5. The ICJ’s Advisory Opinion on the Obligations of States in Relation to Climate Change

In 2025 the ICJ handed down its comprehensive advisory opinion on the obligations of states in relation to climate change. The questions put to ICJ by the UN General Assembly were as follows:^[1]

Having particular regard to the Charter of the United Nations, the International Covenant on Civil and Political Rights, the International Covenant on Economic, Social and Cultural Rights, the United Nations Framework Convention on Climate Change, the Paris Agreement, the United Nations Convention on the Law of the Sea, the duty of due diligence, the rights recognized in the Universal Declaration of Human Rights, the principle of prevention of significant harm to the environment and the duty to protect and preserve the marine environment,

1. What are the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases for States and for present and future generations?
2. What are the legal consequences under these obligations for States where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment, with respect to:
   1. States, including, in particular, small island developing States, which due to their geographical circumstances and level of development, are injured or specially affected by or are particularly vulnerable to the adverse effects of climate change?
   2. Peoples and individuals of the present and future generations affected by the adverse effects of climate change?

While the advisory opinion is not itself binding on states^[2] (in contrast to judgments rendered in contentious proceedings^[3]) it nonetheless carries legal significance. Advisory opinions are usually accepted by states as final and conclusive. Accordingly, the Court’s advisory opinion on climate change should be regarded as an authoritative statement of the relevant law, particularly given that the opinion was adopted unanimously.

The ICJ considered that in ascertaining the obligations of states in respect of climate change a wide range of international law was relevant, including the climate treaties (UNFCCC, Kyoto Protocol and Paris Agreement), UNCLOS, environmental treaties, customary international law, international human rights law and other principles of international law.^[4]

The ICJ did not accept the argument made by Australia and some other governments that the climate treaties constituted the ‘principal source of States’ obligations to protect the climate system’ and that other international treaties or customary rules should not be interpreted ‘as going beyond’ the climate treaties.^[5] The following discussion will focus on the Court’s treatment of the Paris Agreement and customary international law, but the advisory opinion is of much broader scope and now stands as the most wide ranging and influential summary of the international legal obligations of states to address the climate crisis.

In relation to the climate regime, the Court found that the parties to the Paris Agreement have agreed through a decision adopted at COP26, and subsequently reaffirmed, that the 1.5 °C threshold is the parties agreed primary temperature goal for limiting the global average temperature increase under the Paris Agreement.^[6] This is an important conclusion, not only for the interpretation of the Paris Agreement but also for understanding the operation of other obligations under international law to protect the climate system.

Under Article 4(2) of the Paris Agreement, parties must prepare, communicate and maintain successive NDCs that they intend to achieve and pursue domestic mitigation measures to achieve these contributions. The Court found that these are legally binding obligations. The Court considered that the discretion of parties in preparing NDCs is ‘limited’ and that parties must meet a stringent duty of due diligence to ensure that their NDCs meet their obligations under the Paris Agreement and, together, are capable of achieving the temperature goal.^[7] NDCs must become progressively more demanding over time and must reflect a state’s ‘highest possible ambition’. The Court interpreted the latter requirement to mean that a party’s NDC must be capable of making an adequate contribution to the achievement of the temperature goal.^[8] This means that the ambition in a party’s NDC must be referable to the overarching objective of the Paris Agreement under Article 2 to hold the increase in global average temperature to below 1.5 °C, which the Court interpreted to be the primary temperature goal.^[9]

In addition, Article 4(2) of the Paris Agreement ‘establish[es] an obligation of conduct’ such that ‘parties are required to act with due diligence in taking necessary measures to achieve the objectives set out in their successive NDCs’.^[10] The Court explained that what is required ‘is not a guarantee that communicated NDCs will be achieved, but rather that they will make best efforts to obtain such a result’.^[11] The Court found that parties must proactively pursue domestic mitigation measures that are ‘reasonably capable of achieving the NDCs set by them’ and explained that these may include ‘putting in place a national system, including legislation, administrative procedures and an enforcement mechanism, and exercising adequate vigilance to make such a system function effective’.^[12]

Turning from the climate treaties, the Court considered the relevance of the duty to prevent transboundary harm, a longstanding rule of customary international law. The Court found that this duty also applies to the climate system,^[13] concluding that the ‘most significant primary obligation’ for states in relation to climate change under customary international law is the obligation to prevent significant harm to the climate system and other parts of the environment.^[14] The Court noted that the duty arises only where there is risk of significant harm to the environment. Importantly, the Court accepted that the risk of significant harm may be present in situations where the harm results from the cumulative effects of various acts undertaken by states and private actors under their jurisdiction or control, even if it is difficult to identify a specific share of responsibility for any particular state.^[15]

The Court also observed that ‘States must assess the possible cumulative effects of their acts and the planned activities under their jurisdiction or control’^[16] and that while climate change is the consequence of all activities that contribute to GHG emissions it ‘does not mean that individual conduct leading to emissions cannot give rise to the obligation to prevent significant transboundary harm even if such activity is environmentally insignificant in isolation’.^[17]

Having found that climate change entailed a risk of significant harm, the Court then turned to consider the required standard of conduct. The Court reaffirmed that to comply with the obligation to prevent significant harm to the environment, states must act with due diligence. The Court identified several elements which it considered to be particularly relevant for determining what due diligence requires in the context of climate change. These elements include:

• implementing appropriate regulatory measures to prevent significant harm to the climate system;^[18]
• considering relevant scientific information (which may entail the standard of due diligence becoming more demanding over time);^[19]
• taking into account relevant international rules and standards (including certain COP decisions, such as those relating to the 1.5 °C temperature threshold);^[20]
• having regard to the differing capabilities and national circumstances of states (with developed states required to satisfy a more demanding standard of conduct);^[21]
• considering the precautionary approach or principle to guide the determination of the required standard of conduct;^[22] and
• the obligation to undertake certain procedural steps, including the requirement to undertake an environmental impact assessment (‘EIA’) (the Court ‘considers it important that all States provide for and conduct EIAs with respect to particularly significant proposed industrial activities contributing to GHG emissions’).^[23]

The Court specifically observed that the failure

to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies — may constitute an internationally wrongful act which is attributable to that State.^[24]

The Court also considered the potential consequences of a breach of the obligation to protect the climate system. The Court affirmed the accepted view that international legal obligations relating to the climate system as a global environmental commons establish obligations erga omnes (ie owed to all states) such that ‘responsibility for breaches of such obligations, such as climate change mitigation obligations, may be invoked by any State’.^[25] However, while this means that any state may have standing to invoke a breach of another state’s international legal obligations in relation to climate change, only states that establish that they have been injured may make a claim for reparation.^[26]

In order for an applicant state to obtain reparation, it would be necessary to establish that it suffered damage caused by a wrongful act of another state. The Court explained that the general legal standard for causation in international law (‘a sufficiently direct and certain causal nexus between the wrongful act … and the injury suffered by the Applicant’^[27]) can be applied to the climate context and involves two distinct elements: first, establishing whether a certain event or trend can be attributed to climate change; and second, whether damage caused by climate change can be attributed to a state or a group of states.^[28] The Court emphasised that establishing a causal link would need to occur in each specific case based on the evidence raised in those proceedings.^[29]