Can geoengineering buffer ocean acidification?

We’ve looked a lot at mitigation and adaptation responses to climate change – but could geoengineering help reverse ocean acidification?

Which type of geoengineering?

Of the two geoengineering approaches mentioned in the first week, only carbon dioxide removal methods can tackle ocean acidification. Solar radiation management schemes to cool the planet leave CO₂ levels rising, which will continue to result in ocean acidification. Furthermore, cooling the planet actually increases the amount of CO₂ that dissolves in the ocean. Viable geoengineering approaches to tackle ocean acidification involve adding alkalinity to the ocean in different ways.

Adding Limestone Powder

Let’s remind ourselves of some key equations in ocean acidification:

1. CO₂ + H₂O –> H₂CO₃

2. H₂CO₃ –> H⁺ + HCO₃^–

3. CO₃^2- + H⁺ –> HCO₃^–

Carbonate ions (CO₃^2-) act as a natural buffer to acidification in reaction three – which is great, but their supply cannot keep up with the addition of hydrogen ions from dissolving carbon dioxide (reactions one and two). If we add limestone powder (CaCO₃) to the oceans, then it’ll dissolve and improve the strength of the ocean’s natural buffer to acidification.

Ships would be used to add the limestone powder a few times a year across the oceans. There will be an energy cost in mining the limestone and grinding it down to a very small particle size – but this could be achieved using renewable energy to avoid creating a CO₂ source.

The process won’t harm life in the oceans because carbonate weathering happens naturally anyway – it will just be speeded up. The total cost has been estimated at $40-45bn a year, and the process needs to be continued for over 100 years to be truly effective (requiring international co-operation and major investment).

Dissolving Olivine

In a similar principle to adding limestone powder, dissolving olivine (Mg₂SiO₄) could enhance the sequestration of carbon dioxide. For every gram of olivine dissolved, 0.28g of carbon is sequestered. This is equivalent to the natural process of silicate rock weathering. It would help buffer ocean acidification by supplying alkalinity to the ocean (the Mg²⁺ ions ultimately combine with CO₂ to produce MgCO₃).

It would take around 100 dedicated ships to distribute olivine over an ocean surface area large enough to make a difference. Alternatively, olivine particles could be added to coastlines where the natural action of the waves would enhance their dissolution.

^{Could adding olivine partlices to coastlines be the solution to ocean acidification?}

Enhanced Weathering strategies

Rather than adding carbonate or silicate rock powder to the ocean with ships, it could be added to the land instead, where it will dissolve and be washed to the ocean. This approach simply accelerates normal, slow processes of weathering that lead to carbon dioxide being locked up in carbonate rocks on the ocean floor. It works because small rock particles dissolve much faster than natural larger ones.

If we can enhance the weathering across one-third of the world’s tropical regions, then we could ameliorate ocean acidification by 2100. This approach will enable calcifying organisms (such as corals) to continue calcifying and keep ecosystems such as coral reefs from dying out completely under acidification.

Up to 1 PgC could be sequestered per year via this mechanism – around 10% of anthropogenic emissions. But grinding rocks to tiny particles and distributing them across the land costs energy and carries health risks as the particles are breathable. Proposers of the technique suggest it could cost anywhere between $60-600 trillion, noting:

Enhanced weathering is a sobering indication of actions that may be required if fossil fuel emissions are not phased down rapidly.

Enhanced weathering techniques do have an advantage over traditional carbon capture and storage solutions in that carbon dioxide isn’t being stored under pressure as a liquid, but as a rock. This makes it much more stable and unlikely to be leaked again.

What do you think about these solutions to ocean acidification? Are they viable? Are there any alternatives?