Reading though the papers this morning, I came across an Associated Press article stating that 97% of the surface area of the Greenland Ice Sheet was melting in mid July. The melting started between July 8th and July 12th, the area that had melted had gone from 40% to 97%, the greatest extent ever measured since the advent of satellite measurements 30 years ago. By July 16, the heat wave that caused this event began to dissipate. A scientific outpost on top of the ice sheet reported temperatures rising above freezing for several hours.
Should we be alarmed? According to the press release, a melting event on this scale happens about every 150 years, with the last event happening in 1889. So, this kind of event is rare, but not unprecedented. It is only if these sorts of events occur more frequently than once every 150 years or so that we should become concerned.
Yesterday, I attended the sea level change forum, where there was discussions on the amount of carbon dioxide needed to make the Greenland ice sheet stable. One of the speakers, Eelco Rohling, gave an estimate that at 450 PPM carbon dioxide concentration in the atmosphere, the entire Greenland ice sheet would become unstable, and prone to melting. So how will the melting of the Greenland ice sheet happen in this scenario?
As the Earth’s atmosphere attains higher CO2 levels, it is highly likely that these broad melting events will happen more often. As meltwater percolates down into the ice, it makes it weaker and warms it up. When the meltwater starts reaching the base of the ice sheet, it lowers the viscosity of the ice-ground interface, making it easier for the ice to flow. This causes the ice sheet to become thinner, as the ice flows out to the ocean and calves. As the surface of the ice sheet goes down, it increases the likelihood of more melting events, as the ice gets closer to the equilibrium line altitude. As you might guess, this would have a cascading effect, and the rate of ice sheet melting would increase. Given the size of the Greenland ice sheet, even with high levels of greenhouse gasses it would still take thousands of years for the entire ice sheet to melt.