Could spreading basalt dust over farmers’ fields help remove atmospheric carbon? A large multidisciplinary team of scientists is confident it can, and in doing so, boost crop yields and soil health at the same time.
In this issue, David Beerling, a biogeochemist at the University of Sheffield, UK, and his colleagues explore a strategy to enhance rock weathering (DJ Beerling et al. Nature 583, 242–248; 2020). .
It is a continuous natural phenomenon in which carbon dioxide and water react with silicate rocks on Earth’s surface. In this process, atmospheric CO2 is converted into stable bicarbonate which dissolves and then flows into rivers and oceans. The idea of scaling up this process to remove carbon has been considered for some three decades. The team’s results provide the most detailed analysis yet of the technical and economic potential of this approach – and some of the potential challenges, including gaining public acceptance.
The researchers modeled what would happen to atmospheric carbon if basalt dust was added to agricultural land in the world’s largest economies, including Brazil, China, the European Union, India, Indonesia and the United States. According to their calculations, doing so would remove 0.5 billion to 2 billion tons of CO2 from the air each year. The upper limit is more than 5 times the United Kingdom’s annual emissions, and equates to offsetting emissions from about 500 coal-fired power plants.
The team is also conducting field trials in four countries – the only trials of its kind so far. The authors tell Nature that preliminary results suggest the theory is holding up. Application of 20 tonnes of basalt dust to a half-hectare UK plot increased CO2 removal by 40% compared to that seen on an untreated plot and by 15% in another trial, which spread dust on oil-palm plantations in Malaysia . Preliminary results also indicate that the addition of basalt increased yields in these and other crops.
These are spurring growth at a time when governments around the world are struggling to meet their climate commitments. The approach, if successful, could enable high-emitting countries such as the United States and China to remove some of the carbon that has been pumped into the atmosphere in recent decades. In addition, the machines that are needed to disperse basalt dust on the fields already exist: farmers use them to treat the soil from limestone.
Cost of earth
But, like many promising technological improvements, spreading basalt dust across the world’s agricultural fields may prove more complicated than previously thought. Researchers must answer several important questions about the economic cost and environmental impacts. And there are potential questions for regulators as well.
Tampering with the geochemical cycle would inevitably alter ecosystems in soils, rivers and even oceans. Some of this can be beneficial: the right kind of rock dust can strengthen desirable plant communities, for example. And the alkaline material that moves to the oceans could, in theory, counteract acidification, helping to protect corals and other organisms that are at risk from rising levels of atmospheric CO2. But we must be reassured that there are no harmful consequences for land and sea, and any potential impact will need to be carefully monitored.
Furthermore, mining rock on an industrial scale, pulverizing it and spreading dust over crop fields will not be cheap. The current price of carbon on the EU’s emissions trading system is less than €28 (US$31) per tonne. In contrast, Berling and his colleagues estimate that increased rock weathering costs would be between $80 and $180 per tonne of CO2.
That said, such costs are in line with competitive technologies that can be used to take CO2 out of the atmosphere. And although the rock will need to be mined, the Sheffield team is rightly calling for an inventory of suitable waste rock, free from existing mining operations. This will reduce costs, increase carbon content and lead to more efficient use of mined materials.
Citizen science
The project team also studied how members of the public would react to such technologies (E. Cox et al. Nature Climate. Change https://doi.org/10.1038/s41558-020-0823-z; 2020). From research conducted in the United Kingdom and the United States, it is clear that CO2 removal strategies may face skepticism.
Respondents who participated in the survey and workshop discussions feared that they might take too long to develop, and expressed concern that basalt dust could affect ocean ecology. Many also opposed the idea of such technologies becoming an alternative to tackle the root causes of climate change.
Concerns about ecological impacts can be allayed with proper government oversight.
