Assessing ocean alkalinity for carbon sequestration

Abstract

Over the coming century humanity may need to find reservoirs to store several trillions of tons of carbon dioxide (CO2) emitted from fossil fuel combustion, which would otherwise cause dangerous climate change if it were left in the atmosphere. Carbon storage in the ocean as bicarbonate ions (by increasing ocean alkalinity) has received very little attention. Yet recent work suggests sufficient capacity to sequester copious quantities of CO2. It may be possible to sequester hundreds of billions to trillions of tons of C without surpassing postindustrial average carbonate saturation states in the surface ocean. When globally distributed, the impact of elevated alkalinity is potentially small and may help ameliorate the effects of ocean acidification. However, the local impact around addition sites may be more acute but is specific to the mineral and technology. The alkalinity of the ocean increases naturally because of rock weathering in which >1.5 mol of carbon are removed from the atmosphere for every mole of magnesium or calcium dissolved from silicate minerals (e.g., wollastonite, olivine, and anorthite) and 0.5 mol for carbonate minerals (e.g., calcite and dolomite). These processes are responsible for naturally sequestering 0.5 billion tons of CO2 per year. Alkalinity is reduced in the ocean through carbonate mineral precipitation, which is almost exclusively formed from biological activity. Most of the previous work on the biological response to changes in carbonate chemistry have focused on acidifying conditions. More research is required to understand carbonate precipitation at elevated alkalinity to constrain the longevity of carbon storage. A range of technologies have been proposed to increase ocean alkalinity (accelerated weathering of limestone, enhanced weathering, electrochemical promoted weathering, and ocean liming), the cost of which may be comparable to alternative carbon sequestration proposals (e.g., $20–100 tCO2−1). There are still many unanswered technical, environmental, social, and ethical questions, but the scale of the carbon sequestration challenge warrants research to address these.

Assessing carbon dioxide removal through global and regional ocean alkalinization under high and low emission pathways

“…Indeed, it is not clear whether such an effort is even feasible given the cost and the logistical, political, and engineering challenges of producing and distributing such large quantities of alkaline material (Renforth and Henderson, 2017). In the case of RCP8.5, it is unlikely that this level of AOA could be justified given our results.…”

“…We can only speculate on the potential impact of a reduction in aqueous CO 2 and elevated pH levels on marine biota in these regions. For a recent review of the potential impact of rising pH and aragonite saturation state on marine organisms, we direct the reader to Renforth and Henderson (2017).…”

“…Kheshgi (1995) first proposed AOA as a method of CDR. Renforth and Henderson (2017) review the early experimental, engineering, and modelling work undertaken to investigate AOA. From the observational perspective, we draw particular attention to the experimental work of Albright et al (2016) which provided an in situ demonstration of localized AOA to offset the observed changes in ocean acidification on the Great Barrier Reef which have occurred since the pre-industrial period.…”

Assessing Carbon Dioxide Removal Through Global and Regional Ocean Alkalization under High and Low Emission Pathways

“…Indeed, it is not clear whether such an effort is even feasible given the cost, logistical, 625 political and engineering challenges of producing and distributing such large quantities of 626 alkaline material (Renforth and Henderson, 2017). In the case of RCP8.5 it is unlikely that 627 this level of AOA could be justified given our results.…”

“…We can only speculate on the 533 potential impact on marine biota through a reduction in aqueous CO2 and elevated pH levels 534 in these regions. For a recent review of the potential impact of rising pH and Aragonite 535 saturation state on marine organisms we direct the reader to Renforth and Henderson (2017). 536 537…”