Ocean acidification has been prominent in the headlines this month, following the publication in December of a NOAA study that found the pH of surface waters in the California Current ecosystem have been dropping at a rate of 0.021 per decade during the last century, more than twice the global average .
This result is deduced from measurement of the shell thickness (actually the area normalized shell weight or ANSW) of Globigerina bulloides foraminifera samples recovered from radioisotope dated sediment cores taken in the Santa Barbara Basin. ANSW is related to the seawater carbonate ion concentration at the time the organisms grew, and from this, in turn, pH can be determined.
The Pacific coast of North America, from Alaska down to California, is home to a multi-billion dollar per year fishery and shellfish industry, which is already feeling the impact of acidification. The upwelling of low-pH, CO2-rich deep ocean waters into the California Current compounds the acidifying impact of anthropogenic CO2 uptake to create this double whammy for the industry.
Digging a bit deeper into local action plans, I was surprised to see that the main emphasis is on measuring and predicting the downward slide of ocean pH and the impact on ocean ecosystems, without much consideration being given to potential remedial actions that could be developed.
Through my work on negative emissions technologies (NETs) I have become familiar with the options to sustain and increase the ocean carbon sink – the sink that has come to our aid in absorbing close to one third of all historical CO2 emissions – and particularly the technique of ocean alkalinity enhancement or OAE . One of the simplest approaches, proposed in 2008, is simply to disperse powdered limestone in upwelling regions  – including the region that is exacerbating acidification in the California Current!
So why was this not being discussed as a potential option to address the rapid acidification being experienced on the Pacific coast? Why the deafening silence? I posted the following query on the Ocean Acidification Information Exchange (OAIE) to see what the views of that community were.
It goes without saying that rapid decarbonization must be the foundation of our global response to the climate crisis, but even optimistic emissions reduction pathways lock in a lot of additional acidification over the next century and beyond.
Why aren’t we talking about progressing ocean alkalinity enhancement on a massive scale as a positive action to address this, starting regionally and extending globally? Whether it’s enhanced weathering of silicates, limestone weathering in upwelling regions, or the variety of electrochemically enhanced processes (including carbon negative fuel production) that can add alkalinity to the oceans, there are many techniques that could have a positive impact.
Yes, there’s a lot of research to be done, and, yes, it’s gonna come at a high cost (as will inaction.) But so far there seems to be a reluctance to even talk about the first steps that will begin this journey … let alone take them! Or have I missed something?
From the discussion that followed, I learned a lot about shell recycling, which has been investigated to increase alkalinity in estuarine waters subject to high freshwater runoff, where calcite/aragonite is undersaturated (one example being in Puget Sound). But the view on wider scale OAE was that older assessments that ruled out OAE as infeasible had done so by considering OAE as a sole alternative to emissions mitigation (e.g. ), rather than as part of a portfolio of techniques to supplement aggressive emissions reduction and, ultimately, enable us to reach net-negative.
So perhaps it’s time to get OAE back into the mainstream discussion – as a negative emissions technology with potentially billion dollar scale co-benefits. The recently approved US spending bill included for the first time an allocation for research into ocean capture, so perhaps we’ll soon start to see some progress.
 Osborne, E. B., Thunell, R. C., Gruber, N., Feely, R. A., & Benitez-Nelson, C. R. (2019). Decadal variability in twentieth-century ocean acidification in the California Current Ecosystem. Nature Geoscience, 1-7.
 Renforth, P., & Henderson, G. (2017). Assessing ocean alkalinity for carbon sequestration. Reviews of Geophysics, 55(3), 636-674.
 Harvey, L. D. D. (2008). Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions. Journal of Geophysical Research: Oceans, 113(C4).
 Can geoengineering solutions for climate change also help OA? in Frequently asked questions about ocean acidification (pg. 12). Available here.