Why Greenland's darkening ice has become a hot topic in climate science

John Abraham, The Guardian

Darkening causes the snow to absorb more sunlight which in turn increases melting

Last July, a record melting occurred on the Greenland ice sheet. Even in some of the highest and coldest areas, field parties observed rainfall with air temperatures several degrees above the freezing point. A month before, it was as though Greenland expert Jason Box had a crystal ball; he predicted this complete surface melting in a scientific publication. Box's research then got broader public visibility after climate activist and writer Bill McKibben covered it in Rolling Stone magazine.
The basic premise of Box's study was that observations reveal a progressive darkening of Greenland ice. Darkening causes the white snow surface to absorb more sunlight which in turn increases melting. Given that this process is likely to continue, the impact on Greenland melt, and subsequent sea level rise, will be profound.

There are several mechanisms that are known to darken arctic ice, including desert dust, pollen, soot from natural forest fires, and human biomass burning for land clearing and domestic use. Industrial, shipping, and aircraft pollution also play a role. Some of these effects are increasing. As climate change accelerates, more areas are being burned by wildfire each year. Box wondered how much increasing wildfires with resulting soot landing on the northern ice might amplify what scientists call a "positive feedback" - a self-reinforcing cycle - increasing Greenland melting. The cycle starts with initial warming, leading to more fires, more soot, and in turn more warming and more melt. The feedback is important, particularly in polar regions where observed warming is twice the rate of more southerly locations around the globe. Box calculates this effect has doubled Greenland surface melting since year 2000.

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