“It's a train running downhill, and the hill is getting steeper.”
Over the past two decades, Greenland’s snowy white surface has been getting darker, resulting in the absorption of more heat from the sun and increasing snow melt. According to a new study, this trend is likely to continue, with the surface’s albedo, or reflectivity, decreasing by as much as ten percent by the end of the century.
Although soot blowing in from wildfires is contributing to the problem, it is not driving the dramatic change. The real culprits are the feedback loops created by the melting itself; one of these loops, although not visible to the human eye, is having a profound effect.
The findings, which are published in the European Geosciences Union journal The Cryosphere, could have global implications for rising sea-levels, and this inflow of freshwater could also affect ocean ecology and circulation.
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“You don't necessarily have to have a 'dirtier' snowpack to make it dark,” lead author Marco Tedesco, a research professor at Columbia University's Lamont-Doherty Earth Observatory and adjunct scientist at NASA Goddard Institute of Space Studies, said in a press release.
“A snowpack that might look 'clean' to our eyes can be more effective in absorbing solar radiation than a dirty one. Overall, what matters is the total amount of solar energy that the surface absorbs. This is the real driver of melting.”
The feedback loops work like this: During a warm summer with clear skies, lots of solar radiation enters the atmosphere causing the surface to melt. As the top layers of the fresh, powdery white snow disappear, older impurities like dust and soot begin to show, darkening the surface.
At the same time, as the snow melts and refreezes, the grains of snow get larger. Meltwater acts like glue, sticking grains together when the surface refreezes and these larger grains create a less reflective surface that allows more solar radiation to be absorbed. The impact of grain size on albedo is strong in the infrared range, something humans can't see, but satellite instruments can detect the change.
“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.”
The study used satellite data in order to compare summertime changes in Greenland’s albedo from 1981 to 2012. The first decade showed little change, but around 1996 the ice began absorbing two percent more solar radiation per decade. Not only that, summer temperatures in Greenland increased at a rate of 0.74 degrees Celsius per decade.
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The researchers concluded that the likely cause for this shift was a change in atmospheric circulation. The North Atlantic Oscillation, a large-scale natural weather cycle, went into a phase in which summer atmospheric conditions favored more incoming solar radiation and warmer-moist air from the south.
However, these conditions shifted in 2013-2014 to favor less melting, but the damage was already done. “While new snowfall can make the ice sheet brighter again, the dark material built up during the melt years is waiting just below the surface, preconditioning the surface to future melting,” Tedesco said.
The researchers also ran a computer model to simulate the future of Greenland’s snow and ice cover, and conservatively found that over the current century the albedo for the entire ice sheet could fall by as much as eight percent, and by as much as 10 percent on the western edge. However, the change could be twice that, according to Tedesco.
The feedback loops could be stopped with lots of snowfall and less melting, but that is not likely given the continued increase of greenhouse gases being emitted into the atmosphere. “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.”