There was one more crisis to be survived before mammals could take over the Earth. That was the Paleocene-Eocene thermal maximum (PETM, for lovers of acronyms), a short, sharp shock of greenhouse warming at the border between the Paleocene and Eocene epochs, about fifty-six million years ago. It was damaging enough to drive many species to extinction. The PETM is of interest today because it is the most recent period of rapid greenhouse warming in Earth’s history, so it may help us understand climate change today. The parallels are eerie. The amounts of carbon dioxide released into the
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