After a couple of hours of climbing on wide backcountry skis, the scientists and their assistants reach the highest point in the basin, a 13,500-foot ridge marked by a cairn. Behind lies the rugged bulk of the San Juans, and far below their ski tips is the arrow-straight main street of Telluride. Beyond are the deserts of the Colorado Plateau, spiked with the Abajos, the La Sals, and the hazy, distant outline of the Henry Mountains.

"Look at all the damn dust!" says Painter, gesturing at the slopes. To a casual observer, the snow looks white, but on closer inspection, it is covered with streaks of red and pink dust. The next question: Where is all the dust coming from?

From the top of Senator Beck Basin, we can’t see Jayne Belnap, but she is less than 100 miles away on the dusty horizon, studying how soil moves around the world. Belnap, a researcher with the U.S. Geological Survey, has spent most of her career investigating the soils of the Utah deserts (HCN, 1/19/04: Getting under the desert’s skin), and she has gathered several years of data on dust.

On desert grasslands that have never seen grazing, "there’s barely any dust production, no matter what"; the dust traps she posts in those areas collect perhaps a tablespoon every six months. Most years, traps in formerly grazed grasslands collect about twice as much, and currently grazed lands collect even more, about nine times as much. But the most dramatic differences, says Belnap, emerge during severe drought years. While the ungrazed grasslands stay more or less the same, formerly grazed ground produces as much as 20 times the amount of dust as in wetter years. Currently grazed lands "just go bonkers," with the dust traps sometimes filling faster than Belnap and her coworkers can empty them.

Grazing, development, off-road vehicle use, and military training activity form what Belnap calls a "background signal" of dust in the Southwestern deserts.That signal seems to be strengthening. Jason Neff, a researcher at the University of Colorado at Boulder who works with both Belnap and Painter, studies historic dust in sediment cores from lakes in the Senator Beck Basin. He says most dust in the San Juans comes from northern Arizona, and that "there’s a hint that things have changed in the past 100 to 150 years," with dust deposits appearing to have increased significantly.

One prime suspect is heavy grazing on the Navajo Reservation in northern Arizona and New Mexico, though road-building and other construction are clearly on the rise. "Whether it’s cows, people or vehicles, we don’t know, and we may never know," says Neff. "But over the last 150 years, so much of the landscape has been used. It just makes sense that there would be more dust."

The researchers hypothesize that if dust events do continue to increase, mountain snow will melt earlier in the spring, and the summer droughts that may ensue could lead to — you guessed it — more dust, further eroding the mountains’ ability to store water. "I hate to use the word catastrophe, but that’s probably the right word," says Belnap.

Throw climate change into the mix, and the forecast gets even more grimly interesting. So far, the effects of global warming on the higher, colder Rockies are not as marked as those in lower coastal ranges, where even a small rise in winter temperatures can turn snow into rain (HCN, 3/6/06: Save our snow). But if dust keeps crowding into the mountains, says Painter, it could amplify the effects of warmer temperatures, boding ill for high-elevation snow in the Rockies and elsewhere. "If you put dust and warming together — watch out," says Painter.

The effects of dust aren’t limited to the San Juans, or the Colorado Plateau. Many major mountain ranges, in the West and beyond, are downwind from deserts, and they collect dust just as the San Juans do. And dust, of course, knows few boundaries. Every so often, Westerners are showered with tiny particles from the Gobi Desert, or from the Takla Makan Desert of far western China.

It’s enough to make Painter think big. On the downhill slide to Red Mountain Pass, after a long day of discussions and data collection on the sun-crusted snow, the researcher pauses again to look at the slopes’ streaks of dust, which appeared earlier than usual this year. He shakes his head at their bold appearance, and their global implications. "The next thing we need to do," he says with a grin, "is send a grad student to the Hindu Kush."

Michelle Nijhuis is HCN’s contributing editor. Her series on global warming in the West won the 2006 Walter Sullivan Award for Excellence in Science Journalism.