Environment: 'It's very, very alarming. It's a warning signal.'

As six months of winter darkness descend over the top of the world, temperatures are falling and the Arctic Ocean is again freezing. But the damage has been done. The National Snow and Ice Data Center reports that just 1.65 million sq. mi. (about 4.3 million sq km) of Arctic sea ice survived the summer's annual melt. That's the smallest amount left over after September since scientists began keeping records. And they estimate the Arctic ice may have shrunk by half since the 1950s. Worse, the melt is accelerating. The Arctic may be ice-free by the summer of 2030, far earlier than previous predictions. Though melting sea ice doesn't cause oceans to rise, it can speed global warming by exposing more dark water to sunlight. The machinery of the Arctic climate is mysterious, but this much seems clear: what begins in the north will eventually touch all of us.