Greenland Is Getting Darker

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Greenland Ice Sheet

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The bright surface of the Greenland ice sheet¹ reflects more than half of the sunlight that falls on it. In the past, this property helped stabilize the ice sheet: less absorbed sunlight meant lower ice-sheet temperatures and less melting. Recent observations, however, indicate that the reflectivity of Greenland's ice-sheet surface declined over the preceding decade. The darker surface now absorbs more light, accelerating Greenland's ice loss, or ablation.²

The accompanying map shows the difference between the amount of sunlight reflected from Greenland's surface in the summer of 2011 and the average percentage reflected between 2000 and 2006. Changes can be seen across almost the entire ice sheet; in some areas, sunlight reflectivity is nearly 20 percent lower than the average of a decade earlier. The map was prepared from observations by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Terra and Aqua satellites, as part of a U.S. National Oceanic and Atmospheric Administration (NOAA) report concerning the Arctic.

Climate scientists had long predicted a reduction in sunlight reflectivity in polar regions as a result of increasing global temperatures. Rising temperatures melt snow and ice, exposing darker underlying land, vegetation, water, or ice-sheet surfaces that absorb more light. This, in turn, reinforces further warming.

Most of the patterns observed on this map are consistent with those predictions. Lower and warmer areas are darker than the colder, higher interior. This occurs each summer as winter snow retreats from the edge of the ice sheet. Dark pools of meltwater form on its surface, while winds collect dust particles on the water, further reducing the reflectivity of incoming sunlight.

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Snow Ice Crystals at Two Different Temperatures

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But the darkening of the interior of the ice sheet is just as noteworthy as the changes along its margins, and it has attracted the attention of Jason Box of Ohio State University, who was analyzing data on the ice sheet's reflective properties.

The high interior lies nearly three kilometres above sea level, and little summer melting is observed there. So why are these regions becoming darker?

As Box explains, the darkening of the interior is caused by changes in the size and shape of snow ice crystals. As temperatures rise, snow grains become denser and reflect less light than smaller, many-sided grains. Higher temperatures also round the sharp edges of the crystals; consequently, rounded particles absorb more sunlight than jagged particles. See the accompanying electron-microscope image.

¹ An ice sheet is a mass of glacial ice that covers the surrounding land and is larger than 50,000 square kilometres.

² The reduction in the volume of an ice sheet through evaporation or melting is termed ablation.