Spreading rock dust on farmland could suck billions of tonnes of carbon dioxide from the air every year, according to the first detailed global analysis of the technique.
The chemical reactions that degrade the rock particles lock the greenhouse gas into carbonates within months, and some scientists say this approach may be the best near-term way of removing CO2 from the atmosphere.
The researchers are clear that cutting the fossil fuel burning that releases CO2 is the most important action needed to tackle the climate emergency. But climate scientists also agree that, in addition, massive amounts of CO2 need to be removed from the air to meet the Paris agreement goals of keeping global temperature rise below 2C.
The rock dust approach, called enhanced rock weathering (ERW), has several advantages, the researchers say. First, many farmers already add limestone dust to soils to reduce acidification, and adding other rock dust improves fertility and crop yields, meaning application could be routine and desirable.
Basalt is the best rock for capturing CO2, and many mines already produce dust as a byproduct, so stockpiles already exist. The researchers also found that the world’s biggest polluters, China, the US and India, have the greatest potential for ERW, as they have large areas of cropland and relatively warm weather, which speeds up the chemical reactions.
The analysis, published in the journal Nature, estimates that treating about half of farmland could capture 2bn tonnes of CO2 each year, equivalent to the combined emissions of Germany and Japan. The cost depends on local labour rates and varies from $80 per tonne in India to $160 in the US, and is in line with the $100-150 carbon price forecast by the World Bank for 2050, the date by which emissions must reach net zero to avoid catastrophic climate breakdown.
“CO2 drawdown strategies that can scale up and are compatible with existing land uses are urgently required to combat climate change, alongside deep emissions cuts,” said Prof David Beerling, of the University of Sheffield, a lead author of the study. “ERW is a straightforward, practical approach.”
Prof Jim Hansen, of Columbia University in the US and one of the research team, said: “Much of this carbonate will eventually [wash into] the ocean, ending up as limestone on the ocean floor. “Weathering provides a natural, permanent sink for the carbon.”