The Arctic has warmed more than the rest of the planet, and its ice cover has thinned and shrunk. Last September, at the end of the summer melt season, the ice extent was about 825,000 square miles smaller than the long-term average—a loss equal to the size of Alaska and California combined. That loss speeds the warming, as sunlight is absorbed by dark ocean instead of reflected into space by ice.

Extent of Arctic ice, September average, millions of square miles

Melting sea ice doesn’t raise sea level—it’s already in the water—but melting land ice does. Mountain glaciers are in global retreat. The total sea level rise of eight or nine inches since 1900 has contributed to a sharp increase in flooding along coasts. During Superstorm Sandy, for example, floods and winds caused $68 billion in damage on the U.S. East Coast.

The big threat is the ice sheets covering Greenland and Antarctica. They hold enough ice to raise seas more than 200 feet—and they’re losing it. When Earth was just a bit warmer, 125,000 years ago, they seem to have lost a lot: Sea levels were 20 to 30 feet higher. Such a rise today would swamp coastal cities.