Scientific American: The climate challenge just became a lot more challenging. We know that man-made carbon dioxide emissions are accelerating global warming. But intrepid research has revealed an additional sinister threat: methane. As Sarah Simpson reports, the warming of the Arctic is releasing vast quantities of methane that has been locked away for centuries in formerly frozen soil. Once released, methane traps 25 times more heat in the atmosphere than carbon dioxide does. So it is more imperative than ever …