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On the menu of geoengineering options, one has always stood out as a saner choice: enhanced weathering. No need to deploy giant mirrors in space to deflect sunlight or to risk food riots by converting crops to fuel. Just scatter crushed rock far and wide.

It sounds simple: rock mops up carbon dioxide – it’s a slow but significant part of the carbon cycle – so if we speed up natural weathering we could get enough of this greenhouse gas out of the atmosphere to dampen climate change.

Ordinarily, common silicate minerals react slowly with CO₂ in air and water. In the process, much of the CO₂ is bound up in stable alkaline compounds, which eventually wash into the oceans.

It’s reasonable to assume that if silicates were spread more widely over Earth’s surface as crushed rock, more CO₂ would bind to them more rapidly, offsetting emissions from human activities. This would have two benefits: the CO₂ would be stored more or less permanently; and the end product is alkaline, so run-off has the added bonus of countering ocean acidification.

But, like many other geoengineering ideas, enhanced weathering has been beset by problems. Most such propositions have suggested dusting cropland and other surfaces with olivine-rich rock, because it weathers quickly and can capture a tonne of CO₂ per tonne of dissolved rock. The olivine, however, would need to be mined on a vast scale, with huge environmental and energy costs. And, as it weathers, olivine that could get into the food chain. That limits its use on farms, which are ideal places to deploy this technique.

Country rock

Now, a team of researchers led by David Beerling of the University of Sheffield, UK, have laid out with far fewer drawbacks, by applying basalt dust to croplands in place of olivine. The boon from basalt is that it’s much less toxic, and far richer in phosphorus, one of nature’s great plant promoters.

Although basalt has only one-third of the carbon-capture potential of olivine, applied to two-thirds of the world’s most productive croplands at a rate of 10 to 30 tonnes per hectare per year it could capture 0.5 to 4 billion tons of carbon per year by 2100. Given a to help keep global warming below 2°C, this is really significant. What’s more, this scheme could boost crop yields, lessen the need for fertiliser, production of which creates carbon emissions. In addition, this dust could be spread in place of limestone, which is widely used as a soil additive, using the same farm equipment widely used to apply limestone.

Still, huge quantities of rock would be required. But there are ways around this. One is to source the same type of mineral from vast amounts of industrial waste such as basalt dust from construction, ash from the sugar cane industry or slag from China’s steel plants, a lot of which is discarded. As well as saving resources, this would also save energy. Another refinement is to target only the most productive croplands, where this method is likely to be most effective, reducing the amount of basalt needed. These, conveniently, are located in the most carbon-polluting nations and regions – China, India, the European Union, the US and Russia.

Get into the field

There is a growing interest among scientists in understanding the pros and cons of various geoengineering options. Those in the know sense the need to firm up plan B. As with all such schemes, what’s needed is a field trial. In this case, one promising location – with the existing infrastructure and machinery – is the UK.

What’s uplifting about this latest proposition is that it not only revitalises the possibility of enhanced weathering but offers hope for the efficacy of geoengineering more generally.