International legal obligations of States in respect of climate change

1. The request for an advisory opinion

On 29 March 2023, the United Nations General Assembly (‘UNGA’) adopted resolution 77/276. The resolution requested the Court to issue an advisory opinion on the legal obligations of States in respect of climate change. The resolution was initiated by the archipelago of Vanuatu, whose existence is under threat due to rising sea levels. In an earlier blog post, we explored the context of this request for an advisory opinion and the various countries’ position in the proceedings, including those of Vanuatu and the Netherlands.

In its advisory opinion, the Court addresses the following two key questions: (1) what obligations do States have under international law to protect present and future generations from climate change, and (2) what are the legal consequences if States fail to fulfil these obligations?

2. Sources of international climate law

First, the Court addresses the question which sources of international law contain obligations with regard to climate change. According to the Court, the most directly relevant applicable law is found in the following sources:

• Charter of the United Nations;
• Climate change treaties, such as the United Nations Framework Convention on Climate Change (‘UNFCCC’), the Kyoto Protocol to the United Nations Framework Convention on Climate Change (‘Kyoto Protocol’) and the Paris Agreement;
• United Nations Convention on the Law of the Sea (‘UNCLOS’);
• other environmental treaties such as the Vienna Convention for the protection of the ozone layer (‘Ozone Layer Convention’);
• customary international law, including the duty of States to prevent significant environmental harm and the duty to co-operate for the protection of the environment;
• international human rights; and
• other principles, including the principle of sustainable development, the principle of common but differentiated responsibilities and respective capabilities, (intergenerational) equity, the precautionary principle and the polluter pays principle.

The Court concludes that the obligations arising from these sources do not conflict with each other. Instead, they coexist harmoniously. The Court sees no inconsistencies between the various sources, noting that the preambles of the UN climate change treaties contain references to other rules and principles. According to the Court, the principle of lex specialis does not result in climate change treaties generally excluding other rules of international law. This principle applies when there is a genuine inconsistency between two provisions, or when one provision is clearly intended to take precedence over another.

3. Obligations under international climate law

The Court formulates various obligations for States in its opinion. The Court categorises the most significant of these obligations under the headings of mitigation, adaptation and co-operation. Due to the scope of the opinion, this blog post will only discuss the mitigation obligations formulated by the Court based on UN climate change treaties and customary international law. The Court also discusses adaptation and co-operation obligations in more detail in its opinion. 

According to the Court, mitigation lies at the heart of the objectives set out in the UNFCCC. The Court interprets the most significant (legally binding) mitigation obligations of the UNFCCC in Article 4, which emphasises the joint and individual responsibilities of parties to combat climate change. Among other things, the Court highlights the result-based obligations set out in this article, such as preparing national inventories of anthropogenic emissions by sources and removals by sinks, as well as the effort-based obligations, including the obligation to co-operate in developing and disseminating technologies and practices that control, reduce or prevent greenhouse gas emissions. 

The Kyoto Protocol and the Paris Agreement provide further specifications of the obligations of the UNFCCC. For instance, the Kyoto Protocol imposes quantified emission reduction obligations on the parties listed in Annex B to the Protocol. Article 2 of the Paris Agreement sets out the objective of limiting the increase in the global average temperature to well below 2°C above pre-industrial levels, while also pursuing efforts to limit the temperature increase to 1.5°C. The Court assumes that, during the 26^th Conference of the Parties (‘COP’), the parties agreed to continue their efforts to limit the global average temperature increase to 1.5°C. Consequently, the Court takes this 1.5°C target into account when interpreting the obligations under the Paris Agreement, including those relating to nationally determined contributions (‘NDCs’). 

The Court finds that States must prepare, communicate and maintain successive NDCs (Article 4(2) of the Paris Agreement), and that the content of the NDCs is also relevant in determining whether the obligations of the Paris Agreement have been fulfilled. For instance, the Court interprets Article 4(3) of the Paris Agreement not ‘merely hortatory’, but as an 'obligation' for each subsequent NDC to reflect the highest possible level of ambition and to be more ambitious than its predecessor. The Court therefore concludes that NDCs must become increasingly ambitious over time, with the ambition of each party’s NDC relating to limiting the increase in the average global temperature to below 1.5°C. In this regard, the Court refers to Article 3 of the Paris Agreement, which stipulates that parties must make efforts 'with a view to achieving the purpose of this Agreement as set out in Article 2'. 

Furthermore, the Court believes that States have only limited discretionary powers in the preparation of their NDCs. However, the standard applied in assessing the NDCs of different parties will vary based on due diligence and the principle of common but differentiated responsibilities and respective capabilities. This standard depends on a range of factors, including the historical contribution and level of development and national circumstances of the party in question. The Court clarifies that, while States are not required to guarantee the full realisation of their NDC’s, they must make every effort to achieve these objectives. This means that States must act proactively and take reasonably appropriate measures to achieve their NDCs. 

Based on customary international law, the Court rules that States must prevent significant environmental damage by acting with due diligence. When assessing whether a State has fulfilled this duty of care, the Court considers the following elements to be important: 

1. States must adopt appropriate rules and measures;
2. States must obtain and analyse scientific and technological information;
3. States must take into account binding and non-binding standards, including COP decisions and recommended technical standards;
4. the expected level of diligence depends on economic development and available resources. Developed countries with greater capacity have higher obligations, but all States must act according to their capabilities;
5. even in the face of scientific uncertainty, States should not delay or omit preventive measures;
6. States must assess the risks and consequences of activities that cause greenhouse gas emissions based on the best available scientific knowledge; and
7. States must inform and consult with other States in good faith when planned activities pose a risk of significant damage to the climate system or affect collective climate efforts.

4. The consequences of non-compliance with international climate obligations

The Court discusses the consequences of failing to comply with climate obligations under international law, based on three elements: (1) the applicable law, (2) State responsibility in the context of climate change, and (3) the legal consequences of an international wrongful act. 

4.1 Applicable law

The Court finds that States can be held responsible not only for breaching their obligations under the climate change treaties, but also for breaching the customary international law obligation to prevent significant damage to the climate system. In such cases, compliance hinges on the standard of due diligence.

The Court rejects the argument that the framework of the climate change treaties constitutes a lex specialis that excludes the general rules on State responsibility. The Court concludes that the text, context and purpose of the climate change treaties demonstrate no intention to deviate from the general rules on State responsibility. Consequently, responsibility for breaches of obligations under the climate change treaties is to be determined by applying the well-established rules on State responsibility under customary international law.

4.2 State responsibility in the climate-change context

The Court has found that the unprecedented nature and scale of harm resulting from climate change raises questions about how the customary rules on State responsibility should be applied. The Court identifies attribution and causation as the main issues requiring clarification in view of the special features of climate change.

In the context of determining State responsibility, attribution means attaching a given action or omission to a State. Some States have suggested that applying the rules of State responsibility in the context of climate change is difficult since the emission of greenhouse gases is not, in itself, an internationally wrongful act. The Court emphasises that, while greenhouse gas emissions do not constitute an internationally wrongful act in themselves, a breach of the obligations previously established by the Court in this opinion may constitute an internationally wrongful act which is attributable to that State (e.g. a State's failure to take appropriate measures to protect the climate system from greenhouse gas emissions). The Court also notes that, while climate change is caused by cumulative greenhouse gas emissions, it is scientifically possible to determine each State’s total contribution to global emissions by taking into account both historical and current emissions. The Court therefore considers that the rules on the responsibility of States under customary international law are, in principle, suitable for addressing situations involving multiple injured or responsible States.

Regarding causality, the Court notes that causing damage is not a requirement for the determination of responsibility as such, and therefore causation only plays a role in determining reparation. The Court refers to the existing legal standard for establishing causation, as developed in its case law. This standard requires the existence of a sufficiently direct and certain causal nexus between the wrongful act and the injury suffered. The Court considers that this standard can also be applied in this case. In the context of climate change, the Court notes that causality involves two elements: (1) whether a given climatic event or trend can be attributed to anthropogenic climate change, and (2) to what extent damage caused by climate change can be attributed to a particular State or group of States. While acknowledging that the causal link between a State’s unlawful acts or omissions and climate damage is more tenuous than in the case of local sources of pollution, the Court argues that establishing such a link is not impossible. Whether a causal link exists must be assessed in concreto, taking into account the relevant factors.

Finally, the Court addresses the erga omnes nature of certain obligations. It considers that all States have a common interest in protecting global common environmental goods. Consequently, the Court deems the obligations of States under customary international law to protect the climate system and other environmental elements from anthropogenic greenhouse gas emissions to be erga omnes obligations. As a result, any State can hold another State responsible for failing to fulfil its obligations when these obligations originate from customary international law. When such obligations arise from climate change treaties, all parties to the treaty can hold a State responsible for failing to fulfil its obligations under that treaty.

4.3 Legal consequences of an internationally wrongful act 

The Court notes that the legal consequences that may be attached to an internationally wrongful act depend on the nature of the violation and the type of damage caused. The various legal consequences include the obligation to comply, the obligation to cease and guarantees of non-repetition, and the obligation to provide full reparation. 

States have a continuing obligation to perform their obligations despite their breaches thereof. For instance, if an inadequate NDC is identified in a State that is party to the Paris Agreement, that State may be required to adopt an NDC that complies with the NDC obligations.

In the case of a continuing internationally wrongful act, customary international law also imposes an obligation to put and end to the wrongful act. The Court indicates that this obligation may require a State, for instance, to withdraw all administrative, legislative and other measures falling under the internationally wrongful act. Furthermore, a State may be required to provide appropriate assurances and guarantees that the wrongful act will not be repeated.

If a causal link has been established between the wrongful act of a State (or group of States) and the specific damage suffered by an affected State or individuals, there may be an obligation to provide reparation in the form of restitution, compensation and/or satisfaction. This must be 'full reparation', meaning the consequences of the wrongful act must be undone and the situation as it would have if the act had not been committed must be restored. 

5. How does the Court's opinion differ from previous climate cases? 

Climate change and the responsibilities of States have long been part of the legal agenda. Important rulings have now been handed down in the Netherlands and across Europe. On 20 December 2019, the Supreme Court ruled in the Urgenda case that the Netherlands must offer adequate protection to its residents against the dangers of serious climate change. On 9 April 2024, the European Court of Human Rights reached a similar conclusion in Verein KlimaSeniorinnen Schweiz e.a. v. Switzerland. For a detailed discussion of the latter case, we refer you to this earlier blog. Additionally, the ongoing climate case brought by Bonaire and Greenpeace against the Dutch State is discussed in more detail in this blog. 

The Urgenda judgment, the KlimaSeniorinnen case and the Greenpeace case all address the vertical relationship between States and their citizens. In these cases, the court is considering whether citizens can expect protection from their government based on (among other factors) human rights. Additionally, the case brought by Milieudefensie against Shell is currently pending before the Supreme Court. This case addresses the question of whether a company is obliged to reduce its CO2 emissions based on human rights and the unwritten duty of care. Further information on this case can be found in these blogs: one about the appeal proceedings and another about the court's ruling. 

Conversely, the Court's opinion focuses on the horizontal relationship between States. It clarifies the climate obligations that States have towards each other under international law, and the grounds on which States can hold each other accountable for violations of these climate obligations. In doing so, the opinion establishes a legal basis in international law that goes beyond current case law on the climate obligations of States.

6. Conclusion

Although the Court's opinion is not legally binding, we expect it to play a significant role in the future development of international law concerning climate obligations and State responsibility. By clarifying the obligations of States and elaborating on the legal framework for State responsibility, the Court is establishing a foundation upon which States can hold each other accountable. If States fail to reach effective agreements through political channels on how they will fulfil their responsibility to combat climate change, this is likely to lead to an increase in interstate climate cases in the coming years. Such agreements must also include provisions for compensation for States that are already experiencing significant climate-related damage.