Solar radiation (sunlight) is the main provider of energy for the melting of snow and ice in Greenland. As the solar radiation reaches the ice sheet, a large percentage is reflected back up; fresh snow reflects up to 90% of sunlight. The ice reflectivity (a.k.a. albedo) therefore exhibits a strong control on the melting of the ice sheet. However, Greenland ice albedo has been decreasing since the beginning of satellite observations in 1981.

The darkening has several causes. Firstly, snow crystals undergo metamorphism which makes them poorer reflectors, a process that accelerates with increasing temperature. So a warmer summer will cause a darker ice sheet surface. Also, since some of the previous summers were record-breaking in terms of temperature and melt, areas that were snow covered now reveal darker bare ice, and areas that were ice before, now are exposed earlier in the melt season as the winter snow blanket disappears earlier. In addition to these factors, the ice sheet albedo is also impacted by dust, transported from close by and far away, particles released into the atmosphere by increasing amounts of wild fires in North America, pollen, and industrial emissions, such as those by ships taking a northerly route.

The darkening of the ice sheet is a self-reinforcing mechanism called the melt-albedo feedback. The darker the ice sheet gets, the more solar radiation is absorbed, which in turn facilitates melting and snow metamorphosis. This results in more exposed ‘dark’ ice, or a reduction in snow albedo.

Change in summer albedo, blue colors showing darkening. Data were derived from MODIS (Moderate Resolution Imaging Spectroradiometer) observations.

A freshly fallen snow crystal has numerous facets to reflect sunlight (left). Warming causes the grains to round at the edges and clump together (right). Scanning electron microscope photos courtesy the Electron and Confocal Microscopy Laboratory, USDA Agricultural Research Service.