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All you need to remove a lot of carbon dioxide from the atmosphere is an open field and piles of crushed-up rock. That is the promise of what may become one of the primary ways to durably remove billions of tonnes of COâ‚‚ from the atmosphere, helping the world reach net zero on time and avoid the most dangerous effects of climate change.
Known as “enhanced weathering”, the method involves spreading crushed-up silicate rocks – usually volcanic basalt – on fields, ideally on farms where it can also help crops grow. CO₂ dissolved in water reacts with calcium and magnesium in these rocks to form bicarbonate minerals that store the carbon. Eventually, most of these minerals drain into rivers and flow to the ocean, where they remain for millennia.
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This rock weathering process happens naturally, but only over geological timescales. By crushing and spreading the rock, the idea is to increase the surface area exposed to COâ‚‚ to make the reaction happen faster.
In this edition of Fix the Planet, I’ll look at some of the many enhanced weathering projects scaling up around the world and their biggest challenge right now, which is reliably measuring how much CO₂ they actually capture and store.
Magic dust
From giant fans that blow air over absorbent chemicals to sinking seaweed buoys into the deep ocean, researchers are exploring an impressive variety of ways to remove CO₂ from the atmosphere. This is no replacement for slashing emissions, but the Intergovernmental Panel on Climate Change says somewhere between 5 billion and 16 billion tonnes of CO₂ will have to be removed each year by the middle of the century to keep the global temperature rise below 1.5°C.
Just a few years ago, spreading rock dust on farms was just one of these ideas yet to be tested. Now, companies on every continent except Antarctica have enhanced weathering projects under way, with plans to scatter hundreds of thousands of tonnes of rock dust over tens of thousands of hectares of farmland.
“It’s getting more and more obvious that enhanced rock weathering will soon be one of the leading carbon dioxide removal technologies,” says at InPlanet, a company headquartered in Germany that has just started what it says is the first large-scale enhanced weathering project in South America. Last year, the firm spread 50,000 tonnes of basalt on Brazilian farmland, with plans to spread 200,000 tonnes more this year.
Others are expanding as well. UNDO, a UK firm, has put down nearly 170,000 tonnes of basalt rock so far at sites in Scotland, England and Australia and has trial sites in Kenya and Tanzania. It also has a in Canada testing a rock called wollastonite that weathers faster than basalt.
In December 2023, a consortium of big tech companies that has pledged to spend more than a billion dollars to support COâ‚‚ removal made its largest purchase yet from an enhanced weathering company. The group paid California-based Lithos Carbon $57 million to remove more than 150,000 tonnes of COâ‚‚ over the next five years by spreading rocks on US farms.
Those projects make up the lion’s share of the roughly of CO₂ removal purchased from enhanced weathering projects so far, just a fraction of which have yet been delivered. But the potential is enormous. As a high-end estimate, one found that placing 10 tonnes of basalt rock each year on every hectare of farmland on Earth would remove more than 200 billion tonnes of CO₂ over 75 years, an amount roughly equivalent to our entire remaining carbon budget. Another study, factoring in limits to cropland and other considerations such as the energy needed to grind rocks, found it would be possible to scale up to remove between 0.5 billion and 2 billion tonnes of CO₂ by mid-century. Spreading rock dust just on UK farms could get the country almost halfway to its 2050 net-zero target, according to work by at the University of Sheffield, UK, and his colleagues.
Improved yields
Proponents of enhanced weathering also point to the beneficial effects it can have on soil. Many farmers already spread limestone on fields to make them less acidic, which makes nutrients more accessible to crops. Spreading crushed silicate rocks like basalt instead can have a similar effect, as well as replenishing other metals and nutrients and reducing some fertiliser-related emissions. Researchers are still working out the precise effects, but early results are encouraging.
at UNDO says the company’s recent tests at a trial field near Newcastle, UK, found a 15 per cent increase in the average yield of spring oats after a single treatment with basalt rock compared to untreated crops. A by Beerling’s team found a 12 to 16 per cent increase in yields of corn and soybeans in the US Midwest after spreading basalt rock compared with traditional liming. “Ultimately, farmers may want to do this just because it improves their soil and yields,” says Beerling.
There are several issues that could complicate enhanced weathering, though. For instance, depending on its source, rock dust could contain minerals that might contaminate crops, such as asbestos. Many of the companies working now use rock powder produced as a byproduct of other work at quarries (one is even using ), but at larger scales these existing supplies could run out, requiring more rocks to be mined and crushed or shipped from elsewhere, adding associated emissions. Trucking in rocks and spreading them on fields also generates emissions that must be factored in.
A bigger challenge at the current stage, however, is reliably measuring exactly how much CO₂ such projects remove and store over the long term, a requirement for selling trustworthy carbon credits to companies seeking to count removals against their emissions. “There is sometimes the perception that you just need to chuck rock around the field,” says Mann. “The real difficulty is not getting rock to the field, but measurement.”
Measuring up
Once rock dust is spread over a field, a host of factors affects how rapidly the weathering process occurs and how much mineralised carbon ultimately makes it to the ocean for storage. These range from the type of rock and the size of particles in the dust to the acidity of the soil and the climate at a given site. The hot and humid climate in the tropics is associated with than cool, dry places, for instance: InPlanet claims weathering rates in Brazil are fully 50 times faster than in temperate sites. A substantial and difficult-to-measure amount of carbon can also be lost to the atmosphere as the bicarbonate flows from the field to the sea.
at Yale University says carbon removals can be measured directly by looking at gas fluxes above the soil, the composition of water in soil and how much of the rock dust is dissolved over time. By combining enough of these measurements, reaching a reliable estimate of removals at a given site is possible, he says. The hard part is doing it across huge areas of farmland without breaking the bank. “This is going to remove carbon,” he says. “The question is, can you actually quantify this at a reasonable cost?”
This challenge is exacerbated by the lack of a widely agreed-upon standard for measuring enhanced weathering. That leaves companies to decide on their own methodologies for monitoring and reporting any CO₂ removed by their efforts. “They’re not independent protocols,” says Beerling, likening it to students grading their own homework.
That doesn’t necessarily mean firms overstate their removals – for instance, Mann says UNDO generally assumes around 20 per cent of potential removals are lost before long-term storage. But as more projects with varying standards spring up, it could lead to less reliable removals. “I fear we’ll have a lot of rock hitting the ground that can’t be measured,” says Mann.
There are to establish better standards for weathering projects. One third-party already exists, published by a carbon registry called Puro.earth, although the company says no credits have yet been verified with it. But even with agreed-upon standards in place, using them at larger scales will require finding ways to accurately model rates of weathering based on just a few choice measurements, says Planavsky.
To that end, Beerling, Planavsky and their colleagues are working on a to see how predictions from different models of rock weathering stack up in experiments. Better models will also require a lot more data on what happens when the right rocks are spread on a field, whether in Britain or Brazil.
