Cryoscience: Snowfall brightens Antarctic future * Charles S. Zender<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#auth-1> 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<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref1>, 2<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref2>. Pristine snow is a valuable shield against global warming as it reflects up to 85% of sunlight and traps only the remainder as heat1<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref1>, 3<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref3>, 4<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref4>. This is why almost imperceptible reductions in snow reflectance owing to warming and pollution3<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref3>, 5<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref5> have become a great concern. Increased heat trapping by darker snow triggers a vicious feedback cycle that speeds the greying of snow5<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref5>, 6<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref6>, 7<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref7>. 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<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref8> 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<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref1>, 9<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref9> (Fig. 1<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#f1>). 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<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref1>, 5<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref5>, 9<http://www.nature.com/nclimate/journal/v2/n11/full/nclimate1730.html#ref9>. 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. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
