January 21, 202514 min readCrushed Rocks Could Be the Next Climate SolutionSpreading crushed stone across farm fields could inexpensively pull CO2 from the air while also increasing yields. But it would require a mountain of miningBy Douglas Fox edited by Mark Fischetti Jared Unverzagt/Getty ImagesThe scene that unfolded on a cold November day in central Illinois might seem commonplace, but it was part of a bold plan to pull billions of tons of carbon dioxide from the atmosphere and stuff it into the ocean.A few miles south of Urbana a dump truck trundled past bare fields of dirt before turning into an adjacent lot. It deposited a cottage-size mound of grayish-blue sand190 metric tons of a crushed volcanic rock called basalt. Farmers spread the pulverized basalt across several fields that they sowed with corn months later. This was the fourth year of an ambitious study to test whether the worlds farmlands can be harnessed to simultaneously address three global crises: the ever rising concentration of planet-warming CO2 in the atmosphere, the acidification of the oceans and the shortfall in humanitys food supply.The trial results, published in February 2024, were stunning. David Beerling, a biogeochemist at the University of Sheffield in England, and Evan DeLucia, a plant physiologist at the University of Illinois Urbana-Champaign, led the study. They found that over four years, fields treated with crushed basalt and planted with alternating crops of corn and soy pulled 10 metric tons more CO2 per hectare out of the air than untreated plots. And crop yields were 12 to 16 percent higher. In other research, they found that adding crushed basalts to soils improved the harvest of miscanthus, a tall grass that is used to make biofuels, by 29 to 42 percent, and the fields captured an estimated 8.6 metric tons of CO2 per hectare of land each year, compared with untreated fields. It was exciting, Beerling says. We were pleasantly surprised.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Their findings added to positive results elsewhere. In 2020 researchers in Canada reported that adding the mineral wollastonite to fields growing lettuce, kale, potatoes and soy sequestered CO2 in the soil at rates as high as two metric tons per hectare per year. And last spring Kirstine Skov, a natural geographer at the start-up company UNDO Carbon in London, showed that crushed basalts improved the yields of spring oats by 9 to 20 percent while reducing soil acidity in several fields in England.Scientists, start-up companies and large corporations are experimenting with elaborate technologies to slow global warming: High-altitude planes that release sulfur dioxide into the stratosphere to block some incoming sunlight. Machines on Earths surface that pull CO2 out of the atmosphere. Iron sprinkled across the sea that enhances the growth of algae that absorb CO2. These deployments could buy humanity some extra time to transition from fossil fuels to clean energy while preventing the climate from crossing dangerous thresholds in a permanent way. But the exotic approaches require gobs of money and energy or could threaten ecosystems. Simply spreading crushed rock on fieldsas farmers have done for centuries with limeseems refreshingly low tech. Thats part of its elegance, Beerling says.The basalt in Illinois came from a quarry in southern Pennsylvania, where it is mined for roofing and building materials. Basalt is the most abundant rock in Earths crust. As it naturally weathersgradually dissolving in soil waterit captures CO2, converting it into bicarbonate ions in the water, which cannot easily reenter the atmosphere. The reaction also releases into the soil nutrients that are important for plant health, including calcium, magnesium and silicon. Grinding and spreading basaltan approach known as enhanced rock weathering (ERW)speeds up those processes greatly. It could help cash-strapped farmers around the world by increasing crop yields, reducing fertilizer use and potentially allowing them to sell carbon credits.Seeing how this landed with the public and press strengthened our belief that this was the right way to go. David Beerling, University of Sheffield If ERW were to be scaled up globally, it could remove up to two billion metric tons of CO2 from the air every year, according to Beerling. That would cover a significant share of the atmospheric carbon humanity must draw down to keep temperature rise to 1.5 degrees C, widely acknowledged as the necessary goal to prevent widespread catastrophe. But ERW would require mining and crushing billions of tons of rock every yearenough to build a mountainand transporting it to farms, all of which would release CO2. Still, calculations suggest that those emissions would pale in comparison to the amount of CO2 that the rock stores away for centuries or longersequestered more permanently than it could have been in a forest of trees.ERW is newer than the other so-called negative emissions strategies, and so far only a few trials have been fielded. Yet companies are already looking to sell carbon credits tied to the technique. Noah Planavsky, a biogeochemist studying enhanced weathering at Yale University, sees promise in these unsettled circumstances. But he worries that if ERW expands too quickly, before the technique is refined, it could produce disappointing results and generate a backlash. This has the potential to be something truly impactful, he says. And there are so many ways you can imagine it going poorly.The idea of ERW is based on a fundamental insight about how Earth naturally functions. Across geological time, lava eruptions spewed huge amounts of CO2 into the atmosphere, heating the planet. Subsequent weathering of the erupted rock over millions of years pulled the gas out of the atmosphere, cooling the planet back down. Basalts are effective in capturing CO2 because they are high in calcium and magnesium from deep in the planet. Today vast swaths of North and South America, Africa, Asia, and other areas are covered in these solidified lavas.Scientists have long wondered whether humans could accelerate CO2 removal by speeding up rock weathering. In 1995 Klaus Lackner, a physicist then at Los Alamos National Laboratory in New Mexico, proposed heating basalts to absorb CO2 more quickly. Over time this basic idea fermented into other forms: injecting concentrated CO2 into hot layers of basalt underground where they would form carbonate minerals, or spreading powdered basalt across the ocean, which would absorb CO2, sinking the carbon.A worker spreads pulverized basalt on a recently harvested cornfield in central Illinois.Jordan Goebig/University of IllinoisIn the late 2000s Phil Renforth, a Ph.D. candidate at Newcastle University in England, noticed that the demolished remnants of steel mills in his area accumulated white crusts of carbonate minerals on the ground. Fragments of steel slag and concrete, both high in calcium, were reacting with CO2. In 2013 he and Jens Hartmann, a geochemist then at the University of Hamburg in Germany, published a paper suggesting that calcium-rich rocks could be crushed and spread on farmland to capture CO2 while also improving soils.At about that time, Beerling was studying how grasslands influence the weathering of bedrock and the natural capture of CO2. When he read Renforth and Hartmanns paper, he realized he could use his model to predict how basalt weathering would unfold on farmlands. In 2016 Beerling published calculations predicting that a millimeter or two of basalt dust spread annually over the worlds tropical lands could reduce CO2 levels by 30 to 300 parts per million (ppm) by 2100. Atmospheric carbon dioxide is currently around 425 ppmup from 280 ppm before the industrial revolutionand is expected to hit 500 to 1,200 ppm by 2100. The modeling suggested that ERW could prevent 0.2 to 2.2 degrees C of warming by that date.Common climate scenarios predict that if humans are going to limit warming to two degrees C, we need to remove five to 10 gigatons of CO2 from the atmosphere annually by 2050. In 2018 Beerlings team published updated calculations predicting that if crushed basalt were spread yearly across 700,000 square kilometers of corn and soy croplands in the U.S., it could remove 0.2 to 1.1 gigatons of CO2 from the atmosphere annually.In 2020 Beerling and his collaborators, joined by Renforth, published a refined analysis in Nature. They estimated that if two gigatons of CO2 a year had to be captured worldwide through ERW, China, India, the U.S. and Brazil could cover 80 percent of that amount, even after accounting for the CO2 emitted while mining, crushing and transporting the rock. Obviously a combination of carbon capture methods would be needed to reach 10 gigatons a year. But, Beerling says, If you can do two [gigatons] of it with enhanced weathering and improve food security and soil health, thats 20 percent of the way there.The Illinois trial provided strong validation. Farming of corn and soy typically releases CO2 through the respiration of roots and soil microbes, but the basalt-treated corn-soy fields released 23 to 42 percent less CO2. Multiplied across the U.S., thats 260 million tons of CO2 potentially avoided each year.Unlike geoengineering approaches such as hoisting sulfur into the sky or scattering iron across the sea, which people often view as risky tinkering with nature, ERW was well received when papers were published, Beerling says. It was important to see how this landed with the public and the press, he says. The reactions strengthened our belief that this was the right way to go.ERW is fundamentally different from two other soil-based carbon strategies that have been around longer. In a method called biochar, farmers partially burn leftover plant matter, turning it to charcoalnearly pure carbonwhich is plowed into the dirt for long-term storage. In the second method, leftover plant material is plowed back into the soil without being charcoaled; this stores carbon as organic molecules that can nourish crops, although the molecules can also return to the atmosphere.ERW traps CO2 as dissolved bicarbonate in soil water, which eventually runs off farm fields into streams that ultimately lead to the sea, storing CO2 in the ocean water as bicarbonate or as solid carbonate minerals on the seafloor. Studies predict that ERW would reliably store bicarbonate in the ocean for 100 to 1,000 years, which could also help reduce climate-related ocean acidification. Whats more, ERW could alleviate another major problem, not addressed by the two other methods, that plagues farmers around the world.One of the most striking examples of how rock weathering has regulated atmospheric CO2 levels over the eons can be found along the western coast of Indiaone reason some of the earliest efforts to roll out ERW by start-up companies are happening in this country. Indias coastal plain, dotted with rice paddies and villages, abruptly rises 1,000 meters through a chaotic maze of sharp ridges, V-shaped canyons, rushing rivers and waterfalls to a high plateau. The canyon walls are striped in alternating layers of yellow and brown basalt, marking the edge of the Deccan basalts, formed from a massive series of lava flows that started around 66 million years ago. By 50 million years ago Earth was unusually warm, with CO2 levels nearly four times what they are today. Around that time, the Deccan basalts began altering the planets climate in a slow but potent way. Continental drift carried them into the equatorial belt, where abundant rainfall and warm temperatures caused the rocks to weather more quickly. The weathering minerals pulled CO2 from the air and washed it down rivers to the sea, trapping it there.Over the next 30 million years, estimates indicate, weathering basalts drew more than one million gigatons of CO2 from the atmosphere, some of it becoming buried as carbonate on the seafloor. Atmospheric CO2 declined, temperatures cooled, and an ice sheet began growing across Antarctica.Ben Gilliland; Paul Nelson/James Cook University (scientific reviewer)The village of Sarekha Khurd, in central Indias Madhya Pradesh state, sits near the eastern, inland edge of the Deccan basalts. The people there have farmed rice for centuries, in a patchwork of paddies divided by rows of teak and red-blossomed gum trees. Many of the farmers live tenuously, working little plots the size of one to two soccer fields. They earn an average of $1,500 a year, spending up to 30 percent of that on fertilizers and other chemicals. And they face constant hazards. Heat waves as high as 48 degrees C (118 degrees Fahrenheit) can stunt crops and disrupt needed monsoon rains. Constant agriculture has slowly acidified the dark, rich soils, depleting their stores of calcium and magnesium, as farmers harvested plants rather than leaving them to decay and return their minerals to the soil. The average pH of soils in this area is slightly acidic, around 6.4 (7.0 is neutral), similar to saliva. This is not ideal for growing rice because acidification impairs the plants absorption of nutrients, such as phosphorus, and it may even alter the mix of soil microbes, allowing pathogenic bacteria or fungi to spawn disease outbreaks that can damage crops.Farmers worldwide have dealt with soil acidity since long before they understood it. Dozens of pits found in the forests north of Paris suggest that as early as 6,000 years ago, farmers dug into the limestone bedrock and scattered pieces of it on the fields where they grew wheat, barley and peas. Later on, Romans would scatter chalky calcium carbonate rocks onto croplands to reverse sour soil. For centuries farmers in Europe and North America neutralized acidity by sprinkling fields with crushed limestone, rich in carbonate.But people in many areas, including India, dont have easy access to limestone. And the process of neutralizing acidic soil with lime can potentially release CO2 into the air. In such places, ERW is appealing because it can reverse that dynamic, converting airborne CO2 into dissolved bicarbonate in soil.Last May farmers in Sarekha Khurd started trying ERW. Workers with Mati Carbon, an ERW start-up based in Houston, Tex., trucked in 1,250 metric tons of crushed rock from nearby quarries that mine the Deccan basalts for road construction materials. The company is currently providing basalt, free of charge, to more than 180 farm villages in Madhya Pradesh and its neighboring state of Chhattisgarh. They plan to add more basalt each year. Rice yields have increased by 15 to 20 percent on average, and in some cases by up to 70 percent.Imagine the farm of the future. Part of the farmers view of their mandate is carbon dioxide removal. Noah Planavsky, Yale University Mati Carbon recently expanded its operations to a handful of villages in Tanzania and Zambia. Our mission is the farmer, says Mati founder Shantanu Agarwal, especially these smaller, climate-vulnerable farmers. The company hopes to earn money by selling carbon credits. Agarwal and Jacob Jordan, Matis lead scientist, estimate that improved soils, increased crop yields and reduced spending on fertilizers could raise poor farmers income by 10 to 30 percent, making them less vulnerable.As promising as early trials have been, a large-scale rollout of ERW would have to overcome some stark realities, starting with the staggering amount of rock it would require. Beerlings calculations suggest that if ERW were used to capture two gigatons of CO2 a year, it would consume 13 gigatons of basalt annuallyabout 4.5 cubic kilometers of rock, roughly equal to the volume of the Matterhorn. That would require 30 percent more mining than the 40 gigatons or so of sand, gravel and crushed rock that are now quarried worldwide annually for industry. Such an increase might not be possible for some kinds of rock, but the worlds reserves of basalt are truly vast, distributed widely across the planet.Crushed basalt thats already produced in quarries as an unused by-product could pick up some of that slack. So could calcium-rich industrial by-products, such as crushed concrete, mine tailings, ash from sugarcane milling and coal burning, and wastes from cement, aluminum and steel production. But many of these by-products contain chromium, nickel, cadmium, and other toxic elements, so they could maybe be used to capture CO2 in factory yards or tailings piles at mines but not on croplands. When additional basalt mining and crushing is needed, it will cost about $10 and emit around 30 kilograms of CO2 per ton. Beerlings team considered these factors when it estimated that ERW would cost $80 to $180 per ton of CO2 captured, after emissions are subtracted.Two farmers harvest rice from paddies in India that had been treated with ground-up rock. Rice yield was about 25 percent higher than in the past, when no rock was spread.Deepak Kushwaha/Mati CarbonBut there will be other costs. In China and Indiatwo countries with the most agricultural potential for ERWthe thriving rock-quarrying industries have been criticized for poor protection of human rights. Indias sandstone-quarrying industry, for example, employs more than three million people. A 2020 report published by the Washington, D.C.based Center for Human Rights found that many of them are bonded laborerspeople who work at low wages to repay loans with annual interest rates up to 20 percent, making it difficult to ever repay debts and trapping them in the job. Such workers may face dangerous temperatures, rock collapses and swirling mineral dust.A 2022 study found that quarry workers in northeastern India suffer poor lung and heart health, with low levels of blood oxygen, high pulses and poor lung airflow. If a quarry worker is injured, dies or falls ill, wives or children may be forced into work to repay the debt. These problems arent limited to India, says Bhoomika Choudhury, a lawyer and labor researcher with the Business & Human Rights Resource Center in Dubai, who wrote the 2020 sandstone report: We are seeing these patterns everywhere in countries across Asia, Africa and South America.Any large increase in quarrying would also translate into more landscapes being torn upsome of them in potentially sensitive areasalthough this is also true for other materials that will have to be mined to support the broader transition to renewable energy, such as lithium, cobalt, graphite and rare earth elements. It is also possible that even if mining challenges are surmounted, ERW wont work as well worldwide as it has in the small trials that have been done thus far. For example, many scientists assumed ERW would work best in the warm, wet tropics, where basalt weathers more quickly. But two recent studies complicate that picture.A 2022 trial that Beerlings group supported in Malaysia, where basalt dust was spread across parts of a palm oil plantation, produced inconclusive results. Beerling suspects that the benefits are being temporarily masked by local conditions. The dark, pungent soils contain more decaying organic matter and more clay than the soils in Illinois; those charged materials can latch on to the breakdown products of basalt, keeping them from converting CO2 into bicarbonate. Theres a delay in capturing carbon dioxide, Beerling says. It doesnt happen until the soils capacity to bind the dissolving minerals has been saturated, which may take a year or take five years, he says. This remains to be seen.Acidity is the other complicating factor, according to a trial on tropical sugarcane fields in northeastern Australia. The soil there is acidic, so it can potentially consume the basalt before it has a chance to react with CO2. Initial results, published last October, show that CO2 capture rates are only about 1 percent of those in Illinois. Paul Nelson, a soil scientist at James Cook University in Cairns who helped lead the study, says it may be hard to fix the problem just by neutralizing acidic soils before adding basalt because in wet tropical areas the acidity may extend many meters down, to the bedrock.Right now researchers are just trusting that wherever ERW is done, from Illinois to Australia, the CO2 that is captured as dissolved bicarbonate will seep into streams, flow through rivers and reach the ocean without encountering a highly acidic environment. If it does flow through an acidic environment, Nelson says, some of it could be converted into CO2 along the way, returning to the atmosphere.Despite the uncertainties, some two dozen companies have emerged to try to exploit ERW. Many are selling anticipated carbon-capture credits, in some cases to companies such as Microsoft and Stripe that hope to zero out their carbon footprint. This activity makes Planavsky, the Yale biogeochemist, uneasy. Hes aware of lessons learned in another carbon market that grew too quickly. In recent years companies have sold more and more voluntary carbon offsets for protecting forests, but some of the projects have subsequently been revealed as worthless. ERW is a potentially really valuable opportunity to remove CO2, Planavsky says, but its not going to work everywhere. If companies cut corners, he says, ERW could blow up on the launch pad.Yet for ERW to have a large impact by 2050, it will need to expand quickly, says Gregory Nemet, an energy scientist at the University of WisconsinMadison. Last May he and his colleagues published a study analyzing the combined potential of novel CO2 removal methods such as ERW, direct air-capture machines and the use of biofuels with CO2 captured from smokestacks. Between now and 2050 these methods need to grow by something like 40 percent per year, every year, Nemet says. That sounds extreme, although he says that electric cars and solar energy have expanded even more rapidly for 10 or 20 years. And if enhanced weathering ends up costing $80 to $180 per ton of CO2, as Beerlings group predicted, it may be cheaper than direct air capture ($400 to $1,000 per ton right now), and similar to biofuels with smokestack capture ($100 to $300 per ton today).If ERW does pan out on a large scale, Planavskywhose family farmssees potential societal benefits that go beyond CO2 removal. Building machines that capture CO2 from the air or from smokestacks will generate profits for big companies. But with a low-tech approach like ERW, even small farmers could sell carbon credits. Imagine the farm of the future, he says. Part of the farmers view of their mandate is carbon dioxide removal.