Spreading rock dust on farms seems at first like an odd way to lock up carbon.

But consider: It relies on a geological process with a billion-year track record; it could actually boost crop yields; and it could lock up as much carbon as planting a trillion trees.

Suddenly, the prospect of an abiotic solution grows ever more intriguing.

By Dan Ferber

David Beerling’s phone started buzzing soon after the Nature paper came out in July 2020, and it has not stopped since. “I must have had a dozen farmers ringing me up: ‘I’ve seen a piece in the Guardian—how do I get involved?’” says Beerling, a biogeochemist at the University of Sheffield.

nbsp | AnthropocenenbspEnhanced Rock Weathering | Anthropocene Magazine Spreading rock dust on farms aka enhanced rock weathering might boost crop yields and could lock up as much carbon as planting a trillion trees

The miners were calling, too. “Wehavetens of thousands of tons of crushed basalt in the UK,” they said. “Do you want to use it?” Then came the venture capitalists saying, “I can raise money for you.” They no doubt recognized the immense profit potential, Beerling says. “If nations mandate carbon dioxide removal, and if you have a patent on one of the removal technologies, all of a sudden you’re quids in.”

Beerling directs the Leverhulme Centre for Climate Change Mitigation, a large, multidisciplinary research effort to investigate one of the less known strategies for slowing climate change: spreading finely ground basalt rock on farm fields worldwide. He and colleagues from a dozen other institutions had modeled the impact of this method, called enhanced weathering, on a country-by-country basis.

What they found was eye-popping. In their Nature paper, they reported that enhanced weathering could remove up to 2 billion tons of CO2 from the atmosphere each year by 2050—much of it from the highest-emitting countries, including the United States and China. This is the equivalent of the combined annual emissions of Germany and Japan, and it’s on par with the projected contributions of other negative-emissions strategies, such as reforestation and afforestation.

But what really sets enhanced rock weathering apart from these other technologies is its enormous win-win potential. Those farmers were calling Beerling in part because enhanced weathering could offer them lucrative benefits. The powdered basalt used in enhanced weathering would quickly release minerals and alkalinity into moist soils. This could help restore degraded, acidified soil in farm fields around the world, boosting yields of crops that sustain humanity.

In the three decades since the world was first alerted to the dangers of climate change, carbon dioxide levels in the atmosphere have risen relentlessly to 414 ppm, 48 percent higher than pre-industrial levels. “We increasingly look out the window and see climate disaster after climate disaster,” says Noah Deich, president and cofounder of Carbon 180, a Washington, DC–based nonprofit focused on carbon drawdown. “We need to have the option to clean up legacy carbon at a large scale, so we’re not stuck with the carbon we have today that’s causing immense suffering around the world.”

That’s where negative-emissions technologies—also called carbon dioxide removal (CDR) or carbon-drawdown methods—come into play. Until the past decade, most scientists and policymakers had banked on nature-based, biotic strategies such as planting forests, cultivating kelp, or restoring wetlands, while large-scale technological interventions in the climate system, or geoengineering, strategies were often dismissed as reckless and ill-advised. And indeed, “Photosynthesis is one of the world’s oldest and most technologically mature carbon-removal strategies,” says Deich. “But planning and replanting forest alone will not draw down all the carbon we need to remove.”

That makes the prospect of an abiotic solution ever more intriguing. It is, after all, a solution with a history even longer than that of photosynthesis—on the order of several billion years.

Drawdown through the Ages

Often referred to as our sister planet, Venus has roughly the same size and the same mass as Earth. Both planets condensed out of the planetary nebula around the same time, roughly 4.5 billion years ago, and both planets are volcanically active, periodically belching carbon dioxide from the planet’s interior. But while carbon dioxide makes up just 0.04 percent of Earth’s atmosphere, it makes up 96 percent of the atmosphere on Venus. This creates a thick blanket that traps enough of the sun’s heat to bake Venus’s surface to 475 degrees Celsius—hot enough to melt lead.

What saved Earth from becoming a similar hellscape was a single type of chemical reaction, possibly the most under-appreciated reaction in chemistry. Over millions of years, this reaction, known as chemical weathering, has pulled down almost all the carbon dioxide that ever existed in our atmosphere and locked it up in the Earth’s crust and mantle. That carbon dioxide reacted with silicate minerals, the most common mineral in Earth’s rocks. On land, and especially on the sea floor, this reaction formed carbonate-rich rocks such as limestone and dolomite. Geological processes then buried this rock deep beneath Earth’s surface, locking away carbon for eons and cooling the planet.

Chemical weathering, first described by the French geochemist and metallurgist J.J. Ebelman in 1845, was brought to broad scientific attention when Nobel Laureate Harold Urey, a University of Chicago geochemist, reported it in Proceedings of the National Academy of Sciences in 1952. Geochemists have since worked out the details. Carbon dioxide in the atmosphere dissolves in raindrops, forming a dilute solution of carbonic acid (H2CO3). “That’s sparkling water, essentially,” says Lennart Bach, a marine biogeochemist at the University of Tasmania who studies enhanced weathering.

What saved Earth from becoming a hellscape was a single type of chemical reaction, possibly the most underappreciated reaction in chemistry.

When that rain falls on igneous rocks such as basalt and peridotite, which are rich in silicate minerals, the carbonic acid dissolves the minerals into water-soluble ions, including calcium (Ca2+), magnesium, and bicarbonate (HCO3-), which can travel downstream to the ocean via groundwater, streams, and rivers.

There, marine creatures by the billions, many of them microscopic, build their shells by combining calcium with the bicarbonate. When they die, those shells sink to the ocean floor, pile up as ooze, and ultimately turn into limestone and other carbonate rocks. A geological process called subduction buries those rocks deep in the Earth’s crust and mantle, locking away carbon for millions of years.

A different type of geological process complements such chemical weathering. During mechanical weathering, wind and flowing water split, crush, and grind the rocks to form cobbles, pebbles, and sand, dramatically increasing the surface area available for chemical weathering. Mining and industrial processes that grind basalt do this as well, which means that finely ground basalt—an abundant waste product of mining that has been piling up worldwide for decades—is available to be spread on farms.

In theory, chemical weathering could remove all the carbon dioxide that our smokestacks and tailpipes have spewed into the atmosphere since the beginning of the Industrial Revolution, but that would take about 7,000 years. Given the poky pace of natural chemical weathering, scientists realized about three decades ago that the process could use a boost.

Rocks to the Rescue

In 1990, humanity was just beginning to wrap its mind around the reality of climate change. Just two years had passed since climate scientist James Hansen of NASA made international headlines when he testified to the US Senate that global warming had begun. The United Nations had formed the Intergovernmental Panel on Climate Change (IPCC), and the group had just
issued the first of its far-reaching assessment reports. And Walter Seifritz, a little-known physicist and nuclear engineer at Switzerland’s Paul Scherrer Institute, penned a letter to Nature suggesting a solution.

“It would be advantageous,” he wrote, “if there were an abundant mineral to which CO2 could be bound chemically via an exothermic reaction to form a stable, permanent substance.” He suggested that silicate minerals would serve that purpose well, and that an industrialized process based on chemical weathering could remove CO2 from the atmosphere.

For 15 years, little happened. “The idea lay dormant until people started to realize that we need negative emissions to get us out of trouble,” Beerling says. By the mid-2000s, the first scientists had begun proposing controversial geoengineering schemes to head off climate change, such as fertilizing the open ocean to make tiny ocean plants called phytoplankton gobble more carbon dioxide, or tinkering with clouds to reflect sunlight and cool the planet. Then, in 2006, Dutch geochemist Olaf Schuiling of the University of Utrecht proposed the large-scale use of olivine, a green silicate mineral found in basalt, peridotite, and other common igneous rocks, to draw down carbon dioxide.

Geologists have long known that olivine and similar minerals, which crystallize first from cooling magma, also weather faster and more completely than other minerals. They also knew that water was essential for weathering. For these reasons, Schuiling and a colleague proposed spreading ground olivine over forests, especially in tropical areas, to neutralize soil acidity from acid rain while simultaneously drawing down carbon dioxide.

In theory, chemical weathering could remove all the carbon dioxide that our smokestacks and tailpipes have ever spewed into the atmosphere . . . but that would take about 7,000 years. The process, scientists realized, could use a boost.

Schuiling also proposed spreading crushed olivine on and near beaches, where natural wave action could grind the rock down into even finer clay and silt-sized particles, which would mineralize quickly. (Others have suggested adding olivine gravel a little further offshore, where currents and waves would grind it to sand on the seafloor.) Schuiling’s thinking was that, as the olivine particles weathered, they would restore silicon for diatom skeletons, restore calcium for coral skeletons, draw down large amounts of carbon dioxide, and counter ocean acidification.

In the years that followed, critics challenged Schuiling, claiming the method was impractical, and he countered with additional research papers. He has since retired, and others have picked up his baton. For example, Project Vesta, a nonprofit spun out of a climate-change think tank called Climitigation, is conducting lab and field studies testing the idea. They claim that deploying it on less than 2 percent of the shallow seas covering continental shelves—an area about the size of France—would capture all the carbon humanity emits in a year, and that the process would be cost-effective. But researchers are still debating whether these calculations are realistic, and they agree that field studies in coastal environments will be needed to answer the question.

A Move to Basalt

Even if the promise of olivine materializes, the method has one major drawback, Beerling says. The mineral is usually contaminated with nickel and chromium—both potentially toxic metals. “If the world is up in arms about plastics in the ocean and coral reefs dying and ocean acidification, how are you going to get people to accept tons of chromium- and nickel-rich rocks in the ocean?”

For these reasons, around 2014 Beerling and his team began testing a different approach. Rather than olivine, they proposed using basalt, a common igneous rock, to take up CO2. Basalt is rich in pyroxene, a blocky, dark mineral that, like olivine, weathers quickly but does not contain toxic heavy metals.

Because weathering happens especially quickly in warm, humid environments, Beerling and Lyla Taylor, a senior research fellow at the University of Sheffield, initially modeled how well-ground basalt, applied to tropical forests, would draw down carbon dioxide. They reported in Nature Climate Change in 2016 that applying ground basalt over less than a third of all tropical land could dramatically reduce atmospheric CO2 (by up to 300 ppm in the atmosphere) and go a long way toward averting ocean acidification as well.

As Beerling was presenting the results at a conference on photosynthesis, Steve Long, a professor of crop sciences and plant biology at the University of Illinois, Urbana-Champaign, was in the audience, listening carefully. Afterward Long, who helps run experimental farms to better understand how climate change is affecting Midwestern agriculture, buttonholed Beerling. “He was talking about putting them in a forest,” recalled Long. “But in the Midwest, we have 90 million hectares of corn and soybean land, and all those farms are equipped with lime spreaders. So I said, ‘Why not try it here?’”

Beerling and Taylor soon embarked on another modeling study. Earlier models of enhanced weathering had ignored agricultural processes, and models of cropland soil processes had ignored weathering. But in real life, the two processes interacted.

When the pair modeled them together, the results looked promising. So Beerling pulled together a multidisciplinary team to put enhanced weathering to the test, and he applied for funding to the Leverhulme Trust, the UK-based charity that was looking to fund high-risk, high-reward research. In 2015, they won a ten-year, £10 million (US$12.5 million) grant. “It’s a dream ticket,” Beerling says. “You’ve got ten years, and you don’t have to worry about applying for grants, and you can drive it forward.”

The problem isn’t a lack of rocks— there’s more than enough of those. Enhanced weathering faces the same question that haunts so many other climate solutions—will it scale?

Ground basalt has been piling up as an unwanted waste product worldwide for decades.

Down on the Energy Farm

In the flat farmland of East Central Illinois, giant corn and soybean fields stretch to the horizon. But one 320-acre plot south of the University of Illinois campus in Urbana is a field like no other. This is the Energy Farm, a living laboratory dedicated to testing future methods of sustainable production for corn and soybeans, as well as switchgrass and Miscanthus—perennial tall-grass crops grown as feedstock for biofuels.

It also turned out to be an ideal site for Beerling and his UK colleagues to test whether applying basalt to farmland could ever be practical at scale. Given that farm fields with corn and soybeans are the dominant ecosystem in the Midwest, if it works there, it could work over a vast region, says Ilsa Kantola, a soil ecologist who has worked at the Energy Farm for eight years.

nbsp | AnthropocenenbspEnhanced Rock Weathering | Anthropocene Magazine Spreading rock dust on farms aka enhanced rock weathering might boost crop yields and could lock up as much carbon as planting a trillion treesKantola, Long, and several other UIUC colleagues have been applying ground basalt to half of a 50-acre section of the Energy Farm, using the fields in the other half as a control. They’ve monitored a wide variety of soil, plant, and atmospheric parameters to answer questions about carbon dioxide drawdown and how the basalt affects the soil and crops that grow in it.

When ground basalt is chemically weathered—in moist soil or anywhere else—it consumes protons, or acidity, and produces magnesium and calcium, along with small amounts of zinc and copper—nutrients that plants need. “They need their vitamins to perform to the best of their ability, like the rest of us,” Kantola says.

To determine whether ground basalt can cut net carbon dioxide emissions from corn and soybean fields, Carl Bernacchi, an environmental plant physiologist at the USDA Agricultural Research Service in Urbana; Evan DeLucia, a UIUC plant biologist; and Long are using a sophisticated field measurement called eddy covariance. As air moves across a farm field, invisible currents cause it to tumble with updrafts from the plants’ canopy and downdrafts from the atmosphere, just like a tumbleweed. Instruments monitor CO2 levels above the plants’ canopy around the clock, then feed that data into a model akin to the computational fluid-dynamics models that engineers use to design aircraft. The method details how gases are exchanged between the atmosphere, soil, and plants. This in turn will reveal whether basalt helps keep carbon dioxide emissions from farm fields in check.

The Illinois field trials are now in their fourth year, and the researchers plan to report later this year how the treatment affects CO2 uptake and how the soil and crops respond, both above and below ground. They’ve held off so far because weather can vary a great deal from one growing season to the next. “No serious agronomist would hang their hat on one or two years of data,” Beerling says.

Nevertheless, interesting results are trickling in, including one surprising fringe benefit of ground basalt. Typically, Midwestern farmers spread nitrogen-based fertilizer in their fields, and soil microbes convert some of what plants leave untouched into nitrous oxide, a greenhouse gas 300 times as potent as carbon dioxide. “Because we put so much nitrogen on cornfields, agriculture is a major source of N2O,” Kantola says.

When Kantola and her colleagues fed data from three years of field measurements into a sophisticated model, they were pleasantly surprised. Corn and Miscanthus fields reduced nitrous oxide emissions 16 percent and 9 percent, respectively, they reported recently in Global Change Biology—Bioenergy. “Nitrous oxide is where we see the biggest difference,” Kantola says.

Dollars and Sense

While the jury remains out with regard to enhanced rock weathering, small field tests have recently raised hopes, and new trials are being launched. In experiments in small rooftop plots, researchers at the University of Guelph in Ontario showed that ground wollastonite, another common rock rich in silicate minerals, underwent weathering in soil, drew down significant amounts of carbon dioxide, and improved the growth of soybeans and alfalfa. The Future Forest Company, a British firm focused on carbon-drawdown methods, is funding field tests comparing basalt weathering, biochar, and reforestation on the Isle of Mull off the coast of Scotland. And the Leverhulme Centre is funding basalt-weathering field tests on a sugar-cane farm in Australia and an oil-palm plantation in Malaysian Borneo.

Even if numbers keep coming in on the right side of the carbon ledger, when it comes to scaling the technology, the devil is in the details. It takes energy to transport ground basalt from mines and other depots to farms, and that typically requires burning fossil fuels and generating CO2. In a recent modeling study, University of Salzburg researchers showed that basalt had to be ground to fine, silt-like particles, which can take so much energy it can cancel out the CO2 drawdown benefits from weathering.

Ultimately, the prospects for enhanced rock weathering come down to economics, Long says. Mining, grinding, milling, and transport cost money. But the method also offers economic advantages. Mine owners benefit because ground basalt has been piling up as an unwanted waste product worldwide for decades. Farmers benefit from both improved crop yields and the fact that they can spread ground basalt on their fields by using the same equipment that they use to spread lime. And in the Midwest, a network of railroads already crisscrosses the region, which eliminates the need to build more rail lines or truck the basalt in. For all these reasons, “The Midwest is one of the areas where it’s easiest to get the basalt down on the ground,” Kantola says.

Nevertheless, Long says, “I think the direct benefits to farmers will be small.” For that reason, policies that reward carbon drawdown will likely be needed. “We’ll probably have to pay farmers to do this to get their carbon credits,” he says.

Investors are also starting to envision financial rewards from negative-emissions technologies. Among those, enhanced rock weathering may be the upstart, but its potential is attracting a lot of attention. In 2021, a startup called Heirloom Carbon Technologies received an infusion of private capital for its semi-industrial version of enhanced rock weathering, which involves spreading a magnesium-rich mineral in a field to capture carbon dioxide, then applying an industrial process at the end of the year to capture pure CO2 and regenerate the mineral. The investors included Breakthrough Energy, founded by Bill Gates, and two other climate-focused venture capital firms. Such investments, along with similar investment in 44.01, a firm focused on a direct air-capture technology, demonstrate a “huge appetite” for carbon-drawdown technology, says Deich, the president of Carbon 180.

In the case of enhanced weathering, there is no lack of material to capture all the CO2 needed to satisfy that appetite. “There are way more rocks that can be mineralized to sequester carbon than we would ever need to remediate the legacy carbon in the atmosphere,” Deich says, “but enhanced weathering is still very nascent in terms of engineering and economics.” In other words, it faces the same question that haunts so many other climate solutions—will it scale?

If it does, the impact would be global, and it could benefit not just the atmosphere and food production, but the ocean as well. Most of the Midwest lies in the Mississippi River watershed, and Illinois and Leverhulme researchers are analyzing water that runs off the Illinois farms. If basalt treatment works as intended on farm fields, then calcium and silica, along with the bicarbonate, could end up in the ocean—where it would neutralize acidity building up in the ocean, and where marine creatures could use it to build their shells.

“Can we cure the corals, starting with the corn?” Kantola wonders. “We don’t know yet, but that’s the dream.”

____________________________

Dan Ferber is a science writer and editor based in New Jersey. He’s the coauthor of Changing Planet, Changing Health, the first book on the human health impacts of climate change. 

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