Dust dampens albedo effect, spurs snowmelt in the heights of the
Himalayas
Specks blowing in from Africa, Middle East exert enormous effect on
climate

Date:
October 5, 2020
Source:
DOE/Pacific Northwest National Laboratory
Summary:
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. That's because dust - lots of
it in the Himalayas - absorbs sunlight, heating the snow that
surrounds it.



FULL STORY ==========================================================================
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.

"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.

Much of the data for the study comes from satellite images obtained
by multiple NASA instruments, including NASA's Cloud-Aerosol
Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), OMI
(Ozone Monitoring Instrument), and MODIS (Moderate Resolution Imaging Spectroradiometer). These instruments can detect dust and other aerosols
in the atmosphere, and measure snow coverage and albedo, from hundreds
of miles above Earth. Equipped with data from these and other sources,
the PNNL team did extensive computer modeling of the processes at work.

Dust with staying power Dust particles usually stay in snow longer than
black carbon, the scientists noted. Dust is usually a little bit bigger;
it's not as easily blown off the snow and it doesn't fall through snow
as easily. There's also a lot more of it.

"The snow in the western Himalayas is receding rapidly. We need
to understand why this is happening, and we need to understand
the implications," said Sarangi. "We've shown that dust can be a big contributor to the accelerated snowmelt. Hundreds of millions of people in
the region rely on snow for their drinking water -- we need to consider
factors like dust seriously to understand what's happening." Qian notes
that as the climate warms and snow lines move higher, scientists expect
the role of dust to become even more pronounced in the Himalayas --
a region that, aside from the Arctic and Antarctic regions, contains
the biggest mass of snow and ice on the planet.


========================================================================== Story Source: Materials provided by
DOE/Pacific_Northwest_National_Laboratory. Original written by Tom
Rickey. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Sarangi, C., Qian, Y., Rittger, K. et al. Dust dominates
high-altitude
snow darkening and melt over high-mountain Asia. Nat. Clim. Chang.,
2020 DOI: http://dx.doi.org/10.1038/s41558-020-00909-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201005112133.htm

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