For a few days in July of 2012, it was so hot in the Arctic that nearly the entire surface of the Greenland ice sheet turned to slush.

It was so uncharacteristically warm that scientists, emerging from their tents high on the peak of the ice sheet, sank up to their knees in the suddenly soft snow. And then, that snow started melting.

Near the edge of the ice sheet, bright blue puddles collected on the flat white surface. Rivulets of melt trickled down, braiding into fat, gushing rivers. The meltwater punched through gullies and spilled down crevasses. One river near the edge of the ice sheet was so swollen that it swept away a bridge that had been there for