SEATTLE (AP) — Soot on snow causes winter snowpacks in the Cascades and other Western mountain ranges to melt faster, leading to runoffs earlier in the spring, scientists say.

Researchers at the Pacific Northwest National Laboratory in Richland, who studied the effect of pollution on snowpack, found soot can melt as much as a couple inches of snow in some areas of the western U.S.

“These changes can affect the water supply, as well as aggravate winter flooding and summer droughts,” said Yun Qian, an atmospheric scientist with the laboratory and author of the computer modeling study. The lab is part of the federal Department of Energy.

Soot from diesel trucks, smoke stacks and other fossil fuels lands on snow and darkens it. The dirty snow, compared with pristine white snow, reflects less sunlight and soaks up more heat, melting snowpacks as much as a month early in the spring in the West.

The timing of the snowmelt has the biggest implication for water resource managers, farmers and others who depend on snowpack for water, Qian said.

“You may have more runoff in the winter time based on our figures and less in the spring and summer,” he said. Runoff is most needed in spring and summer for farms, cities and fish.

Results from computer modeling show that the soot warms up the snow and the air above it by as much as 1.2 degrees Fahrenheit.

Soot landing on snow begins a cycle that melts snow and exposes more bare ground to snow, leading to more snowmelt.

“Compared to the temperature changes, this plays a much smaller role (on snowmelt) but it is still an important role,” said William Gustafson, an atmospheric scientist and the study’s co-author. Previous studies have looked at the effect of dirty snow on climate, but this is the first to look at it on a regional scale.

The study will be published in the Journal of Geophysical Research-Atmospheres.

Phil Mote, Washington state’s climatologist, questioned how big an effect soot on snow could have on increasing snowmelt in the Northwest.

The role of soot isn’t well known and so its impact on the observed changes in snowpack and streamflow over time “cannot be quantitatively determined,” said Mote, who is also a research scientist at the Climate Impacts Group at the University of Washington.

“There are too many unknowns and their comparison is designed to give a big answer,” he added.

Qian agreed that the computer modeling study simplifies reality but it “can help us understand the mechanism and the cause.

“This is just a first step,” Qian added.