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By Adam Vaughan

Dr Dimitar Epihov

Spreading rock dust on cropland around the world could save around a tenth of humanity’s “carbon budget”, the amount of carbon dioxide we can afford to emit without triggering catastrophic levels of global warming.

Earth’s three biggest CO2 emitters – China, the US and India – have the most to gain from the strategy, which is known as enhanced rock weathering (ERW). Rocks naturally absorb CO2, but ERW accelerates the process by grinding them up to increase their surface area.

David Beerling at the University of Sheffield in the UK says his team’s modelling of ERW’s potential is the most realistic yet because it limits how much rock is available and the energy countries would be willing to use for grinding. Factoring in countries’ climate, cropland area and evolving energy systems, they found that rock dust could remove between 0.5 and 2 gigatonnes of CO2 annually by 2050. Humanity’s fossil fuel use emits around 35 gigatonnes of CO2 each year.

“If you can extract a gigatonne a year, it’s significant. Two gigatonnes is the combined CO2 emissions of aviation and shipping, and those two are going to be very difficult to decarbonise. I would say it’s got very exciting potential for transforming how we manage the agricultural landscape,” says Beerling.

His team calculated that if 2 gigatonnes of CO2 were removed annually over half a century, it would save up to 12 per cent of the world’s carbon budget.

Rock dust may hold appeal over other CO2 removal options because it doesn’t require changes to land use – such as growing energy crops for bioenergy with carbon capture and storage – and there is growing evidence that it has the side effect of boosting crop yields too, says Beerling.

“We need to clean up the [climate change] mess in sensible ways, over a time scale of decades to centuries,” says team member James Hansen at Columbia University in New York. “One of the ways with multiple benefits is rock dust farming.  I particularly like it because it is more permanent than most CO2 draw-down schemes.”

There is still a long way to go. The amount of CO2 extracted by ERW today is effectively nothing, says Beerling. At $80 to $180 per tonne of CO2 removed, it is pricier than planting trees. Public attitudes may also need to be overcome: research published on Monday found that people in the US and the UK found it the least appealing of three ways to remove CO2.

Journal reference: Nature, DOI: 10.1038/s41586-020-2448-9

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