The sand of the deserts accelerates the thaw of the highest mountains

Image taken by a NASA satellite showing the dust column over India heading towards the Himalayas.

It was known that climate change, with its global warming, is melting the glaciers of the great Asian mountain ranges.

Also, that the soot of the Industrial Revolution had reached the Himalayas, the Karakorum or the Hindu Kush (HKH).

And it was known that the smoke from the cars of the populous Chinese, Indian or Pakistani cities reached so high.

What was not known, at least its real dimension, is that the sand of the deserts also does it and in enormous quantities.

A study now shows how Arabian and even Saharan dust darkens the snow on the highest mountains on the planet, facilitating its thaw.

Much of the beaches in the Caribbean or the land on which the Amazon rainforest grows comes from the African deserts.

Every so often, the news shows images of huge clouds of dust, the haze, crossing the Atlantic to the west.

But in spring, in these latitudes, the winds go predominantly eastward and end at the wall that forms the HKH complex.

The African sands reach its westernmost part, but also those of the Arabian desert or the Thar desert, less known, but the largest in India, with its 200,000 km².

The region has been losing ice for at least three decades.

Climate change was taking most of the blame.

But this study shows that all this desert dust also plays a role.

By settling on snow or ice, sand reduces its albedo, facilitating its thaw.

In the same way that dark clothes give more heat than light ones, clean snow reflects solar radiation better (albedo effect) than dirty ones.

Scientists believed that carbon black or soot from industrial and combustion engine emissions was what darkened snow the most.

But the dust has overtaken him.

Impact

"This is the first time that we have quantified the role of long-distance dust in melting snow," says the researcher at the Indian Institute of Technology in Madras and study co-author Chandan Sarangi in an email.

"Thaw trends in the western Himalayas have to do with elevation, with a maximum intensity of reduction in areas located between three and five kilometers," he adds.

The authors of the study, published in

, conclude that human emissions from the region do not go beyond the first 3,000 meters.

From here, the dust takes over.

To estimate the impact of desert dust, the researchers relied on images taken by a series of satellites.

Missions like NASA's Calypso use instruments like lasers to measure the presence of particles in the atmosphere and radio waves to measure changes in the snow cover.

Soot traps more solar radiation than sand.

But at these altitudes there are between 100 and 1,000 times more of the latter than of the former.

The authors of the study recall that the optical properties of snow depend on its own characteristics, such as its shape or age (how long it has been on the ground) and external characteristics, such as the concentration of particles or its thickness.

When melting, the first layers of snow carry away the smaller carbon black, but leave behind the sand, which accelerates the thaw.

Albedo

"Dirty or aged snow absorbs sunlight better than fresh white snow," says Yun Qian, a scientist at the Pacific Northwest National Laboratory and a co-author of the study, in a note.

Newly fallen snow reflects up to 90% of solar radiation.

At this center under the US Department of Energy, Qian was the one who modeled the small-scale interaction of snow and different particles.

His Indian colleague puts the average reduction in snow albedo at between 4% and 8% and, at mid and high altitudes, more than half is due to dust.

"It may seem little in absolute terms but, to have a perspective, 3% of the incident solar radiation (with a total of 1,000 watts per square meter) translates into 30 W / m², which is a substantial climatic forcing", Sarangi details.

The impact of desert dust on snow is not limited to the HKH complex.

The sand of the Sahara also melts the ice in the Alps and, as Quian says, “these results are likely to be repeated in other large mountain ranges such as the Rockies, the Cascades or the mountain ranges in the US, and several mountain ranges in Asia. , like the Caucasus or the Urals ”.

The authors fear that as climate change raises the snow line, dust will take a greater role in the thaw.

"In general, temperature and rainfall are the main determinants of the amount of snow that melts each year," recalls Taylor Smith, a researcher at the Institute of Geosciences at the University of Potsdam (Germany).

"But there are other relevant factors to consider, such as the time of the rains or [the increase] in temperatures," adds this scientist, who has not intervened in the research.

One of these factors is now being desert dust.

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