Spreading crushed rock on farms could absorb 1 billion tons of CO2

Spreading crushed silicate rocks such as basalt on fields could remove up to 1.1 billion tons of carbon dioxide from the atmosphere annually while increasing crop yields, according to an analysis of the method’s global potential. But some scientists question whether this number is really achievable.

This technique, known as enhanced rock weathering, accelerates the breakdown of rocks by rainwater, a natural process that over millions of years transferred CO2 from the atmosphere to the ocean and helped cool the planet during the Greenhouse and Earth periods. For centuries, farmers have spread limestone soil over fields to improve nutrient uptake by crops.

“The main benefit is the resolution of atmospheric CO2 through chemical reactions,” he says Chuan Liao at Cornell University in New York. “And there are some side benefits as well, like adding … magnesium, possibly calcium, to replenish soil nutrients.”

As emissions continue to rise, the UN climate body said humanity will require carbon removal to limit global warming to 1.5°C above pre-industrial levels. Countries like Brazil have encouraged increased rock weathering to reduce emissions as well as fertilizer costs. Last year, a climate-enhancing start-up in India called Mati Carbon won the $50 million Elon Musk XPRIZE grand prize for its potential for large-scale carbon removal.

Atmospheric CO2 dissolves in rain to form carbonic acid. In silicate rocks, it reacts with silica and metals to lock CO2 into bicarbonate ions. Bicarbonate washes into rivers and oceans, where it can remain dissolved for millennia or become incorporated into the calcium carbonate exoskeletons of clams, corals, and sea urchins. Crushing rocks exposes more surface area to rain, which increases CO2 removal.

Based on how much rock could fit in agricultural fields, studies estimate that increased rock weathering could reduce 5 billion tons of CO2 per year this century. Liao and his colleagues did a “reality check” on these estimates by including how quickly farmers adopted other innovations, such as irrigation, and how effective weathering might be in different regions.

They modeled scenarios of limited and widespread adoption of enhanced weathering and found that the technique could remove 350 million to 750 million tons of CO2 per year by 2050 and 700 million to 1.1 billion tons per year by 2100. By comparison, global CO2 emissions from fossil fuels in 2025 were approximately 38 billion tons.

While Europe and North America would initially do most of this removal, they would be overtaken by Asia, Latin America and sub-Saharan Africa as silicate rock supply chains were established and costs were reduced. Warmer temperatures and precipitation accelerate weathering in these regions, potentially allowing farmers there to sell more carbon removal credits per ton of spilled rock.

“[For] there will be fewer barriers for farmers in the Global South to do this decades from now,” says Liao.

However, Marcus Schiedung at the Thünen Institute of Climate-Smart Agriculture in Germany and his colleagues argue recently paper that projections such as these obscure major uncertainties about increased rock weathering. For example, if it doesn’t rain and the soil stays dry, carbon removal can be up to 25-fold slower. The estimate of 1.1 billion tons of carbon removal is likely to be inflated, Schiedung says.

In high pH soils, rainfall may weather carbonates in the soil rather than crushed rock. These are eventually converted back to carbonates in the ocean, releasing CO2 and causing no net carbon removal, he says. In low pH soils, naturally occurring acids can react with the crushed rock and carbon is not removed from the precipitation. As soil acidity decreases, CO2 emissions from microbes increase.

What’s more, in some cases, mining and transporting the rock to the farm could release more carbon than is removed, Schiedung says.

“I’m a skeptic,” he says. “We have to be sure that the CO2 is absorbed. Otherwise we run the risk of measuring something.” [removing carbon]but somewhere else it gets released again, which is likely to happen in this geochemically complex system.”

Some also worry that increased rock weathering could introduce toxins into the food supply. Olivine, the rock on which Liao’s projections are based, contains heavy metals such as nickel and chromium.

The remaining rock in most existing mines is also contaminated with metals, they said David Manning at Newcastle University in Great Britain. Instead, countries would likely have to open massive amounts of basalt quarries, which would require time and money.

“One gigatonne of CO2 removed per year requires 5 gigatons of rock per year, and that’s a problem because nobody knows where that rock came from,” says Manning. “That’s the main obstacle to growth.”

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