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Soot and glaciers

January 12, 2010

January 11, 2010  -

Graham Cogley

A little soot can make a big difference to the brightness of snow. Freshly
fallen snow, when clean, is one of the brightest of substances, reflecting
well over 90% of incident sunlight and presenting the risk of snow blindness
to ill-equipped travellers on glaciers.

As the snow ages, the snowflakes collapse and become rounded. Opportunities
for photons to bounce off and head back into the sky become fewer.
Opportunities for absorption become more frequent because the photons spend
more of their time passing through grain interiors. Eventually, as the snow
turns into glacier ice, the reflected fraction of incoming radiation drops
to as low as one half or less.

There is more than this to the radiative physics of snow and ice. For
example the wavelength of the impinging photon makes a difference, and so
does the angle at which it strikes the surface (more reflection when the
angle is closer to horizontal). When a thaw begins, some of the snow turns
into liquid water, which, ironically, is one of the darkest of substances.
So wet snow is not particularly bright. Dirt also makes a difference.

If the dirt is black enough, even a small amount reduces significantly the
brightness, or albedo, of the snow. This was shown dramatically as long as
30 years ago by Warren and Wiscombe. The more soot, the more darkening, but
as little as a few parts per billion by weight reduces the albedo of pure
snow (that is, collections of grains of ice) by a few percent in the visible
part of the spectrum. We also get significant sunlight in the (invisible)
near-infrared, but the effect of soot is much reduced there because ice is
itself very dark in the near-infrared. All the same, soot makes a

Photon for photon, exposed glacier ice yields two or more times as much
meltwater as does clean snow, assuming both are at the melting point. So we
are very interested in anything, such as soot, that reduces the radiative
contrast between the ice and the overlying snow. What with
industrialization, growth of the human population, and more intense
clearance of forests by burning, there is more soot about now than there
used to be. How much of it actually reaches the glaciers, and precisely how
large its contribution is to the faster rates of mass loss observed in
recent decades, are open questions. But it would be surprising if we were to
look for evidence of a link and failed to find it.

Evidence of a link is just what Xu Baiqing and colleagues, writing in a
recent issue of the Proceedings of the National Academy of Sciences, appear
to have found. They measured soot concentrations in ice cores from five
Tibetan glaciers, and found radiatively significant amounts in all but one,
with evidence for recent increases in at least two. These glaciers are
downwind of two of the world's largest sources of airborne soot, India and
western Europe. (Yes, Tibet is a long way from Europe, but the soot
particles are tiny and once they are aloft they can travel thousands of
kilometres before being washed out.)

And at the recent Fall Meeting of the American Geophysical Union, Bill Lau
of NASA drew attention to another way in which soot can affect glacier mass
balance. While the soot is still in the atmosphere it constitutes what he
calls an "elevated heat pump". It heats the air (rather than the surface),
the heated air rises, and new air is drawn in from elsewhere to replace it.
In the Himalayan-Tibetan region, the new air comes from the south and is
warm and moist, so this amounts to an induced intensification of the summer
monsoon. Warmer air means more melting, but moister air means more
precipitation and therefore, where the temperature is right, more snowfall.
Working out the net impact on the glaciers, then, will be a challenge.

These studies leave us a long way from nailing down soot as one of the
reasons for more negative glacier mass balance, which will require
concurrent measurements of sootfall, incident radiation, temperature and
rates of snowfall and melting. But at the very least the soot concentration
measurements show that the soot is there, and the most solid part of the
deductive chain - the fact that soot makes snow absorb more radiation - is
already firmly in place. Greenhouse gas is not the only pollutant we should
be worrying about.
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