Scientific American: Based on its long-term orbit, Earth should be heading into an ice age. But instead of continuing to cool–as it had been for at least the past 2,000 years–the Arctic has started to warm. And the reason is humans’ impact on the composition of the atmosphere, new research suggests. To look at this trend, geologist Darrell Kaufman of Northern Arizona University and a consortium of colleagues reconstructed Arctic temperatures decade by decade over the past two millennia by pulling …