Carbon Dioxide Removal and Conversion to Ocean Alkalinity: Why and How

Abstract:

Drastic reduction in anthropogenic CO2 emissions is the most obvious way to stabilize atmospheric CO2. However, there is growing risk that effective emissions reduction policies and technologies will not engage soon enough to avoid significant CO2-induced climate and ocean acidification impacts. This realization has lead to increased interest (e.g., IPCC AR5, 2014; NRC/NAS, 2014) in the possibility of pro-actively increasing CO2 removal (CDR) from the atmosphere above the 55% of our emissions that are already removed from air by natural land and ocean processes. While a variety of biotic, abiotic, and hybrid CDR methods have been proposed, those involving geochemistry have much to recommend them. These methods employ the same geochemical reactions that naturally and effectively remove excess planetary CO2 and neutralize ocean acidity on geologic time scales. These reactions proceed when the hydrosphere, acidified by excess air CO2, contacts and reacts with carbonate and silicate minerals (>90% of the Earth’s crust), producing dissolved bicarbonates and carbonates, i.e., ocean alkalinity. This alkalinity is eventually removed and the excess carbon stored via carbonate precipitation. So while the importance and global effectiveness of such reactions are not in question, it remains to be seen if this very slow, natural CDR could be safely and cost-effectively accelerated to help manage air CO2 levels on human rather than geologic time scales. Various terrestrial and marine, geochemistry-based CDR methods will be reviewed including: 1) the addition of minerals to soils and the ocean, 2) removal of CO2 from waste streams, esp. from biomass energy, via wet mineral contacting, and 3) the production and use of mineral derivatives, e.g. oxides or hydroxides, as CDR agents. The additional potential environmental benefits (e.g., reversal of ocean carbonate saturation loss) and impacts (e.g., increased mineral extraction), as well as potential economics will also be discussed.