The Fishy Fix to a Methane-Spewing Crop
Rice has the biggest
carbon footprint of any grain. Bite by bite, bacteria-guzzling minnows can make
it much smaller.
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.
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.
For Oswald Schmitz, an
ecologist at the Yale School of Forestry and Environmental Studies (he's not
involved in the project), the big lesson of Devlin's research is that “animal
diversity drives the carbon cycle,” he says. With a large predator present,
carbon from decaying plant material gets shunted into fish meat instead of
flowing into the atmosphere as methane. Animals might even be useful in
conservation generally, helping ecosystems wilder than rice paddies sequester
carbon and aiding in the fight against climate change. “We view animals as
passengers on a sinking ship,” Schmitz says, “when in fact, they're drivers of
the ship.”
On a wet, drizzly December
day in California's Sacramento Valley, the Resource Renewal Institute launched
its most ambitious test yet of this idea. Chance Cutrano, the organization's
director of programs, emptied buckets of golden shiner minnows, their
underbellies flashing silver, into a 7-acre paddy. Between 2,500 and 3,000 fish
would enter the turbid water; later 13,000 more fish were plopped into two
other locations. “This is as good as it gets for fish,” Cutrano said. “Go forth
and grow!”
For the rice farmers, Fish
in the Fields offers the additional enticement of a second possible income
stream from paddies that don't have much use in the winter. The paddies provide
all the food the fish need, and the fish are harvested before rice planting
season, so they don't interfere with the summer crop.
Where could farmers sell
minnow protein? That winter day, Moskowitz had a proof of concept: pouches full
of dried-fish dog treats. She'd baked them at home. “My kitchen smelled
interesting after I made them,” she told me with a smile. But her dog loved them.
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