Silicate rocks, such as olivine, react with atmospheric carbon dioxide as they are battered by rain and snow, locking up carbon in the bicarbonate produced by the weathering process. The theory goes that this process could be sped up by mining olivine, grinding it into a powder, and sprinkling it across the world's oceans. Bicarbonate ions are produced when the olivine reacts with either atmospheric carbon dioxide or is dissolved in the surface ocean. This process would increase the alkalinity in the oceans, further enhancing carbon dioxide uptake from the atmosphere. In addition the iron-rich olivine would stimulate phytoplankton production in some regions, absorbing yet more carbon dioxide.

So how might the theory play out in reality? By coupling a marine ecosystem and biogeochemistry model to a general circulation model, Judith Hauck and her colleagues from the Alfred Wegener Institut, Germany, explored the amount of carbon dioxide removal this technique could achieve in theory. Assuming that 3 gigatons (equivalent to around one-third of present day coal extraction) of olivine could be mined, powdered and sprinkled on the oceans every year, they found that 2.1 tons of carbon dioxide would be drawn down for every ton of olivine.

Most of this drawdown (57%) was due to the increased alkalinity, whilst the remainder was associated with a temporary drawdown caused by the fertilization of the ocean. "Only the carbon dioxide uptake due to increased alkalinity is (quasi) permanent, while those uptakes related to ocean fertilization reduce carbon dioxide only for some decades to centuries," explained Hauck.

However, the researchers also showed that the ocean is quick to re-equilibrate with the atmosphere, meaning that olivine couldn't be used to counteract ocean acidification unless it was done in tandem with massive reductions in carbon dioxide emissions. Ultimately, for every 3 gigatons of olivine sprinkled on the oceans, their model shows that a maximum of 1.7 gigatons of carbon would be locked up each year – equivalent to 17% of the anthropogenic emissions during 2014.

In reality it would almost certainly be less, in particular due to uncertainties about how much iron would be released from olivine. "This was a sensitivity study with differing iron solubilities," said Hauck. "More laboratory and possibly small-scale field studies would be necessary to determine olivine dissolution rates and the amount of iron that is set free under natural conditions."

For now the idea is only a thought experiment – existing international agreements would not allow this kind of geoengineering experiment to be implemented, and even testing it in small-scale field experiments is difficult under current regulations. But if countries are really committed to keeping global warming within 1.5°C, as agreed at COP21 in Paris last December, then these kind of carbon dioxide removal methods may have to be considered seriously.

The findings are published in Environmental Research Letters (ERL).

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