Seasonal snow and snow melt are sources of fresh drinking water for two billion people living in mountainous regions, but those critical resources are threatened in a changing climate. Now a new study finds cause for optimism in the Northern Hemisphere, depending on how much cleaner snow becomes with reduced carbon emissions.

Here’s why: Fossil fuels and wood create black carbon when they’re burned, as with vehicle emissions or forest clearing. Dust arises from both industrial emissions and the natural process of soil erosion. The air pollution contributes to a host of problems including a warming climate, but the particles also settle and become dirty snow. It behaves differently than clean snow, which reflects 80% to 90% of sunlight, because the dark particles absorb more sunlight and cause snow to warm and melt faster.

“Snow is not just snow,” says Dalei Hao of Pacific Northwest National Laboratory (PNNL) in the United States, first author of the study published this month in the journal Nature Communications. “There’s clean snow and there’s dirty snow, and how they respond to sunlight is very different.”

The researchers studied trends from 1995 through 2014, primarily in the Tibetan Plateau in Asia and the western United States where rapidly rising temperatures and dirty snow led to snow melt. They then modeled trends from 2015 through 2100 under two scenarios, one where carbon dioxide emissions rise and one where they decline.

In some scenarios, the researchers predict snowpack loss will be less than half of what has been feared. That would be welcome news to people at risk of flooding when snowpack melts too quickly, and who rely on snow melt for drinking water and agriculture. It also means tourism and winter sports may be less affected.

“There have been a lot of alarming projections about the future snowpack. It’s a critically important issue,” said PNNL scientist Ruby Leung, also a corresponding author of the study. “The Himalayas, for instance, are the headwaters for several major rivers in southeast and eastern Asia. The condition of the snowpack in mountains has a direct effect on the quality of life for millions of people.”

Results showed that with cleaner snow and reduced emissions, snowpack loss of about 15% is cut in half by fewer black carbon particles. If emissions rise, and snowpack loss reaches 58%, the cleaner snow from less pollution still reduces the loss but only by about 8%.

To be clear, the black carbon particles aren’t the only reason for a melting snowpack. A warming climate is an obvious cause of snow melt, but also of increased rain that might otherwise have been snow if it were colder. What’s likely is that the warming temperatures and cleaner snow will coincide as the climate changes.

“Warming temperatures and cleaner snow are competing effects,” said Leung. “Our paper indicates that the warming effect is dominant, but that cleaner snow will cancel out some of the effect. We are not saying that snow will increase in the future. We’re saying that snow will not decrease in the future as much as it otherwise might.”

In a separate study, Hao and colleagues found that the shape of snow grains also changes the amount of sunlight they can absorb. Real snow, packed on the ground with its many tiny odd shapes, should melt more slowly than many models suggest because they assume spherical shapes for flakes of snow.

Other factors in snowpack loss include increasing wildfire activity, and changes to bacteria, lichens, algae and other organisms that form the biological soil crust. Increased dust is expected with loss to this layer.

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