The Fishy Fix to a Methane-Spewing Crop

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Next time you sit down to a bowl of steaming rice, consider this: Of all the grains humans eat, rice has the biggest carbon footprint. Of course, rice is a staple for half of humanity, which partly explains the outsize footprint. The big problem, though, is that rice is usually grown in water, and there’s not much oxygen in the muddy bottom of a rice paddy. That low-oxygen muck is a happy place for a type of bacteria that produces methane. And each methane molecule can do far more harm to the climate than a carbon dioxide molecule, contributing nearly 30 times more warming over a 100-year span.

In other words, when you grow rice, you also grow a lot of climate-heating bacteria.

One potential solution: fish. Experiments by the nonprofit Resource Renewal Institute suggest that introducing fish to rice paddies kicks off a cascade of events that changes the water’s bacterial communities and ends with less methane leaking into the atmosphere. The fix also offers up a different way of thinking about how living systems contribute to climate change.

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Illustration: Alvaro Dominguez

If the project pans out, it could change rice cultivation around the world. So it’s notable that the effort began almost accidentally. The organization started its Fish in the Fields project in 2012 to reduce overfishing in the wild. “It was going well,” says Deborah Moskowitz, the institute’s president. But in 2015, the outdoor-wear company Patagonia, a major funder of the institute, raised concerns about the climate impact of rice. The company asked Moskowitz if her group could do anything about the methane rising off the fields.

Moskowitz began to scour the scientific literature. She found evidence from Asia suggesting that fish grown in rice fields—an ancient practice in that region—could substantially reduce methane. But the findings weren’t consistent, and no one could explain how fish pulled this off. So when Moskowitz came across a paper in the journal Nature Communications that investigated the fish-methane relationship in a different context—a lake—she was ecstatic.

For three summers, Shawn Devlin, the lead author of that paper, had divided a small Finnish lake in two with a curtain-like barrier. Because it was shallow and covered in ice every winter, the lake naturally lacked fish. Devlin introduced perch on one side and left the other side fish-free. Then, once a month, he measured the greenhouse gases coming off the lake. The side with fish produced 90 percent less methane than the side without.

How? The food chain. Aquatic ecosystems host a veritable Serengeti of microscopic organisms: Some microbes, like the problematic methane producers, grow fat on dead plant material. Others, however, eat methane. These methane lovers are known as methanotrophs.

When the perch showed up, they feasted on the methanotrophs’ main predator, little creatures called zooplankton. With fewer zooplankton around, the methane-eating bacteria proliferated, capturing much of the lake’s emissions before they could bubble into the atmosphere.

“It set me back on my heels,” Moskowitz says of the paper. “I thought, ‘Why should this lake be that much different from a flooded rice paddy?’ ”

Devlin, who’s an ecologist at the University of Montana, didn’t initially see how his findings might turn into a method for reducing emissions. He thought of the research as purely descriptive of certain lake dynamics. So when Moskowitz called him with her pitch to apply the idea to rice paddies, he was gobsmacked. “As an ecologist,” Devlin told me, “to have the concept applied somewhere is such a rarity that it’s mind-blowing.” He headed to California.

The results of the collaboration so far have been promising. In California rice fields, golden shiner minnows introduced by the project have reduced methane coming off fallowed rice paddies by 64 percent. By trying different species and densities of fish, Devlin thinks he can get that number closer to the 90 percent he saw in Finland.

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