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snow and the future of polar ice

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Rau, Greg

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Dec 3, 2012, 4:42:16 PM12/3/12
to geoengineering
Cryoscience: Snowfall brightens Antarctic future

Nature Climate Change

2,

770–771

(2012)

doi:10.1038/nclimate1730

Published online

26 October 2012

Snowpacks absorb more sunlight as they warm. The Antarctic Plateau may buck this trend over the twenty-first century as increased snowfall there inhibits the snowpack from dimming.

The colour of snow tells a remarkable story. To the human eye, snow appears white because its reflectance of visible light is uniformly high. However, its reflectance changes with astonishing abruptness at other wavelengths, and is a complex function of the exact ice crystal size and shape1, 2. Pristine snow is a valuable shield against global warming as it reflects up to 85% of sunlight and traps only the remainder as heat1, 3, 4. This is why almost imperceptible reductions in snow reflectance owing to warming and pollution3, 5 have become a great concern. Increased heat trapping by darker snow triggers a vicious feedback cycle that speeds the greying of snow5, 6, 7. With temperatures increasing globally, what, if anything, will oppose the self-reinforced darkening of snow and keep it from melting even faster? Writing in Nature Climate Change, Picard et al.8 use snow-colour measurements to deduce that fresh snowfall inhibits the seasonal greying of snow on the Antarctic Plateau by up to 3%, and reduces summertime temperatures there by up to 4 °C. On climate timescales, the increase in Antarctic snowfall expected with twenty-first-century warming may be enough to prevent the surface from further darkening.

Antarctica's reprieve from darker snow is a welcome surprise, because the enemies of snow reflectance are time and temperature, which is projected to rise by about 3 °C this century. Much like ice cubes in a home freezer, snow crystals lose their sharp facets to duller, rounder shapes as they age1, 9 (Fig. 1). Heat accelerates this metamorphism so that pristine, sharply faceted fresh crystals quickly grow during summer to become larger, rounder, aged snow, which absorbs more heat and reflects less sunlight1, 5, 9. Snow reflectance also changes during wind events (which shatter and sublimate crystals) and as a result of surface crusts and ripples. The findings reported by Picard and colleagues suggest that these secondary contributors explain less than one-third of changes in summer snow reflectance on the Antarctic Plateau. Temperature and snowfall are the main players.


The Antarctic Plateau endures long periods of polar night, during which its visible reflectance cannot be measured, so Picard and colleagues focused on the seasonal behaviour of a reflectance proxy — the snow grain size. First they teased grain-size information from the wavelength-dependent surface microwave emissions measured daily by meteorological satellites. A sophisticated model of the microwave signal travelling from the surface through the atmosphere best matches the measured signal when the snowpack is modelled as smaller, younger surface grains atop larger, inactive snow grains deposited in previous seasons.

 


Andrew Lockley

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Dec 3, 2012, 5:03:49 PM12/3/12
to ra...@llnl.gov, geoengineering

That leads me to wonder if we could use cloud seeding to darken Greenland, or the Arctic sea ice.

A

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Charlie Zender

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Dec 4, 2012, 10:08:27 AM12/4/12
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Hi Andrew, 
I assume your goal in cloud seeding would be to brighten not darken the surface to cool climate. In any case, inducing precipitation via cloud seeding has never worked reliably but if such a method is developed and demonstrated to work in the Arctic (rather than the semi-arid regions where most seeding attempts are made), then one effect of the induced snowfall would be to brighten the surface, whether Greenland or sea ice. Nothing is brighter than fresh snow so the more we have the better for a cooler climate.


cz

Andrew Lockley

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Dec 4, 2012, 10:25:10 AM12/4/12
to Charlie Zender, Greg Rau, geoengineering

A good dump of late spring snow could do quite a bit to help retain permafrost, too.  There would be both an albedo and a thermal insulation effect.

The effect would be more marked, although more temporary, than inducing snow over permanent ice cover.

I'm really surprised this hasn't been.mentioned before, as it's pretty simple (unless I'm missing something)

A

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