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From sugar plantations in Brazil to tea estates in India, crushed rock is being sprinkled across large stretches of farmland globally in a novel bid to combat climate change.<\/p>\n
The technique is called Enhanced Rock Weathering (ERW) and aims to speed up the natural capture and storage of carbon dioxide — a planet-warming greenhouse gas.<\/p>\n
It is potentially big business with tech giants, airlines and fast fashion firms lining up to buy carbon credits from ERW projects to “offset” or cancel out their own emissions.<\/p>\n
– What is ERW? –<\/p>\n
ERW aims to turbocharge a natural geological process called weathering.<\/p>\n
Weathering is the breakdown of rocks by carbonic acid, which forms when carbon dioxide in the air or soil dissolves into water.<\/p>\n
Weathering occurs naturally when rain falls on rocks, and the process can lock away carbon dioxide from the air or soil as bicarbonate, and eventually limestone.<\/p>\n
ERW speeds the process up by using quick-weathering rocks like basalt that are ground finely to increase their surface area.<\/p>\n
– How effective is ERW? –<\/p>\n
ERW is still a fairly new technology and there are questions about how much carbon it can remove.<\/p>\n
One US study found applying 50 tonnes of basalt to a hectare of land each year could remove up to 10.5 tonnes of carbon dioxide per hectare over a four-year period.<\/p>\n
But scientists applying basalt to oil palm fields in Malaysia and sugarcane fields in Australia measured much lower removal rates.<\/p>\n
“Field trials are showing that there have been overestimates of the amount and rate captured,” said Paul Nelson, a soil scientist at James Cook University who has studied ERW.<\/p>\n
Rates depend on variables including rock type and size, how wet and hot the climate is, soil type and land management.<\/p>\n
And measuring the carbon captured is difficult.<\/p>\n
The most popular technique measures “cations”, positively charged ions that are released from the rock during weathering.<\/p>\n
But those cations are produced regardless of which acid the rock has reacted with.<\/p>\n
“If there are stronger acids than carbonic, then it will react with those,” said Nelson, so measurable cations are produced even when carbon dioxide is not captured.<\/p>\n
That doesn’t mean ERW is pointless, said Wolfram Buss, a researcher on carbon dioxide removal at the Australian National University, just that it needs to be carefully calibrated and measured.<\/p>\n
“There is no doubt that this technique works,” he said.<\/p>\n
“However, to be sure how much carbon dioxide we actually remove, more funding is required to do fundamental studies.”<\/p>\n
– Are there other benefits? –<\/p>\n
The added rock increases soil alkalinity, which can boost crop growth, soil nutrients and soil formation.<\/p>\n
Basalt is both naturally abundant and often available as a byproduct of quarrying, lowering the costs of the process.<\/p>\n
Experts note that even if the rock reacts with other acids in the soil, failing to lock away carbon dioxide at that stage, it can still have planetary benefits.<\/p>\n
That is because acids in the soil would otherwise eventually wash into rivers and the sea, where acidification leads to the release of carbon dioxide.<\/p>\n
If the rock neutralises that acid in the soil, “you’ve prevented carbon dioxide being released from the water into the atmosphere downstream”, said Nelson.<\/p>\n
The scale of those possible “prevented” emissions is not yet clear, however.<\/p>\n
– What are the risks? –<\/p>\n
ERW is broadly considered safe since it merely speeds up an existing natural process. However, some quick-weathering rocks have high levels of potentially poisonous heavy metals.<\/p>\n
Scattering finely ground rock also requires appropriate protective gear for those involved.<\/p>\n
But the main risk is that incorrect measurements overestimate captured carbon.<\/p>\n
Some projects are already selling carbon credits from ERW. If a company buys an ERW credit to “offset” its emissions but the process captures less than projected, it could result in net higher carbon dioxide put into the atmosphere.<\/p>\n
– Where is ERW being done? –<\/p>\n
Projects are happening in most parts of the world, including Europe, North America, Latin America and Asia.<\/p>\n
Earlier this year, a project in Brazil announced it had delivered the first-ever verified carbon-removal credits from an ERW project.<\/p>\n
The process is being used or trialled in agricultural settings from tea plantations in India’s Darjeeling to US soy and maize fields.<\/p>\n
– What investor interest is there? –<\/p>\n
An ERW startup — Mati Carbon, working in India — won the $50 million X Prize for carbon removal projects earlier this year.<\/p>\n
In December, Google announced what was then the world’s biggest ERW deal, for\u00a0200,000 tons of carbon removal credits, to be delivered by the early 2030s by startup Terradot.<\/p>\n
The cost of the deal was not disclosed but a separate agreement by Terradot with a company representing firms including H&M sold 90,000 tons for $27 million.<\/p>\n
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