The Greenland Ice Sheet covers an area about the size of all the land in the United States east of the Mississippi River. This huge mass of ice averages a thickness of 2.3 kilometers (1.4 miles), and contains roughly 8 percent of the world’s fresh water. If all of it melted, it would increase global sea level by about 7.4 meters (24 feet).
Except for a narrow perimeter near the coasts, the ice sheet covers most of Greenland’s land surfaces year round. In winter, snowfall blankets the coastlines as well, making the whole island appear white in satellite imagery. But as temperatures rise in the summer, Greenland’s appearance begins to change. Melting exposes rocky coastlines, where glaciers pour out through narrow fjords to the sea. Farther inland, the smooth expanses of white are replaced by bands of darker, bare ice pockmarked with melt ponds and streams.
That underlying “dark” ice is part of the permanent ice sheet, and it is of great interest to climate scientists Johnny Ryan of Aberystwyth University and Jason Box of the Geological Survey of Denmark and Greenland. Ryan and Box spent much of the summer of 2014 mapping dark ice and studying its composition in a campaign they called the Dark Snow Project.
Fresh snow has an albedo of about 0.86, meaning it reflects about 86 percent of the sunlight that hits it. The darker underlying ice can have an albedo as low as 0.3. Since it is much less reflective than fresh winter snow, dark ice absorbs a much higher percentage of incoming sunlight, warms the surface faster, and hastens melting.
Around their camp in southwestern Greenland, Ryan and Box observed that snow and ice were darkened by a combination of dust, algae, and soot from wildfires. Most of the soot and dust was likely deposited thousands of years ago, Ryan noted, “so what happened in the past is having a direct effect on how the ice sheet behaves today.”
The image at the top of this page was captured by an unmanned aerial vehicle (UAV) on August 5, 2014, and offers a view of the tents and scientific equipment in the Dark Snow Project. (The camp was located at 67.078 north latitude and 49.402 west longitude.) Dark ice, rich with impurities, was particularly visible east of the melt stream near the camp. The second image was taken by the UAV on August 6 and shows a closeup of one of the tents, as well as research equipment such as a spectrometer and reference targets of pure white (albedo of 1) and pure black (albedo of 0) to calibrate the sensors on the UAV. Several water-filled cylindrical melt holes known as cryoconites are also visible.
The goal of the 2014 Dark Snow expedition was to measure the albedo of the surface using cameras and pyranometers. Box and Ryan hope to compare the albedo measurements they made with the UAVs to the ground-based albedo measurements, and then to satellite observations of albedo. The images below were captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on February 25, 2014, and on August 5, 2014.
Understanding the albedo of the ice sheet—and the composition of the impurities that darken it—is critical for scientists trying to understand how climate change will affect the rate of Greenland ice sheet melting. Though most scientists estimate that it will take several hundred or even thousands years for the ice sheet to melt completely, there are still a lot of questions about the process and timing.
“There’s still a great deal we don’t understand about the materials that darken the underlying ice sheet and how their presence will affect how the ice sheet behaves,” said Box. “We don’t, for instance, have good measurements of the relative abundance of dust, algae, and black carbon (soot) that we find in the areas with dark snow,” he added.
NASA images by Jeff Schmaltz, LANCE/EOSDIS Rapid Response. UAV images courtesy of Johnny Ryan and the Dark Snow Project. Caption by Adam Voiland, with assistance from Jason Box, Johnny Ryan, and Alun Hubbard.
- Terra - MODIS