The world has seen some extraordinary winter conditions in both hemispheres over the past year: snow in Johannesburg last June and in Baghdad in January, Arctic sea ice returning with a vengeance after a record retreat last summer, paralyzing blizzards in China, and a sharp drop in the globe's average temperature. It is no wonder that some scientists, opinion writers, political operatives and other people who challenge warnings about dangerous human-caused global warming have jumped on this as a teachable moment. "Earth's