09 January 2012

GREENLAND'S ICE IS DARKENING

Greenland melt map by NOAA’s climate.gov team, based on NASA satellite data processed by Jason Box, Byrd Polar Research Center, the Ohio State University.
  
Not only is Greenland's ice melting, it's also become darker and therefore more absorbent of light—accelerating its own thaw. 


The map above shows the difference between the amount of sunlight Greenland reflected in the summer of 2011 versus the average percent it reflected between 2000 to 2006. Virtually the entire ice sheet shows some change, with some areas reflecting close to 20 percent less light than a decade ago.

Melting atop the Greenland ice sheet. Image via The Big Picture.
  
As expected, rising temperatures melt snow and ice to uncover water, vegetation, and bare ground. These darker substrates absorb more sunlight. 

As predicted, the loss of reflectiveness amplifies the initial warming.

Greenland glacier. Credit: Ville Miettinen via Wikimedia Commons.

Most of the melt patterns on the map (top) fit these expectations. 

But what's unexpected here is that the reflectivity of Greenland's ice is diminishing not just at the coasts but far inland as well. 

Even at the highest point of the ice sheet, nearly two miles above sea level, where there's no visible summertime melting, the ice is darkening.


Smaller, colder snow crystals, left. Warmed ice crystals, right. Credit: NASA Earth Observatory.

So what's going on?

Well, according to Jason Box at Ohio State University the inland darkening is a result of changes in the ice crystals themselves.

As temperatures rise, the snow grains clump together, reflecting less light than the many-faceted smaller crystals (above left). 

The warmed—but not melted—crystals become rounded (above right), and these shapes absorb more sunlight than jagged crystals.


Greenland glacier from the air. Credit: Mila Zinkova via Wikimedia Commons.
  
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