Study shows dust causes early snow melt

Bill Scanlon, Rocky Mountain News

Published June 26, 2007 at midnight

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Grandpa may have been right — there was more snow in his day, or at least it lasted longer.

A new study by Colorado scientists shows that dust stirred up by farming, grazing, mining and recreation in the Four Corners area blew onto the San Juan Mountains and caused the snow to melt 30 days earlier this spring.

The study has implications for mountains from the Front Range to the Himalayas, said Tom Painter, lead investigator for the study done by the University of Colorado's National Snow and Ice Data Center.

It could mean less water for farming, shorter rafting and skiing seasons, and less potential for hydroelectric energy. However, good management of reservoirs could lessen the negative consequences.

The wind-blown dust came from the Colorado Plateau, 200 miles southwest of the San Juan Mountains, which are in southwestern Colorado, said Painter, whose study appears in the current edition of Geophysical Research Letters.

"The amount of impact measured and modeled in this system stunned us," said Painter. "The fact that dust can reduce snow cover duration so much – a month earlier — transforms our understanding of mountain sensitivity to external forces."

The CU team collected snow at precise locations. They scraped the dust from some of the snow, but left the dust on other samples, then compared results.

Global warming also may be playing a part in creating the conditions that send dust to the mountains in the spring, say the CU researchers.

The white snow reflects the sun, sending a lot of its rays back into the atmosphere, said Stephanie Renfrow, of the CU center, who also worked on the paper. The darker dust takes in more of the sun's energy and causes temperatures to increase, melting snow faster.

Prior to large-scale human settlement in the American Southwest, the grasses on the deserts and plateaus held most of the dust in place, says Painter.

But after a century of breaking up the desert's crust, the dust blows freely in storms, he said.

Painter talked to the Rocky by telephone from the base of Mont Blanc in France, which he said has its own coating of dust.

This summer, so does the Front Range, the Sawatch Range, the Gore Range and the Elk Range, among other places in Colorado, he said.

It doesn't happen every year, he said. But while dust-covered mountains were a rarity 100 years ago, these days it happens every year when there is a drought in the desert and several sustained wind events, he said.

That's what happened in 2006 — eight significant wind events in the Four Corners area, versus three or four a year the previous few years, he said.

Most global warming models predict increased drying and warming in the American Southwest, Painter noted. If that's the case, the dust storms could get stronger and more frequent.

The key factor is how long the dust stays on the surface of the snow. If it collects early in the year, but then is covered by a lot of snow that's not accompanied by a big wind event, the snow won't melt very fast.

In 2006, the blizzards were enormous early on, but scant later in the winter, so that dust-covered snow was exposed to the spring sun.

"There are reports of this phenomena happening all over the place, wherever there are deserts fairly close to mountains," Barrett said.

scanlon@RockyMountainNews.com or 303-442-8729

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