Ocean-Based Carbon Dioxide Removal (CDR)

Ocean Alkalinity Enhancement

Our planet is being transformed by climate disruption, with some of the worst impacts occurring in the ocean. Currently, most efforts to address climate change are focused on reducing emissions of greenhouse gases. While those efforts remain vital, they’re no longer enough. We must also clean up the “legacy” greenhouse gas pollution already in our atmosphere.

Ocean Visions believes that we may be able to harness the power of the ocean to restore the climate and the ocean itself. The ocean already holds more carbon than any other part of the biosphere and has the potential to contribute even more. The sheer scale of the ocean means that any ocean-based carbon dioxide removal (CDR) solutions proven to be viable and safe would have the potential to clean up billions of tons of CO2.

A number of ocean-based CDR approaches are being explored—including ocean alkalinity enhancement.

Graphic © National Academies of Sciences, Engineering, and Medicine. 2021. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. https://doi.org/10.17226/26278. Adapted and reproduced with permission from the National Academy of Sciences, Courtesy of the National Academies Press, Washington, D.C.

Ocean Alkalinity Enhancement Overview

Ocean alkalinity enhancement is an approach to carbon removal that involves adding alkaline substances to seawater to accelerate the ocean’s natural carbon sink. Adding alkalinity to the ocean converts dissolved inorganic CO2 in seawater into bicarbonates and carbonates, which are stable forms of carbon with a lifetime of approximately 10,000 years. The resulting CO2 deficit in surface waters is quickly rebalanced via a net movement of atmospheric CO2 into the ocean, to reestablish equilibrium. 

This process occurs naturally over geologic time as alkalinity is produced through the slow weathering of rocks, such as limestone or basalt, and subsequent runoff of the alkaline molecules into the ocean. We may be able to greatly enhance this process via human-caused additions of alkaline material. Billions of tons of alkaline minerals such as olivine, basalt, and carbonate are readily available to be mined and could be applied to beaches or the open ocean. This “enhanced weathering” could theoretically increase CO2 sequestration in the ocean.

Technical Potential

There are different ways to add alkalinity to the ocean. These include spreading finely ground alkaline substances over the open ocean, depositing alkaline sand or gravel on beaches or coastal flats, and reacting seawater with alkaline minerals inside reactors before releasing the modified seawater back into the ocean.

Environmental Co-Benefits and Concerns

Ocean alkalinity enhancement may counter ocean acidification — the change in seawater chemistry that has been caused by absorption of anthropogenic CO2 pollution into the upper layer of the ocean. Increasing ocean pH via alkalinity enhancement may then be directly beneficial for organisms that are vulnerable to ocean acidification.

However, weathering of alkaline minerals like olivine releases byproducts such as silica, magnesium, and trace metals. These could have both positive and negative impacts on the marine ecosystem and change the composition of phytoplankton communities. An array of lab and field experiments are needed to fully understand the environmental impacts of, and species sensitivities to, alkalinity enhancement.

Cost Considerations

The cost of ocean alkalinity enhancement will vary depending on the alkalinity source, transport, and distribution methods of alkalinity in the ocean. Raw material production (e.g., mining, grinding, transport) and the energy sources which are used will greatly influence the net CO2 removal. Distance between alkalinity sources and optimal distribution points could also be an influential factor.

Dive Deeper

Join the Ocean-Based CDR Community

Ocean Visions’ CDR Community brings together stakeholders to advance the state of knowledge, build bridges across disciplines, and help the community move towards safe and equitable testing and piloting of the most promising ocean-based CDR approaches.

Explore Ocean-Based CDR Road Maps

Ocean Visions’ ocean-based CDR road maps provide overviews of potential technologies, obstacles they face, and first-order priorities needing attention to advance the field. The road maps are intended to catalyze global collaboration and engagement and will be updated and refined as advances emerge in science, technology, governance, and policy.