Black carbon from dust storms accelerates Himalayan glacier melt

Regional pollution is speeding up snow melt in the Indian Himalayas. That’s according to a new study from an international group of scientists including Indian Institute of Technology Madras civil engineering Ph.D. student Amit Singh Chandel and Karl Rittger, research associate at the Institute of Arctic and Alpine Research (INSTAAR) at the University of Colorado Boulder.

The study, published in the Journal of Geophysical Research: Atmospheres, reveals how dust storms pick up black carbon from heavily-polluted areas and deposit it in the mountains. Black carbon darkens the ice and snow, causing it to melt faster.

It’s a key insight into a persistent problem. Dwindling ice in the Himalayas has long troubled researchers and decision makers in southern Asia. Nearly two billion people rely on the water produced by Himalayan snowmelt, and glacier loss could lead to water shortages and an increase in extreme floods.

“Three of the four largest river basins in the world lie in this region,” co-author Chandan Sarangi said. “Understanding this snowmelt is a key question.”

The new study is the result of collaboration across scientific disciplines and international borders. The seeds were planted a few years ago. Sarangi’s lab had already published numerous mathematical models mapping perennial dust storms in Northern India. And they knew that black carbon was getting into the dust somewhere along the way.

In order to paint a clearer picture, Sarangi looked to pair his modeling work with long-term measurements from the field. That’s how he found co-author Rakesh Hooda, a senior research scientist at the Finnish Meteorological Institute. Hooda previously collected nine years of atmospheric measurements at a site in the Himalayan foothills.

Hooda’s data, paired with Sarangi’s models, revealed two distinct types of storms. The first type brews in the deserts of western India and Pakistan and travels east at low elevations. These storms mix with pollution from the heavily-populated Indo-Gangetic Plains before reaching the Himalayas.

The second type of storm starts in the Saharan Desert and travels at higher altitudes. These storms pick up very little pollution.

The new model told the researchers where the dust storms were coming from and what was in them. Now the question was how the storms impact snow and glaciers.

That’s where Rittger came in. In previous research, he had developed a model that could tease apart contributing factors to snow-darkening using satellite imagery. The team applied Rittger’s methodology to the dust storm models and once again validated the findings with on-the ground data.

The results were clear. The storms with high concentrations of pollutants melted more snow.

“You get a darkening of 26% versus 58%.” Rittger said. “The polluted dust events have double the impact.”

Previous research on diminishing glaciers in the Himalayas has often focused on global warming, but the new findings reveal a second, more local cause. To Rittger, it’s a good reminder to look for regional environmental problems that might mimic or coexist with global ones.

“It’s still a human-induced impact, it’s just not a climate change impact,” he said.

The success of this study has all of the co-authors thinking about next steps. Rittger hopes to one day expand his Snow Today website, which provides automated readouts of snow characteristics in the Western US, to Asia. Sarangi hopes to establish more field sites.

“We need more measurements to characterize the aerosols reaching the Himalayas.” He said. “It’s increasing every day.”