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

Spreading crushed silicate rocks like basalt on fields could remove up to 1.1 billion tonnes of carbon dioxide from the atmosphere per year while increasing crop yields, according to an analysis of the method’s global potential. But some researchers question whether this figure is actually achievable.

Known as enhanced rock weathering, the technique accelerates the degradation 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 greenhouse periods. Farmers have been spreading crushed limestone on fields for centuries to improve nutrient uptake by crops.

“The main advantage lies in the resolution of atmospheric CO2 through chemical reactions,” says Chuan Liao of Cornell University in New York. “And there are some secondary benefits as well, like adding … magnesium, potentially calcium, to supplement the nutrients in the soil.”

As emissions continue to rise, the United Nations climate body has said humanity will need to remove carbon to limit global warming to 1.5C above pre-industrial levels. Countries like Brazil have encouraged increased rock weathering to reduce both emissions and fertilizer costs. Last year, an Indian startup called Mati Carbon won the $50 million top prize in Elon Musk’s XPRIZE competition for its potential for large-scale carbon removal.

Atmospheric CO2 dissolves in rain to form carbonic acid. In silicate rocks, this reacts with silicon dioxide and metals to trap CO2 in bicarbonate ions. Bicarbonate washes into rivers and oceans, where it can remain dissolved for millennia or be incorporated into the calcium carbonate exoskeletons of clams, corals and sea urchins. Crushing rocks exposes a larger surface area to rain, thereby increasing this CO2 removal.

Based on the amount of rock that could accumulate in agricultural fields, studies have projected that increased rock weathering could absorb 5 billion tons of CO2 per year this century. Liao and his colleagues conducted a “reality check” of these estimates by incorporating how quickly other innovations like irrigation were adopted by farmers and the effectiveness of weathering in different regions.

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

While Europe and North America would initially handle the bulk of this extraction, they would be overtaken by Asia, Latin America, and sub-Saharan Africa as silicate rock supply chains become established and costs decline. Higher temperatures and precipitation accelerate severe weathering in these regions, potentially allowing farmers to sell more carbon removal credits per ton of rock spilled.

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

However, Marcus Schiedung of the Thünen Institute for Climate-Smart Agriculture in Germany and colleagues argue in a recent paper that such projections obscure major uncertainties regarding increased rock weathering. For example, if it doesn’t rain and the soil remains dry, carbon removal can be up to 25 times slower. The estimate of 1.1 billion tons of carbon removed is likely inflated, Schiedung believes.

In high pH soils, precipitation can alter the carbonates present in the soil rather than the crushed rocks. These will eventually be converted back to carbonates in the ocean, releasing CO2 and resulting in no net carbon removal, he says. In low pH soils, natural acids can react with the crushed rock and the carbon will not be removed from precipitation. As soil acidity decreases, CO2 emissions from microbes increase.

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

“I’m skeptical,” he said. “We need to be sure that the CO2 is absorbed. Otherwise we run the risk of measuring something [removing carbon]but elsewhere it is released again, which is likely to happen in this complex geochemical system.

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

Leftover rock in most existing mines is also contaminated with metals, according to David Manning of Newcastle University, UK. Countries would likely have to open a large number of basalt quarries, which would take time and money.

“One gigatonne of CO2 removed per year requires 5 gigatons of rock per year, and that’s a tricky point because no one knows where that rock comes from,” Manning says. “It’s a major barrier to growth.”