Crushed rocks could one day draw down billions of metric tons of carbon dioxide from the air.
The idea, often referred to as “enhanced rock weathering” (ERW), is among a handful of negative-emission technologies beginning to gain traction as the world struggles to lower greenhouse gases. The idea is simple: Grinding up certain types of rock and spreading them across a large land area can accelerate the Earth’s natural rate of carbon absorption.
It might sounds far-fetched, but, scientifically speaking, it could work. The remaining questions are ones of cost and scale, but a new study suggests both could be within reach. Cutting fossil-fuel use remains first priority, but the decades-long delay in lowering emissions means we’ll also have to rely on negative-emissions technology to draw down carbon dioxide from the air.
Beerling is the lead author of a paper published last week in Nature showing that, if deployed at scale under the right conditions, ERW could capture between 500 million and 2 billion metric tons of carbon dioxide from the air at a cost of between $80 and $180 per metric ton. That’s many times the current price of carbon in the world’s largest markets, but it’s in line with World Bank estimates for what the price of carbon ought to be in the decades to come if the world is to hit goals set under the Paris climate agreement.
Here’s how it works. Basalt, a common rock variety, contains minerals of silicon, iron, calcium, magnesium and aluminum. By grinding up basalt into granules slightly bigger than table salt and spreading them across a field, you allow those minerals to be absorbed into the ground. Eventually they leach into water bodies, lowering their acidity and allowing the water to absorb more carbon dioxide from the air. That absorbed carbon dioxide is eventually consumed by tiny marine organisms called phytoplankton, which die and settle down at the bottom of the ocean floor—along with the carbon they consumed during their lives.
The benefits don’t just accrue to the climate. Silicon and iron replenish depleted soils and boost crop production (which on its own may increase absorption of carbon dioxide), while calcium, magnesium, and aluminum salts reduce soil acidity. Acidic soil is a problem for as much as 40% of the world’s arable land, and the added minerals make the ground more hospitable for the microbial communities that also help crops grow.