The startups betting on pulverized rocks for carbon removal

To many industry observers, “carbon removal” conjures visions of extreme feats of engineering: whirring fans that suck in air or wells built to inject carbon underground. 

But to some, the industry’s potential looks just like sand.  

Direct air capture is easily the buzziest carbon removal pathway, but it is far from the only one being explored. Forecasts suggest DAC plants will remove less than one gigaton of carbon from the air by 2050. By then, IPCC projections suggest the world may need to be removing a whopping 5.8 gigatons of carbon per year to keep warming below 1.5 degrees Celsius, though that number is hazy and based on how much humans continue to emit. 

By some estimates, enhanced rock weathering — a technique that uses crushed rock to lock away carbon — could remove more than 2 billion tons of carbon annually by 2050. 

So today, weathering startups are working to prove that they can turn those theoretical numbers into actual removal, then win the essential financial backing that is already flowing to other carbon removal technologies. But first, they’ll need to overcome some lofty barriers, including figuring out how to measure, and prove, how much carbon they’re absorbing.  

Enhancing a natural reaction 

Enhanced rock weathering relies on a natural chemical reaction between water and silicate minerals, which make up many rocks on earth. When carbon dioxide reacts with rain and the resulting acidic rain falls, it creates a chemical reaction that locks up carbon. The carbonates it produces eventually travel to the ocean, where carbon can be stowed for millennia. The ocean is the largest carbon sink on the planet. 

Enhanced rock weathering speeds up that natural reaction using rock crushed into tiny pieces, which allows the rock to react more quickly and suck up carbon at a faster rate. 

A growing group of startups aim to capitalize on that process. And while the field is still nascent, several have pilots in the works or small-scale projects up and running. For instance, Vesta focuses on rock weathering in coastal areas and estimates that it will remove 500 tons of carbon from the atmosphere via its pilot project in New York.

The potential for enhanced rock weathering is not solely in its capacity to make a dent in emissions, according to those who hope to commercialize it. Because the process relies on plentiful materials that are already shipped from place to place, they say the industry could scale to bigger project sizes more rapidly than carbon removal options like DAC that require significant engineering and development.

“We are, in many ways, co-opting old supply chains for a new and environmentally-friendly purpose,” says Tom Green, Vesta’s co-founder and CEO.

For instance, many enhanced rock weathering companies get their materials from quarries that already exist; in 2013 there were more than 3,000 producing crushed rock and stone in the United States alone. 

There are potentially side benefits as well. The minerals involved in enhanced rock weathering can also improve soil health and boost crop yields , and plant roots help the chemical reaction happen more quickly.

“It's really considered a very socially-desirable negative emissions technology, in the sense that there [are] a lot of co-benefits,” says Seung Hun Baek, a postdoctoral fellow at Lawrence Livermore National Laboratory, who recently authored a paper on the potential of enhanced rock weathering on global agricultural lands. 

And when washed out to the ocean, carbonates make water more alkaline. If deployed more widely, that could help fight ocean acidification, which contributes to coral bleaching. Vesta’s Green said the company’s coastline-focused process also helps fight erosion and restore shores. 

However, given the current scale of enhanced rock weathering’s deployment, any significant impact from these benefits is still years away.

Carbon removal for $20 per ton?

Enhanced rock weathering also has another, important attribute on its side: it’s durable. 

“One way in which enhanced rock weathering is different from other nature-based solutions is that it's very hard to reverse,” says Peter Olivier, UNDO’s head of new markets. “Once we have put the rock on the field, if we go out of business, that process is still going to happen.”

UNDO uses basalt — a rock that makes up 90% of all global volcanic rock and most of the ocean floor — sourced from aggregate companies that are already producing the stuff. The rock they use is a byproduct, generally too small for other uses. Relying on other producers means UNDO doesn’t need to own its own quarries or equipment; its partners can crush the rock however they choose. It’s an energy-intensive process that today often relies on fossil fueled-electricity or diesel generators. 

Even with those added emissions, though, Olivier says UNDO removes 9 tons of carbon for every ton it emits. And while measuring and verifying these kinds of removals has been a complicated project, the company has already inspired enough confidence to sell carbon credits to large corporate offtakers. This is a big year for the company: UNDO plans to reach 185,000 tons of silicate rock spread by the end of the year, which they calculate will remove about 40,000 tons of carbon by the time it’s completely weathered.

Like many other weathering startups, UNDO focuses on farmland. They pay local companies or landowners themselves to spread the rock, with the aim of selling the carbon credits in the voluntary market. The London-based company already has projects in Europe, the U.S., Australia, Canada, and Kenya. 

Meanwhile, Vesta has thus far focused on the U.S. In 2022, the startup set up a pilot on New York’s Long Island, spreading 650 tons of olivine sand. Olivine is considered the most common mineral on earth, and Vesta sources it from a quarry in Norway. The quarry uses renewable energy to grind the rock into sand-size particles, and Vesta transports it across the ocean for use. 

All told, Green said the company removes 20 tons of carbon for every one ton it emits in that process. They’re now looking at other sites on the Eastern Seaboard, and have identified one, to apply more sand. 

At scale, Vesta estimates it will be able to remove one ton of carbon for $21. 

That is far below currently projected costs. One of the few peer-reviewed papers on the technical and economic potential for enhanced rock weathering puts the current average cost of weathering projects in the U.S. between $160 and $180 per ton. Other carbon removal technologies could cost anywhere from $15 to $1,000 per ton. 

‘Definite uncertainties’

But weathering does face headwinds. 

Measurement is one major uncertainty. Companies claiming that they’re removing carbon dioxide from the atmosphere need to be able to prove it. 

Other engineered carbon removal options like DAC would likely involve pipelines to carry carbon around, and those pipelines can be metered. But measuring rock weathering will never be that precise. And today, there’s no standard for how to measure carbon removal from enhanced rock weathering.

“There are definite uncertainties in this process,” said Olivier. “It's an open ecosystem process, and as a consequence it’s going to be different in different places. There remains an enormous challenge to nail down and drive out the uncertainty in these processes.”

Both UNDO and Vesta are iterating mechanisms to measure carbon removed throughout the weathering process. For instance, Vesta uses proprietary hardware to measure alkalinity in seawater, which Green said is a direct measurement of the carbon removal reaction. 

Scientists are also only beginning to determine the scale that enhanced rock weathering could feasibly reach. Baek said his is only the second paper to estimate the potential for enhanced rock weathering worldwide and, to his knowledge, there’s no peer-reviewed life cycle assessments to bear out the claims that companies make about their net carbon removals.

“The idea for enhanced rock weather has been around,” he added, “but really exploring its potential, I think, is newer in the field.”  

And the field has yet to receive significant government support. Direct air capture has garnered splashy Department of Energy press conferences and promises of billions of dollars in federal funds, along with significant backing from the powerful fossil fuels industry. Green said it’s not clear if enhanced weathering is eligible for carbon removal tax credits like 45Q or certain governmental grant programs. 

That type of support can spur early projects, which can drive down costs more broadly. At this point, Baek said, the uncertainty about measurement, scale, and how cheap the projects can ultimately be is complicating the industry’s growth.  

“Theoretically, it should be easy to scale, but for non-science reasons, it's difficult to,” he said. 

Still, enhanced weathering has already seen some successes. 

In April, Microsoft announced it would purchase carbon removal from UNDO, which the company will carry out over the next couple of decades. And Frontier — an advance market commitment for carbon removal founded by companies including Meta, Shopify, Stripe — has also signed $3.7 million in contracts for enhanced weathering projects in collaboration with eight startups. (Neither Vesta nor UNDO are involved in these contracts.) 

Olivier calls these “catalytic purchases” that show confidence that weathering startups can bear out their removal claims. And much like with renewable energy, corporate demand for carbon removal could help build interest among more conservative entities, like the government. 

In late October, the Department of Energy said it would disburse $36 million to projects focused on marine carbon dioxide removal, including measurement techniques, which Green sees as an encouraging signal. And the department includes enhanced weathering in the suite of ideas it's pursuing as part of its Carbon Negative Shot program.  

“We're going to need more than one solution,” Green said. ”The scale of the problem is enormous.” 

“What we need to make sure is that the energy and the dollars and everything flows towards approaches [that] are able to demonstrate that they work: that they're scalable, that they're durable, and of course, critically, that they’re cost effective,” he added.

Editor's note: This story was corrected on November 27; UNDO does not currently pay growers to use a portion of their farms to spread rock.