AUDIE CORNISH, HOST:
Each summer, about half of the surface of the Greenland ice sheet melts. That's on average, but this month, in just four days, an estimated 97 percent of the ice sheet's surface has already melted. That rate is abnormally fast and it comes just a week after a huge iceberg broke off a glacier in Greenland.
For more now, I'm joined by Tom Wagner. He's a scientist at NASA. Hi there, Tom.
TOM WAGNER: Hello.
CORNISH: Now, to start, how'd this come to NASA's attention?