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.

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In 1990, the oceanographer John Martin proposed that the Southern Ocean is starved of iron, and that deliberately seeding its waters with the nutrient would allow phytoplankton to grow. The blooming plankton would soak up carbon dioxide, Martin argued, and cool the planet and slow the pace of global warming. Researchers have since tested this idea in 13 experiments, adding iron to small stretches of the Southern and Pacific Oceans and showing that plankton do indeed flourish in response.

Such iron-fertilization experiments have typically been billed as acts of geoengineering—deliberate attempts to alter Earth’s climate. But Savoca and his colleagues think that the same approach could be used for conservation. Adding iron to waters where krill and whales still exist could push the sputtering food cycle into higher gear, making it possible for whales to rebound at numbers closer to their historical highs. “We’d be re-wilding a barren land by plowing in compost, and the whole system would recuperate,” says Victor Smetacek, an oceanographer at the Alfred Wegener Institute for Polar and Marine Research, in Germany. (Smetacek was involved in three past iron-fertilization experiments and has been in talks with Savoca’s group.)