Dramatic declines in ocean oxygen have played a role in many of the planet’s worst mass extinctions, and this process by which dead zones grow—choking off marine life and wiping out fisheries—is already quite advanced not only in the Gulf of Mexico but just off Namibia, where hydrogen sulfide is bubbling out of the sea along a thousand-mile stretch of land known as the Skeleton Coast. The name originally referred to the detritus of wrecked ships, but today it’s more apt than ever. Hydrogen sulfide is also one of the things scientists suspect finally capped the end-Permian extinction, once all
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