As the snow atop Greenland grows darker, which it has done over the past 20 years, it absorbs heat from the sun and ramps up the snow melt.
That trend is on course to continue, says a new study. The albedo, or surface reflectivity, of the snow and ice will likely decrease by as much as 10 percent toward the end of the century.
The study was led by a researcher with NASA and Columbia University's Lamont-Doherty Earth Observatory, and based its findings on analysis of satellite data for two decades. Like a previous study, this one says that soot from wildfires is not driving the change. But the new research says that soot is still contributing to the problem. Essentially, the problem is that melting is in a feedback loop: Melt causes darkening, which causes more melt.
"You don't necessarily have to have a 'dirtier' snowpack to make it dark," Marco Tedesco, lead author, said. "A snowpack that might look 'clean' to our eyes can be more effective in absorbing solar radiation than a dirty one. Overall, what matters, it is the total amount of solar energy that the surface absorbs. This is the real driver of melting."
The process is this: The surface begins to melt in a warm summer with open, fairly cloudless skies and lots of solar exposure. When fresh snow's upper layers disappear, the entire surface starts melting. That's when impurities below the surface begin to reach the top -- dust from erosion, soot from previous years. The detritus of a suburban lawn during snowmelt, but in the Arctic. If the summer is warm enough and lasts a long enough time, several years of these dark specks can appear in concentrations on the surface.
While all that is happening, the process of snow melting and refreezing results in larger grains of snow -- a bit like the process of bulky grains of ice clogging a kitchen freezer whose contents melt and then refreeze. The meltwater seals it all together as it refreezes, so that the overall surface is less reflective.
"It's a complex system of interaction between the atmosphere and the ice sheet surface. Rising temperatures are promoting more melting, and that melting is reducing albedo, which in turn is increasing melting," Tedesco said. "How this accumulates over decades is going to be important, because it can accelerate the amount of water Greenland loses. Even if we don't have a lot of melting because of atmospheric conditions one year, the surface is more sensitive to any kind of input the sun can give it, because of the previous cycle."
"As warming continues, the feedback from declining albedo will add up," Tedesco said. "It's a train running downhill, and the hill is getting steeper."
The findings were published in the European Geosciences Union's journal The Cryosphere, and have global implications because of the sea rise that can result from Greenland's melting.
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