The Greenland ice sheet is the second-largest single piece of ice on the planet (second only to the one in Antarctica). Nearly two miles thick in places, it contains some 684,000 cubic miles of ice. And because that is all above sea level — as opposed to floating, like the northern ice cap — if all of the Greenland sheet were to melt, it would raise the world sea level about 20 feet.
So it’s a bit worrisome that the annual melting season has begun a month earlier than the previous record start — and in spectacular fashion, immediately leaping to a melt extent not usually seen until June. It’s a clear and present danger to any low-lying cities, but also a reminder that the uncertainty of future predictions is one of climate change’s most threatening aspects.
This chart shows the percentage of the melt extent over the ice sheet by month. Today’s outlier is extremely stark: