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 annually while increasing crop yields, according to an analysis of the method’s global potential. But some researchers question whether that figure is really achievable.

Known as enhanced rock weathering, this technique accelerates the breakdown of rocks by rainwater, a natural process that, over millions of years, has transferred CO2 from the atmosphere to the ocean and helped cool the planet in hothouse-Earth periods. Farmers have been spreading ground limestone on fields for centuries to improve nutrient uptake in crops.

“The main benefit is through sort of solving atmospheric CO2 through chemical reactions,” says Chuan Liao at Cornell University in New York. “And there are also some side benefits, such as adding… magnesium, calcium potentially, to supplement soil nutrients.”

As emissions continue to increase, the United Nations climate body has said humanity will require carbon removal to limit global warming to 1.5°C above pre-industrial levels. Countries like Brazil have encouraged enhanced rock weathering to cut both emissions and fertiliser costs. Last year, an enhanced weathering start-up in India called Mati Carbon won the top prize of $50 million in Elon Musk’s XPRIZE competition for large-scale carbon removal potential.

Atmospheric CO2 dissolves in rain to form carbonic acid. In silicate rocks, this reacts with silicon dioxide and metals to lock the CO2 away in bicarbonate ions. The bicarbonate washes into rivers and the ocean, where it can remain dissolved for millennia or be incorporated into the calcium carbonate exoskeletons of clams, corals and sea urchins. Crushing the rocks exposes more surface area to rain, boosting this CO2 removal.

Based on how much rock could fit onto farm fields, studies have projected that enhanced rock weathering could draw down 5 billion tonnes of CO2 a year this century. Liao and his colleagues did a “reality check” on these estimates by incorporating how fast other innovations like irrigation have been embraced by farmers and how efficient weathering could be in different regions.

They modelled scenarios with both limited and widespread adoption of enhanced weathering and found that the technique could remove 350 million to 750 million tonnes of CO2 per year by 2050 and 700 million to 1.1 billion tonnes per year by 2100. For comparison, global fossil fuel CO2 emissions in 2025 totalled around 38 billion tonnes.

While Europe and North America would initially do most of this removal, they would be surpassed by Asia, Latin America and sub-Saharan Africa as supply chains of silicate rocks became established and costs decreased. Higher temperatures and precipitation speed up weathering in these regions, potentially allowing farmers there to sell more carbon-removal credits per tonne of rock spread.

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

However, Marcus Schiedung at the Thünen Institute of Climate-Smart Agriculture in Germany and his colleagues argue in a recent paper that projections like this are glossing over major uncertainties about enhanced rock weathering. For instance, 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 tonnes of carbon removal is likely to be inflated, says Schiedung.

In high-pH soils, rainfall can weather carbonates in the ground rather than the crushed rock. Those will eventually be converted back into carbonates in the ocean, releasing CO2 and resulting in no net carbon removal, he says. In low-pH soils, naturally occurring acids can react with the crushed rock and carbon won’t be removed from the rainfall. As soil acidity diminishes, CO2 emissions from microbes increase.

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

“I’m a sceptic,” he says. “We need to be sure that the CO2 is taken up. Otherwise, we get into the risk that we measure something [removing carbon], but somewhere else it’s released again, which is, in this geochemical complex system, likely to happen.”

Some also fear that enhanced rock weathering could introduce toxins into the food supply. Olivine, the rock that Liao’s projections are based on, contains heavy metals like nickel and chromium.

The leftover rock at most existing mines is also contaminated by metals, according to David Manning at Newcastle University, UK. Countries would probably have to open huge numbers of basalt quarries instead, which would take time and money.

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