The proposed climate fix tech companies just spent millions on? Rocks.

To try to counteract the impact their pollution has on the climate, Google and other big companies have bought into a plan to trap carbon dioxide using rocks. They recently announced multimillion dollar deals with a Sheryl Sandberg-backed startup called Terradot.

Google, H&M Group, and Salesforce are among a gaggle of companies that collectively agreed to pay Terradot $27 million to remove 90,000 tons carbon dioxide from the atmosphere. The deals were brokered by Frontier, a carbon removal initiative led by Stripe, Google, Shopify, and McKinsey Sustainability.

Separately, Google announced its own deal to purchase an additional 200,000 tons of carbon removal from Terradot. Both companies declined to say how much that deal is worth. If the cost is similar to the Frontier agreement — roughly $300 per ton of CO2 captured — it could add up to $60 million, although Google says it expects the price to come down over time for this larger deal.

Google says it’s the biggest purchase yet of carbon removal through enhanced rock weathering (ERW), the strategy Terradot uses to try to slow climate change. It’s a relatively low-tech tactic for taking carbon dioxide out of the atmosphere that now has significant backing from some big names.

“I mean, it’s a big deal, says Oliver Jagoutz, a professor of geology at MIT. “I think it should go a little out of the academic world into the industrial world. And I wish these guys all the best.”

Terradot grew out of a research project at Stanford, where CEO James Kanoff and CPO Sasankh Munukutla were undergraduate students at the time. Shortly before graduating in 2022, they co-founded the company along with Kanoff’s former professor, Scott Fendorf, who is now Terradot’s chief scientist and technical advisor.

Before starting that research project, Kanoff had briefly dropped out of Stanford during the covid pandemic to co-found a nonprofit called the Farmlink Project that connects food banks to farms with excess produce. Kanoff met Sandberg through that initiative, which is how he was able to get the former Facebook COO’s support for Terradot as an investor.

“I’ve known James, the CEO, since long before this company started,” Sandberg said in a press release. “These are proven leaders, which is rare to find in an early-stage company. They have the drive, the right technology and a strong focus on execution to succeed.”

Enhanced rock weathering attempts to speed up a natural process that might otherwise take thousands of years. Rainfall naturally “weathers” or breaks down rock, releasing calcium and magnesium and triggering a chemical reaction that traps CO2 in water as bicarbonate. Groundwater carrying that bicarbonate eventually makes its way to the ocean, which stores the carbon and keeps it out of the atmosphere.

Accelerating the process, in theory, is simple: crush up rock and spread it out over a large area, increasing the surface area of exposed rock that reacts with CO2. Terradot has a 2029 deadline to make good on the 90,000-ton Frontier deal. It’s supposed to capture the additional 200,000 tons for Google by the early 2030s.

Terradot takes basalt from quarries in southern Brazil to nearby farms. Farmers can use the finely-ground basalt to manage the pH of soil, and carbon removal is a bonus. Terradot struck up a partnership with Brazil’s agricultural research agency (EMBRAPA), allowing the startup to use this strategy on more than one million hectares (roughly 2,471,054 acres) of land. Another perk in Brazil is a hot, humid climate that also helps to speed up the weathering process.

The tricky part will be trying to count how much CO2 Terradot actually manages to trap. Google admits this in its announcement:

Right now, it’s hard to measure with precision how much CO2 this process removes from the atmosphere. But the only way to develop highly rigorous measurement tools is to deploy this approach widely in the real world. That’s why our support aims to help Terradot’s solution get out of the lab more quickly.

Terradot says it’ll take soil samples to assess how much CO2 is captured based on how the rock degrades over time. But it’s harder to figure out how much calcium, magnesium, and bicarbonate makes it to the ocean to permanently sequester CO2. Fertilizer in the soil can also potentially limit how much carbon is captured through enhanced rock weathering.

“How much they sequester is still the outstanding question,” Jagoutz says. But he doesn’t think that uncertainty needs to stop trials in the real world. “I also think, why not try? … I don’t think we have the luxury to overthink it right now.”

Carbon dioxide emissions from fossil fuels are already making heatwaves, droughts, wildfires, storms, and other climate disasters more dangerous. And Google’s carbon footprint has grown as it builds out energy-hungry AI data centers. The company has recently announced plans to help develop advanced nuclear reactors and new solar and wind farms to power its data centers with carbon pollution-free electricity. When it comes down to it, switching to clean energy is the only effective way to stop climate change.

Carbon removal, at best, is just an attempt to counteract some of a company’s legacy of pollution while they make that energy transition. And even though Google says it signed the biggest ERW deal to date, 200,000 tons of carbon removal is still a small fraction of the 14.3 million metric tons of carbon dioxide pollution it was responsible for last year.

“It’s very clear that this is not a substitute for emissions reductions at all … we need both of these tools,” Kanoff says. “Any of the partners we’re even thinking about working with, they have some of the most aggressive emission reduction strategies of any of the companies really in the world. And those are the groups that we really want to partner with to advance carbon removal.”