Painter says last year's levels were five to 20 times higher than other years measured -- unusually high even in the longer term, based on his conversations with locals. He and his colleagues are tracing the spike back to an overall lack of spring "greenup" in northeastern Arizona during 2009, which they attribute to poorly timed rains. With only sparse grasses holding down soil and most of the biological crusts long gone, strong winds blowing from Nevada carried tons of dirt off in clouds bound for Utah and Colorado.

The situation in northeastern Arizona has become dire, with sand dunes mobilizing across the Navajo Reservation and dried-up mud flats south of I-40 spewing dust, says USGS scientist Margaret Hiza Redsteer, based in Flagstaff. "Last year was a particularly dry year after many, many dry years during this current drought," she says. Unusually strong winds added to the problem, she notes. "We're also having warmer temperatures than we've had in the past, and that increases aridity." Many climate models indicate that the increased aridity of recent years will become the future norm as the air continues to warm and raise evaporation rates.

Urban dust -- concentrated around cities and drifting into mountains along with desert dust -- consists not only of soil particles but also pollution from fireplaces and car exhaust, including the small particles that are particularly dangerous to human health. Tens of thousands of Americans die prematurely every year from breathing in small particles, Reynolds notes, because the dust weakens the lungs, which in turn stresses the heart.

The dust also makes snow more vulnerable to an early meltdown. Soot -- produced by burning wood, coal and diesel fuel -- and soil particles are the worst offenders, heating up the air while they remain afloat. When they come to rest on snow, their dark colors cause the snow to heat much faster.

The early melting of snow, in turn, has been linked to an increase in the number of big Western wildfires since the mid-1980s. It means problems with the system of reservoirs dotting the West and less available water throughout the Colorado River Basin.

Dust has other, more subtle, effects. Scientists are finding that it can change rainfall patterns much the way global warming itself does. In a double whammy for Southwesterners, particulate pollution can increase the likelihood of both more extreme drought and more severe storms.

"It turns out that if you put too many particles in a cloud, it can actually prevent rainfall," says Meinrat Andreae, an atmospheric scientist and air pollution expert with the Max Planck Institute in Germany. The small particles from car exhaust and coal-powered electrical plants can shape water vapor into droplets too tiny to overpower the wind currents keeping them afloat. Sometimes the drops evaporate without ever reaching the ground.

On the other hand, dust and other mineral particles can seed ice crystals in clouds. When those crystals crash into each other, they create the electrical charges that power lightning bolts. Ice crystals also make storms more intense. Rain falls in heavier drops, sometimes even as head-pummeling hailstones the size of baseballs. These intense storms cause more flooding and erosion, yet less long-term water storage, than the drizzles from clouds without ice crystals.

Particles darken the sky in China and India, where people in heavily populated areas burn coal, dung and firewood for cooking and heating. Satellite images suggest that the air over cities holds 20 to 70 times as many cloud-seeding particles as the air over pristine regions, with Asian cities at the higher end. And these images and chemical fingerprinting show Gobi Desert dust and Shanghai smog sometimes mingle with the local particles darkening California and Colorado.

Rangwala, who grew up near Delhi in central India, says the effects of pollution and local land disturbance on the region's rainfall patterns helped inspire him to become a climatologist. He currently works for the Western Water Assessment, a National Oceanic and Atmospheric Administration program based in Boulder, Colo.

His research in the San Juans indicates that dust may be heightening the temperature rise documented in his analysis of dozens of weather and snow stations. The early melting of dusted snow leaves the landscape with less moisture to temper the sun's rays.

Things are even more extreme in the area where he grew up. His hometown of Udaipur, known as the City of Lakes and Palaces, has become a city of empty promises. The many small reservoirs dotting the landscape have failed to fill during most of the past 20 years, Rangwala says. Like Phoenix, Udaipur sits at about 1,000 feet in elevation, ringed by mountains. The area has even begun to resemble the U.S. Southwest, with the cacti that once lurked in the background becoming increasingly prominent. Decades of deforestation of the native teak trees have contributed to the problem, boosting the loads of dust that warm the air and intensify rainfall.

Like the monsoons of the American Southwest, India's summer rains are often accompanied by thunder, lightning and intense downpours. Rangwala says his friends and colleagues back home are concerned because most of their region's rainfall is now coming almost exclusively as intense storms. "When I go back to India, I hear that people have not seen drizzle for a long time," he says. "What we get are these intense events. We don't get enough, and what we get comes in big chunks. The landscape is becoming more like Tucson now."

Melanie Lenart is an environmental scientist and writer whose previous freelance publications include Nature Reports: Climate Change and Landscape Architecture Magazine. Her work on climate issues includes a 2010 book, Life in the Hothouse: How a Living Planet Survives Climate Change.

This story was funded by a grant from the David and Lucile Packard Foundation.