Most of California's water comes from the snow stored in the Sierra Nevada each winter. In the spring, melting snow helps fill the state's reservoirs for the dry summer. (Read "When the Snows Fail" in National Geographic magazine.)

As the state's historic drought drags on, scientists are watching the Sierra snow with intense interest—and they're worrying that even tiny airborne particles of dust may have a big effect on water supplies.

Here's how: As California gets drier, it's getting dustier, and at least some of that dust is landing in the Sierra. Dusty snow, with its darker surface, absorbs more solar radiation than clean snow does, meaning it heats up faster and melts more quickly.

That earlier spring snowmelt could mean that spring runoff will happen when the reservoirs are still full from winter rains, sentencing the state to a longer, drier—and dustier—summer. Rising global temperatures are already speeding snowmelt, and dust can create a positive feedback loop that makes the problem worse.

It's Happening in the Rockies

Thomas Painter, a snow hydrologist with NASA's Jet Propulsion Laboratory in Pasadena, California, has already documented the dramatic effects of dust on snow in the Rocky Mountains. He and his colleagues have found that the increasingly severe spring dust storms from the Colorado Plateau are causing snow in the southern Rockies to melt as much as 50 days earlier than clean snow would have.

Dust has an even bigger effect than warming on melt rates: Raising the temperature by 4°C (7.2°F) accelerates the melt by only 18 days.

Painter estimates that the earlier snowmelt and longer summers have reduced average annual runoff in the Colorado River by more than 5 percent—no small matter for the seven states, including California, that use the river's already overtapped flow.

He and his colleagues are now studying the effects of dust on snow in the Sierra Nevada through the Airborne Snow Observatory, which uses remote-sensing technology to take detailed measurements of snowpack size and reflectivity. Dust isn't expected to have as powerful an effect in the Sierra as it does in the Rockies, partly because some of the dust in the Sierra comes from the light-colored soils of Central Valley farm fields, not the red-rock deserts of the Southwest.

But Painter and his team are already seeing more dust in the Sierra than they anticipated. "Even if the acceleration of melt is only half of what we've seen in the Rockies, that's a profound effect," he says.

But Dust Could Increase Snowfall Too

It's not only domestic dust that affects the Sierra snowpack—and the effect is not necessarily all bad. Kim Prather, an atmospheric chemist at the University of California, San Diego, researches the effects of airborne dust on precipitation. She and her team have found that dust from deserts in the Middle East and North Africa is being carried across Asia and high over the Pacific all the way to California.

Dust is one of several kinds of particles that can "seed" clouds, allowing water vapor to condense to form ice crystals or raindrops. In 2009, Prather studied two back-to-back storms that were fortuitously similar in every way—except that the second storm contained dust from across the Pacific. Because of all the long-distance dust, Prather found, the second storm dropped 40 percent more precipitation over California. "We refer to it as Mother Nature's perfect cloud-seeding system," she says.

Prather and her team are now using satellite data to study changes in the amount of dust transported into California over time. It's possible that recent droughts in North Africa and the Middle East, which have increased the amount of dust being blown into the air there, are leading to a rise in natural cloud seeding in the Sierra. That would mean that as California's water supplies are being depleted by local climate change, they're being boosted by climate change on the other side of the world.

But even if Sahara dust is increasing Sierra snow, it's unlikely to relieve California's drought, says Prather. One reason: Snow seeded with the dust may melt faster than snow seeded with other, lighter-colored particles such as sea salt or marine bacteria. The effect is probably smaller than that of local dust storms, which deposit dark layers right on top of the snowpack, but it could still accelerate the spring melt.