The world is buzzing with new technologies to reduce emissions of carbon dioxide, even if getting them to widespread use is often painfully slow. On the other hand, there is a desperate shortage of options to remove the carbon already in the atmosphere, other than the distinctly low-tech organisms known as trees. One possibility could be even more ancient – crushed rocks. A new study suggests there is more to the idea than might at first appear, with some substantial side-benefits.
For billions of years, the weathering of silicate rocks has removed carbon dioxide from the atmosphere. Silicates rich in calcium or magnesium react with carbon dioxide from the atmosphere to produce carbonate and bicarbonate ions that become contained in soil or washed to the oceans, where the carbon is stored for geological periods of time. This is, however, an excruciatingly slow process. Although weathering speeds up in the conditions we are creating, it is still far too slow for humanity's needs.
Professor David Beerling of the University of Sheffield noted weathering rates depend on surface area, and therefore increase dramatically when rocks are crushed into tiny particles.
Moreover, there could be other advantages. "Human societies have long known that volcanic plains are fertile, ideal places for growing crops without adverse human health effects,” Beerling noted in a statement. This fertility comes from the nutrients the local rocks release as they break down, which should also accelerate if the rocks are crushed.
In Nature Plants, Beerling provides an overview of studies where crushed basalt and other volcanic rocks were added to croplands as a form of fertilizer. Mauritian soils treated in this way produced 30 percent more sugar cane over the next five years than similar fields without basalt addition. Although silicate dust has been used since 1871 to boost crops in the United States, there have been few studies on its effectiveness in temperate regions, but one trial in New Jersey found benefits for several different crops lasting 3-4 years.
Some crops benefit from the silicon itself, including its role as a disease suppressant. For others, the advantage lies in the release of trace minerals from the rocks, such as phosphorus.
Encouraging as these trials have been, Beerling argues we need far more studies, across diverse crops and soil types, before we can generalize about the benefits.
Even in the best-case scenario, silicate rock weathering is unlikely to be a panacea. Beerling calculated the potential carbon storage if two-thirds of the world's croplands were regularly supplied with silicates crushed to a size of 10-30 micrometers (0.0004-0.0012 inches) in diameter. Even the best case is a fraction of the current rate of emissions from human activity.
Moreover, as the paper notes; “A key issue affecting the efficiency of carbon capture is the energy cost associated with mining, grinding and spreading the ground rock, which could reduce the net carbon drawdown by 10-30 percent, depending mainly on grain size.” Some of the silicates could be drawn from waste products in various industries, but if the idea takes off sufficiently mining will be needed. Beerling acknowledges the dangers this can pose, both to the local environment and to miners in the form of silicosis.
Nevertheless, given the shortage of opportunities to take any significant amount of carbon out of the air at an affordable price, one that does so while also providing other benefits certainly deserves exploration.
Previous efforts at using crushed silicates to remove carbon dioxide have mostly focused on olivine. Beerling points out that while olivine captures a lot of carbon per tonne crushed, it usually contains metals that suppress plant growth, making basalts a safer bet.
Beerling notes that farms with acidity problems already use crushed limestone to balance the soil pH, providing infrastructure that could be converted to using silicates instead.
The television announcer in The Hitchhiker's Guide To The Galaxy advised pre-technological sentient beings, “The secret is to bang the rocks together, guys.” Maybe the advice extends to those with the capacity to pick up the signal.