Rising sea levels already cause flooding in many American cities. Increasingly high tides at coastal military bases pose a threat to national security. Now the timetable for disaster has been speeded up by a shocking prediction: sea ice will be melting ten times faster than previously estimated. Sea levels will rise by ten feet in just 50 years, according to James Hansen, who warned that climate change was caused by human behavior when he was working for NASA. How much of civilization will be under water? We talk with Hansen and others.