Selected Potential Considerations with Respect to Marine Carbon Dioxide Removal: In Brief

Contents

• Overview of Marine Carbon Dioxide Removal Approaches
• Biological Pathway Approaches
• Chemical Pathway Approaches
• Considerations for Congress
• Permitting for Marine Carbon Dioxide Projects
• Federal Research for Marine Carbon Dioxide Removal

The Intergovernmental Panel on Climate Change (IPCC) states in its Sixth Assessment Report that "human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming."¹ The IPCC projects average global warming "more likely than not" will continue to increase in the near term (through 2040) under the lowest greenhouse gas (GHG) emission scenarios considered.² Both the IPCC's Sixth Assessment Report and the U.S. Global Change Research Program's Fifth National Climate Assessment state that carbon dioxide removal (CDR) approaches, in addition to other efforts, are likely needed to mitigate rising atmospheric carbon dioxide (CO₂) and the associated impacts of climate change (e.g., increasing average global temperature, rising sea levels, increasingly severe weather events).³ According to experts, to be effective, CDR approaches must remove CO₂ from the atmosphere and durably store carbon for prolonged periods of time.⁴

The ocean is a major component of the global climate system, serving as a planetary reservoir of heat, water, and carbon. The IPCC estimates that the ocean has taken up and stored between 18% and 28% of anthropogenic CO₂ emissions over the past 60 years.⁵ Scientists have investigated how marine CDR (mCDR, also referred to as ocean or ocean-based CDR) approaches may augment the ocean's natural ability to absorb atmospheric CO₂ and store carbon for extended periods of time in coastal and ocean environments.⁶

Interest from various stakeholders in the use of mCDR to mitigate rising atmospheric CO₂ levels is growing. For example, the National Oceanic and Atmospheric Administration's (NOAA's) Office of National Marine Sanctuaries reports that it has received an increased number of external inquiries about mCDR deployment in national marine sanctuaries.⁷ To this end, in 2023, the Biden Administration identified obtaining new information about the "safest and most effective approaches" to mCDR as a high priority and established the Marine Carbon Dioxide Removal Fast Track Action Committee (mCDR FTAC).⁸ The mCDR FTAC's charter acknowledged the "urgent need to resolve key knowledge gaps for different [mCDR] techniques."⁹ In the 118^th Congress, congressional interest in mCDR primarily focused on advancing mCDR research, development, and demonstration (RD&D).¹⁰ No legislation related to mCDR had been introduced in the 119^th Congress as of March 10, 2025.

The study of mCDR is a relatively new field. In light of growing interest in mCDR, this In Brief provides an overview of different mCDR approaches and discusses some outstanding related research questions. These questions may be informed by additional RD&D, including controlled field experiments. Congress also may consider laws applicable to the various mCDR approaches and the potential challenges, if any, researchers may encounter in conducting RD&D, including field experiments in the U.S exclusive economic zone (EEZ).¹¹

Overview of Marine Carbon Dioxide Removal Approaches

mCDR approaches generally are categorized into those that enhance or accelerate marine biological pathways or chemical pathways.¹²

Biological Pathway Approaches

Approaches that manipulate marine biological pathways enhance the uptake of atmospheric CO₂ by increasing the growth of marine plants (i.e., micro- and macroalgae). These mCDR biological pathway approaches include the following:

• Artificial Upwelling. This approach relies on a technological intervention to transport cold, nutrient-rich ocean waters from the deep ocean to the surface. Cold, nutrient-rich waters can stimulate primary producers (i.e., phytoplankton) to take up CO₂ via photosynthesis. Once dead, the primary producers may sink into the ocean, transporting carbon out of the surface ocean. A small portion of this carbon may be buried in ocean sediments for up to 100 years.¹³
• Artificial Downwelling. This approach relies on a technological intervention to transport relatively warm and CO₂-rich surface waters, as well as the primary producers (i.e., phytoplankton) living in the surface waters, to water depths below the pycnocline (generally between 500 and 1,000 meters below the ocean surface) for the purpose of storing downwelled CO₂ in the deep ocean and burying carbon associated with dead primary producers in ocean sediments for up to 100 years.¹⁴
• Macroalgal (Seaweed) Cultivation. This approach uses fast-growing marine plants to take up CO₂ via photosynthesis.¹⁵ In order for this to be a successful mCDR technique, these plants cannot be harvested for food or other uses (e.g., fertilizers, feed and fuel for land activities), and, once dead, these plants must sink and be buried in ocean sediments.¹⁶ Depending on the offshore environment, such carbon may be stored in ocean sediments for decades to millennia.¹⁷
• Ocean Fertilization. This approach adds micro- (iron) or macronutrients (nitrogen or phosphorous) to the surface ocean. These nutrients can stimulate primary producers (i.e., phytoplankton) to take up CO₂ via photosynthesis. Once dead, the primary producers may sink into the ocean, transporting carbon out of the surface ocean. A small portion of this carbon may be buried in ocean sediments for centuries to millennia.¹⁸

Chemical Pathway Approaches

Approaches that manipulate marine chemical pathways alter the natural air-sea gas exchange by changing seawater alkalinity (i.e., the ocean's ability to resist pH changes). These approaches aim to change seawater alkalinity so the ocean can absorb more CO₂.¹⁹ mCDR approaches that augment marine chemical pathways include the following:

• Direct Ocean Removal. This approach relies on technologies to remove and capture CO₂ directly from seawater by changing the pH of the treated water. The treated (decarbonized) water is returned to the surface ocean, where it can absorb more CO₂.²⁰
• Ocean Alkalinity Enhancement (OAE). OAE alters seawater alkalinity either by adding alkaline minerals to increase the ocean's ability to absorb atmospheric CO₂ or by using electricity to split seawater into its acidic and basic components.²¹ In the absence of processes that remove the added alkalinity, some have proposed OAE can sequester CO₂ on timescales of millennia or longer.²²

Considerations for Congress

Limited experimentation, including a lack of controlled field experiments,²³ for many mCDR approaches creates uncertainties regarding the efficacy, cost, potential co-benefits, and potential negative impacts of full-scale deployment of mCDR approaches.²⁴ On February 23, 2024, the National Science Foundation (NSF), acting through the mCDR FTAC, sought input from all interested stakeholders to inform the development of an implementation plan to advance mCDR research.²⁵ In its notice, NSF stated, "As of yet, however, no marine CDR methods are considered ready for full-scale deployment or commercial application."²⁶ Some stakeholders contend that small-scale proof-of-concept field testing is necessary to quantify CDR potential (i.e., efficacy) as well as the potential co-benefits and negative impacts associated with different mCDR approaches.²⁷

Federal RD&D and federally funded research may elucidate whether mCDR approaches are effective and at what scale. Some stakeholders may question the relative priority of mCDR RD&D compared with other federal activities, such as existing programs that support more technologically mature CDR options or GHG emissions reductions.²⁸ Others may invoke a moral hazard argument against CDR because they prefer policies and actions to reduce GHG emissions prioritized over those aimed at removing emitted GHG from the atmosphere.²⁹ If Congress is interested in directing federal agencies to conduct and advance mCDR RD&D or to support nonfederal RD&D, it could consider several policy approaches, including identifying a permitting framework for mCDR RD&D in the U.S. EEZ.

Permitting for Marine Carbon Dioxide Projects

Some stakeholders contend that scaling and accelerating mCDR RD&D is unlikely to happen without a regulatory framework that facilitates controlled field experiments.³⁰ Either a field-based demonstration or a fully deployed mCDR project likely would require compliance with certain state, federal, and/or international regulations, depending on the mCDR approach and the project's location. Proposals for mCDR projects within the U.S. EEZ could be subject existing laws, such as the Marine Protection, Research, and Sanctuaries Act (MPRSA; P.L. 92-532); the Coastal Zone Management Act (P.L. 109-58); Section 10 of the Rivers and Harbors Act of 1899 (33 U.S.C. §403); the Clean Water Act (33 U.S.C. §1251 et seq.); and the Magnuson-Stevens Fishery Conservation and Management Act (MSA; 16 U.S.C. §1801 et seq.), among others. For example, the MSA requires federal agencies to consult with NOAA on activities that may adversely affect essential fish habitat.³¹ As another example, the MPRSA, also known as the Ocean Dumping Act, regulates the transportation and "dumping" of any material—which may include alkalinity enhancement materials—into ocean waters (see textbox, below).³²

Congress has not authorized mCDR projects in law. Should Congress support testing of mCDR projects, it could take different approaches. Congress could amend existing laws to specifically address mCDR broadly (or selected mCDR approaches). Alternatively, Congress could consider new legislation that provides a framework for mCDR projects deployed within the U.S. EEZ or regulates the scale and siting of a project to limit the potential impact on waters adjacent to the EEZ. For example, H.R. 5457/S. 2812 in the 118^th Congress would have directed NSF, in collaboration with other federal agencies, to "award funding for research into governance frameworks for safe and sustainable experimentation" with mCDR.³³

Federal Research for Marine Carbon Dioxide Removal

Several federal agencies have existing mandates, programs, and activities that intersect with mCDR research. NOAA has acknowledged that it has "capabilities that can be applied to understand and assess CDR."³⁴ For example, NOAA's Ocean Acidification Program conducts and funds research, monitoring, and other activities to develop mitigation strategies to address ocean acidification, among other purposes.³⁵ Some mCDR approaches, such as OAE, have the potential to mitigate ocean acidification.³⁶ The Department of Energy's (DOE's) Office of Fossil Energy and Carbon Management (FECM) identified CDR as a priority area.³⁷ For FY2025, DOE requested $237.5 million for CDR activities.³⁸ On June 6, 2024, DOE and NOAA signed a memorandum of agreement to collaborate on mCDR RD&D.³⁹

Within agencies, Congress may consider designating an existing program (or office) or creating a new program to lead mCDR RD&D activities for the agency.⁴⁰ For example, within DOE, Congress could designate FECM, designate another office, or create a new office to lead mCDR activities. Several Members introduced legislation in the 118^th Congress that would have directed certain agencies to establish mCDR RD&D programs (e.g., H.R. 5457/S. 2812, H.R. 7797) or amend existing programs to include mCDR approaches as part of their RD&D programs (e.g., S. 1576). Additionally, H.R. 10471/S. 5629 in the 118^th Congress would have established a research and competitive grant program in NOAA to advance the science and understanding of mCDR. It also would have established mCDR-related programs at NSF, the National Aeronautics and Space Administration (NASA), and for the National Institute of Standards and Technology.

Congress also could designate a federal agency to coordinate federal mCDR RD&D activities or establish an interagency working group (IWG) for this purpose. Some stakeholders have proposed NOAA lead on federal mCDR research and coordination.⁴¹ In the 118^th Congress, S. 2002 and H.R. 9212 would have established the Interagency Group on Large-Scale Carbon Management in the National Science and Technology Council for the purpose of developing a federal strategic plan for CDR RD&D and coordinating federal CDR activities.⁴² H.R. 10471/S. 5629 in the 118^th Congress also proposed the creation of an interagency working group for mCDR that would have been cochaired by NOAA and DOE. Alternatively, Congress could amend existing laws to broaden the scope of an existing IWG, such as the IWG on Ocean Acidification, to explicitly include mCDR research activities.⁴³ In addition to reducing potential duplication across federal agencies, improved federal coordination on mCDR activities could yield research results that inform whether mCDR approaches are effective.⁴⁴

The private sector may be reluctant to fund mCDR activities until the risks and benefits are better understood. In the absence of private sector support, agencies such as DOE, NOAA, and NSF have provided funding to support external and/or joint (federal and nonfederal) projects aimed at advancing mCDR RD&D.⁴⁵ In addition to H.R. 10471/S. 5629, some Members introduced other legislation in the 118^th Congress that would have directed certain federal agencies to provide funding for nonfederal research. For example, Section 101 of H.R. 5457/S. 2812 would have directed DOE's FECM to provide competitive demonstration awards for mCDR projects within the U.S. EEZ, and Section 903 would direct NSF, in collaboration with other federal agencies, to provide funds for mCDR research. Some stakeholders contend that recipients of federal funding should be required to abide by a code of conduct designed to ensure researchers avoid, minimize, mitigate, and monitor the environmental impacts of mCDR activities,⁴⁶ as with DOE's Responsible Carbon Management Initiative.⁴⁷

What Is the Effectiveness of Marine Carbon Dioxide Removal?

Results of some laboratory and modeling studies indicate that mCDR may be effective at absorbing atmospheric CO₂ and storing it in the ocean for prolonged periods of time.⁴⁸ Some stakeholders contend that controlled field experiments are needed to determine whether mCDR approaches can lead to a net increase in ocean carbon storage.⁴⁹ A controlled field experiment may include (1) quantifying the rate at which carbon is taken up by the ocean (i.e., sequestration rate); (2) monitoring and verifying carbon sequestration; (3) determining the permanence of carbon sequestration; and (4) monitoring ecological and environmental shifts to demonstrate that the approach is effective while minimizing impacts. This research may be challenging, because "extremely precise measurements are required to detect a small [carbon] removal signal."⁵⁰

Several federal agencies have established programs to collect ocean data that may be applicable to monitoring and verifying carbon removal (e.g., NOAA, NASA, U.S. Geological Survey).⁵¹ To accelerate the development of "marine carbon sensor technologies operating across spatial and temporal scales broad enough to sufficiently understand mCDR," among other research questions, the Advanced Research Projects Agency-Energy established the Sensing Exports of Anthropogenic Carbon Through Ocean Observation (SEA-CO2) program.⁵² SEA-CO2 has supported at least 11 projects.⁵³ Congress may consider whether the current federal investment in ocean monitoring and observational instruments is sufficient to provide accurate information about the efficiency of mCDR.

What Is the Cost of Marine Carbon Dioxide Removal?

Experts have estimated the cost per ton of carbon removal ($/tCO₂ removed) for future at-scale deployment for the different mCDR approaches.⁵⁴ Approaches proposed to have a low-to-moderate cost include macroalgal cultivation ($25-$125/tCO₂ removed), ocean alkalinity enhancement ($25-$160/tCO₂ removed), and ocean fertilization ($50-$125/tCO₂ removed).⁵⁵ Artificial upwelling and downwelling are proposed to have a moderate cost ($100-$150/tCO₂ removed), and direct ocean removal is proposed to have a high cost ($400-$600/tCO₂ removed).⁵⁶ Direct air capture CDR is estimated to cost $40-$1,000/tCO₂ removed.⁵⁷ Some stakeholders may contend the cost for full-scale deployment of mCDR should be compared with the costs of a no-action alternative, such as the economic costs associated with climate-related risks and impacts.⁵⁸

Some stakeholders have proposed goals for cost per ton of carbon removal for CDR. H.R. 7054, introduced in the 118^th Congress, would have directed DOE to determine if technologies to remove CO₂ directly from ambient air or seawater are economically feasible and provide a roadmap for decreasing the cost per ton of carbon removal to no more than $150 by 2035, among other research objectives.⁵⁹ DOE's Negative Carbon Shot aims to advance technologies that reduce the cost of CDR approaches to less than $100/tCO₂-equivalent removed.⁶⁰

Related to elements of cost, S. 5369 in the 118^th Congress would have amended the Internal Revenue Code of 1986 to establish a technology-neutral tax credit available to those investing in U.S. next-generation CDR technologies (including "marine-based" CDR) among its proposed provisions.

What Are Potential Co-benefits and Negative Impacts of Marine Carbon Dioxide Removal?

Limited experimentation for many mCDR approaches provides uncertainties regarding the potential ecological and environmental co-benefits and impacts associated with mCDR.⁶¹ For example, over the past 30 years, at least 16 ocean fertilization experiments have taken place in the open ocean.⁶² No field experiments have taken place for other mCDR approaches.⁶³ Results from these experiments led some stakeholders to assert that such efforts will have "unknown, unpredictable, and potentially highly damaging impacts on the food web in marine ecosystems."⁶⁴ The specific interactions and degree of impact of potential deleterious effects on the marine food web and other ecological interactions remain unknown.

Some scientists, primarily through laboratory and modeling studies, have identified a range of effects associated with mCDR approaches. Depending on the approach, these effects include potential reduction in deep-sea oxygen levels,⁶⁵ reduction in ocean acidification,⁶⁶ and changes to biochemical cycling with potential repercussions on marine ecosystems (Table 1). A study by NOAA's Office of National Marine Sanctuaries evaluated the level of risk associated with mCDR approaches based on their level of potential negative environmental impacts. The study identified ocean fertilization and OAE via accelerated weathering as having medium environmental risk. It identified macroalgal cultivation, artificial upwelling and downwelling, and OAE via electrochemical acid removal as having high environmental risk.⁶⁷ Some bills introduced in the 118^th Congress would have directed federal agencies to either conduct research examining the potential impacts of mCDR or provide funding to nonfederal researches investigating these impacts (e.g., H.R. 5457/S. 2812, S. 1576, H.R. 7797, and H.R. 10471/S. 5629).

Footnotes