this, the authors said it is difficult to tease out what portion of the trend is warming and what portion is natural variability.

Betancourt noted that in some cases, climate change will likely amplify natural climate signals, and it may dampen them in other cases.

In the paper, the authors wrote that from 20 to 50 percent of the spring snowpack decline since the 1980s may be due to natural variability.

Betancourt said since the Rockies are such a big area, it is still hard to tease out how climate change may be affecting snowpack regionally.

"I think we are learning quite a bit about this now, but if people were to ask you, well, how much of this is anthropogenic and how much of this is natural, you have to take into account the regional complexities, and the answer may be different for one region than another," he said.

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