Can Rock Dust Soak Up Carbon Emissions? A Giant Experiment Is Set to Find Out

Mary Yap has spent the last year and a half trying to get farmers to fall in love with basalt. The volcanic rock is chock full of nutrients, captured as its crystal structure forms from cooling magma, and can make soil less acidic. In that way it’s like limestone, which farmers often use to improve their soil. It’s a little more finicky to apply, and certainly less familiar. But basalt also comes with an important side benefit: It can naturally capture carbon from the atmosphere.

Yap’s pitch is part of a decades-long effort to scale up that natural weathering process and prove that it can lock carbon away for long enough to make a different to the climate. “The bottleneck is getting farmers to want to do this,” Yap says.

On Thursday, Yap’s young startup, Lithos Carbon, got a $57.1 million boost for its quest to turn basalt dust into a viable climate solution. It came from Frontier, a benefit corporation backed by a consortium of companies aiming to finance promising approaches to carbon dioxide removal, or CDR. Lithos says it will use the funds to soak up 154,000 tons of CO₂ by 2028, by sprinkling basalt dust on thousands of acres of US farmland. The average US car emits about 4 tons of CO₂ each year.

The carbon removal purchase is the largest yet by Frontier, which was formed last year with nearly $1 billion from its tech-dominated members. Many of those companies, which include Meta, Alphabet, and payments processor Stripe, which owns Frontier, have made climate pledges that require not only reducing the emissions from their operations and supply chains but also “negative emissions”—sucking up carbon from the atmosphere to cancel out other emissions.

That accounting trick has been easier to prove out on paper than in practice. Many companies would have once turned to buying carbon offsets from activities like protecting forests that would otherwise be felled. But some have been trying to move away from those scandal-plagued and often short-lived approaches and into more durable techniques for carbon removal.

The current options for companies seeking negative emissions are limited. Frontier’s purchases are essentially down payments on ideas that are still in their infancy—generally too hard to verify or too expensive, or both, to attract a significant customer base. “What we're trying to evaluate the field on is whether it’s on the trajectory to get to climate-relevant scale,” says Nan Ransohoff, who leads Frontier and also climate work at Stripe. The group starts with small “prepurchases” meant to help promising startups, and then moves on to “offtake” agreements for larger amounts of carbon that its members can count toward their emissions goals.

The Lithos purchase is one of those larger deals. It prices carbon removals at $370 per ton, about a quarter of which will pay for field monitoring and modeling to verify that carbon is being sequestered away from the atmosphere for the long term. Ransohoff says Frontier believes that Lithos is on a path to its goal of removing CO₂ for customers at a cost of less than $100 per ton, and at a rate of at least a half a billion tons per year.

Lithos, founded in 2022, is developing a technology called enhanced rock weathering. It involves spreading a fine dust of basalt across fields before planting. As the rock further weathers from rainfall, it reacts with CO₂ in the air. That forms bicarbonate, which locks away the carbon by combining it with hydrogen and oxygen atoms. Ultimately, the compound is washed into the ocean, where the carbon should stay put.

The strategy has the benefit of piggy-backing on things that humans already do, Yap says. That’s in contrast with techniques like direct air capture, which involves building industrial plants that suck carbon out of the atmosphere. It’s easy to measure carbon removed that way—it’s all captured there onsite—but critics say it will be difficult to scale up because removing enough carbon to make a difference will require thousands of dedicate, resource-intensive facilities.

Using basalt dust to capture carbon should be more easily scaled up. There are plenty of fields to dump rock dust onto, and plenty of water for carbon to end up in. But the distributed nature of the process also makes measuring how much carbon was actually removed from the atmosphere more difficult.

The first challenge is knowing precisely how much carbon dioxide has been trapped in the soil thanks to the added basalt. That can vary depending on local weather conditions, soil chemistry, the exact properties of the basalt, and other factors. Lithos currently monitors how its basalt dust is dissolving using instruments placed on every 2.5 acres of land. But that’s expensive, even across a relatively small number of farms. Yap hopes that Frontier’s purchase will make it possible to collect enough data to eventually replace some of that testing with computer models.

Another challenge is that carbon sucked up by basalt reactions doesn’t necessarily stay locked up for long. It’s a long journey from, say, an Illinois farm to the Gulf of Mexico, through groundwater, rivers, and wetlands. A lot can happen on that journey, including some carbon dioxide getting released back into the atmosphere, says David Ho, an oceanographer at the University of Hawaii. (Ho was texting with WIRED from a ship in the northeast Atlantic, where he is investigating related questions about air and water interactions.)

Simulations of ocean chemistry are generally good at predicting the fate of bicarbonates. But the process of getting to the sea is significantly murkier, given the complexities of water flows and different chemical environments. Ho, who cofounded a nonprofit called [C]Worthy that is developing tools for validating CDR techniques, calls the uncertainty “huge.” Lithos is among those working on simulations of how the bicarbonate ions behave in rivers, part of what Yap describes as a “cradle-to-grave” model of the weathering process.

Ho calls the startup's approach among “the most promising” of carbon removal strategies. Yet he also questions the appropriateness of corporations like those behind Frontier using purchases of nascent CDR techniques to meet their climate commitments. Ho thinks there’s too much uncertainty in calculating the exact amount of carbon they remove, and a lack of independent verification. “It’s kind of like marking their own homework,” says David Beerling, director of the Leverhulme Centre for Climate Mitigation and a biogeologist who has worked with Lithos’ scientific partners but isn’t involved with the company.

Still, Beerling and Ho both say the funds from Frontier’s Lithos deal will help push enhanced rock weathering toward the point where companies and governments feel confident that it works, by jump-starting data collection on the technique. “The great thing about these large deployments is that it forces them to solve the logistical challenges,” Beerling says. “How do you get the rock down? How do you get farmer buy-in? How do you sample the fields? How do you process all that data?”

Those logistical questions loom large for Yap. To farmers, Lithos is essentially a fertilizer supplier, selling a dusty substance that is claimed to improve fields and replace limestone, with side benefits to the climate. “The way we position ourselves is actually as an agriculture company,” Yap says.

Lithos is initially enticing customers by offering to sprinkle basalt on farmland for free. The company collects basalt waste from quarries—there isn’t much market for the fine dust—and transports it by truck to the nearby fields of its farmer partners. After the basalt has been applied, fields are monitored carefully not just for carbon uptake but also for changes in crop productivity, which depends on the specific plants and soils in a location, and the chemical composition of local basalt. Lithos currently operates in seven states across the US.

Yap says she already has a lengthy waiting list of farmers, representing enough acreage to capture potentially hundreds of thousands of tons of carbon. The proposition can be appealing because it means no longer paying for limestone to add to soil. Lithos expects to get more cost-efficient as it scales up, and Yap says that costs are already falling, but to reach the $100 per ton goal set by Frontier, farmers may need to be willing to pay for basalt deliveries themselves.

If Lithos can get through that bottleneck, scaling up will inevitably bring other challenges. Among them will be ensuring that large amounts of bicarbonate-rich runoff don’t disturb local ecosystems, and grappling with the environmental and health impacts of moving large volumes of basalt dust over greater distances. Some analyses have found that if the transportation demands grow too high, the carbon benefits of weathering can be erased entirely. Ho says that’s all the more reason to proceed with caution, noting that in the near term, CDR techniques should generally take a backseat to the more immediate challenge of cutting carbon emissions.

Ranshoff of Frontier says that’s the goal: to identify by 2030 the kinds of companies that will be ready to scale in time to reach levels of carbon removal required to meet net zero goals by midcentury. “We're in the process of learning about something that no one's ever done,” says Yap. “We're learning as we go.”