Dust takes a toll

Imtiaz Rangwala studies climate change on the Tibetan Plateau and, more recently, in the American West. In mid-May, the climatologist flew into Durango, Colo., through skies darkened by dust. The next day, the dust had settled on the San Juan Mountains, where Rangwala was checking on the snow. "As far as my eye could see -- up to 11- or 12,000 feet -- there was dust all around," he says.

Much as they did last year, billowing gusts of dust from the Arizona desert have blanketed southwestern Colorado. The dusty air that clouded visibility during Rangwala's flight forced the closure of a 22-mile stretch of I-40 from Winona to Winslow. In late December, similarly thick dust clouds along I-10 in the Phoenix area caused a 20-car pileup that killed three people.

Dust is nothing new in the Southwest. Sediment records show that settlers and their livestock started kicking up dust in the region 150 years ago. Despite a reduction in grazing, continued disturbance by grazing animals, off-road vehicles and plows -- exacerbated by drought -- is sending the soil flying again. The recent onslaught of dust in Southwestern skies affects human health and safety -- and also, as scientists are discovering, the region's climate.

In the San Juans, University of Colorado researcher Jason Neff and his colleagues are piecing together a 5,000-year-long history of Southwestern dust. By studying sediment cores from high-elevation lakes, they found that little dust moved around the region until cattle arrived in the West in the second half of the 19th century. By last century's end, the amount of dust landing in the San Juan Mountain sediments had increased fivefold.

Seeking to find out why so little dust moved around before cattle arrived, Neff measured soil dynamics at an ungrazed site, one that retains the biological crusts that once coated desert soils throughout the West. The mystery was solved. "You can blow 80-mile-an-hour winds across the crust environment," Neff says, "and no soil will move." These biological crusts -- communities of bacteria, algae, fungi and lichens that take decades to form -- quickly crumble and disappear under pressure from hooves, vehicles and farm machinery.

Many changes since the mid-1800s have broken the protective crusts. Utah's deer population -- currently managed for trophy hunting -- has expanded from about 15,000 to 430,000, says U.S. Geological Survey scientist Jayne Belnap, who tracks dust and its sources. The explosion in human population, with Arizona, Nevada and Utah among the fastest-growing states, means that a lot more vehicles are driving and four-wheeling across the desert. Farming that was once confined to small plots near local streams now covers massive fields. And soil on dry plowed lands is easily picked up by strong winds, as farmers found during the 1930s Dust Bowl drought.

Biological crusts quickly vanish under such disturbance. Yet these crusts, which once filled the spaces between desert plants, excel at holding down soil. Just how effective they are was shown in a nine-year study led by Belnap and USGS researcher Richard Reynolds that compared dust emissions from undisturbed versus grazed sites. "The most striking finding," Reynolds says, "was the different responses among the sites during the drought of 2002."

In years of average rainfall, the disturbed sites produced about three times more dust than the non-grazed sites. In drought years, the most disturbed site produced some 5,600 times more dust than a site that was never grazed. "It got to the point where we couldn't keep up with the dust collectors," even changing them every week, Belnap says. "Basically, what we've been finding is that desert surfaces are pretty stable unless they've been disturbed. It's D-cubed: dust, drought, disturbance."

In 2009, drought and land disturbance abetted by high winds combined to produce an unusually dusty year, according to five years of snow studies by scientists in the San Juans.

"In 2005 through 2008, we were measuring concentrations of dust in parts per million," says Thomas Painter of California's Jet Propulsion Laboratory, who led the San Juan studies. "In 2009, we had to retool our measurements to be measuring in parts per thousand."

• Wildlife