It’s no surprise that as the planet warms, we’re losing snow. What is surprising is that this loss isn’t just a consequence of more greenhouse gases heating the atmosphere, but of more particulate pollution from fossil fuels. When tiny bits of black carbon fall on snow, they darken it. The snowpack absorbs more of the sun’s energy, warms, and melts faster.
New modeling suggests that by burning less fossil fuels, the ailing snowpack will get a two-for-one benefit: lower temperatures on the snow’s surface and in the surrounding air. “You will start seeing a reduction of these tiny particles in the air, and they would have pretty immediate effect on the snowpack,” says Pacific Northwest National Laboratory climate scientist Ruby Leung, coauthor of a recent paper describing the modeling in Nature Communications. “We expect the air to be cleaner, and therefore the snow to be whiter and cleaner.”
Cleaner snow is less melty. Freshly fallen snow is one of the brightest natural surfaces on Earth, reflecting 90 percent of sunlight. “Black carbon deposition on snow is essentially making it dirty,” says climate scientist Lawrence Mudryk, who studies snow at Environment and Climate Change Canada but wasn’t involved in the new paper. “And that increases the amount of snow melt that occurs, just because darker surfaces absorb more light and get warmer faster.” (Think about how hot you feel outside when wearing a black shirt, which absorbs the sun’s energy, versus wearing a white shirt, which reflects it.)
That melting is of particular concern for the 2 billion humans who rely on the world’s snowpack for a steady source of water. Unlike rain, which immediately flows into reservoirs, a snowpack slowly releases water as winter rolls into spring and summer. This tends to provide more water than periodic rainfall, a lot of which is lost when it soaks into the ground. (Unless you’re purposefully recharging an aquifer with stormwater to tap into later for drinking.)
“People don't necessarily know where their water comes from, because they're downstream of where the snow and ice accumulates and then melts,” says snow hydrologist S. McKenzie Skiles, who studies the impact of pollutants at the University of Utah but wasn’t involved in the new paper. “In the western US, up to 80 percent of water resources can come from snow melt, depending on how close you are to the mountains.”
Globally, climate change means warmer air and less snowfall—between 1955 and 2020, spring snowpack declined by 20 percent across the American West. With less snow and ice, those areas warm more, and faster. “Snow cover is melting out by days to weeks earlier, due to climate change,” says Skiles. “There's sort of a double whammy here: Snow is getting darker, and that's absorbing more sunlight. But then it's also melting out earlier and exposing darker ground cover underneath, and that absorbs a lot more sunlight.”
Warmer temperatures also mean that more precipitation falls as rain instead of snow. If it all falls fast, it can lead to catastrophic floods, like the ones that hit the Himalayas in August.
To do their new modeling, Leung’s group laid out two scenarios focused on the Tibetan plateau. The first was business as usual; it assumed some improvement in energy technology, but not a drastic change in the use of fossil fuels. Making only a modest tech improvement would lead to the loss of nearly 60 percent of the snowpack in the Tibetan plateau by the end of the century. But the slight reduction in particulates that would stem from this tech upgrade would help a bit: It would ameliorate the loss by 8 percent, so overall the snowpack would shrink by about 55 percent.
The second scenario assumed a future with more clean energy technology, and so less warming than in the first scenario. This emissions reduction would still lead to a loss of 15 percent of the Tibetan plateau’s snowpack by the end of the century. (The unfortunate reality of climate change is that much of the pain is locked in: Even if we were to stop emissions tomorrow, carbon dioxide will stick around the atmosphere for centuries.) But factoring in the corresponding reduction in pollution, by the year 2100 that snowpack loss would be only around 8 percent.
One bit of hope, says Leung, is that “when we start reducing the burning of fossil fuels, the tiny particles in the air don't last that long.” The effect on snow quality can be almost immediate. For example, in 2021, Skiles published a paper that showed how air pollution fell during the 2020 Covid lockdowns and also reduced pollution on snow and ice in the Indus River Basin.
But there’s a complicating factor that humanity can’t always control: dust. As the planet warms, drier areas shed more dust, which can blow great distances in the wind and land on snow. Saharan dust regularly coats the ski resorts of Europe and floats clear across the Atlantic, settling in the Americas. That material can have the same effect as fossil fuel particulates, darkening and heating snow to hasten its melting.
However, people do have more control over the dust stirred up by land use. When humans cut down forests like the Amazon and turn them into agricultural land, that disturbs soil that would normally be held down by tree roots. Protecting forests, then, will have all sorts of benefits: sequestering carbon in trees, protecting biodiversity, and keeping dust from lofting into the atmosphere.
Bigger and more intense wildfires, too, are a growing problem for the snowpack. Smoke particles have the same effect as fossil fuel pollutants. In places with permanent snow cover, summer wildfires can taint the snowpack. Ancient glaciers, too, are getting dusted with wildfire smoke, accelerating their decline. “The Canadian wildfires are a great example of that from this past summer, where Canadian glaciers saw rapid decline in cover,” says Skiles. “People still need to study it, but it may be due in part to deposition from those Canadian wildfires. Here in the western US, it's the same thing. We have rapidly declining glaciers, where deposition from forest fires is landing on those glaciers and contributing to darkening.”
The wildfire season is also getting longer in the American West: In 2021, for instance, a massive fire broke out in late December in Boulder, Colorado, destroying more than 1,000 buildings. “Increasingly, fires are overlapping with snow season,” says Skiles.
That’s bad news, because it means that even as humanity cuts fossil fuel use, the snowpack is still in danger from these other processes. “Both dust emission and fires are becoming more intense in the future, and that could unfortunately offset any gains from reductions in black carbon emissions,” Skiles says. These changes are still worth making—but humanity will also have to get better at managing its increasingly sparse and erratic water supply. “We're just going to have to come to terms with the fact that we're going to have to use the water that's available now more efficiently,” Skiles says.
