If you had lived some 50 million years ago and taken a trip to the poles, you would have found lush forests and creatures like crocodiles instead of miles-thick ice sheets. That’s because during the Eocene, greenhouse gas concentrations were much higher than they are today, leading to a natural period of global warming. Levels of methane, which is 80 times as potent a planet-warmer as carbon dioxide, were especially high, ratcheting up temperatures and allowing plants and animals to migrate toward the poles—just as they’re slowly doing once again.
Methane may have been heating the Eocene poles in another more subtle, fascinating way: by creating a blanket of invisible
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