A new study has found that tiny, agile dust particles deposited at high altitudes in the Himalayas are having a greater effect on snow-darkening than previously thought. Black carbon, an industrial pollutant, was generally believed to have a larger darkening effect than dust, but this study overturns that belief. The team of scientists, led by Chandan Sarangi of the Indian Institute of Technology Madras, collaborated across three other institutions in the United States. They gathered data from satellites and ran a sophisticated climate model to confirm that natural dust carried in winds is having a dominating darkening effect amongst the highest snowy peaks of High Mountain Asia, which includes the Tibetan Plateau and surrounding mountain ranges.
Dust is considered a natural air pollutant. It is lifted into the atmosphere by winds blowing over deserts and other arid areas, and can be carried long distances. Dust originating in Saudi Arabia and Pakistan travels thousands of kilometers to be deposited in the Himalayas. Dust is transported within elevated aerosol layers at 2,000-5,000m altitude in the spring and summer months. These layers are distinguishable bands within the atmosphere that sit high above the surface layer of air that usually carries aerosols such as smoke and soot. One elevated aerosol layer is present across northwest India, carried the prevailing westerly winds, and deposits dust above 3,000m when it reaches the tips of the western Himalaya.
Dust is not to be confused with black carbon, which is generated by human activities. Black carbon is also present within High Mountain Asia, emitted by burning materials such as crop stubble and worn tires on the agricultural Indus and Gangetic plains. However, black carbon is carried within the surface layers of the atmosphere and is therefore deposited at lower elevations, between 0 and 3,000m altitude.
The main finding of the study was that dust-induced snow darkening had a greater effect above 4,000m altitude than that of black carbon, which has long been seen as the dominant threat. As temperatures rise, snow at lower elevations disappears, but dust’s role remains with high elevation snow. “With global warming, the snow line rises and the snowpack in low-elevations disappears. Black carbon’s role decreases, but dust’s role remains,” lead author Sarangi told GlacierHub. The contribution of dust to snowmelt will therefore likely increase in the future. “We should be worried about the effects of dust because it is hard to limit the dust that ends up in the Himalayan snowpack,” Sarangi said.
Both black carbon and natural dust have an effect on albedo — the reflectance of solar radiation from a surface. Snow, being shiny and white, has a high albedo. It reflects a large proportion of radiation from its surface and thereby keeps cool. But surfaces covered in black carbon and dust, being dark-coloured, absorb more solar radiation, warming the surface and causing melting.
“This was the first time we quantified the role of long-distance transported dust in snow melting in the Himalayas,” Sarangi told GlacierHub. His team’s complex, high-resolution model is able to simulate the distribution of dust transport and deposition to High Mountain Asia.
Sarangi suggests that better land and water usage in Pakistan and western India can limit the amount of dust suspended into the atmosphere. Some forms of agriculture leave the surfaces of fields bare, allowing for dust production during windy periods. Lekhendra Tripathee, research associate professor at the Northwest Institute of Eco-Environment and Resources, of the Chinese Academy of Sciences in Lanzhou, explained to GlacierHub that enhancing vegetation cover in source regions may be a method of land management that could reduce the production of dust.
Despite the new results of Sarangi’s paper, there are still uncertainties that remain. Tripathee told GlacierHub that “model results need to be validated and supported by continuous observations and analysis of snow samples from the region.” He continued, saying that a current lack of measurements are limiting models’ ability to draw robust conclusions. “More studies are needed in the future to understand the respective roles of these impurities [dust and black carbon],” said Tripathee, concluding that “policymakers and scientists must sit together to prepare strict regulations to reduce the sources of these impurities to overcome future damages.”
Accelerated snowmelt may have significant implications for the large human populations downstream of glacier-fed rivers in the coming years. Dust deposition in spring can induce an early influx of snowmelt water in Himalayan rivers and result in a reduction of the peak flow in summer. “This timing shift in water availability due to dust-induced early snowmelt can cause water stress on the huge population dependent on these rivers in the driest summer months,” warned Sarangi. Pressure on water resources may worsen the water management challenges already facing High Mountain Asia communities. They might even exacerbate the resource-led political conflicts in this region, which is already marked by geopolitical tension.