Time Magazine: Back in the Pliocene era, between 5 million and 3 million years ago, the average global temperature was about 7°F warmer than it is today, yet atmospheric carbon dioxide levels were about the same. If carbon dioxide were the sole factor in warming, that wouldn’t make any sense. It isn’t, of course; there are several other contributors, including the brightness of the sun and the location of the continents (whose positions dictate, among other things, where ice caps can form) — but these …