Spreading rock dust on the ground could pull carbon from the air, researchers say

Known as enhanced rock weathering, the process involves layering crushed rock onto soil. When silicate or carbonate minerals in the dust dissolve in rain water, carbon dioxide is drawn from the atmosphere into the solution to form bicarbonate ions. The bicarbonate ions are eventually washed by runoff into the ocean, where they form carbonate minerals, storing their carbon indefinitely.

The study published Thursday is the first analysis that used modeling and simulations to estimate the potential of individual countries to capture carbon through enhanced rock weathering.

As nations around the globe strive to mitigate climate change, it’s not enough to reduce the carbon dioxide, methane and other greenhouse gases produced when humans burn fossil fuels, experts say.

Carbon already in the atmosphere has to be removed, scientists say, if nations are to prevent the average global surface temperature from crossing a perilous threshold: 2 degrees Celsius (3.6 Fahrenheit) above preindustrial levels. That is the point beyond which experts warn about dangerous and irreversible changes to the planet. The 2015 Paris accord, supported by every major economy except the United States under President Trump, calls for severe cuts in greenhouse gas emissions, as well as the removal of between 2 billion and 10 billion metric tons of carbon dioxide each year to reach net-zero emissions by 2050.

“Spreading rock dust on agricultural land is a straightforward, practical CO2 drawdown approach with the potential to boost soil health and food production,” said David Beerling, director of the Leverhulme Centre for Climate Change Mitigation and lead author of the study. “Our analyses reveal the big emitting nations — China, the U.S., India — have the greatest potential to do this, emphasizing their need to step up to the challenge.”

The researchers point out that the countries where enhanced rock weatherization has the greatest potential also have much to lose as the planet warms. “China, the United States and India are all vulnerable to climate change and resultant sea-level rise,” the study said. “Their high risks of economic damage and social disruption provide impetus for creative co-design of agricultural and climate policies.”

European countries have less capacity to remove CO2 through enhanced rock weatherization than China, the United States and India because they have less farmland. The European countries with the greatest potential to get rid of carbon dioxide through enhanced rock weatherization are also the largest emitters: Germany, Spain and Poland, the study said.

The researchers say the cost of enhanced rock weatherization is comparable to other proposals to remove carbon dioxide, including sequestration, in which CO2 is captured as it is emitted from a power plant or a factory and is absorbed into a liquid or a solid and stored. But spreading rock dust over farmland carries the added benefit that it could help rebuild deteriorating agricultural soils in many parts of the world, they argue.

Still, researchers acknowledged their theoretical analysis requires long term real-world trials. And they raise concerns that demand for rock dust may increase mining or grinding operations, which in turn could consume enough energy to account for 10 to 30 percent of the amount of CO2 captured. Layering dust on agricultural soils also requires careful management to make sure metals and other organic compounds are not added to crop fields, they said.

Farmers have long applied limestone to their fields to reduce acidity and provide nutrients in Africa, Brazil and Malaysia. Enhanced rock weathering replaces lime with crushed calcium and magnesium-rich silicate rock.

Spreading volcanic rock on farmland not only has the potential to remove carbon dioxide but has several side benefits, the scientists said. The practice does not compete with the use of agricultural land — in fact, it can enhance the quality of the soil, improving crop yields while reducing the need for artificial fertilizers. Rock left over from mining or construction — “waste rock” — can be processed for this use, creating a reason to recycle it, the researchers said.

Indonesia and Brazil, which emit a small fraction of the carbon dioxide produced by the United States and China, would also have great potential to remove CO2 this way because they have a large percentage of farmland and because their tropical climates would speed up the processes involved in weathering, the scientists said.

“We have passed the safe level of greenhouse gases,” said James Hansen, a partner in the study and director of the Climate Science, Awareness and Solutions Program at Columbia University’s Earth Institute. “Cutting fossil fuel emissions is crucial, but we must also extract atmospheric CO2 with safe, secure and scalable carbon dioxide removal strategies to bend the global CO2 curve and limit future climate change.”