Tuesday, July 3, 2012

Greenland Something Less than Snow White

12 August 2005, 8 PM local time, Photo from a helicopter flying over the ice sheet surface at ~1500 feet altitude. This is how much darker the Greenland ablation area is than a fresh snow surface that blankets it in wintertime. Along much of the southwestern ice sheet at the lowest 1000 m in elevation, impurities concentrate near the surface and produce this dark surface. Not all of the ice sheet is this dark, only the lower ~1/3 of the elevation profile of the ice sheet is. However, as melting increases on the ice sheet, so does the area exposed that is this dark.

The following provides detail to a story run by NOAA entitled Greenland Ice Sheet Getting Darker

Freshly fallen snow under clear skies reflects 84% (albedo= 0.84) of the sunlight falling on it (Konzelmann and Ohmura, 1995). This reflectivity progressively reduces during the sunlit (warm) season as a consequence of ice grain growth, resulting in a self-amplifying albedo decrease, a positive feedback. Another amplifier; the complete melting of the winter snow accumulation on glaciers, sea ice, and the low elevations of ice sheets exposes darker underlying solid ice. The albedo of low-impurity snow-free glacier ice is in the range of 30% to 60% (Cuffey and Paterson, 2010). Where wind-blown-in and microbiological impurities accumulate near the glacier ice surface (Bøggild et al. 2010), the ice sheet albedo may be extremely low (20%) (Cuffey and Paterson, 2010). Thus, summer albedo variability exceeds 50% over parts of the ice sheet where a snow layer ablates by mid-summer, exposing an impurity-rich ice surface (Wientjes and Oerlemans, 2010), resulting in absorbed sunlight being the largest source of energy for melting during summer and explaining most of the inter-annual variability in melt totals (van den Broeke et al. 2008, 2011).

The photo below shows how dark the ice sheet surface can become in the lowest ~1000 m elevation in the “ablation area” after the winter snow melts away and leaves behind an impurity-rich surface. This dark area is where the albedo feedback with melting is strongest.

 Dirty ice surrounds a meltwater stream near the margin of the ice sheet. Compared to fresh snow and clean ice, the dark surface absorbs more sunlight, accelerating melting. © Henrik Egede Lassen/Alpha Film, from the Snow, Water, Ice, and Permafrost in the Arctic report from the U.N. Arctic Monitoring and Assessment Programme.

Satellite observations from the NASA Moderate-Resolution Imaging Spectroradiometer (MODIS)  indicate a significant Greenland ice sheet albedo decline (-5.6±0.7%) in the June-August period over the 12 melt seasons spanning 2000-2011. According to linear regression, the ablation area albedo declined from 71.5% in 2000 to 63.2% in 2011 (time correlation = -0.805, 1-p=0.999). The change (-8.3%) is more than two times the absolute albedo RMS error (3.1%). Over the accumulation area, the highly linear (time correlation = -0.927, 1-p>0.999) decline from 81.7% to 76.6% over the same period also exceeds the absolute albedo RMS error.

Greenland ice sheet average reflectivity or albedo (multiply by 100 to get % units) for 12 summer (June-August) periods.

According to Jason Box, the lead author of the Greenland chapter of the 2011 Arctic Report Card and the analyst of the reflectiveness data, the darkening in the interior is just as remarkable than the changes at the margins. The interior is the high-point of the dome-shaped ice sheet, rising to nearly two miles above sea level. There is no visible melting there in the summer, so why is the area becoming darker?

Map of changes in the percent of light reflected by the Greenland Ice Sheet in summer (June-July-August) 2011 compared to the average from 2000-2006. Virtually the entire surface has grown darker due to surface melting, dust and soot on the surface, and temperature-driven changes in the size and shape of snow grains. Map by NOAA’s climate.gov team, based on NASA satellite data processed by Jason Box, Byrd Polar Research Center, the Ohio State University.

The darkening in the non-melting areas, says Dr. Box, is due to changes in the shape and size of the ice crystals in the snowpack as its temperature rises. Snow grains clump together, and they reflect less light than the many-faceted, smaller crystals. Additional heat rounds the sharp edges of the crystals. Round particles absorb more sunlight than jagged ones do.

A freshly fallen snow crystal has numerous facets to reflect sunlight (left). Warming causes the grains to round at the edges and clump together (right). Scanning electron microscope photos courtesy the Electron and Confocal Microscopy Laboratory, USDA Agricultural Research Service.

On the Kangerdlugssuaq Glacier -- one of Greenland's largest ice fields -- scientists measure the movement of the ice sheet as it transports frozen water to the ocean. They discover that the speed of the glacier's march to the sea has tripled in just ten years. Alarm bells sound because at the current melt rate, within a few decades rising seas will have a profound effect on the low-lying countries of the world.

Once considered an inexhaustible source of food, the oceans are now in danger of being significantly depleted. Matt Damon hosts "The State of the Planet's Oceans" as award-winning filmmakers Hal and Marilyn Weiner investigate the health and sustainability of the world's oceans and the issues affecting marine preserves, fisheries, and coastal ecosystems worldwide.

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