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Gigatons Under the Sea

Science tells us that, in addition to making severe cuts in carbon emissions, carbon dioxide removal (CDR) is essential to achieving net-zero emissions by 2050. CDR approaches range from highly technological like direct air capture to natural solutions via enhancing natural processes in forests and soils. But there is another critical component to the carbon cycle that calls for serious CDR attention: oceans.

According to the National Oceanic and Atmospheric Administration (NOAA), the oceans are one of the world’s most effective carbon sinks, capturing around one-third of anthropogenic carbon dioxide emissions since the industrial revolution. Per unit volume, seawater can store 150 times more carbon than air, suggesting that there is enormous potential to utilize the oceans as a means of mitigating the effects of climate change. Oceans are also providers of global and local economic benefit. They provide direct output through fishing and agriculture, facilitate trade and transport, and provide for productive coastlines through conservation activities, tourism, and local jobs. The World Wildlife Foundation values these activities to be over $24 trillion in combined assets.

However, the rising atmospheric carbon dioxide level is causing the oceans to absorb more carbon than they can handle, and is subsequently harming sensitive marine life such as corals and shellfish. Compared to preindustrial times, the overall surface water pH has declined by approximately 25%, a process known as ocean acidification. These impacts to ocean health could cost a global economic decline on the order of $428 billion annually by 2050.

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Figure 1: Schematic representation of  six different oceans CDR pathways taken from the NASEM 2021 report.

Considering how productive of a carbon sink the oceans are, ocean CDR presents tremendous opportunities to enable our net-zero goals. There are many ocean CDR strategies, ranging from increasing biomass in coastal ecosystems, to direct carbon removal from seawater, that warrant serious consideration. Each of these solutions differs in terms of readiness, cost, and carbon-removal capabilities. Nonetheless, the sheer carbon removal potential of the following strategies indicate that ocean carbon removal strategies must be considered as a part of any climate solution moving forward. Fortunately, the National Academies for Science, Engineering, and Medicine recently released a report that details a research agenda to assess the benefits, risks, and potential for responsible scale-up of ocean CDR approaches. It provides important research recommendations and considerations for six distinct forms of using oceans for carbon removal, shown in figure 1.

Ecosystem Recovery (Blue Carbon)

Blue carbon is defined as carbon that is naturally stored in marine and coastal ecosystems but generally refers to mangrove forests, salt marshes, and seagrass beds. Blue carbon removal is reminiscent of terrestrial plant-based carbon removal strategies. Coastal aquatic ecosystems do an especially good job of sequestering and storing carbon. Through photosynthesis, these plants can store carbon in their biomass, but are even more effective at storing carbon in their dense soils for potentially thousands of years. Thus, a relatively straightforward CDR approach is to restore these habitats.

One such project is the Kaimana Coastal Conservation and Community Development Project in Indonesia, where a team of experts estimate that around 30 million tons of emissions could be reduced by maintaining the mangrove forests in Indonesia alone.

Nutrient Fertilization

Enhancing biological potential allows plants like algae to absorb more carbon by increasing their volume and stimulating biological processes. Possible pathways for this include algae cultivation and iron fertilization. Greater exposure to necessary nutrients, such as phosphorus or iron, can drastically increase the rate of photosynthesis and remove more carbon. To manage increased algae volumes, more developed aquaculture practices must be developed to utilize excess algae as food, fuel, or for long-term storage products.

Seaweed Cultivation

Leveraging photosynthesis, macroscopic biomass like seaweed can be cultivated and introduced into the ocean to draw down CO2 and store it in the deep sea or sediments. While a lot of knowledge exists for optimizing carbon draw down into seaweed – through optimal biomass densities, nutrient ratios, and optimal farming techniques — less is known about the impacts and implications of depositing the seaweed into the deep oceans. These concerns range from unwanted, but benign accumulation of sediment on the sea floor to drastic disruption of marine ecosystems.

Given the relative simplicity of the technique, companies like Running Tide are beginning to demonstrate how the process of growing and transporting seaweed and kelp for CDR purposes can be automated. Companies like Stripe have signed advanced contracts to pay $250/ton for demonstrating this innovation CDR technique.

Artificial Upwelling & Downwelling

Artificially pumping water through vertical columns can recreate the natural upwelling and downwelling process. Upwelling brings nutrient-rich cold water to the surface, spurring algae growth, which subsequently sequesters more carbon. Downwelling processes pump surface water to the deep ocean and brings the now carbon-rich algae with it, finding long-term storage in the deep ocean. Artificial downwelling has been used to combat coastal hypoxia, in effect ventilating the ocean with oxygen-rich surface water.

Ocean Alkalinity Enhancement

Ocean alkalinization attempts to counteract the increasing acidity by artificially making the oceans more alkaline by adding alkaline substances to the ocean. This effectively speeds up the reaction process between surface water and air, allowing the oceans to store more carbon while lessening the effects of acidification. While the potential scale of these strategies may be enormous, there is large uncertainty about both the environmental impacts and the technology readiness.

Electrochemical Ocean CDR (Direct Seawater Capture)

This artificial strategy utilizes electrochemical conversion, in conjunction with desalination plants, to turn seawater more alkaline, while directly removing CO2 as a part of a solid mineral and allowing for long-term storage on the seafloor. An example is the single-step carbon sequestration and storage project at UCLA. Although this technology is in its infancy, researchers remain confident that small-scale projects will be beneficial in confronting climate change.

The following table outlines the carbon removal potential for each strategy, as identified by the NASEM report.

Chart Adapted from NASEM 2021 Report. Low, Moderate, and High Potential Scale corresponds to <0.1 Gt/year, ~0.5 Gt/year, and >1 Gt/year potential capacity for CDR, respectively. Low, Moderate, and High Potential Cost corresponds to <$50/ton, ~$100/ton, and >$150/ton, respectively. Environmental risk and co-benefits were assessed with qualitative considerations – see the NASEM report for more details.


It is important to remember that emissions reduction remains the priority, as it is cheaper to avoid emitting CO2 into the atmosphere in the first place. However, science now tells us that carbon dioxide removal at the gigaton scale will also be essential to achieve our net zero goals. Ocean CDR can play an important role and the best path forward will be a combination of the above-mentioned strategies, but there are several dimensions to consider before scaling up these projects that must be addressed first:

  1. Environmental Considerations: Each of these strategies will require a rigorous environmental assessment. Strategies such as enhancing the ocean carbon sink and ocean alkalinization risk changing the ecological properties of aquatic ecosystems which could have global impacts. Moving forward, strategies with high co-benefits and low environmental risks should be prioritized for scale up.
  2. Technological Innovation: Far more research must be conducted to understand the risks and benefits of each ocean CDR approach. Research and innovation is also needed to build up the monitoring and verification capacity needed to ensure the efficacy of these approaches. Understanding how different strategies can work together will also be extremely important in maximizing carbon removal potential.
  3. Demonstration: For projects where we have a good understanding of the risks and tradeoffs, demonstration efforts will help bring down costs and contribute to further understanding of opportunities and challenges. The government plays a major role in demonstration efforts and recent legislation like the bipartisan Sea Fuel Act has begun allocating funds ($10 million/year through 2023) toward deploying ocean carbon management and direct air capture. While this is a good start, more funding is needed in order to adequately research the systems and develop the necessary technology.
  4. Governance & Community Impact: Both national and international governance frameworks must be considered when drafting legislation about ocean carbon removal. Local and regional considerations are also important, as many communities will be impacted when considering scaling up these projects. Maintaining clarity with relevant stakeholders will be crucial.

In this latest report by NASEM, a broad research and development agenda for Ocean CDR is outlined to expand the knowledge base and governance structures required for implementing and accurately quantifying Ocean CDR approaches in service of our climate goals. The estimated cost for the research agenda is $475 million over 10 years, and spans both foundational and technology-specific recommendations. For a more detailed description of the research agenda, see: A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. For more details on the challenges and opportunities presented by Ocean CDR, see the Aspen Institute’s recent Guidance for Ocean-Based Carbon Dioxide Removal Projects.

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