Key International Legal Obligations and Principles Relevant to Marine Geoengineering

[Stephanie Prufer J.D. is a Legal and Policy Officer at OceanCare and holds a Master of Environmental Management from Yale Law School. She was involved in negotiations at the London Convention and London Protocol and the BBNJ Agreement. She previously interned at ITLOS.

Dr James Kerry is Chief Scientist at OceanCare and an adjunct Senior Research Fellow at James Cook University. He was involved in negotiations at the London Convention and London Protocol.

Prof Kristina Gjerde J.D. (in absentia) was IUCN’s High Seas Policy Advisor and was instrumental in the BBNJ Agreement process.

Marine geoengineering refers to a diverse and contested set of proposed interventions in ocean and coastal systems, including techniques intended to enhance carbon uptake, alter ocean chemistry, manipulate marine ecosystems, or modify the reflectivity of clouds or the sea surface. Although such proposals are presented in climate-mitigation terms, their defining feature is not that they offer a proven response to climate change, but that they would deliberately intervene in complex marine systems that are already under severe pressure from warming, acidification, pollution, overfishing, and biodiversity loss. This makes marine geoengineering first and foremost a question of ocean governance: how international law should assess, restrict, or prevent activities that may introduce new risks to the marine environment in the name of addressing existing ones.

Although it remains inchoate, the relevant legal framework is not a permissive gap waiting to be filled by experimentation and commercialization, but an evolving framework that must be based on precaution, prevention of transboundary harm, protection and preservation of the marine environment, and environmental impact assessment that must shape how States respond to these proposals.

Here, we highlight the key international legal norms and principles applicable to marine geoengineering under the United Nations Convention on the Law of the Sea (“UNCLOS”), the London Convention and Protocol (“LC/LP”), and the Biodiversity Beyond National Jurisdiction (“BBNJ”) Agreement, and interpretation of these laws in the 2025 International Court of Justice and 2024 International Tribunal for the Law of the Sea Advisory Opinions on Climate Change. This piece responds, in part, to proposals for “re-interpretation” of some of these key principles in the White Paper by the Sabin Center for Climate Change Law on the:

“International Legal Guidelines for Marine Carbon Dioxide Removal Governance under the London Convention and London Protocol” (hereinafter “White Paper”).

Here, we offer a different account of how those principles should be understood in light of the broader international legal framework governing marine environmental protection and marine geoengineering.

Parties Cannot Move Forward with Climate Mitigation that has the Potential to Cause Significant Environmental Harm

State Parties cannot enable climate mitigation actions that have the potential to cause significant environmental harm. This would be inconsistent with marine environmental protection under already existing international instruments. 

In accordance with UNCLOS, States have a general duty in Article 192 to protect and preserve the marine environment. States must also prevent, reduce, and control marine pollution resulting from the use of technologies under their jurisdiction or control under Article 196.

Under the London Convention and London Protocol, precaution must shape Parties’ actions. In 1991, Parties to the London Convention adopted a resolution clarifying the application of the precautionary approach to the Convention. Parties agreed that:

“in implementing [the London Convention] the Contracting Parties shall be guided by a precautionary approach to environmental protection whereby appropriate preventative measures are taken when there is reason to believe that substances or energy introduced in the marine environment are likely to cause harm even when there is no conclusive evidence to prove a causal relation between inputs and their effects.”

This resolution directs Parties to the London Convention to consider and “take all necessary steps” to abide by the precautionary approach (art. 2).

The precautionary approach is also an explicit obligation of the London Protocol:

“Appropriate preventative measures are [to be] taken when there is reason to believe that wastes or other matter introduced into the marine environment are likely to cause harm.”

(art. 3)

This remains an obligation “even when there is no conclusive evidence to prove a causal relation between inputs and their effects.” (art. 3).

More recently, in 2023, Parties to the LC/LP agreed that:

“each of the four marine geoengineering techniques prioritized for evaluation [including ocean alkalinity enhancement, marine geoengineering with biomass, marine surface albedo enhancement, and marine cloud brightening]has the potential for deleterious effects that are widespread, long-lasting or severe”

and

“there is considerable uncertainty regarding their effects on the marine environment, human health, and on other uses of the ocean.”

Potential climate benefits do not displace the duty to apply the precautionary approach; the purpose of which is to ensure that actions taken in the name of environmental protection do not create new or greater harms.  By upholding this approach, States ensure that credible but uncertain risks of harm are met with care, restraint, and protective action, particularly where impacts on marine ecosystems and their biodiversity may be difficult to detect, reverse, or attribute.

States are Under an Obligation not to Transfer Harm

Climate mitigation measures must comply with States’ duties to prevent environmental harm, including the obligation not to transfer damage or hazards from one area to another, or transform one type of pollution into another.

States that take actions which carry a risk of causing transboundary harm have an international law obligation to take all appropriate measures to prevent transboundary harm and to take measures to reduce said risk. The White Paper correctly highlights that:

“[t]he principle that parties must prevent activities within their control from causing transboundary harm is a bedrock principle of international environmental law.”

With respect to the marine environment, Parties to the LC/LP that are also Parties to UNCLOS are more specifically under an obligation “to protect and preserve the marine environment” and:

“act so as not to transfer, directly or indirectly, damage or hazards from one area to another or transform one type of pollution into another.”

(arts. 192, 195).

Furthermore, Parties to UNCLOS are under an obligation to:

“take all measures necessary to prevent, reduce and control pollution of the marine environment resulting from the use of technologies under their jurisdiction or control.”

(art. 196).

The ITLOS Climate Change Advisory Opinion clearly states that the obligation not to transform one type of pollution applies to marine geoengineering:

Marine geoengineering would be contrary to article 195 if it has the consequence of transforming one type of pollution into another. It may further be subject to article 196 of the Convention which requires States, inter alia, to take all measures necessary to prevent, reduce and control marine pollution resulting from the use of technologies under their jurisdiction or control. (para. 231).

As the ITLOS Advisory Opinion further notes, obligations under the Paris Agreement or the United Nations Framework Convention on Climate Change (UNFCCC) are separate and UNCLOS may demand more. (paras. 219-224) Fulfilling one’s Paris and UNFCCC obligations does not automatically satisfy UNCLOS obligations. UNCLOS is an independent source of duties that may thus condition how nations regulate climate change technologies. As stated by Professor Cymie Payne:

“[w]hen States become parties to UNCLOS, they agree to protect and preserve the marine environment, and they also make the more specific commitment to prevent, reduce, and control pollution of the marine environment.”

Coherence between UNCLOS, the LC/LP, and the climate regimes is best found in identifying approaches that can protect the marine environment while reducing climate change impacts.

Marine geoengineering should therefore not be permitted merely because it is framed as a response to climate change. Any decision to authorize such activity must also ensure that it does not cause, transfer, or transform environmental harm, as required under UNCLOS.

Due Diligence does not Require the Application of Untested Technologies that may Pose Further Risks to the Marine Environment

The ICJ Climate Change Advisory Opinion states:

“[w]here a risk can be addressed with readily available technologies, States are expected to use them. However, when technologies pose further risks, States are expected to use them with prudence and caution.”

(para. 286).

At present, according to the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection, the marine geoengineering proposals under discussion are not “readily available” and have not been assessed with prudence to determine their feasibility, effectiveness, and potential for harm at scale. 

The burden of proof remains on proponents of marine geoengineering to demonstrate that a proposed technique is not only theoretically capable of meaningful climate mitigation, but is feasible, effective, durable, monitorable, environmentally safe, and capable of being halted or remediated if harmful effects arise under the conditions in which it would be deployed. Due diligence does not require States to pursue untested or speculative technologies simply because they are presented as climate mitigation measures. Rather, prudence and caution require early assessment of whether proposals are likely to fail on basic grounds, including life-cycle emissions, material and energy requirements, ecological risk, monitoring limitations, or the inability to demonstrate durable sequestration. Where such an assessment shows that a technique is infeasible, ineffective, unsafe, or incapable of delivering a genuine climate benefit, the appropriate outcome may be an exit ramp rather than expanded research or progressive scaling.

Equity considerations are also relevant in the context of due diligence. Marine geoengineering proposals should not be evaluated solely by asking whether they are technically possible for a handful of technologically advanced States or private actors. Their governance must also consider who controls the technology, who bears the risks, who has access to information and decision-making, and whether affected and vulnerable States have the capacity to assess, monitor, challenge, or respond to potential impacts. UNCLOS obligations concerning scientific and technical assistance, preferential treatment for developing States, and the transfer of marine technology provide an important basis for this inquiry, as do the advisory opinions’ emphasis on cooperation, capabilities, and the special position of vulnerable developing States. A proposal that depends on highly unequal access to technology, expertise, finance, or monitoring capacity should not be treated as a readily available or equitable climate response.

Due diligence, therefore, requires more than the development of international guidance or reliance on proponents’ claims. It requires States to adopt and implement effective national measures capable of identifying when proposed marine geoengineering activities should not proceed, regulating private actors under their jurisdiction or control, and ensuring accountability where such activities risk harm to the marine environment.

Systemic Integration of Environmental Impact Assessments for Marine Geoengineering Requires Consideration of the BBNJ Agreement

The requirement for States to prepare environmental impact assessments is a recognized obligation under international law, by which States give practical effect to precaution, due diligence, and the obligation to prevent harm to the marine environment. In the context of marine geoengineering, EIA requirements are particularly important because proposed activities may involve novel interventions, uncertain effects, cumulative impacts, and risks that extend beyond national jurisdiction. The question is therefore not whether climate-related objectives can substitute for environmental assessment, but how proposed activities must be assessed before they are authorized, including under the LC/LP, UNCLOS, and the BBNJ Agreement.

The White Paper asserts that Parties should analyze the projects’ climate mitigation benefits alongside their potential environmental harms. However, the fact that there may be some climate mitigation benefits does not obviate the need to ensure that the project will not cause transboundary harm or cause other significant adverse impacts. The BBNJ Agreement’s provisions on EIAs underscore this requirement.

Under the BBNJ Agreement, marine geoengineering activities in areas beyond national jurisdiction or within national jurisdiction that have the potential to “cause substantial pollution of or significant and harmful changes to the marine environment in areas beyond national jurisdiction” must be assessed through either a Party’s national process or the Agreement’s Part IV EIA requirements. (art. 28). These obligations include an initial screening, consultation requirements, notification requirements as part of the Clearinghouse Mechanism, and requirements to consider:

“associated impacts, such as economic, social, cultural and human health impacts, including potential cumulative impacts and impacts in areas within national jurisdiction, as well as alternatives to the planned activities.”

(art. 31).

Parties may rely on an assessment under another framework only where that assessment is equivalent. Accordingly, for Parties to both the LC/LP and the BBNJ Agreement, reliance on LC/LP assessment frameworks for marine geoengineering would require those frameworks to satisfy the BBNJ Agreement’s equivalency standard; otherwise, the BBNJ Agreement’s EIA requirements would continue to apply.

Conclusion

The ITLOS Climate Change Advisory Opinion makes clear that States’ obligations to respond to climate change do not displace their independent obligations under UNCLOS to protect and preserve the marine environment. That point is particularly important for marine geoengineering.  The ocean must not be treated as a canvas for technological interventions that would reduce it to a carbon sink to be exploited, but as an environment with intrinsic value requiring protection, refusing to subordinate marine ecosystems to climate interests. This recognition of the intrinsic value of the marine environment is expressed clearly in the BBNJ Agreement’s preamble:

Desiring to act as stewards of the ocean in areas beyond national jurisdiction on behalf of present and future generations by protecting, caring for and ensuring responsible use of the marine environment, maintaining the integrity of ocean ecosystems and conserving the inherent value of biological diversity of areas beyond national jurisdiction.

The path forward must therefore be one in which responses to the climate crisis are consistent with, rather than exceptions to, the protection and preservation of the marine environment. Precaution, equity, and stewardship are not obstacles to climate action; they are conditions for its legitimacy.

Photo by NighthawStudio on Unsplash