Enhanced Rock Weathering : Good for Farmers, the Planet and the Economy
By Michael Gregson
Vegetable growers and tea planters in Sri Lanka have long used crushed rocks to improve crop yields, with limestone in the form of ground dolomite improving the soil by reducing acidity and adding nutrients. Now scientists have shown that a similar practice could also save the planet.
A technique known as enhanced rock weathering (ERW) replaces lime with crushed calcium and magnesium-rich silicate rock to suck carbon dioxide out of the air.
Spreading volcanic rock on farmland not only has the potential to remove CO2 but has several side benefits. 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 fertilisers. Rock left over from mining or construction, ‘waste rock’, can be used to create a virtuous circle of green recycling.
There is huge potential in Sri Lanka with all the raw material widely and cheaply available on the island. Across the world, the procedure could remove 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 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 US$80 per tonne in India to US$160 in the US, and is roughly 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 Professor David Beerling, of the University of Sheffield, a lead author of the study. “ERW is a straightforward, practical approach.”
Professor 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.”Hansen, who famously warned the US Senate about global warming in 1988, said improving soil could also underpin food security for billions of people.
Other proposed ways of pulling CO2 from the atmosphere at similar rates include using chemical solvents to capture it directly from the air, or growing energy crops, burning them to produce electricity and then burying the CO2 emissions. The new research suggests ERW will be less expensive than either and, unlike energy crops, does not compete with food for land. But the scientists said all approaches may be needed to beat the climate crisis.
Planting trees and adding charcoal to soil also remove CO2 from the air, and these approaches could potentially be used in combination with ERW to maximise the impact. “Planting trees is an excellent option for CO2 removal but is not sufficient on its own,” said the scientists.
Basalt is preferred for ERW as it contains the calcium and magnesium needed to capture CO2, as well as silica and nutrients such as potassium and iron, which are often deficient in intensively farmed soils. Some farmers in south-east Asia already use it to boost depleted silica in rice fields.
Basalt is one of the most common rocks on Earth, and waste dust from mining could be used for ERW, as could waste from cement and steel manufacturing. This would remove the need to grind the rocks into fine particles, which requires energy.
Professor Beerling said ERW did not require new technology, and farmers could get behind it, adding: “If you could demonstrate to farmers in China and India, for example, that they are going to get crop yield increases and get paid $100 a tonne for removing CO2, then it becomes really attractive.”