the basic rule of thumb for staple cereal crops grown at optimal temperature is that for every degree of warming, yields decline by 10 percent. Some estimates run higher. Which means that if the planet is four degrees warmer at the end of the century, when projections suggest we may have as many as 50 percent more people to feed, we may also have 50 percent less grain to give them. Or even less, because yields actually decline faster the warmer things get. And proteins are worse: it takes eight pounds of grain to produce just a single pound of hamburger meat, butchered from a cow that spent
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