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International Tribunal for the Law of the Sea renders major decision on climate change

Posted on 23 May 2024

In Brief

  • This week, the first ever ruling on climate change under a global treaty was handed down by an international court, the International Tribunal for the Law of the Sea (ITLOS).
  • The decision clarified the obligations to combat climate change incumbent on States who are party to the United Nations Convention on the Law of the Sea (the Convention).
  • The Convention is a global treaty adopted by 169 States governing crucial matters in relation to the ocean, such as States' maritime boundaries, international shipping, and the exploitation of marine resources.
  • The Tribunal held that the discharge of greenhouse gases (such as carbon dioxide and methane, which have the effect of trapping heat from the Sun in the Earth's atmosphere thereby causing global warming) from human activities constitutes "pollution of the marine environment" for the purposes of the Convention. 
  • The Tribunal found that the Convention therefore requires States to take "all necessary measures" to reduce greenhouse gas emissions from all sources in order to keep global warming within 1.5°C above pre-industrial levels.
  • The decision follows after the European Court of Human Rights rendered a landmark judgment on climate change under a regional treaty, the European Convention on Human Rights last month. 
  • A copy of the ITLOS decision is available here.


ITLOS' decision takes the form of an "advisory opinion". Such an opinion may be rendered where States who are party to the Convention ask for an interpretation of the treaty in relation to a particular issue. Unlike a judgment, an advisory opinion does not make findings of liability or order remedies against any party. However, it establishes an authoritative statement of the law that may be relied upon in subsequent contentious proceedings. 

The request for an advisory opinion was submitted by six Small Island States (Antigua and Barbuda, Tuvalu, the Republic of Palau, Niue, the Republic of Vanuatu and Saint Lucia) through an international organisation they had created for this purpose, the Commission of Small Island States on Climate Change and International Law (COSIS). 

The applicants asked ITLOS to clarify States' obligations under the Convention to prevent, reduce and control pollution to the marine environment caused by climate change, as well as to protect and preserve that environment from the impacts of climate change.

More than 50 States, international organisations and NGOs participated in the proceedings, which included a live-streamed hearing over two weeks in September 2023.


The Tribunal made the following key findings:

  • The discharge of greenhouse gases from human activities constitutes "pollution of the marine environment" for the purposes of the Convention. The harmful effects of greenhouse gas emissions include "ocean warming" and "ocean acidification" due to the increased uptake of heat and carbon dioxide by the sea from the air. The resulting marine heat waves and altered chemical balance in the water endanger marine biodiversity. Climate change also causes sea level rise due to Arctic melting, which contributes to more frequent and severe tropical storms and poses an existential threat to some Small Island States.
  • The Convention places obligation on States to take "all necessary measures" to prevent, reduce and control pollution of the marine environment. In the context of climate change, this amounts to a duty to take measures to reduce greenhouse gas emissions from all sources, not just maritime activities such as shipping.
  • Which measures are "necessary" must be determined objectively, taking into account the best available science, as well as international rules and standards. The UN Framework Convention on Climate Change (UNFCCC) and the Paris Agreement are particularly relevant, notably the global temperature goal of limiting the temperature increase to 1.5°C above pre-industrial levels and the timeline for emission pathways to achieve that goal. In this regard, the Paris Agreement provides that Parties should "reach global peaking of greenhouse gas emissions as soon as possible" followed by "rapid reductions thereafter" so as to achieve a "balance" between greenhouse gas emissions and their removal by sinks (such as plants, soils and oceans that are capable of absorbing and storing a certain amount of carbon dioxide) "in the second half of this century".
  • States must take measures to reduce greenhouse gas emissions "in accordance with their capabilities".  This reflects considerations of equity between developed and developing States and is consistent with the principle of "common but differentiated responsibilities" under the Paris Agreement.
  • Developed States, moreover, have a duty to assist developing States in their efforts to address marine pollution through capacity building, the sharing of scientific expertise, technology transfer, and preferential access to funding and support from international organizations.
  • The Tribunal characterised States' obligation to reduce greenhouse gas emissions as a stringent obligation of "due diligence", rather than an obligation of result. This means States have an ongoing "obligation of conduct" to adopt the legislation and enforcement mechanisms necessary to regulate activities in their jurisdictions with a view to reducing their greenhouse gas emissions. This includes an obligation to keep the effects of those activities under "continuing surveillance" and to conduct environmental impact assessments for any planned projects which may cause significant greenhouse gas emissions.
  • In addition to States' obligation to reduce emissions, they must also take measures to "protect and preserve" the marine environment against the impacts of climate change. This duty applies especially to rare or fragile ecosystems and the habitats of endangered marine species. In particular, this means States must restore ecosystems that have already suffered degradation.


The ITLOS Advisory Opinion is significant because it establishes a clear obligation for States to reduce greenhouse gas emissions in line with the 1.5°C target that is complementary to – but independent of – the Paris Agreement. The Tribunal rejected the argument of large emitters in the proceedings that the Paris Agreement establishes an exclusive legal regime in relation to climate change, at the exclusion of the Convention.  

While the decision relies on the Paris Agreement in several respects, the Tribunal made clear that States' commitments under that agreement may fall short of discharging their duty to reduce greenhouse gas emissions under the Convention. This is because the Paris Agreement allows Parties to determine their own emission reduction plans (so-called Nationally Determined Contributions) that may fail to achieve the collective goal of keeping global warming to 1.5°C. Indeed, the First Global Stocktake of progress under the Paris Agreement concluded in 2023 that States' combined national pledges would result in warming of between 1.7 and 2.1°C by the year 2100.

The Advisory Opinion therefore opens the possibility of a State being sued by another State (or a grouping such as COSIS) under the Convention for failing to take adequate steps to reduce emissions in its territory in line with the 1.5°C target and thereby causing a transboundary harm.  

Such an action would not be possible under the Paris Agreement for an additional reason: unlike the Convention, the Paris Agreement does not have a mandatory dispute resolution procedure, but only an optional one. So far, only one State (the Netherlands) has elected to apply those provisions.    

More immediately, the ITLOS decision is likely to influence States' second-round submissions, due in June, to the International Court of Justice, the highest judicial authority for international law, which is in the process of preparing its own advisory opinion on climate change.   


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