The Arctic is warming at an unprecedented rate, new research suggests. Last year was the warmest on record for the Arctic, and sea ice extent was at an all-time low since record keeping began in 1979. And more than half of the Greenland ice sheet melted in 2015. The tundra is also browning, with less vegetation on land. While one year of browning isn't cause for concern, "we're looking at, depending on where in the Arctic, a two- to four-year consistent drop in vegetation. It's something that's actually just gotten on our radar from a terrestrial ecology perspective,"...