The researchers say “It’s likely that these results translate to other high mountain chains”. Above 4,500 metres dust is found to be ahead of other forms of pollution.
– – –
Dust blowing onto high mountains in the western Himalayas is a bigger factor than previously thought in hastening the melting of snow there, researchers show in a study published Oct. 5 in Nature Climate Change.
That’s because dust—lots of it in the Himalayas—absorbs sunlight, heating the snow that surrounds it, reports Phys.org.
“It turns out that dust blowing hundreds of miles from parts of Africa and Asia and landing at very high elevations has a broad impact on the snow cycle in a region that is home to one of the largest masses of snow and ice on Earth,” said Yun Qian, atmospheric scientist at the U.S. Department of Energy’s Pacific Northwest National Laboratory.
Qian and Chandan Sarangi, formerly a postdoctoral associate at PNNL and now at the Indian Institute of Technology Madras in India, are corresponding authors of the study.
More than 700 million people in southeast Asia, as well as parts of China and India, depend on melting snow in the Himalayas for much of their freshwater needs in summer and early fall, driving the urgency of scientists ferreting out the factors that influence earlier snowmelt in the region.
In a study funded by NASA, scientists analyzed some of the most detailed satellite images ever taken of the Himalayas to measure aerosols, elevation, and surface characteristics such as the presence of dust or pollution on snow.
Of dust, soot, sun and snow: The albedo effect
Dark objects on or in snow absorb sunlight more effectively than pure white snow, whose reflectivity fends off sunlight so forcefully that snow can be blinding on a bright, sunny day. But snow near an object that absorbs sunlight—like snow on a dark-colored car where some of the roof is exposed—heats up and melts faster than pristine snow.
Scientists use the word “albedo” to discuss how well a surface reflects sunlight. Dirty snow has a low albedo, while pure snow has a high albedo. Dust and soot lower snow’s albedo, causing the snow to absorb more light, heating up and melting snow faster.
The albedo effect at high elevations is crucial to life for millions of people who rely on snowmelt for their drinking water. Darker, dirtier snow melts faster than pure snow, changing the timing and amount of snowmelt and affecting agriculture and other aspects of life.
The powerful effect of dirty snow
The team found that dust plays a much larger role melting snow than soot and other forms of pollution, known as black carbon, at elevations above 4,500 meters. Below that, black carbon dominates.
It’s a surprise for scientists, who note that far more studies have explored the role of black carbon than dust in snowmelt.
The dust blows into the western Himalayas from the west—from the Thar Desert in northwestern India, from Saudi Arabia and even from the Sahara in Africa. The dust comes in winds thousands of feet high, at what scientists call elevated aerosol layers.
While desert dust is natural, the scientists say that its prevalence in the Himalayas is not without human influence. Increasing temperatures have changed atmospheric circulation, affecting the winds that can carry dust hundreds or thousands of miles. Changing land-use patterns and increasing development have reduced vegetation, liberating dust that otherwise would have been tied to the land.
Qian was one of the first scientists to develop sophisticated modeling tools to analyze how impurities like dust and soot affect the rate at which snow melts. He did that early work more than a decade ago in the mountains of the U.S. West.
“It’s likely that these results translate to other high mountain chains, including the Rockies, Sierras and Cascades in North America and several mountain chains in Asia, such as the Caucuses and Urals,” Qian said.
Full report here.
via Tallbloke’s Talkshop
October 5, 2020 at 11:39AM