International Court of Justice issues landmark opinion on state obligations in respect of climate change

The International Court of Justice's (ICJ) advisory opinion on the obligations of states in respect of climate change issued on 23 July marks a pivotal moment in the governance of and accountability for greenhouse gas (GHG) emissions. It concluded that states have obligations under international law to prevent significant harm to the environment, combat climate change and protect human rights and that a failure to act may constitute an “internationally wrongful act” entailing responsibility under international law.

Although the opinion is not directly enforceable, it still holds significant legal weight and is expected to have far-reaching implications not only for states and international organisations but also for the global community as a whole. It could prompt more robust climate-related policies and regulation and serve as an inspiration for a fresh wave of international climate litigation, including inter-state actions as well as actions brought by individuals, groups and non-governmental organisations challenging state inaction.

Organisations across the public and private sectors may consider themselves to have an increased exposure to climate-related litigation risks, as the opinion is likely to be used as a reference in enforcing corporate due diligence and responsibility. It will foreseeably be relied upon in arguments around the interpretation of rights and obligations under international agreements such as investment protection treaties.

Who requested the opinion and why?

The United Nations General Assembly made the request for the advisory opinion in March 2023 to obtain clarification on states' legal obligations and their responsibility for the consequences for climate-related harms. The request was rooted in a campaign led by students from the University of the South Pacific in Fiji and an international network of activists whose daily lives were affected by climate change.

The request was for the ICJ to opine on two questions.

• 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 GHGs for states and for present and future generations?
• 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 states which are affected or particularly vulnerable to the adverse effects of climate change; and people and individuals of the present and future generations affected by the adverse effects of climate change. 

The ICJ heard submissions from over 90 states and organisations at the hearing in December 2024, the highest level of participation in any proceeding before the court, following which it adopted the opinion unanimously, only the fifth time that it has done so in its nearly 80-year history.

Scientific background

The ICJ's opinion was based upon its assessment of the relevant scientific background, in which it relied above all upon the reports of the Intergovernmental Panel on Climate Change as the best available science on the causes, nature and consequences of climate change. 

It noted, in particular, that it is scientifically established that the climate system has undergone widespread and rapid changes, including an increase in global surface temperatures. 

The opinion stated that climate change is caused by the accumulation of GHGs, leading to a global warming effect, primarily due to human activities, including the burning of fossil fuels or the weakening or destruction of carbon reservoirs and sinks. 

It also declared that the consequences of climate change are severe and far reaching, affecting natural ecosystems and human populations – and the threat posed by climate change is urgent and existential.  

Principal findings 

The ICJ’s stated aim in giving the opinion is to establish general principles to guide decisions on state responsibility by the ICJ itself and other international tribunals, with regard to the following international frameworks which it considers to be the directly relevant applicable laws on climate change and human rights:

• The principal climate change treaties, namely the UN Framework Convention on Climate Change, the Kyoto Protocol of 1997 and the Paris Agreement 2015 which regulate the international response to climate change.
• The United Nations Convention on the Law of the Sea and other environmental treaties including the Biodiversity Convention and the Desertification Convention.
• The International Covenant on Economic, Social and Cultural Rights 1966, the International Covenant on Civil and Political Rights and the human rights recognised under customary international law.

The ICJ also addressed the applicability and relevance of customary international law, concluding that the following guiding principles should support the interpretation and application of the most directly relevant laws:

• The duty to prevent significant harm to the environment and the duty to cooperate for the protection of the environment.
• Principles of sustainable development, common but differentiated responsibilities and respective capabilities, equity, intergenerational equity and the precautionary approach or principle.

Climate change treaty obligations

The ICJ concluded that the principal climate change treaties impose legally binding obligations on states to prevent climate harms, to adopt mitigation and adaption measures to reduce greenhouse gases and to cooperate in good faith, including by sharing information, financial resources and technology. Developed countries are also obliged to take the lead in combatting climate change.

On the Paris Agreement, the ICJ underlined the 1.5°C temperature limit to be the parties' agreed primary temperature goal for limiting global warming. Parties must ensure that they make an adequate contribution to achieving this temperature goal and that these measures must be outlined in their nationally determined contributions (NDCs) – the climate action plans prepared by parties to the Paris Agreement. Parties should do their upmost to ensure that their NDCs reflect their highest ambition to achieve the goals of the Paris Agreement.

Obligations under customary international law

Duty to prevent significant harm to the environment

The ICJ considered that a customary duty of states to prevent significant harm to the climate system and other parts of the environment arises as a result of from the general risk of significant harm to which states contribute through activities undertaken within their jurisdiction or control. 

It considers the duty of prevention to be the most important primary obligation. This is because states are under a customary law obligation to act with due diligence to prevent significant harm to the environment, not just in cases of “cross-border harm” but also in cases of “transboundary harm" –that is, environmental damage that originates in one state but causes significant adverse effects in another.  

The obligation to prevent significant harm comprises several elements but includes states taking precautionary measures which account for scientific and technological information, as well as relevant rules and international standards; and undertaking risk assessments and environmental impact assessments.

The standard of due diligence for preventing significant harm to the climate system was described as a “stringent” standard that requires “a heightened degree of vigilance and prevention”.

Determining what is required by way of due diligence requires an assessment in concreto of what is reasonable for a state under its specific circumstances, while also identifying a required standard of conduct based on the environmental risk.

Due diligence as the required standard of conduct

The ICJ considers that:

• States must act with due diligence to prevent harm to the environment, both domestically and within other states. Such measures include regulations to achieve significant and sustained reductions of GHG emissions and robust enforcement and monitoring mechanisms to ensure their implementation.
• Due diligence should account for current standards contained in climate treaties and customary international law, and other sources such as decisions of the Conference of the Parties.
• States should actively pursue scientific and technological information as well as knowledge-sharing co-initiatives to assess the probability and seriousness of possible climate harms. This information provides evidence to assess these harms and inform the highest standard of due diligence, which may become more demanding with new scientific or technical knowledge. However, a state's failure to take preventative measures may not indicate a lack of due diligence if it lacks the resources and capacity to access and act on scientific information.
• The standard required in each case will be specific to each state's respective circumstances. Developed states will be expected to take more demanding measures to prevent environmental harm and satisfy a more demanding standard of conduct. That said, the fact that a state may have fewer resources and capabilities cannot be used to justify undue delay or any general exemption from such obligations. Scientific uncertainties should also not delay states from taking preventative measures.

Duty to cooperate for the protection of the environment

The ICJ considers the duty to cooperate for the protection of the environment forms part of customary international law. While it recognises that this duty gives states some discretion around regulating their GHG emissions, it does not excuse them from co-operating with the requisite level of due diligence or absolve their efforts from any scrutiny.

Customary law and specific treaty obligations

Fulfilling treaty obligations alone does not mean that states have also fulfilled their customary law obligations. Treaties and customary international law inform each other – they establish independent obligations that do not necessarily overlap. Therefore, a state may not evade customary law obligations by exiting certain treaties. 

Customary obligations are the same for all states and exist independently regardless of whether a state is a party to the climate change treaties. A non-party state that cooperates with the community of states that are parties to the three climate-change treaties may similarly be seen to be fulfilling its customary obligations through such practice. However, if it does not cooperate in this manner, it must demonstrate that its policies and practices conform to its customary obligations.

Obligations under International human rights law

The ICJ also addressed the connection between international human rights law and climate change. It views the right to a clean, healthy and sustainable environment as a human right, concluding that the adverse effects of climate change may impair enjoyment of human rights.

Further, it noted that under international law, the human right to a clean, healthy and sustainable environment is essential for the enjoyment of other human rights, such as the right to health and an adequate standard of living.

'Legal consequences'

The latter half of the opinion addresses the second question, namely the "legal consequences" arising from states' acts and omissions that cause significant harm to the climate system and other parts of the environment.

Significantly, the ICJ concluded that, in general, such legal consequences are identified and addressed through the application of the secondary rules of international law concerning the responsibility of states for internationally wrongful acts..

In accordance with these well-established rules, the possible legal consequences for such breaches include:

• Cessation of the wrongful acts or omissions, if they are continuing.
• Providing assurances and guarantees not to repeat the wrongful actions or omissions, if required.
• Giving full reparation to the injured states in the form of restitution, compensation and satisfaction, provided that the general conditions of the law of state responsibility are met, including a sufficiently direct and causal nexus between the wrongful act and injury.

Recognising the erga omnes nature of climate change obligations – that is, that they are the concern of all states, and the potential for multiple states to contribute to climate change harms, albeit to significantly different degrees – the ICJ sought to address head on the thorny issues of legal attribution and causation.

Attribution

The ICJ considered that, in the context of climate change, it is appropriate to apply the well-established rule that the conduct of any organ of a state must be considered acts of that state. For example, a state's failure to protect the climate from GHG emissions may be deemed an internationally wrongful act attributable to that state, whether the act causes harm or not. Accordingly, each injured state may separately invoke the responsibility of every state whose failure to act has resulted in climate-related damage. Further, where several states are responsible for the same act, the responsibility of each state may be invoked in relation to that act.

Whether or not individuals are entitled to hold a state legally accountable or make a claim in relation to matters involving injury or harm arising from climate change is dependent on the relevant primary obligations of states. Factual questions as to attribution and apportionment of responsibility are to be resolved on a case-by-case basis.

Causation

The ICJ considered that determining causation in the context of climate change harm involves two distinct elements: whether a climatic event or trend can be attributed to anthropogenic climate change, and to what extent damage caused by climate change can be attributed to a state or group of states. The former element is addressed by recourse to science, and the latter by considering the facts and evidence available to the court. 

The ICJ acknowledged that, while the causal link between a state's wrongful actions or omissions and climate-related harms is more tenuous than in the case of local sources of pollution, it does not follow that the identification of a causal link is impossible. A causal link must be established in each case through an in concreto assessment.

Business horizon

• Expect more robust regulation. With the opinion offering clarity on states’ legal obligations, governments will be under increasing pressure to take action against emissions generated by the private sector.
• Focus on renewable energy investment. The establishment of legal obligations on states to proactively mitigate against the risk of climate change reduces the investment risk for major solar, wind and storage projects – indeed, underscoring the case for renewable energy investment and green technologies as well as the adoption of sustainable business practices.
• Increased litigation and compliance risk. The opinion builds upon a growing body of both national and international case law, such as the Netherlands judgment last year against a multinational energy group and the KlimaSeniorinnen ruling by the European Court of Human Rights, in which states have been challenged for failing to take adequate action on climate change and climate commitments. The trajectory of climate litigation is therefore likely to accelerate in light of the opinion, with courts increasingly being called upon to rule on state and corporate responsibility.
• Persuasive legal authority. Although the opinion addresses the obligations of states only, it is likely to be relied upon by claimants as a persuasive legal authority in private litigation and enforcement actions. It will almost inevitably impact standards of appropriate corporate governance and compliance. The opinion may also foreseeably affect the interpretation of rights and obligations under international agreements, such as investment protection treaties and intergovernmental agreements.

Osborne Clarke comment

For the first time, the ICJ firmly establishes that states' climate obligations are no longer political declarations or targets which can be cited merely as guidance – they are duties whose non-observance may entail responsibility as internationally wrongful acts under the secondary rules of international law.

With climate inaction risking legal consequences, the opinion is likely to act as a catalyst for more robust climate governance. Policymakers globally will now be expected to embed climate-related targets, including the 1.5°C temperature threshold, into enforceable domestic frameworks that align with the principal climate treaties, or risk accountability.

Looking ahead, states will be expected to exercise stringent due diligence when preparing their 2025 NDCs (due September 2025) to ensure that they are aligned with the principal climate treaties and in particular reflect their “highest possible ambition” to achieve the goals of the Paris Agreement.