Greenland’s white snow is getting darker. Scientists have generally attributed that darkening to larger, slightly less white snow grains caused by warmer temperatures. But researchers have found a new source of darkening taking hold: impurities in the snow.

“It can increase the speed of melting,” says Marie Dumont, a remote sensing scientist at Météo France in Grenoble, who publishes today with her colleagues in Nature Geoscience.

Scientists have known for years that Greenland’s snow is getting darker, based on satellite observations that have revealed lower albedos, or reflectivity. That’s a problem because the darker the snow is, the more sunlight it absorbs, and the faster it melts. Greenland’s melting ice sheets are already predicted to raise sea levels by 20 centimeters by 2100.

But Dumont and her colleagues have found that, since 2009, there has been a darkening that cannot be explained by larger snow grain size alone. Using satellite observations, they found lower albedos at elevations and at times of the year that are too cold for larger snow grains to form.

The researchers instead propose that impurities in the snow—dust, soot, and microorganisms—are responsible. Using satellite observations, they found higher levels of impurities in the snow and atmosphere between 2009 and 2013—a time in which impurity levels over Antarctica stayed constant. They suggest that dust could be arriving from snow-free land areas in Greenland and nearby in the Arctic that are experiencing earlier melting of seasonal snow cover due to climate change. Volcanic eruptions in Iceland in 2010 and 2011 are also important sources of material, they say.

Using a model, the researchers found that impurities could be responsible for melting 27 billion tonnes of ice a year—roughly 10% of the average annual mass loss over the last decade.

The new model is the first to document and quantify this new feedback—one that is not accounted for in climate models, says Jason Box, an ice scientist at the Geological Survey of Denmark and Greenland in Copenhagen, who has documented rising impurities at a local scale during field campaigns. Dumont says the new darkening effect could easily add 2 centimeters to the projections of sea level rise by 2100—and perhaps more if impurity levels grow with time.