Dumping alkaline green sand into shallow seas could increase ocean absorption of carbon dioxide by 8 per cent this century, according to a modelling study.

Oceans take up almost a third of the CO₂ we emit into the atmosphere and researchers are exploring ways to boost that amount. Dissolving 1 gigatonne a year of ground-up olivine, a common, greenish mineral made mainly of magnesium, iron, silicon and oxygen, in shallow seas could reduce atmospheric CO₂ levels by 10 parts per million (ppm) by 2100, Andrew Yool and Julien Palmiéri at the UK’s National Oceanography Centre have found. This would lower Earth’s average temperature by 0.06°C, they estimate.

Atmospheric CO₂ is currently at 423 ppm and would reach 1200 ppm by 2100 in the high-emissions scenario the pair modelled. But the study suggests that if we cut emissions from most sectors, olivine deployment in a few key locations could help compensate for hard-to-abate sectors like steel-making or air travel, says Yool.

“It’s doable. [Olivine] works at a kind of a scale approximately similar to what you could realistically mine,” he says. “It’s something which I think could help with the ticket to achieve net zero.”

The idea is to speed up the natural weathering of rocks by the sea. Olivine would react with water to form bicarbonate (HCO₃), turning seas more alkaline, just like an antacid does to your stomach. That allows more CO₂, a weak acid, to dissolve in the water.

A 2023 study found that olivine sand deployed across the oceans would mostly sink into the depths before it could raise pH. Yool and Palmiéri modelled an alternative scenario, in which the olivine would be spread over coastal shelves less than 100 metres deep to keep it close to the surface. The study found that currents would then redistribute the more alkaline water, with half of atmospheric CO₂ absorption happening off the shelves.

Alkalinity would increase most in shallow, warm seas like the Yellow Sea, Gulf of Mexico and Persian Gulf and around the Indonesian and Malaysian archipelagos. “Those would be regions where I would be starting to think: what are they like ecologically and would they be able to tolerate this sort of perturbation?” says Yool.

The ecological impact of alkalinity enhancement was flagged as a concern by the 2023 study. It found that olivine sand could harm zooplankton, potentially disrupting the food chain.

In 2022 in Cornwall, UK, magnesium hydroxide was added to waste water flowing into the sea as a trial of alkalinity enhancement. The firm responsible, Planetary Technologies, is seeking regulatory approval to carry out a larger follow-up trial, despite local opposition.

While overall ocean alkalinity would change little after the additional CO₂ is absorbed, we don’t fully know if discharging magnesium hydroxide would harm organisms near the waste water pipe, or if adding olivine would smother organisms on the seafloor, says Paul Halloran at the University of Exeter, UK.

Yool and Palmiéri’s study shows that it is theoretically possible to remove a significant amount of CO₂, says Halloran. “But the open questions are really, could you actually practically do this? And what would the ecosystem impacts be?”

Journal reference: Earth's Future DOI: 10.1029/2023EF004018