On the edge: Tourists scramble over the melting edge of Greenland’s ice sheet near the town of ¬Kangerlussuaq. The rate at which some glaciers are sliding into the ocean has doubled or more in a decade, outpacing estimates that inform official predictions of sea-level rise. Credit: David Talbot

From the air, the western edge of Greenland's ice sheet looks like an aging elephant's skin--gray and cracked, as melting at the margins exposes dirt accumulated over tens of thousands of years. A few kilometers inland, however, the gray gives way to blazing white that goes on for hundreds of miles. These vast glaciers contain snow that has fallen and compacted over millennia. The ice sheet--roughly four times the size of California, and more than three kilometers thick in places--is, in essence, a vast frozen reservoir of fresh water.

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