The ICJ’s landmark Climate Change Advisory Opinion

The court interpreted states’ obligations with reference to the “agreed primary temperature goal” of 1.5°C, rather than 2°C above pre-industrial levels,¹² citing the Glasgow Climate Pact of 2021 and the outcome of the First Global Stocktake of 2023 to “pursue efforts” and limit global warming to 1.5°C as "subsequent agreements" within the meaning of Article 31(3)(a) of the Vienna Convention on the Law of Treaties (VCLT).¹³  The court interpreted the 1.5°C target not as an aspirational goal, but a legal benchmark to assess the content of other obligations contained in the Paris Agreement,¹⁴ as well as those of custom.

In doing so, the court placed the temperature goal at the heart of the Paris Agreement, referring to it as the “context” relevant for the interpretation of other obligations.¹⁵  The court also stressed that the temperature goal provided “a means for achieving [the] object and purpose of the Paris Agreement”, namely “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system”.¹⁶

The court opined that a state’s Nationally Determined Contribution (NDC) must be capable of making an adequate contribution to the achievement of the temperature goal, and there is limited discretion afforded to states in preparing NDCs.¹⁷  In turn, states also have stringent due diligence obligations (best efforts) to take domestic measures to achieve the objectives of their NDCs.¹⁸  Other than the preparation and submission of NDCs, states’ obligations in respect of implementing measures to achieve their NDC commitments are obligations of conduct. The court clarified that this means that “what is required of parties under Article 4(2) is not a guarantee that communicated NDCs will be achieved, but rather that they will make best efforts to obtain such a result”.¹⁹

Adaptation measures are a core objective of the Paris Agreement. The court acknowledged that states parties have specific obligations in relation to adaptation under Article 7 of the Paris Agreement, the fulfilment of which is to be assessed against a due diligence standard. It is therefore incumbent upon states to enact appropriate measures (eg implementing national adaptation plans) that are capable of “enhancing adaptive capacity, strengthening resilience and reducing vulnerability to climate change”.²⁰  The court stressed that adaptation obligations are inherently linked to states’ mitigation obligations, whereby “greater levels of mitigation can reduce the need for additional adaptation efforts”.²¹

In addition to the temperature goal, Article 2 of the Paris Agreement makes clear that financing flows are critical to strengthening the global response to climate change. The court concluded that developed states have clear legally binding obligations to provide financial resources to developing states for both mitigation and adaptation.²²  However, the court observed that the Paris Agreement “does not specify the amount or level of financial support that must be provided”. Parties must implement their financial assistance obligations “in a manner and at a level that allows for the achievement of the objectives listed in Article 2”.²³  The court, however, did not provide further clarification as to the level of financial commitment required to meet such obligation.  

Developed states’ obligations to engage in transfer of technology and capacity-building with developing states were also acknowledged, however, the court did not elaborate on the content of those obligations.²⁴  The court’s remarks on financial assistance and technology transfer were significantly shorter than its conclusions on states’ mitigation obligations.

CBDR-RC is an interpretative principle for obligations within the Regime which “reflects the need to distribute equitably the burdens of the obligations in respect of climate change, taking into account, inter alia, states’ historical and current contributions to cumulative GHG emissions, and their different current capabilities and national circumstances, including their economic and social development”.²⁵  The court clarified that CBDR-RC “guides the interpretation of obligations under international environmental law beyond its express articulation in different treaties” but “does not establish new obligations”.²⁶  The court concluded the principle of CBDR-RC therefore informs the standard of due diligence applicable to states.²⁷

Importantly, when considering the references to CBDR-RC in the Climate Change Regime, it was acknowledged that “the status of a state as developed or developing is not static”,²⁸ meaning a state’s climate obligations may increase over time, in line with its level of development.

In relation to the duty to prevent significant harm, the court confirmed that this obligation is applicable to the climate system, and addressed it in two parts:

1. Risk of harm: Whether an activity constitutes a significant risk depends on the foreseeability of the occurrence of harm and the severity of the harm, taking into account the risks an activity might pose in the future.²⁹  States must also assess the possible cumulative effects of their activities.³⁰  Notably, individual conduct producing emissions can give rise to the obligation to prevent significant harm, even if the conduct is itself environmentally insignificant in isolation,³¹ including private actors subject to a state’s jurisdiction or control.³²
2. Due diligence standard: The content of due diligence is variable subject to the national circumstances of the state, which may evolve over time.³³ Generally, the standard requires states to use all means at their disposal to avoid activities which cause significant harm which may include the adoption of regulations for “deep, rapid and sustained reductions of GHG emissions”.³⁴  States are required to take appropriate measures to prevent significant harm where realisable scientific evidence of a risk of significant harm exists,³⁵ and should not refrain from or delay taking actions of prevention in the face of scientific uncertainty.³⁶

The court found that the customary legal obligation,³⁷ to cooperate, in good faith,³⁸ for the protection of the environment that “reinforces the treaty based co-operation obligations under the Paris Agreement”.³⁹  Whilst the duty does afford states some discretion to determine their means for regulating GHG emissions, it does not serve as an excuse for states to (i) refrain from cooperating with the required level of due diligence or (ii) present their efforts as entirely voluntary and exempt from scrutiny.⁴⁰

In terms of the relationship between states’ treaty obligations and customary international law the court acknowledged that “as it is difficult to determine in the abstract the extent to which the climate change treaties and their implementation practice influence the proper understanding of the relevant customary obligations and their application, the court considers that, at the present stage, compliance in full and in good faith by a state with the climate change treaties, as interpreted by the court (see paragraphs 174-270 above), suggests that this state substantially complies with the general customary duties to prevent significant environmental harm and to co-operate”.⁴¹

The court stressed that customary obligations are the same for all states, and “exist independently regardless of whether a state is a party to the climate change treaties”.⁴²  It is therefore possible that a non-party state, which cooperates with the states parties to the Climate Change Regime in a way that is equivalent to a state party, “may in certain instances, be considered to fulfil its customary obligations through practice that comports with the required conduct of states under the climate change treaties”.⁴³

The court affirmed the findings in the International Tribunal for the Law of the Sea’s (ITLOS) Advisory Opinion on Climate Change,⁴⁵ that anthropogenic GHG emissions constitute “substances or energy” introduced by humans, either “directly or indirectly” “into the marine environment”.⁴⁶

It was confirmed that Article 192 of UNCLOS includes a positive obligation to take measures to protect and preserve the marine environment,⁴⁷ and Article 194(1) requires states to take all necessary measures to reduce and control pollution.⁴⁸  Such measures must be adopted in accordance with the obligations incumbent upon states under the UNFCCC and the Paris Agreement, in so far as the states concerned are parties to those instruments.⁴⁹  Like ITLOS, the court found that the applicable due diligence standard for those obligations is "stringent",⁵⁰ however, as did ITLOS, the court acknowledged states “are not required to ensure an immediate cessation of marine pollution caused by anthropogenic GHG emissions”.⁵¹

You can read our summary of the findings in the ITLOS Advisory Opinion here, and the resultant implications of that Opinion are analysed here.

The court confirmed that the provisions of UNCLOS do not require states parties, in the context of physical changes resulting from climate-change related sea level rise, to update their charts or lists of geographical co-ordinates that show the baselines and outer limit lines of their maritime zones once they have been duly established in conformity with UNCLOS.⁵²

Further, the court opined that once a state is established, “the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood”.⁵³  The court also confirmed that states have a customary obligation to cooperate in the face of sea level rise, and that such a duty is founded on the recognition of the interdependence of states.  As such, cooperation in the context of sea level rise is not a choice, but a legal obligation.⁵⁴

To outline the legal consequences, the court relied on the International Law Commission’s Articles on state responsibility as reflective “in many respects” of customary international law.⁵⁸  In light of the significant divide on this point amongst participants in the proceeding, the court observed that it could not find any “clearly expressed lex specialis” that would exclude the application of the general rules and customary international law on state responsibility.⁵⁹

At the outset, the court made clear that its findings would not be on the responsibility of individual states, noting that this assessment must turn on the facts of a specific case.⁶⁰  Generally, however, breach of an obligation identified in response to question (a) would constitute an internationally wrongful act and attract the responsibility of that state.  Importantly, the court made clear that “the internationally wrongful act in question is not the emission of GHGs per se” but the breach of the obligations discussed above in Part 3 of this article, causing significant harm resulting from anthropogenic emissions of such gases.⁶¹  It follows that breaches of obligations in question (a) may give rise to cessation and non-repetition, full reparation, including restitution where possible, compensation and/or satisfaction.⁶²

Importantly, the court also confirmed that certain obligations of states pertaining to the protection of the climate system, including the obligation to prevent significant transboundary harm under customary international law, are obligations erga omnes.⁶³  This means that the obligations of states are obligations protecting states’ common interests and are the concern of all states for which responsibility for breaches may be invoked by any state.  However, while a non-injured state may pursue a claim against another state in breach of an erga omnes obligation, it may not claim reparation for itself.⁶⁴

The court noted that under customary international law “the most significant primary obligation for states in relation to climate change is the obligation to prevent significant harm to the climate system and other parts of the environment … which applies to all states, including those that are not parties to one or more of the climate change treaties”.⁶⁵  Here, the court determined that a state only incurs liability if it fails to take all measures which were within its power to prevent the significant harm, and that this must be assessed on a case-by-case basis.⁶⁶

The court confirmed that, despite the cumulative impact of GHG emissions on climate change, it is scientifically possible to determine the contribution of each state on a historical and current basis,⁶⁷ and that the rules on state responsibility are capable of addressing a plurality of injured or responsible states.⁶⁸  However, any exercise of attribution would need to be undertaken on the facts of the particular case. While multiple states have contributed to climate change, increasing the difficulty of “whether and to what extent an individual state’s breach of an obligation identified in question (a) has caused significant harm to the climate system”, the court considered that the rules on state responsibility are capable of addressing such a situation.⁶⁹

Causation of damage is not a requirement for the determination of state responsibility, but causation is relevant in determining reparation.⁷⁰  The court’s existing standard for determining causation, which requires the existence of “a sufficiently direct and certain causal nexus” between the wrongful act and the alleged damage, is applicable to wrongful acts and damage related to the obligations set out at question (a).⁷¹

The court outlined that in the context of climate change, it will need to be considered, first, whether a given climatic event or trend can be attributed to anthropogenic climate change, and second, to what extent the damage can be attributed to a particular state or group of states.⁷²  The court considered that the causal link between the wrongful act or omission and the harm might be “more tenuous” compared to local pollution, but that “this does not mean that the identification of a causal link is impossible in the climate change context”.⁷³  The court stressed that the “causal link must be established in each case through an in concreto assessment while taking into account the aforementioned elements outlined by the court”.⁷⁴

States are due to submit their third NDCs in advance of COP30 in November 2025. The court clearly acknowledged that states have limited discretion on the determination of their NDCs as it “must be capable of making an adequate contribution to the achievement of the temperature goal” and represent a state’s "highest possible ambition" in the context of the temperature goal. Some states will be looking at their own NDCs (and others’) with far greater scrutiny to determine compliance and whether the targets set are sufficiently ambitious to achieve the 1.5°C benchmark. 

Following the submission of NDCs ahead of COP30, the court’s Opinion is likely to have significant influence on the COP30 agenda. Climate vulnerable states are likely to refer to, and rely on, the court’s findings regarding mitigation and adaptation initiatives, including the legally binding obligation for developed states to provide financial assistance, to drive forward negotiations. This may result in the Loss & Damage Fund moving from a voluntary scheme to instead require mandatory contributions from developed states, in line with the court’s findings under both the Climate Change Regime and customary international law regarding all states’ obligations to cooperate. While the court recognised the binding nature of developed states’ obligations to provide financial assistance to developing states, no clarification was provided as to what that might look like. The COP’s implementation of this obligation in practice would therefore provide important clarification to implementing the financial assistance obligation. The reluctance to provide further clarification on this obligation may suggest the court was unwilling to interfere with the ongoing negotiations on the New Collective Quantified Goal which aims to set a new global climate finance target for developing countries post-2025.

The importance of the forthcoming decisions at COP30 cannot be understated.  The court acknowledged the important legal effects that COP decisions can have. COP decisions can, in certain situations, create legally binding obligations for states parties to the Climate Change Regime (in so far as they reflect state practice and if they express an opinio juris (sense of legal obligation) of states) and can also constitute subsequent agreements under Article 31(3)(a) of the VCLT, which can be used to show parties’ agreement regarding the interpretation of a relevant treaty.⁷⁶ For example, COP decisions helped to inform the court’s assessment that the states parties to the Climate Change Regime had subsequently agreed that 1.5°C, not 2°C, was the agreed primary temperature goal, despite the express language contained in the UNFCCC.⁷⁷  As the first COP following the release of all three climate change advisory opinions, the COP30 decisions may offer an important opportunity for states to provide further clarification on the interpretation of obligations under the Climate Change Regime. However, states may be equally cautious to agree certain language in light of this finding.

At the outset of assessing state responsibility, the court made clear that “the internationally wrongful act in question is not the emission of GHGs per se” but the breach of the obligations discussed above in Part 3 of this article, causing significant harm resulting from anthropogenic emissions of such gases.⁷⁸  Therefore, while the court’s findings do not prohibit the approval and development of new projects or expansions to existing projects that are likely to generate GHG emissions, it may be that the Opinion will be used to ensure that such projects are subject to more stringent approvals, including environmental approvals under domestic legislation.

For example, the court acknowledged the “failure of a state 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”.⁷⁹  The court also acknowledged that states’ evaluation of such activities does not exclude that “possible specific climate-related effects must be assessed as part of Environmental Impact Assessments at the level of proposed individual activities, eg for the purpose of assessing their possible downstream effects”.⁸⁰

Projects and project expansions that are not aligned with the realisation of a state’s NDC and contribute to cumulative impacts may face greater hurdles. The court also made clear that in relation to the conduct of private actors, “a state may be responsible where, for example, it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction”.⁸¹  A state’s failure to exercise regulatory due diligence, including for emitting projects, is therefore capable of attributing private sector actions to the state and may result in an internationally wrongful act. 

The court’s findings may push some states to implement greater national regulation that calls for project proponents to complete more stringent environmental impact assessments or adhere to an emissions budget/allocation for a particular project or company. These regulatory initiatives may cause project delays or may cause companies to re-evaluate whether the project’s expected returns support the investment and time commitment to clear these regulatory hurdles. This may, in turn, slow the pipeline of projects for the private sector, and provide a greater push for only "green" projects.

Emitting states, in particular perhaps high-emitting states, or corporations may see increased litigious activity in both international and domestic forums. The Advisory Opinion now completes an extensive body of international jurisprudence on states’ obligations with regards to climate change, as articulated in the ICJ, ITLOS, and the Inter-American Court of Human Rights (IACtHR), particularly regarding obligations sourced in the Climate Change Regime, customary international law, and human rights. Armed with these findings, states (and private actors) may now feel they have sufficient clarity from these bodies to explore contentious test cases, giving potential actions a more solid legal footing.

It is likely climate cases in response to these opinions will start to appear in international, regional, and domestic fora. The deadline for submission of states third round of NDCs could present the starting point for such cases to be benchmarked against the obligations clarified in these opinions.

State-to-state litigation

The above said, the widening of avenues for state-to-state climate litigation does not come without challenges. Several factors may limit the availability, or challenge the success, of future claims seeking to rely on the ICJ Advisory Opinion.

First, putting aside the question of whether interstate litigation is politically astute, the availability of forums with jurisdiction over interstate disputes concerning climate change must be considered. The dispute settlement provisions of the UNFCCC and the Paris Agreement limit the forums in which such state-to-state cases can be commenced. States have chosen conciliation as the default mechanism to address climate change disputes arising under Article 14 of the UNFCCC and Article 24 of the Paris Agreement. That said, the mechanism for such conciliation has yet to be established.  Only the Netherlands has accepted the jurisdiction of the ICJ under these agreements. Cuba has accepted negotiation. Solomon Islands and Tuvalu have accepted binding arbitration.⁸²  Further, only 74 states have accepted the ICJ’s "optional jurisdiction" which allows states to recognise the ICJ’s jurisdiction as compulsory in certain legal disputes with other states that have also accepted this clause. In some cases, states have placed significant conditions on the acceptance of that jurisdiction.⁸³  Therefore, as it stands, the availability of the ICJ’s jurisdiction under the Climate Change Treaties and the "optional clause" is ultimately limited for inter-state cases.⁸⁴

Similarly, as discussed in our article on implications arising from the ITLOS Advisory Opinion, the Tribunal’s recognition that anthropogenic GHG emissions are "pollution of the marine environment" means that UNCLOS now offers another avenue for states to seek redress against other states for the adverse effects of climate change, including through the use of ITLOS as a relevant forum.  States may also make modifications to their declaration under Article 287 of UNCLOS as to their preferred dispute settlement forums. Under UNCLOS, the default forum is arbitration (a forum where ad hoc arbitrators, some of which are chosen by the parties themselves, may promote a more predictable outcome). However, some states have elected the ICJ and/or ITLOS as their preferred dispute settlement forum under UNCLOS. Proceedings under UNCLOS may therefore provide an alternative, and in some cases easier, avenue to access the ICJ.

Second, states may withdraw or amend their declaration to the ICJ Statute accepting the court’s compulsory jurisdiction to exclude climate change related cases. We will closely follow whether high-emitting states in particular take such steps to minimise the potential impact of a claim through forum shopping. This however remains to be seen. 

Third, while the court confirmed the application of the general rules on state responsibility, including for breaches of obligations under the Climate Change Regime,⁸⁵ the practical realities of implementing those rules may prove challenging for claimant states. The court’s assertion that causation “may be addressed by recourse to science” does not go so far as to assist claimant states in identifying other factual or legal principles that may be relevant to apportioning causal responsibility amongst states. Similarly, the court considered the remedy of restitution (which involves the re-establishment of the situation that existed before the wrongful act was committed) “may prove difficult or unfeasible in the case of environmental harm, since such harm is often not easily reversible”,⁸⁶ other than in instances of ecosystem restoration. That said, climate vulnerable states could bring actions with the aim of achieving restitution for ecosystems damaged by emissions, many of which (coral reefs, mangroves) are essential to adaptation and the guarantee of human rights in those countries. The court observed that “causation of damage is not a requirement for the determination of responsibility as such”,⁸⁷ and where reparation implies the existence of damage “causation must be established between the wrongful act of a state — or group of states — and particular damage suffered by the injured state”.⁸⁸  The court’s observations suggest it may be easier to contend that a state has failed to meet its climate obligations, rather than prove “a sufficiently direct and certain causal nexus” resulting in damage.

Domestic litigation against governments and corporations

The ICJ Advisory Opinion, along with the ITLOS and IACtHR opinions, are also likely to provide further impetus to individuals or community groups to pursue claims against their governments for a failure to act in relation to emissions reductions and impacts on the climate, for decisions to approve emissions intensive projects, or for failure to implement appropriate adaptation measures. Similar claims could be pursued against corporations that fail to align their operations with a state’s NDC. The effect of the Opinion in domestic legal systems may vary, but it is likely to be cited as informative of states’ international law obligations concerning climate change. 

In this regard, closer to home, the developments from any appeal to decision of Justice Wigney in Pabai v Commonwealth of Australia (No 2) would likely provide an interesting case study for the impact on the Australian common law regarding duties of care.⁸⁹  Justice Wigney’s judgment, concluded that the Commonwealth Government of Australia did not owe a duty of care to the Torres Strait Islanders (or in fact any citizen), concerning the setting of emissions reduction targets,⁹⁰ nor alternative duties of care to protect against marine inundation and erosion.⁹¹  This judgment may be appealed to the Full Federal Court, and it is not unreasonable to expect the Advisory Opinion to feature in a potential appeal, particularly in relation to Justice Wigney’s finding that Australia’s emissions were relatively minor on the global scale, which would have independently been insufficient to cause the relevant harm.

States have already started to implement policies and regulations that seek to limit and reduce greenhouse gas emissions. However, many states have obligations under bilateral or multilateral investment treaties which offer foreign investors certain rights and protections, including protection against expropriation without fair compensation. The court made clear findings that states must adopt a national system of legislation, administrative procedures, and enforcement mechanisms to regulate activities that are likely to generate adverse effects in relation to climate change for both public and private sector actors. This in turn may create tension with states’ commitments to foreign investors under such treaties.

The court expressly acknowledged its decision to focus on the obligations that were "most directly relevant" when assessing the corpus of applicable international law. In doing so, the court did not address the relationship between states’ climate change obligations and international investment law.⁹²  Therefore, in the absence of the court’s clarification on the interaction between the two areas, a state’s adoption of more robust climate legislation may increase the risk of potential claims from foreign investors under bilateral and multilateral investment treaties for expropriation of an investment or violation of fair and equitable treatment.  This is important in the context of “first generation” and “new generation” investment treaties. While “new generation” investment treaties contain provisions that recognise the right to regulate and pursue climate action, including obligations under international agreements, “first generation” treaties do not.  The court may have empowered states to take greater regulatory action in furtherance of its climate change obligations, but the jury is still out on how such ambition may be assessed against existing investment treaty commitments and whether a state could rely on the court‘s findings as a “defence” to such claim.

The court’s findings may, as a result, also influence the drafting of new investment treaties. States may seek to rely on the court’s findings to advocate for the inclusion of a "carve-out" in new investment treaties.⁹³  The carve-out would prevent an investor from commencing a claim under the investment treaty against a state for measures related to the reduction of greenhouse gas emissions or compliance with a states’ international climate change obligations. Alternatively, states may choose to exclude or create special rules for emitting industries.

The court acknowledged that “while states are not required to conclude treaties, they are required to make good faith efforts to arrive at appropriate forms of collective action”.⁹⁴  However, it may be that states consider the negotiation of new climate agreements, with obligations that go beyond those articulated in the current Climate Change Regime, as an important form of collective action.  

The UNFCCC, for example, is a framework agreement, with the Paris Agreement having been negotiated under its umbrella. The nature of such treaties suggests that the agreement marks the start of cooperation, with further arrangements contemplated to be arrived at under the treaty. The court’s clarification of states climate change obligations may help strengthen climate action in ensuring that the agreements negotiated are purpose-built to respond to the unfolding and evolving complexities of regulating GHG emissions. States may also put forward a resolution that goes beyond the court’s findings, calling for greater commitments such as the negotiation of new treaties, or increased cooperation on finance, adaptation, and technology transfers, evaluated against concrete benchmarks.