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There’s a lot we don’t know about ocean CO₂ removal

There are some intriguing ideas, but big questions remain about all of them.

Scott K. Johnson – | 33
Credit: National Academy of Sciences

It’s clear from climate science that we need to drop greenhouse gas emissions to zero as quickly as possible. But it’s also clear from our slow progress that we could use some help with those emissions. One thing that can help is carbon dioxide removal, as it allows us to reach net-zero emissions even as some difficult-to-solve emissions remain.

Carbon removal on land—including obvious techniques like reforestation—gets a lot of attention. Carbon removal in the ocean, on the other hand, has seemed a bit pie-in-the-sky, even though the ocean already soaks up more CO2 than land ecosystems do. A new National Academies of Sciences, Engineering, and Medicine report takes up the challenge of outlining what we would need to learn to make some theoretical techniques for boosting ocean uptake a reality—or to rule them out. The report follows 2015 and 2019 reports that set the stage for carbon dioxide removal science more broadly.

Boosting productivity

The report's goal is to provide some direction, both for scientists designing studies and for funders (like the National Science Foundation) setting priorities. The report is the work of a sizable group of scientists organized by the National Academies, with funding provided by a sponsorship from the ClimateWorks Foundation.

The scientists were asked to assess six different approaches, including manipulations of nutrients, seaweed cultivation, ecosystem recovery, and manipulation of seawater carbonate chemistry.

Nutrient fertilization is pretty straightforward (and has already seen some controversial tests). By adding limiting nutrients—like iron, phosphorus, or nitrogen—it might be possible to stimulate phytoplankton growth. Phytoplankton are part of the ocean’s biological carbon pump, taking up carbon as they photosynthesize near the surface and taking it down to the bottom with them when they die and sink. Juicing this pump a bit could move more CO2 from the atmosphere to the deep ocean or into seafloor sediment.

A related technique is managed upwelling and downwelling, which is a bit like stirring a drink to mix its ingredients. Pumping nutrient-rich water up toward the surface can act like fertilization. And in some shallow areas that see an excess of nutrients, pumping water downward could make the biological carbon pump more efficient.

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Seaweed cultivation for carbon dioxide removal would entail somehow carrying the seaweed carbon down into the deep ocean, where the low oxygen concentration limits microbial decay. This process could be helped along by the fact that growing plants continually release some carbon in a form that is hard for microbes to break down.

Rebalancing

Ecosystem recovery could obviously produce a wide array of benefits—and is unlikely to come with negative side effects. The reason it’s included here is that healthy ecosystems can mean a healthy carbon cycle. The biological carbon pump, for example, relies a great deal on bigger fish that eat plankton and excrete the waste in larger particles that sink much more readily than individual plankton would.

The last category of approaches in the report relates to inorganic carbon in the ocean, known as alkalinity. There is an equilibrium between dissolved carbonate—which critters can use to build calcium carbonate shells, for example—and dissolved CO2. There are ways to play with this equilibrium and push more CO2 into chemical forms that stay in the ocean. Boosting the weathering of rock by dumping pulverized rock into the ocean, for example, could convert dissolved CO2 into bicarbonate and carbonate, instead—which would also help with ocean acidification.

Each of these techniques gets a detailed treatment of both the general state of knowledge and the areas of research that are most needed. But the report also notes a few challenges that show up repeatedly. For one, many of these ideas only exist on the drawing board, with lab experiments taking place on certain aspects of the processes. Real-world pilot studies are necessary to move our understanding forward.

But as we’ve seen with iron fertilization, these experiments can be controversial. Large-scale operations in the future would be even more so.

One reason for trepidation is that many of these techniques could obviously come with unintended consequences. Monitoring those consequences—and even monitoring the intended carbon outcomes—is no small task in the ocean. And that brings us around, inevitably, to money. Research costs money (the report suggests almost $1.5 billion is needed for the highest priorities), and we’ll need that research to determine how much the techniques themselves might cost.

Overall, the report highlights the need for more information to guide our decisions about whether to pursue these techniques. “At present, society and policy makers lack sufficient knowledge to fully evaluate ocean [carbon dioxide removal] outcomes and weigh the trade-offs with other climate response approaches, including climate adaptation and emissions mitigation, and with environmental and sustainable development goals," the report says. "Research on ocean [carbon dioxide removal], therefore, is needed to decide whether or not society moves ahead with deployment, and to assess at what scales and locations the consequences of ocean [carbon dioxide removal] would be acceptable.”

Listing image: National Academy of Sciences

Photo of Scott K. Johnson
Scott K. Johnson Associate Writer
Scott has written about geoscience and energy at Ars as a freelancer since 2011.
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