This browser is not actively supported anymore. For the best passle experience, we strongly recommend you upgrade your browser.

Freshfields Sustainability

| 5 minutes read

On land and at sea: States’ climate change obligations

On 21 May 2024, the International Tribunal for the Law of the Sea (ITLOS) issued an advisory opinion (the Opinion) on the climate change obligations of States under the UN Convention on the Law of the Sea (UNCLOS). While not strictly binding on States, advisory opinions may still be persuasive and carry legal weight in later contentious disputes in which the interpretation of the relevant provisions and the resulting scope of obligations is at issue. The Opinion comes on the heels of a historic decision by the European Court of Human Rights on the scope of States’ positive obligations to combat climate change. Two further advisory opinions are expected from the International Court of Justice and the Inter-American Court of Human Rights in the months to come. 

The Opinion was issued at the request of the Commission of Small Island States on Climate Change and International Law, which includes – for instance –Antigua and Barbuda, Tuvalu and Palau. The Commission asked ITLOS to clarify the specific obligations incumbent upon States parties to UNCLOS in relation to the protection and preservation of the marine environment. The practical highlights of the Opinion include:

  • States parties to UNCLOS are under an obligation to adopt all necessary measures in order to prevent, reduce and control marine pollution caused by greenhouse gas (GHG) emissions. 
  • This obligation goes beyond what is already required of States under specific climate change treaties such as the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement. 
  • The obligations under UNCLOS affect how States should regulate and monitor all activities or projects, and not just those that take place at sea. 
  • At the same time, ITLOS indicated that there are still obstacles to tort-like claims that GHG emissions by one State have caused damage to another State.

Obligation to reduce GHG emissions

The Opinion’s main focus was on Articles 192 and 194 of UNCLOS, which concern the protection and preservation of the marine environment generally, and measures to prevent, reduce and control pollution of it specifically. ITLOS found that these provisions entail an obligation to reduce GHG emissions. This obligation is one of conduct, not result. UNCLOS does not require States immediately to cease all GHG-emitting activities. Rather, States are under a due diligence obligation to adopt “all necessary measures” aimed at eventually preventing marine pollution caused by GHG emissions from occurring at all.

This due diligence obligation extends beyond maritime activities. ITLOS considered that all anthropogenic GHG emissions meet the UNCLOS definition of “pollution of the marine environment”. Consequently, while the UNCLOS obligation to reduce GHG emissions is aimed at the protection of the marine environment, States are required to implement measures that also address land-based activities and projects, even if they have no direct connection with the sea, insofar as they may nevertheless lead to pollution of the marine environment.

Moreover, the due diligence obligation is “stringent” due to the potentially catastrophic effects of climate change. States have some discretion in determining what “necessary measures” must be adopted. However, the assessment of whether the adopted measures comply with UNCLOS must be based on objective criteria, including: 

  • Best available science and the precautionary approach

According to ITLOS, the best available science regarding climate change and its effects on the ocean can be found in the reports of the Intergovernmental Panel on Climate Change (IPCC), which reflect scientific consensus. At the same time, any lack of scientific certainty cannot be used as a reason not to adopt measures to reduce GHG emissions, as States must apply the precautionary approach.

  • International rules and standards

Other climate-related treaties and international instruments (such as the UNFCCC and the Paris Agreement) must be taken into account when assessing whether a State has met its due diligence obligation. For example, whether a State adopted all necessary measures should be assessed against the global temperature goal and the timeline set forth in the Paris Agreement. However, ITLOS added that compliance with the Paris Agreement (and the nationally determined contribution set by a State thereunder) does not, in and of itself, mean that a State has also complied with its due diligence obligation under UNCLOS. Consequently, UNCLOS may oblige States to do more than what is required of them under the Paris Agreement. This may provide an additional pressure point for States to adopt more stringent GHG regulations.

  • The means and capabilities of each State

ITLOS added that the content of “necessary measures” must be determined on a State-by-State basis, ie by taking into account the means and capabilities of each particular State. This is a reflection of the principle of common but differentiated responsibilities included in several environmental treaties (eg the UNFCCC and the Convention on Biological Diversity). In other words, mitigation measures are required of all States, but States with greater means are expected to do more.

States do not fulfil their due diligence obligation merely by adopting relevant laws and regulations aimed at reducing GHG emissions. They must also ensure that non-State actors under their jurisdiction or control adhere to the adopted measures. Therefore, although the Opinion only addresses States’ obligations, businesses should be aware that it may give additional impetus for States not only to regulate, but also to effectively enforce GHG-reduction regulations. This may also have a direct impact on how GHG-reduction regulations are interpreted and applied.

Limits to engaging State responsibility

Determining the existence and scope of State obligations is very different from establishing liability. In the Opinion, ITLOS hinted at this distinction and the practical difficulties it poses for climate change disputes. Among these difficulties, the issue of causation is one of the most prominent obstacles to successful climate-change related claims for compensation – in inter-State and, even more, in private disputes.

Given the complex and diffused causes as well as the global effects of climate change, it remains to date impossible to prove, with sufficient certainty, that GHG emissions from one State (or one particular emitter) have caused a specific damage to another State (or person(s) therein).


The Opinion is an important development for climate change law. It establishes yet another avenue through which States may find themselves under pressure to do more to address climate change. The obligations interpreted by the Opinion fall on States only, and do not bind businesses. Businesses should nevertheless be aware of the potential indirect effects of the Opinion, such as the potential adoption and enforcement of stricter emission restrictions due to the (now clarified) climate change obligations under UNCLOS.

More generally, domestic and international courts continue to grapple with novel questions of law as well as the application of the law to complex questions of fact that come with climate change litigation. States and corporations are being tested by claimants, such as specialised NGOs, before all available fora (including mechanisms such as individual complaints under the OECD Guidelines for Multinational Enterprises on Responsible Business Conduct). To minimise the risks arising from multiple proceedings in different fora against the same respondent, States and corporations should have a comprehensive strategy in place to ensure that their defence in one case does not negatively affect their interests in another.


climate change, energy transition, environment, green energy, litigation, energy and natural resources