SOME 250 MILLION years ago, the organisms of Earth were having a very bad time—the very worst time, you might say. The Permian-Triassic extinction event was unfolding, in which 70 percent of land species and 96 percent of marine species disappeared. Runaway global warming had raised equatorial ocean temperatures to 104 degrees Fahrenheit. The seas rapidly acidified, so shelled critters struggled to build their protective homes. Indeed, the fossil record shows these species got it the worst—strong evidence that the extinction’s culprit was CO₂ mucking with the oceans’ pH balance, and the rest of the planet, for…