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EDITORIAL NOTE *
The manuscript contained three consecutive sections addressing the relationship between the Convention on the Law of the Sea and the Paris Agreement, of which the second reproduced the argument of the first, retaining a heading and internal numbering from an earlier draft. That section has been removed; the argument it made is preserved in the two sections retained.
ABSTRACT
The advisory hearings before the International Tribunal for the Law of the Sea, initiated by the Commission of Small Island States, created a crucial precedent, as they interpreted state obligations under the Convention on the Law of the Sea regarding transboundary environmental damage caused by climate change, and specifically the cross-border effects of greenhouse gas emissions on the marine environment. Article 194 states that states should take all necessary steps, according to their capabilities, to prevent, reduce and manage the pollution of the sea, including pollution within their jurisdiction that can spread to other states or regions under their control. In its recent opinion, the Tribunal applied these obligations clearly to transboundary harm, holding that the prevention of actual damage is not enough, but that potential damage must also be anticipated and prevented before it arises.
This obligation is more demanding than the principles of the Stockholm and Rio Declarations, because the Convention imposes proactive obligations: states have to act where harm is only probable, and not only after it has occurred. The Tribunal confirms that states are obliged to ensure that their activities do not cause ecological damage to other countries or to the wider seas, because emissions from fossil fuels travel through atmospheric and oceanic routes. This method is indicative of a precautionary legal approach, which underlines the primary environmental law principle of avoiding transboundary pollution, and holds states responsible where their policies or actions jeopardise biodiversity and human rights, and may expose vulnerable coastal and island societies to cross-border climate effects.
KEYWORDS
Intersection, climate change, United Nations Convention, transboundary harm, Rio Declaration, Stockholm Declaration
INTRODUCTION
Climate change caused by human beings endangers the life of the oceans
Anthropogenic climate change is threatening the oceans, which are a habitat for life on Earth. The most noticeable effects include warming, acidification, and sea level rise, which not only pose a threat to the survival and biodiversity of marine life but also jeopardise the lives of coastal and island communities. These are environmental changes that cross boundaries and hence constitute transboundary environmental loss that defies existing legal orders. Climate change is an existential and transboundary threat to the integrity of the world’s oceans, manifested in the degradation of ecosystems and even the displacement of whole communities. This crisis points to the need to establish a clear position on international legal responsibility and state accountability.
The advisory opinion: redefinitions of environmental and maritime law
The Tribunal issued a rare kind of advisory opinion in May 2024, reconsidering the outlines of international environmental and maritime law for the future. In it, the Tribunal clearly expressed the obligations of states under the Convention in relation to transboundary damage to the oceans occasioned by climate change. This has taken the law of the sea to a new stage that is not directly connected to traditional maritime regulation, but rather rooted in modern demands of climate and environmental governance, and is a manifestation of the relationship between the Convention and international climate instruments such as the Paris Agreement.
The Convention as ocean constitution and its environmental mandate
The Convention, adopted in 1982, is commonly referred to as the constitution of the oceans, since it provides the fundamental framework for how the seas of the world will be managed and cared for. Part XII is a crucial one because it is concerned with protecting and conserving the marine environment. It imposes binding obligations on states in their own interest as well as on behalf of the international community in general, so that states have a duty erga omnes to ensure that the oceans are left clean and free of pollution and other perilous practices.
High standards of state due diligence and Article 194(1)
One of the key provisions of this framework is Article 194(1), which obliges states to adopt every measure necessary to avert, mitigate, and manage any type of marine pollution. The Tribunal has construed this obligation to imply that a high standard of due diligence should be adopted by states. Such a standard is not only forward-looking, in that it prevents harm before it happens, but also backward-looking, in that it provides remedies where harm has already occurred. The legal problem of primary interest to the Tribunal was whether the emission of greenhouse gases within the sovereign territory of a state, causing environmental pollutants to reach the oceans and whose damaging impacts extend internationally, could give rise to international responsibility.
MAIN CORPUS
In its advisory opinion, the Tribunal provided a far-reaching interpretation of Article 1(4) of the Convention, which stipulates what constitutes pollution of the marine environment. The Tribunal stated that human-made greenhouse gas emissions, after being absorbed by the oceans, should be considered as marine pollution. This is transformative in the sense that it applies the full body of environmental requirements of Part XII to climate change. It carries the implication that issues such as the warming of the oceans, acidity and depletion of oxygen are not merely scientific facts but legally cognisable negative impacts of human action.
Another matter that the Tribunal clarified is the standard of conduct under which states must operate. It emphasised that states should practise a high standard of due diligence in their handling of greenhouse gas emissions, an obligation strongly connected with the precautionary principle of international environmental law. The duty is preventive in character, which implies that states must act before the damage is done. They have a duty to take proactive measures to reduce likely damage, particularly since the scientific community agrees that climate change is a reality. States also need to base their decisions on the best available scientific evidence, and constantly revise their policies as science progresses. Any failure to comply with this commitment constitutes a violation of international law, and it opens the way to state accountability for transboundary damage resulting from climate change.
This is a significant change in international law. Previous instruments, such as the Stockholm Declaration of 1972 and the Rio Declaration of 1992, were more concerned with responsibility once damage to the environment had been done. The Tribunal went a step further by making the law progressive through obligatory, preventive adherence by states; it is thus clear that it is no longer appropriate to wait until it is too late.
The principle of transboundary harm and the International Court of Justice
Among the most significant rules of international environmental law is the principle that states must not cause transboundary harm. It derives from the Latin sic utere tuo ut alienum non laedas, meaning use your property in such a manner as not to harm another. This concept has been confirmed in multiple landmark cases and international declarations. The well-known Trail Smelter Arbitration between the United States and Canada concluded that no state may use its territory to inflict severe harm on another state. On the same note, Principle 21 of the Stockholm Declaration and Principle 2 of the Rio Declaration affirmed that states have the sovereign right to use their resources, but must ensure that they do not cause environmental damage outside their territories.
The International Court of Justice has also contributed significantly to the development of this principle. In the Corfu Channel case of 1949, the Court pointed out that a state should not knowingly allow its territory to be used in a manner detrimental to other states. In the Pulp Mills case of 2010, the Court reiterated that states are under a responsibility of due diligence in order to prevent significant transboundary harm, and that this responsibility includes the conduct of environmental impact assessment and adequate monitoring. Through these developments, the principle of transboundary harm has become customary international law and is regarded as an obligation binding on all states irrespective of any particular treaty commitments.
The most important aspect of this principle is the due diligence standard. It does not make states liable for all cross-border damage; rather, states must apply the utmost care to avoid risks that can be foreseen. Proper regulation of activities that may be harmful, vigilance, and preventive measures are part of due diligence. The Court in Pulp Mills emphasised that this duty includes prior evaluation of the environmental impact of a project and the implementation of ongoing monitoring of its consequences.
The advisory opinion expands this idea of due diligence and puts it into an even more intensive application. Since climate change is a worldwide and existential issue, the Tribunal required a higher degree of care among states in handling greenhouse gas emissions. Through this, it incorporated the principle of transboundary harm alongside the obligations of the Convention to establish a new standard in the area of state responsibility in environmental law.
Legal issues raised by the advisory opinion
The case presented to the Tribunal by the Commission of Small Island States was based on two main and closely interrelated legal issues. Collectively, these questions were the focus of the Tribunal’s advisory opinion on climate change under the Convention.
First legal question: the current state of obligations under the Convention. The initial question required the Tribunal to clarify the current requirements of states under the Convention to safeguard the marine environment against the impacts of climate change. Notably, the Tribunal did not focus on past liabilities or compensation but rather on the nature of current responsibilities. In this way, it reinforced the point that the Convention is a living instrument, one that should develop in line with the latest scientific understanding and global environmental changes.
In its answer, the Tribunal stated that a state’s duties are not optional. They are obligations that must be performed with strict due diligence. This benchmark requires states to take proactive, effective, and science-based measures nationally. Articles 207 and 213 require states to create and implement rules that regulate marine pollution, especially land-based pollution. As greenhouse gas emissions are mainly land-based and their harmful effects occur in the ocean, these articles are directly related to the harm caused by climate change.
Second legal question: protection of the marine environment under Part XII. The second question required the Tribunal to give meaning to the protection and preservation of the marine environment within Part XII, particularly in a climate change context. Article 1(4) contains a broad definition of pollution of the marine environment, which includes the introduction of substances that could cause harm. The Tribunal confirmed that this definition ought to be read in context and in general terms so as to incorporate contemporary problems such as climate change.
The Tribunal acknowledged that greenhouse gas emissions into the atmosphere and, consequently, into the ocean are directly associated with the most evident implications of climate change: warming of the oceans, acidification, and rising sea levels. These effects therefore fall squarely under marine pollution within the meaning of the Convention. The Tribunal also emphasised that under Article 194(5) there is a responsibility to conserve rare and fragile marine ecosystems. This combines biodiversity conservation with climate change control, demonstrating that states are responsible not only for limiting pollution but also for protecting ecosystems, particularly those endangered by climate change.
The connection between the two questions. The two legal questions are closely connected, though they are not framed as such. The elucidation of existing state obligations to prevent climate damage cannot be detached from the interpretation of marine environmental protection under Part XII. Both conclude that greenhouse gas emissions should be considered a type of marine pollution and that pre-emptive, precautionary actions should be legally required of states in order to avoid damage.
The Convention and the Paris Agreement: complementarity, not suspension
One of the recurrent arguments advanced by various states was that the Paris Agreement of 2015, being the treaty specifically devoted to climate change, should act as lex specialis and thus override the framework of the Convention when dealing with climate-related obligations. The Tribunal firmly rejected this claim and made clear that the connection between the two instruments is not one of superiority but of complementarity. Each has a particular normative scope and purpose, and they should be interpreted together in accordance with the principle of systemic integration expressed in Article 31(3)(c) of the Vienna Convention on the Law of Treaties.
The Tribunal highlighted the differences in the legal structures of the two regimes. The Paris Agreement operates mainly through nationally determined contributions, which are procedurally binding and substantively voluntary, and leaves states considerable room as to their level of ambition and their implementation of mitigation targets. In comparison, the Convention, and specifically Article 194, places a categorical obligation on states to take all possible action to prevent and regulate marine pollution, which specifically includes greenhouse gas emissions. This is a mandatory obligation operationalised by the due diligence and precautionary standards that are the cornerstone of Part XII.
To maintain the binding normative content of the Convention, it was therefore necessary to reject the lex specialis argument. Were the Paris Agreement to override it, states could use the voluntary and minimalist nature of their contributions as a barrier to liability for failure to take sufficient action to reduce marine pollution. The Tribunal held that the Paris Agreement cannot be instrumentalised to dilute existing obligations under the Convention, insisting instead that the two must be harmonised rather than one displaced by the other. The Paris Agreement enhances progressive political aspiration, whereas the Convention attaches duties that are legally binding in nature.
This clarification has considerable jurisprudential implications. It makes the Convention a strong legal instrument capable of bringing the climate policies of states under judicial scrutiny by reference to due diligence, as opposed to the essentially political review framework of the Paris regime. In doing so, the Tribunal reinforced the normative framework of international environmental law, ensuring that climate change mitigation cannot be immunised against binding duties to protect and preserve the marine environment. The ruling thus fosters coherence across regimes and enhances the standing of the Convention in the protection of the marine environment in the context of climate change.
Making the two regimes complementary
Although declining to adopt the lex specialis argument, the Tribunal emphasised that the Convention and the Paris Agreement should not be interpreted in isolation but as complementary instruments within the larger context of international environmental governance. This uses a systemic approach to interpretation under Article 31(3)(c) of the Vienna Convention, which requires that treaties be interpreted in the light of any relevant rules of international law applicable in the relations between the parties.
The Convention provides binding and enforceable obligations to promote and conserve the marine environment under Article 192 and to take action against all types of marine pollution under Articles 194 to 212. These provisions give the normative foundation for state action in connection with marine protection and the duty to control land-based sources of pollution such as greenhouse gas emissions under Articles 207 and 213. Conversely, the Paris Agreement operationalises cooperative and facilitative mechanisms: nationally determined contributions under Article 4, the global stocktake under Article 14, and climate finance under Article 9. Though not binding in quantitative terms, these mechanisms offer states structured processes for pursuing collective climate mitigation, and thus supplement the obligations of the Convention.
This is the strategy by which the Tribunal bridges the gap between hard law and the flexibility of a softer instrument. In doing so, it prevents a piecemeal application of international law and gives coherence across regimes, an interpretive approach long advocated by the International Law Commission in order to avert conflict between overlapping treaty regimes.
In addition, this reading supports climate justice: whereas the Paris Agreement gives states the freedom to determine their emissions reduction targets, the Convention asserts that this freedom should not come at the cost of marine environmental quality or of the entitlements of climate-vulnerable states, including small island developing states. The Tribunal affirmed that the obligations of the Paris Agreement operate as a complement and not a dilution of the duties under the Convention, establishing a two-tier structure of cooperative action on climate change alongside binding marine protection. Overall, the Tribunal has made clear that the Convention gives marine environmental protection its binding legal floor while the Paris Agreement gives global climate ambition its political and cooperative reach. They can be read together as establishing a comprehensive system of governance capable of addressing the interrelation of climate change and the protection of the ocean.
SUMMARY *
The article’s central legal point is that the Tribunal solved a definitional problem in a way that transferred a whole body of binding obligation onto a subject it was not written for. Once carbon dioxide absorbed by the sea is characterised as pollution of the marine environment within the meaning of a treaty of 1982, everything Part XII requires of states in respect of pollution applies to emissions, and the standard is due diligence rather than best endeavours. The second finding follows and is the one with immediate practical consequence: the rejection of the argument that the Paris Agreement occupies the field means that a state cannot answer a claim about marine pollution by pointing to its nationally determined contribution. What the article thereby identifies is a route by which climate obligations become justiciable, since the Convention provides for compulsory dispute settlement while the Paris regime provides for facilitative review.
CONCLUSION
The advisory opinion on climate change is a landmark development that recasts the relationship between international environmental law and the law of the sea. By applying an interpretative innovation to a treaty concluded more than forty years ago, the Tribunal has demonstrated that legal instruments are dynamic living instruments, able to evolve to address current and unexpected challenges.
Far from being symbolic, this opinion is a significant legal step, setting out clear, well-established principles to inform state action and deepening the legal basis for climate regulation. It underscores the positive, science-based duty of states to reduce greenhouse gas emissions, acknowledging such emissions as a type of marine pollution. By providing this authoritative view of the law, the Tribunal gives a voice to vulnerable states and to civil society in pressing for more ambitious climate action and in holding governments and corporations to account for the harm visited on the oceans. The impact of the opinion on other international tribunals and on national legal systems is likely to be considerable, marking a new era in which international law becomes a key driver of the global response to the climate crisis.
POTENTIAL SOLUTION *
The article’s analysis is careful and one development since the opinion transforms its significance, because the reasoning it describes has now been adopted by the principal judicial organ of the United Nations.
In July 2025 the International Court of Justice delivered its own advisory opinion on the obligations of states in respect of climate change, and it reached conclusions consistent with those set out here: that obligations arise from the whole corpus of international law rather than from the climate treaties alone, that the standard is stringent due diligence, that the climate regime does not displace other applicable obligations, and that a breach engages state responsibility with the ordinary consequences of cessation and reparation. What was in May 2024 the opinion of a specialised tribunal on a specialised treaty is now the settled view of both, and the alignment is what gives the article’s subject its weight. A practitioner advising a state can no longer treat the reasoning as confined to the law of the sea.
The consequence that matters most is procedural rather than substantive, and it deserves emphasis because it is where the article’s argument becomes actionable. The Convention contains compulsory dispute settlement: a state party may bring proceedings against another before the Tribunal, an arbitral tribunal, or the Court, without the respondent’s separate consent. The Paris Agreement contains nothing of the kind. If emissions are marine pollution and the obligations of Part XII apply to them, then the forum exists and is open, which is precisely why the lex specialis argument was pressed and precisely why its rejection matters. Whether any state will use that route is a political question; that it exists is now a legal fact.
Two qualifications strengthen the argument rather than weakening it. An advisory opinion is not binding on anyone, and its authority is persuasive; what gives it force is uniformity, the quality of the reasoning, and subsequent adoption by other courts, all of which are present here. And the hardest question is one the opinion did not resolve: how a breach is established where the harm results from the cumulative emissions of every state over two centuries. Causation, attribution and apportionment remain to be worked out, and they will be worked out in contentious proceedings rather than in advisory ones.
Finally, one observation for readers of this journal. The strategy adopted by the Commission of Small Island States is instructive well beyond its subject. A group of parties with no military weight, no economic leverage, and no prospect of prevailing in a negotiation obtained an authoritative statement of the law from an international tribunal, and thereby altered what every other party must now argue. That is a mediator’s insight applied at the scale of the international system: where the balance of power at the table cannot be changed, the alternative is to change what the table is obliged to discuss. Weak parties in any dispute have more often than they realise the option of establishing the framework rather than contesting the outcome within it.
* Added by the WMO Editorial Team
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Supplementary references added by the WMO Editorial Team
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