The Greenland Ice Sheet (GIS) is the second largest ice cap in the world, containing 10% of earth’s glacial ice – or a volume of water equivalent to a 7m rise in global sea level, if it were to melt. With global sea level rise predictions in the next hundred years in a range that potentially threatens billions of dollars of coastal infrastructure, understanding future melt on the Greenland ice sheet is a problem of pretty big importance to a large number of people.
Melt has been increasing on the ice sheet pretty dramatically in the past few years – in particular last summer when melt occurred almost everywhere on the ice sheet. Normally the higher altitude areas of the ice sheet, more than 50% of the area of the ice sheet, don’t melt at all, even in the height of summer. A steady increase in surface melt on the ice sheet is contributing increasing amounts to sea level rise – but our understanding of how rapidly the ice sheet will melt in the future remains pretty limited.
We do know that melting on the ice sheet is very sensitive to how much sunlight the ice sheet reflects back into space. Just how much sunlight it reflects, however, depends on how white the ice sheet is – which can vary. If you live in areas that get snow, think about the difference between a fresh snowfall (very white) and the snow a few days later, after it has collected some dirt and soot and aged a bit (not so white, in some areas even sort of gray!). The Greenland ice sheet experiences similar changes in its whiteness, though less pronounced than in an urban area, when the snow ages on the surface or when soot and dust blown in from afar deposit on the ice.
The word for how “white” a surface is its albedo. Specifically a surface’s albedo is the fraction of light the surface reflects. A perfectly white surface would be 100% reflective while a black hole (no reflection at all) would have an albedo of 0. The albedo of the ice sheet varies only a few percent, but this difference translates to a tremendous amount of energy in late spring and early summer under 24 hour sunlight.
On the GIS, properties of the snow, including its physical structure, state of melt, and the presence of absorbing contaminants, are important factors influencing the variability in snow albedo. In general, as snow ages or melts, the albedo of the surface drops, enhancing solar absorption and causing a positive radiation balance feedback enhancing melt. Additionally, extremely small quantities of dust or black carbon particles deposited onto the ice surface can significantly increase light absorption within the upper layers of the ice, further reducing albedo of the surfaces where deposition occurs. Understanding exactly what role each of these played in recent melt events, and accurately incorporating them into modeling that predicts future melt, will require taking very careful measurements of the surface albedo, alongside measurements of the snow grain size and shape, and the amount of dust and soot in the snow.
One tantalizing possibility is that wildfires strongly influenced last summer’s melt. Last summer a number of major wildfires in the Northern Hemisphere during late spring and early summer may have resulted in a particularly large depositions of black carbon onto the ice sheet, darkening the ice and enhancing the melt. The possibility that increased fire activity (predicted in a warming climate) could be linked to increased melt on the ice sheet would be a very important discovery – we’ll let you know after we get back to the lab and through our samples!
To get the most value out of the experiment, we’ll be testing several other hypotheses about what processes control sunlight absorption on the ice sheet, including changes in snow grain size and the influence of clouds. Each hypothesis and the methods we’re using to test it will be the subject of various posts over the course of the expedition. We hope you’ll check back!