Global meltdown

Richard Alley’s eyes glint as we sit in his office in the University of Pennsylvania discussing how fast global warming could cause sea levels to rise. The scientist sums up the state of knowledge: “We used to think that it would take 10,000 years for melting at the surface of an ice sheet to penetrate down to the bottom. Now we know it doesn’t take 10,000 years; it takes 10 seconds.”