Greenland's snowy surface has been getting darker over the past two decades, absorbing more heat from the Sun and increasing snow melt, a new study using satellite data has found.
The results show the trend is likely to continue, with the surface's reflectivity, or albedo, decreasing by as much as 10 per cent by the end of the century.
While soot blowing in from wildfires contributes to the problem, it has not been driving the change, the study found.
The real culprits are two feedback loops created by the melting itself. One of those processes is not visible to the human eye, but it is having a profound effect, researchers said.
The results have global implications. Fresh melt-water pouring into the ocean from Greenland raises sea level and could affect ocean ecology and circulation, they said.
"You do not necessarily have to have a 'dirtier' snowpack to make it dark. A snowpack that might look 'clean' to our eyes can be more effective in absorbing solar radiation than a dirty one," said Marco Tedesco from Columbia University in US.
"Overall, what matters, it is the total amount of solar energy that the surface absorbs. This is the real driver of melting," said Tedesco.
During a warm summer with clear skies and lots of solar radiation pouring in, the surface starts to melt. As the top layers of fresh snow disappear, old impurities, like dust from erosion or soot that blew in years before, begin to appear, darkening the surface.
A warm summer can remove enough snow to allow several years of impurities to concentrate at the surface as surrounding snow layers disappear.
At the same time, as the snow melts and refreezes, the grains of snow get larger. This is because the meltwater acts like glue, sticking grains together when the surface refreezes, researchers said.
The larger grains create a less reflective surface that allows more solar radiation to be absorbed, they said.
The impact of grain size on albedo - the ratio between reflected and incoming solar radiation - is strong in the infrared range, where humans cannot see, but satellite instruments can detect the change.
"It is 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," said Tedesco.
The study used satellite data to compare summertime changes in Greenland's albedo from 1981 to 2012. The first decade showed little change, but starting around 1996, the data show that due to darkening, the ice began absorbing about 2 per cent more solar radiation per decade.
At the same time, summer near-surface temperatures in Greenland increased at a rate of about 0.74 degrees Celsius per decade, allowing more snow to melt and fuel the feedback loops.
The findings were published in The Cryosphere journal.
