Marine Geoengineering: Climate obligations are not a blank cheque to experiment with the ocean

Adding crushed rock to seawater so that it takes up more carbon. Sinking algae into the deep sea. Making the surface of the ocean more reflective so more sunlight bounces back into space. These are among the interventions being proposed under the marine geoengineering umbrella, and some are already being trialled at sea.

What might sound like innovative fixes to the climate crisis is in fact a set of unproven and improbable proposals to manipulate the ocean, risking new harm to marine ecosystems while distracting from the urgent need to cut emissions at source.

Can harm be offset against a climate benefit?

Last year the Sabin Center for Climate Change Law at Columbia University published an analysis of legal principles and guidelines for the regulation of marine geoengineering under the LC-LP. A new guest post on the international law blog Opinio Juris argues that their analysis gets a central point of international law wrong: marine geoengineering cannot be treated as a trade-off between claimed climate benefits and potential harm to the ocean. It was written by OceanCare’s Stephanie Prufer J.D. and Dr James Kerry, together with Prof Kristina Gjerde J.D., a renowned lawyer and expert in international ocean governance, who passed away in late 2025.

The disputed point is how a project should be judged. The Sabin Center proposes that States weigh the expected climate benefit of an intervention against the environmental harm it might cause. But OceanCare and Prof Gjerde argue that this risks turning protection of the ocean into a balancing exercise, where damage to marine ecosystems could be acceptable if the claimed climate gain is considered large enough.

No such calculation exists in international law, the authors argue. OceanCare first raised this concern at the meeting of the LC-LP governing bodies last year. The new Opinio Juris guest post sets out the argument in full ahead of the next meeting of member States from October 5 to 9, 2026.

“Marine geoengineering is not taking place in a legal vacuum,” said Dr James Kerry, Chief Scientist at OceanCare. “States are already bound to protect and preserve the marine environment, to act with precaution, and to prevent environmental harm. Those obligations do not disappear simply because an activity is presented as climate action.“

What States have already promised

States have committed to protect and preserve the marine environment. They have committed to prevent pollution from the technologies under their jurisdiction or control, and not to move damage from one place to another, or to turn one kind of pollution into another. Separately from all that, they have made commitments on climate change.

Both sets of obligations apply at the same time. The recent advisory opinions of the International Tribunal for the Law of the Sea (ITLOS) and the International Court of Justice (ICJ) make clear that meeting climate commitments does not release States from their separate duties to protect the ocean.

These proposals involve putting substances into the sea, bringing them within the scope of the LC-LP, which governs dumping and placement of matter at sea. That framework is built around a long-standing rule of precaution: where there is reason to believe that material introduced into the marine environment is likely to cause harm, preventive measures must be taken, even where the causal link has not been conclusively proven.

Three years ago, the member States reached a conclusion of their own at the LC-LP. A review of proposed marine geoengineering techniques stated that their effects could be widespread, long-lasting or severe, and that much about their consequences remains unknown.

Who bears the burden?

If harm cannot simply be balanced against claimed climate benefit, the burden must fall on those proposing marine geoengineering activities. They should have to show that a technique works, that its effects can be monitored, that it is safe, and that it can be stopped or reversed if harm emerges. Where those tests cannot be met, the proper conclusion may be not to proceed.

This also matters for fairness. Many of these proposals are being advanced by a small number of wealthy countries, research institutions and private companies, while the risks may be felt far more widely, including by States and communities with limited capacity to monitor, challenge or respond to harm.

A familiar push to lower the bar

That same trade-off logic appears again in a recent Sabin Center commentary on two new reports: the National Academies’ report on marine carbon dioxide removal and UNEP’s Limiting Overshoot report. The commentary presents both reports as evidence of the urgent need to reform governance so that marine carbon dioxide removal can move forward. But this reading is highly selective and treats the reports as support for a permissive pathway for marine geoengineering.

While the UNEP report urges rapid climate action, it does not endorse marine geoengineering at all costs, nor does it suggest that every speculative climate intervention should now be treated as necessary. On the contrary, it warns that reliance on carbon dioxide removal carries serious risks and uncertainties, and that rapid emissions reductions remain the central task. Presenting UNEP’s warning about overshoot as a call to use “every tool in the toolbox” risks turning a cautionary report into an argument for precisely the kind of risky technological pathway it warns against.

This is why the Sabin Center’s framing is concerning. A commentary that treats cautionary reports as support for accelerating research and development of marine carbon dioxide removal risks reinforcing the same trade-off logic challenged in the Opinio Juris analysis.

What is on the horizon?

Member States of the LC-LP will meet again in a fortnight, as treaty bodies continue their leading role in developing international governance for marine geoengineering activities. OceanCare strongly supports the adoption of the assessment frameworks that are on the table as an important step towards proper governance of marine geoengineering. But those frameworks must be built on international law as it stands.

That means assessment should not become a route for approving activities that States’ existing obligations would otherwise require them to prevent, restrict or reject. Precaution, the prevention of harm and the duty to protect and preserve the marine environment must be the foundation of any decision-making process, not factors to be traded away against claimed climate benefits.

Stephanie Prufer J.D., Legal and Policy Officer at OceanCare, concludes: “The assessment frameworks now under discussion at the London Convention and London Protocol are urgently needed. But they must be adopted in a form that does not disregard States’ other obligations under international law: States must act with precaution, prevent harm, and protect and preserve the marine environment. The frameworks should strengthen those obligations, not dilute them.”

Read the full analysis on Opinio Juris:

Key International Legal Obligations and Principles Relevant to Marine Geoengineering