By a letter dated 12 December 2022, the Commission of Small Island States on Climate Change and International Law (Commission) requested the International Tribunal for the Law of the Sea (the Tribunal) to give an advisory opinion on the following questions:

What are the specific obligations of State Parties to the United Nations Convention on the Law of the Sea (UNCLOS), including under Pt XII:

(a) to prevent, reduce and control pollution of the marine environment in relation to the deleterious effects that result or are likely to result from climate change, including through ocean warming and sea level rise, and ocean acidification, which are caused by anthropogenic greenhouse gas emissions into the atmosphere?

(b) To protect and preserve the marine environment in relation to climate change impacts, including ocean warming and sea level rise, and ocean acidification?

Held: Article 21 of the Statute of the Tribunal and the Agreement conferring jurisdiction on the Tribunal constitute the substantive legal basis of the advisory jurisdiction of the Tribunal in this case. Considering that one of the main objectives of the UNCLOS is the protection and preservation of the marine environment, to which Pt XII is dedicated, it is clear that the Agreement is an international agreement related to the purpose of the UNCLOS.

The UNCLOS, the Agreement, and other relevant rules of international law not incompatible with the UNCLOS constitute the applicable law in this case. The relationship between the provisions of Pt XII of the UNCLOS and external rules is particularly relevant in this case. The UNCLOS contain certain provisions - also rules of reference - that refer to external rules. These rules of reference employ different terms and have different scopes and legal effects. Article 237 of the UNCLOS clarifies the relationship of Pt XII of the UNCLOS with other treaties relating to the protection and preservation of the marine environment. Article 237 reflects the need for consistency and mutual supportiveness between the applicable rules. On the one hand, Pt XIII is without prejudice to the specific obligations of States Parties under special Conventions and agreements concluded previously in this field and to agreements which may be concluded in furtherance of the general principles of the UNCLOS. On the other hand, such specific obligations should be carried out in a manner consistent with the general principles and objectives of the UNCLOS. The rules of reference contained in Pt XII and art 237 of the UNCLOS demonstrate the openness of Pt XII to other treaty regimes.

Subject to art 293 of the UNCLOS, the provisions of the UNCLOS and external rules should be interpreted consistently to the extent possible. There is an extensive treaty regime addressing climate change that includes the United Nations Framework Convention on Climate Change 1992, the Kyoto Protocol 1997, the Paris Agreement 2016, Annex VI to the International Convention for the Prevention of Pollution from Ships of 2 November 1973, as modified by the Protocol of 1978 (MARPOL), Annex 16 to the Convention on International Civil Aviation 1944, and the Montreal Protocol to the Vienna Convention for the Protection of the Ozone Layer 1987, including the Kigali Amendment 2016. In the present case, relevant external rules may be found, in particular, in those agreements.

The Commission asked the Tribunal to identify specific 'obligations' under the UNCLOS; terms such as 'responsibility' and 'liability' did not appear in the request. The Tribunal confined itself to primary obligations. However, to the extent necessary to clarify the scope and nature of primary obligations, the Tribunal may have to refer to responsibility and liability.

The obligation addressed in the second question is broader in scope than the obligation addressed in the first question. The obligation to protect and preserve the marine environment encompasses the obligation to prevent, reduce and control marine pollution. In addition, it extends to the protection of the marine environment from any negative impacts. The Tribunal followed the order of the questions as they were posed in the request, and in its response to the second question, dealt with the obligations not addressed in the first question.

Replies to Question (a): Anthropogenic GHG emissions into the atmosphere constitute pollution of the marine environment within the meaning of art 1.1.4 of UNCLOS.

Under art 194.1, States Parties have the specific obligations to take all necessary measures to prevent, reduce and control marine pollution from anthropogenic GHG emissions and to endeavour to harmonise their policies in this connection. This obligation is one of due diligence. The standard of due diligence is stringent, given the high risks of serious and irreversible harm to the marine environment from such emissions. However, the implementation of the obligation of due diligence may vary according to States' capabilities and available resources.

Necessary measures should be determined objectively, taking into account, among others, the best available science and relevant international rules and standards, in particular the global temperature goal of limiting the temperature increase to 1.5^o Celsius above pre-industrial levels and the timeline for emission pathways to achieve that goal. The scope and content of necessary measures may vary in accordance with the means available to states and their capabilities. The necessary measures include, in particular, those to reduce GHG emissions.

Under art 194.2, States Parties have the specific obligation to take all measures necessary to ensure that anthropogenic GHG emissions under their jurisdiction or control do not cause damage by pollution to other States and their environment, and that pollution from such emissions under their jurisdiction or control does not spread beyond the area where they exercise sovereign rights. This obligation applies to a transboundary setting and is a particular obligation in addition to the obligation under art 194.1. It is also an obligation of due diligence. The standard of due diligence under art 194.2 can be even more stringent than that under art 194.1 because of the nature of transboundary pollution.

In terms of specific sources of pollution, marine pollution from anthropogenic GHG emissions can be characterised as pollution from land-based sources, pollution from vessels, or pollution from or through the atmosphere.

Under arts 207 and 212, States Parties have the specific obligation to adopt laws and regulations to prevent, reduce and control marine pollution from GHG emissions from land-based sources and from or through the atmosphere, respectively, taking into account internationally agreed rules, standards and recommended practices and procedures contained, among others, in climate change treaties. To this effect, States have the specific obligations to take other necessary measures and, acting especially through competent international organisations or diplomatic conference, to endeavour to establish global and regional rules, standards and recommended practices and procedures.

Under art 211, States Parties have the specific obligation to adopt laws and regulations to prevent, reduce and control marine pollution from GHG emissions from vessels flying their flag or of their registry, which must at least have the same effect as that of generally accepted international rules and standards established through the competent international organisation or general diplomatic conference.

Under arts 213 and 222, States Parties have the specific obligation to enforce their national laws and regulations and to adopt laws and regulations and take other measures necessary to implement applicable international rules and standards established through competent international organisations and diplomatic conference to prevent, reduce and control pollution of the marine environment from anthropogenic GHG emissions from land-based sources and from or through the atmosphere, respectively.

Under art 217, States Parties have the specific obligation to ensure compliance by vessels flying their flag or of their registry with applicable international rules and standards established through the competent international organisation or general diplomatic conference and with their laws and regulations for the prevention, reduction and control of marine pollution from GHG emissions from vessels. To this end, they shall adopt laws and regulations and take other measures necessary for their implementation.

Articles 197, 200 and 201, read together with arts 194 and 192, impose specific obligations on States Parties to co-operate, directly or through competent international organizations, continuously, meaningfully and in good faith, in order to prevent, reduce, and control marine pollution from anthropogenic GHG emissions. Under art 197, States Parties have the specific obligation to co-operate in formulating and elaborating rules, standards and recommended practices and procedures, consistent with UNCLOS and based on available scientific knowledge, to counter marine pollution from anthropogenic GHG emissions. Under art 200, States Parties have specific obligations to co-operate to promote studies, undertake scientific research and encourage the exchange of information and data on marine pollution from anthropogenic GHG emissions, its pathways, risks and remedies, including mitigation and adaptation measures. Under art 201, States Parties have the specific obligation to establish appropriate scientific criteria on the basis of which rules, standards and recommended practices and procedures are to be formulated and elaborated to counter marine pollution from anthropogenic GHG emissions.

Under art 202, States Parties have the specific obligation to assist developing states, in particular vulnerable developing states, in their efforts to address marine pollution from anthropogenic GHG emissions. This article provides for the obligation of appropriate assistance, directly or through competent international organisations, in terms of capacity-building, scientific expertise, technology transfer and other matters. Article 203 reinforces the support to developing States, in particular those vulnerable to the adverse effects of climate change, by granting them preferential treatment in funding, technical assistance and pertinent specialized services from international organisations.

Articles 204, 205 and 206 impose on States Parties specific obligations of monitoring, publishing the reports thereof and conducting environmental impact assessments as a means to address marine pollution from anthropogenic GHG emissions. Under art 204.1, States Parties have the specific obligation to endeavour to observe, measure, evaluate and analyse the risks or effects of pollution of the marine environment from anthropogenic GHG emissions. Under art 204.2, States Parties have the specific obligation to keep under continuing surveillance the effects of activities they have permitted, or in which they are engaged, in order to determine whether such activities are likely to pollute the marine environment through anthropogenic GHG emissions. Under art 205, States Parties have the specific obligation to publish the results obtained from monitoring the risks or effects of pollution from such emissions or to communicate them to the competent international organisationsfor their dissemination. Under art 206, States Parties have the specific obligation to conduct environmental impact assessments. Any planned activity, either public or private, which may cause substantial pollution to the marine environment or significant and harmful changes thereto through anthropogenic GHG emissions, including cumulative effects, shall be subjected to an environmental impact assessment. Such assessment shall be conducted by the state party under whose jurisdiction or control the planned activity will be undertaken with a view to mitigating and adapting to the adverse effects of such emissions on the marine environment. The result of such assessment shall be reported in accordance with art 205.

Replies to Question (b): The obligation under art 192 to protect and preserve the marine environment has a broad scope, encompassing any type of harm or threat to the marine environment. Under this provision, States Parties have the specific obligation to protect and preserve the marine environment from climate change impacts and ocean acidification. Where the marine environment has been degraded, this obligation may call for measures to restore marine habitats and ecosystems. Article 192 requires States Parties to anticipate risks relating to climate change impacts and ocean acidification, depending on the circumstances

This obligation is one of due diligence. The standard of due diligence is stringent, given the high risks of serious and irreversible harm to the marine environment from climate change impacts and ocean acidification.

Under art 194.5, States Parties have the specific obligation to protect and preserve rare or fragile ecosystems as well as the habitat of depleted, threatened or endangered species and other forms of marine life from climate change impacts and ocean acidification.

Under arts 61 and 119, States Parties have the specific obligations to take measures necessary to conserve the living marine resources threatened by climate change impacts and ocean acidification. In taking such measures, States Parties shall take into account, inter alia, the best available science and relevant environmental and economic factors. This obligation requires the application of the precautionary approach and an ecosystem approach.

The obligation to seek to agree under art 63.1, and the obligation to co-operate under art 64.1, require States Parties, among others, to consult with one another in good faith with a view to adopting effective measures necessary to coordinate and ensure the conservation and development of shared stocks. The necessary measures on which consultations are required must take into account the impacts of climate change and ocean acidification on living marine resources. Under art 118, States Parties have the specific obligation to co-operate in taking measures necessary for the conservation of living marine resources in the high seas that are threatened by climate change impacts and ocean acidification.

Under art 196, States Parties have the specific obligation to take appropriate measures to prevent, reduce and control pollution from the introduction of non-indigenous species due to the effects of climate change and ocean acidification which may cause significant and harmful changes to the marine environment. This obligation requires the application of the precautionary approach.