Napa Valley is built on a bedrock of cabernet sauvignon. Imported from France around 1853, this varietal now accounts for 51 percent of grapes grown in the region. It thrives in moderate temperatures—below 68 degrees Fahrenheit or so—making it perfectly suited to the Northern California climate. Or at least, it used to be. Climate change now poses an existential threat to cabernet and other quintessential Napa wines. Projections show the area in Napa suited to growing cabernet, pinot noir, and other premium grapes could be cut in half by 2039. In the face of these climbing temperatures, along with potential water shortages and more devastating fires, Napa’s growers are looking to take action: both to protect their own livelihoods and to contribute to global efforts to fight climate change. Lately, many are embracing something called carbon farming—a suite of practices designed to maximize carbon storage in the soil, instead of letting it loose in the air.

The most optimistic calculations suggest that if implemented worldwide, carbon farming could sequester a gobsmacking 100 percent of annual global CO2 emissions. “The climate change mitigation potential of these agricultural practices alone could lower temperature by 0.1 to 0.3 degrees Celsius by the end of the century,” says Whendee Silver, a professor of environmental science, policy, and management at the University of California–Berkeley and one of the scientists most closely associated with carbon farming.

Those fractions of a degree may not sound like much, but according to the Paris accord goals, we can only afford for temperatures to rise by one more degree Celsius, so lowering temperatures by 0.1 to 0.3 degrees would be a significant achievement. It’s an attractive proposition to any farmer who cares about climate change, and Napa winemakers are generally a very green bunch. Miguel Garcia, who works at the Napa County Resource Conservation District and is leading the effort to implement carbon farming in Napa vineyards, said, “There’s really no reason, given enough time, that the entire valley can’t be carbon neutral or even carbon negative.”

Some experts dispute the idea that tweaks to soil management can render any kind of agriculture carbon neutral, let alone negative, and the answers to some important questions about carbon farming are unanswered or disputed. But Napa winemakers, like many California farmers, are forging ahead.

Garcia creates free, customized plans for vineyards that want to try carbon farming. This process begins with testing their soil carbon baseline and culminates in a prescription of changes to improve it. One step is usually to stop or minimize tilling, which removes organic matter (aka carbon) and aerates the soil, releasing carbon dioxide into the air. Others include adding compost and planting cover crops underneath the grape vines. All of these are big changes to implement for a trade driven by tradition and history. Cover crops change the look of a vineyard from neatly combed rows to something more wild, and tilling has been best practice in vineyard management for a long time. But the region’s winemakers have been surprisingly quick to embrace the new way of doing things. Garcia says 15 vineyards have implemented carbon sequestration plans so far, and 13 more are in the works. Napa County is home to approximately 700 wine grape growers, and Garcia’s goal is to create plans for at least 12 of them every year.

“The more traditional people have ways of doing things that they’ve done for years and so they believe that it’s right,” says Kristen Belair, the winemaker at Honig Vineyard & Winery, which has spent years advancing various sustainability efforts and is now working with Garcia to implement a carbon farming plan. “But I think there’s enough of it being done now and people see what’s around them, you know, they don’t want to be left out.” Silver, the UC–Berkeley scientist, visited Napa Valley earlier this year to give a talk on carbon farming to winegrowers. “I wasn’t quite sure how it was going to go over,” she says. “But there were so many producers that had some experience already that they were asking me really good questions about the details of the science.”

Enthusiastic believers in carbon farming see a relatively painless way to undo years of fossil fuel emissions. Most carbon farming methods—adding compost, planting cover crops, rotating crops, mulching, and even eliminating tilling—require relatively little effort for significant benefit. Groups of American farmers and even the U.S. Department of Agriculture have advocated for no-till farming ever since the Dust Bowl of the 1920s, when soil erosion and drought created dust storms that ruined crops and killed people and livestock across huge swaths of the U.S.

Yet skeptics see in carbon farming an under-researched distraction from more effective climate solutions. One such doubter, Ron Amundson, occupies the office next door to Silver’s in the UC–Berkeley biogeochemistry department. In 2018, he argued in a paper in Proceedings of the National Academy of Sciences that both cultural and scientific challenges stand in the way of carbon farming as a successful climate mitigation strategy. Amundson attended the 2000 Conference of the Parties, the meeting where implementation of the Kyoto Protocol was negotiated. He says the claims he heard about the potential of carbon farming at that conference sound a lot like those made today. “In 20 years of advocating, writing scores of papers and so forth, we can’t see any indication in the atmosphere that any activity in soil has actually pulled back CO₂ in climatically significant ways,” he says of the efforts by carbon farming advocates. “It’s still a promise or a premise rather than an actual mitigation remedy.”

Research into carbon farming continues to unearth big questions and uncertainties. Timothy Searchinger, an environmental policy expert at Princeton, co-authored a report with the World Resources Institute (in collaboration with the several U.N. agencies) on sustainably feeding the world’s population. Its chapter on carbon farming was deflating. “We’re very doubtful about the potential to sequester a lot of carbon, and we think there are other things that are more important to do,” he says. Searchinger and his colleagues reviewed the available research and concluded it showed significant uncertainty around how much carbon is actually sequestered with common carbon farming practices such as reduced tilling or adding more manure or compost to the soil. Changes in soil carbon are extremely hard to measure, and scientists don’t always even define it the same way.

Additionally, no-till farming is hard to pull off for long periods of time—detritus from previous years’ crops builds up, which can make it hard to plant new seeds and complicates soil drainage—and plowing a field even once can undo years of sequestration gains. “For these practices to provide substantial carbon sequestration, pretty much everything has to go right, and it never does,” says Dan Blaustein-Rejto, associate director of food and agriculture policy at the Breakthrough Institute, a climate solutions think tank. About 20 percent of farmers in the U.S. say they practice no-till farming, but Blaustein-Rejto says surveys show about 60 percent of these farmers actually do till their land occasionally.

Amundson, Blaustein-Rejto, and Searchinger agree that as carbon emissions continue to rise, it’s essential to focus on reducing emissions rather than trying to soak up carbon in the soil. Some more prosaic avenues, such as increasing efficiency, have already proved to be very effective. The amount of greenhouse gas emissions associated with a gallon of milk from California is half what it was 50 years ago. Livestock are still responsible for most of California’s methane emissions, and the state has invested hundreds of millions trying to address that. But it has also invested in carbon farming—more than $17 million in the Healthy Soils Program, which distributes money to farmers and ranchers for “implementation of conservation management that improve soil health, sequester carbon and reduce greenhouse gas (GHG) emissions,” as well as demonstration projects to showcase these practices. Farmers can now earn carbon credits for spreading compost on their land—a financial incentive that presumes precise knowledge of how much carbon this practice sequesters, though those numbers are often only vaguely known or actively disputed among scientists.

Yet there’s no way for carbon farming to become a tried and tested emissions reduction method unless farmers try it and scientists test it. Ranchers in Marin County, west of Napa, were some of the pioneers of carbon farming in California, and their efforts are studied closely by several scientists. The winemakers’ work isn’t being as closely scrutinized, but perhaps it should be. Napa is in many ways ideally positioned to implement carbon farming, says Garcia. Winemakers also have an economic incentive many farmers don’t—their customers might be willing to pay more for “carbon neutral” wine.

As the Honig winery stops tilling and plants cover crops and hedgerows, Belair says it’s exciting to see that even on a small plot of land here in Napa Valley, they can actually do something. “There are a lot of myths and anecdotes in farming and winemaking and just in the world in general. So things don’t get questioned and they don’t get tested,” Belair says. Basic agricultural questions remain unanswered about most carbon farming practices: Do they increase or decrease yield? Does compost help with water retention and drought resilience? Napa’s grape growers might help answer them.

In the meantime, it remains true that using compost and mulch, planting cover crops, and reducing tilling are relatively low cost, helpful practices to boost soil health—and they can hopefully boost resilience to drought, extreme heat, and the other ravages climate change throws at us. Focusing only on the climate mitigation impacts of carbon farming undermines the importance of adopting these strategies. Mark Bradford, a soil scientist at Yale, recently published a paper in Nature Sustainability with several colleagues, making this point. They argued that the disagreement over the role of soils in climate mitigation erodes confidence in the effort to increase soil carbon for the purpose of protecting and restoring agricultural soil.

Awarding carbon credits or calculating emissions reduction based on carbon farming may be dubious at this point, but carbon farming is really just good farming. And that’s crucial for creating a sustainable future of agriculture, no matter what its role in fighting climate change may be.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.

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