Pelley stood on a huge block of ice that had split off from the glacier and had dropped into the sea — a big iceberg.

"This part of Greenland is melting faster than just about any other. To get a sense of the enormity of what's happening, consider this: The ice that is melting here is the equivalent of all the ice in the Alps," Pelley explained, standing atop the iceberg.

That's more than 105 million acres of melted ice in 15 years. Just four minutes after Pelley cleared off this berg, part of the ice caved in.

60 Minutes got a bird's-eye view of how unstable the ice is becoming on a flight with glaciologist Carl Boggild.

Boggild anchored 10 research stations to the ice. But every time he comes to visit, the ice and his stations have moved.

Flying over the ice, Pelley noticed lots of fissures and crevices breaking through the ice.

Asked what causes this, Boggild explained, "This is actually the ice flow, where you have so much tension in the ice that it cannot stick together. And it breaks and opens a crevice which goes about 150, 200 feet down."

The ice is also melting on the sides, Boggild says.

High overhead, Pelley remarked that one could hear the water running.

"It's like a small river," Boggild said.

A leading theory says those little rivers lubricate the bottom of the ice sheet, helping it move off the bedrock and out to sea.

And there may be no stopping it. Arctic warming is accelerating. It's a chain reaction. As snow and ice melt they reveal dark land and water that absorb solar heat. That melts more snow and ice, and around it goes.

There's long been a debate about how much of this is earth's naturally changing climate and how much is man's doing. Paul Mayewski, at the University of Maine, says the answer to that question is frozen inside an ice core from Greenland.

With funding from the National Science Foundation, Mayewski has led 35 expeditions collecting deep ice cores from glaciers. The ice captures everything in the air, laying down a record covering half a million years.