The surface energy balance and mass balance of the Greenland ice sheet depends on the albedo of snow, which governs the amount of solar energy that is absorbed. The observed decline of Greenland’s albedo over the past decade1,2,3 has been attributed to an enhanced growth of snow grains as a result of atmospheric warming1,2. Satellite observations show that, since 2009, albedo values even in springtime at high elevations have been lower than the 2003–2008 average. Here we show, using a numerical snow model, that the decrease in albedo cannot be attributed solely to grain growth enhancement. Instead, our analysis of remote sensing data indicates that the springtime darkening since 2009 stems from a widespread increase in the amount of light-absorbing impurities in snow, as well as in the atmosphere. We suggest that the transport of dust from snow-free areas in the Arctic that are experiencing earlier melting of seasonal snow cover4 as the climate warms may be a contributing source of impurities. In our snow model simulations, a decrease in the albedo of fresh snow by 0.01 leads to a surface mass loss of 27 Gt yr−1, which could induce an acceleration of Greenland’s mass loss twice as large as over the past two decades5. Future trends in light-absorbing impurities should therefore be considered in projections of Greenland mass loss.