We have a cutaway of the Greenland ice sheet. At the highest elevations we have what we call the dry snow zones. Snow accumulates every winter there. In the summers, it doesn't really get warm enough to melt the snow; the snow just compacts down into what we call firn and eventually in the glacial ice. This layer of firn can be anywhere from 100 to 300 feet thick. At the lowest elevations in the ice sheet we have the ablation zone. Here snow still accumulates in the winter but in the summers it gets warm enough… …to completely melt the snow away, down to bare glacial ice and all of this melt… …causes runoff, which feeds the rivers around Greenland every single year. We have a vast region in between these two that we call the percolation zone. In these regions of the ice, snow still accumulates every single winter but only partially melts in the summer.
That melt percolates down and refreezes into this sponge of firn beneath and it forms little ice layers. When we begin to increase melt in the summers, these layers grow more frequent, they intermix with each other. And if you keep this process up, and add more and more meltwater… …these layers merge together into very thick and continuous ice layers. When they get thick and continuous enough, you can cause a lid of ice over the sponge. The water begins to runoff, just like it does in the ablation zone. This causes ever-increasing areas of Greenland to run off and contribute directly to sea-level rise..