Read the full story in The Atlantic.

In the 20th century, the largest animals that have ever existed almost stopped existing. Baleen whales—the group that includes blue, fin, and humpback whales—had long been hunted, but as whaling went industrial, hunts became massacres. With explosive-tipped harpoons that were fired from cannons and factory ships that could process carcasses at sea, whalers slaughtered the giants for their oil, which was used to light lamps, lubricate cars, and make margarine. In just six decades, roughly the life span of a blue whale, humans took the blue-whale population down from 360,000 to just 1,000. In one century, whalers killed at least 2 million baleen whales, which together weighed twice as much as all the wild mammals on Earth today.

All those missing whales left behind an enormous amount of uneaten food. In a new study, the Stanford ecologist Matthew Savoca and his colleagues have, for the first time, accurately estimated just how much. They calculated that before industrial whaling, these creatures would have consumed about 430 million metric tons of krill—small, shrimplike animals—every year. That’s twice as much as all the krill that now exist, and twice as much by weight as all the fish that today’s fisheries catch annually. But whales, despite their astronomical appetite, didn’t deplete the oceans in the way that humans now do. Their iron-rich poop acted like manure, fertilizing otherwise impoverished waters and seeding the base of the rich food webs that they then gorged upon. When the whales were killed, those food webs collapsed, turning seas that were once rain forest–like in their richness into marine deserts.

But this tragic tale doesn’t have to be “another depressing retrospective,” Savoca told me. Those pre-whaling ecosystems are “still there—degraded, but still there.” And his team’s study points to a possible way of restoring them—by repurposing a controversial plan to reverse climate change.