7.1 Overview
Current concern about the accelerated rate of carbon dioxide (CO2) diffusion from the atmosphere into the surface ocean has prompted the marine scientific community to explore ocean CO2 removal (CDR) approaches. Land-based CDR methods such as afforestation or bioenergy with carbon capture and storage have received much attention recently. However, meeting climate mitigation targets with land-based CDR alone will be extremely difficult, if not impossible, because the ocean governs the atmospheric CO2 concentration and acts as the natural thermostat of Earth, simply because the ocean contains more than 50 times as much carbon as the atmosphere (Sarmiento and Gruber, 2002). One proposed ocean-based CDR technique is ocean alkalinity1 enhancement (OAE) (Figure 7.1), also termed enhanced weathering (EW), proposed by Kheshgi (1995). This approach is broadly inspired by Earth’s modulation of alkalinity on geological timescales. Adding alkalinity via natural or enhanced weathering is counteracted by the precipitation of carbonate, which reduces alkalinity and, in today’s ocean, is driven almost entirely by calcifying organisms. For example, on geologic timescales, the dissolution of alkaline silicate minerals plays a major role in restoring ocean chemistry via addition of alkalinity to the ocean and conversion of CO2 into other dissolved inorganic carbon (DIC) species (Archer et al., 2009). To date, most attention has been paid to terrestrial EW applications (Köhler et al., 2010; Schuiling and Tickell, 2010; Hartmann et al., 2013), with potential co-benefits in addition to CDR including stabilization of soil pH, addition of micronutrients, and crop fertilization (e.g., Manning, 2010). When applied to the ocean, EW of minerals is achieved by adding large amounts of pulverized silicate or carbonate rock or their dissolution products, which adds alkalinity to the surface ocean and thereby “locks” CO2 into other forms of DIC, which is expected to promote atmospheric CO2 influx into the ocean. Specifically, following alkalinity addition, CO2 is converted into bicarbonate ions (HCO3−) and carbonate ions (CO32−), and these chemical changes lead to a rise in pH (Kheshgi, 1995; Gore et al., 2019). Therefore, this approach has the potential to not only remove atmospheric CO2 but also counteract ocean acidification and thus contribute to the restoration of ecosystems threatened by it.
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National Academies of Sciences, Engineering, and Medicine. 2022. Chapter 7 – Ocean alkalinity enhancement. A research strategy for ocean-based carbon dioxide removal and sequestration, pp 181-208. Washington, DC: The National Academies Press. Chapter.
