Negative emissions scenarios can only be accomplished using a combination of mitigation pathways that include 100 percent clean energy as well as methods to draw down and sequester excess carbon dioxide (CO2) from the atmosphere—a process, known as carbon dioxide removal (CDR), that can be either nature- or technology-based. Regardless of the method deployed, however, CDR should never be considered as an offset that perpetuates fossil fuel dependence, but rather as a tool to draw down GHGs that have been or will be emitted despite emissions abatement measures.
The ocean has played an outsize role in keeping the Earth habitable since the industrial revolution, absorbing 90 percent of heat and as much as 30 percent of excess CO2 emitted into the atmosphere. However, CO2 emissions from human activities are outpacing Earth’s capacity to buffer its climate and prevent catastrophic impacts resulting from climate change. Even still, ocean-based solutions represent a powerful opportunity to harness the power of the ocean to remove CO2 from the atmosphere and sequester it for long periods of time.
This column serves as a high-level overview of the current state of ocean-based CDR. It lays out various nature- and technology-based solutions; calls for significant investment in ocean restoration; summarizes environmental justice concerns; and makes recommendations for the Biden administration to equitably implement these methods.
Nature-based solutions: Immediately deployable actions that remove CO2 from the atmosphere, improve environmental quality, and enhance coastal resilience
Nature-based solutions take advantage of the ocean’s natural ability to store vast amounts of carbon, often referred to as “blue carbon.” Ecosystems such as wetlands, seagrass, and mangroves sequester more carbon per unit area than terrestrial systems. Restoring blue carbon ecosystems lost to coastal development and erosion could remove hundreds of millions of tons of CO2 annually by 2050, with the added benefits of improving coastal resilience and water quality.
Moreover, such nature-based solutions can be implemented immediately and do not come with the same environmental justice or governance concerns that are associated with some other proposed CDR methods (discussed in more detail below). Notably, the budget reconciliation package currently being considered by Congress includes a provision for $10 billion in coastal restoration funding, which would enable coastal communities to start implementing blue carbon solutions across the country.
Another nature-based solution to sequester CO2 is large-scale kelp and seaweed farming, also known as “macroalgal cultivation.” The product has several uses, including as food and fertilizer, and may even reduce methane emissions from cattle. Carbon dioxide is captured naturally via photosynthesis as the macroalgae grow; in addition, these macroalgae absorb phosphorus and nitrogen in the surrounding waters, reducing some of the impacts of ocean acidification. Alternatively, the kelp or seaweed may be harvested and sunk to the seafloor to sequester the carbon.
Technology-based solutions: Large-scale engineering actions that may remove CO2 but remain unproven and carry environmental and governance concerns
In addition to the nature-based solutions described above, scientists and others have proposed a series of technology-based solutions designed through engineering to mimic processes that enhance the Earth’s ability to remove and sequester carbon.
In the ocean, these proposals include altering the ocean’s alkalinity, enhancing the carbon cycle by creating phytoplankton blooms fed by nutrients, and pumping CO2 into the deep ocean.
Ocean alkalinity enhancement
Ocean alkalinity enhancement proposals exploit the natural process that occurs when rocks weather and release ions—such as calcium, magnesium, potassium, and sodium—into the water. Over geologic time, those ions have kept the ocean’s chemistry stable and reliably basic or “alkaline,” at a pH greater than 7.0. The ocean’s alkaline chemistry is what has enabled it to absorb so much excess carbon dioxide from the atmosphere, but this has not come without a cost: As the ocean absorbs CO2, it also becomes more acidic, which in turn reduces its ability to absorb additional CO2—not to mention the irreparable harm inflicted on marine life unable to adapt to changes in ocean chemistry.
To counteract this, scientists and others have proposed enhancing ocean alkalinity by directly adding alkaline agents sourced from rocks or electrochemically from seawater, to make the pH more basic. Large-scale in situ experiments have yet to be conducted, which will be critical to understanding ocean alkalinity enhancement’s efficacy at drawing down and storing CO2 as well as potential impacts to the ocean and wildlife. Furthermore, the additive proposals would require both large-scale mining on land for raw materials and shipping to distribute them, both of which are carbon-intensive activities that may also pollute local communities.
Ocean fertilization is a technique that aims to enhance the ocean’s biological carbon pump, a natural process by which marine organisms convert CO2 into organic carbon via photosynthesis. The organic carbon then sinks to the ocean floor, where it may be stored for millions of years. In the most common application, iron—an element needed by microscopic organisms to grow—is added to the upper water column, where it stimulates massive growth of photosynthesizing microorganisms.
Of the technological solutions detailed in this column, ocean fertilization is one of the few that has been studied in large-scale, open-ocean experiments that have largely demonstrated a lack of net carbon export, suggesting that the theory might not actually work in practice or at scale.
Rather than mimicking natural processes, deep-sea injection directly transports CO2 captured from the atmosphere and stores it deep in the ocean, where it may stay for hundreds of years. Once captured, the CO2 can theoretically be delivered to the deep sea by ship, fixed platform, or pipeline, though there has not been enough field research to quantify how long it would remain isolated from the atmosphere. Directly injecting high concentrations of CO2 into the ocean is likely to result in immediate mortality to marine life in the vicinity due to the instantaneous change in ocean chemistry—for instance, pH, oxygen saturation, and more. For this reason, a more commonly discussed method of deep-sea injection involves the direct injection of highly concentrated CO2 into the geologic reserves of the subseafloor. This method, to date, has only been deployed at large scale to facilitate natural gas extraction from the ocean floor.*
Unlike nature-based solutions, there are still a number of outstanding questions that need to be answered before any of the technology-based options described here can or should be deployed on a large scale, including but not limited to whether or not these proposed solutions work, how they might affect surrounding communities on land and/or marine ecosystems, whether the known impacts can be mitigated and what the plan is for mitigating unintended consequences, whether the benefits outweigh the risks, and whether these solutions can be scaled to the extent needed to achieve meaningful emissions reductions.
CDR solutions must consider environmental justice concerns
There are significant environmental justice concerns regarding carbon dioxide removal—specifically centered around research and development, implementation, unintended and intended consequences, and power dynamics. Historically disadvantaged and marginalized communities have borne the brunt of harmful pollution from power plants and industrial facilities for decades. It is therefore vital that any discussions of the future of CDR projects address this legacy of environmental harm and the voices of these communities—and that strategies integrate these communities into future proposals.
Environmental justice advocates have long opposed technology-based CDR solutions. Recently, the White House Environmental Justice and Advisory Council (WHEJAC) released a report that lists geoengineering and techno fixes, carbon capture and storage or carbon capture, utilization, and storage, and research into or development of these methods as examples of projects that will not benefit environmental justice communities.
Indeed, while many of the ocean-based CDR solutions described in this column are envisioned to take place far offshore, the aforementioned mining, shipping and transportation, and other infrastructure needs required to implement these technologies still have the potential to harm local communities by polluting the air and water, destroying nature, and emitting even more CO2 into the atmosphere due to their high energy requirements. Environmental justice advocates argue that the negative consequences of these strategies—unintended or not—would most likely continue to harm low-income communities and communities of color disproportionately.
The National Academies of Sciences, Engineering, and Medicine (NASEM) is currently conducting a study to recommend a research strategy for ocean carbon dioxide removal and sequestration. The study, which will be released this fall, aims to define a research agenda that would address outstanding questions regarding the efficacy, impact, and scale required to deploy ocean-based CDR solutions to reduce the amount of CO2 remaining in the atmosphere, coupled with aggressive mitigation strategies to eliminate fossil fuel emissions. The study will also identify questions regarding the level of governance needed to safely deploy any of the technology-based solutions described above or emerging technologies in the future.
As the Biden administration considers how to keep the planet within an ever-narrowing pathway to prevent warming greater than 1.5 degrees Celsius, carbon dioxide removal will need to be a part of that discussion. The need to implement as many climate-friendly policies as possible during this administration cannot be understated; however, caution and diligence are warranted.
The Biden administration should:
- Develop a research agenda to address outstanding questions and prioritize large-scale experiments to study ocean CDR’s efficacy and impacts. The forthcoming NASEM report should provide recommendations on this issue.
- Implement nature-based solutions immediately to sequester blue carbon, protect coastal communities from extreme weather, improve water quality, and create jobs.
- Using the existing WHEJAC process, closely collaborate with environmental justice advocates and representatives to ensure their perspectives and expertise are incorporated in decision-making.
- Initiate a White House-led interagency process to develop a domestic governance framework to guide how these projects are implemented in the United States and participate in multilateral dialogues regarding an international governance framework for projects in areas beyond national jurisdiction.
By emitting greenhouse gases into the atmosphere unchecked for hundreds of years, humans have conducted a planetary-scale experiment with catastrophic results. Technologies and policies designed to achieve negative emissions should not risk further planetary-scale experiments of unknown consequence; careful research, development, testing, consultation, and regulation is needed.
In the meantime, nature-based ocean solutions should be widely deployed and well-resourced due to their climate, ecosystem, and community benefits. Furthermore, it is critical that the appropriate social and governance guardrails are in place to avoid further catastrophic impacts on communities and ecosystems.
Kelly Kryc is a senior fellow for Energy and Environment at the Center for American Progress. Kat So is a research assistant for Energy and Environment at the Center.
*Correction, September 22, 2021: This column was updated to clarify that a more commonly discussed method of deep-sea injection involves placing CO2 below the surface of the seabed.