The ICJ Advisory Opinion: a new era for climate change accountability

On Wednesday 23rd July, the International Court of Justice (the “ICJ”) handed down its long-awaited Advisory Opinion on the Obligations of States in respect of Climate Change (the “Opinion”). The Opinion provides authoritative clarification of states’ climate obligations and establishes clear legal consequences for breaches - strengthening the foundation for climate accountability litigation. We summarise below the Court’s key observations and reflect on their impact on climate litigation.

Background

The Opinion was requested by the United Nations General Assembly following a campaign initiated by law students at the University of the South Pacific, supported by over 130 states.

The ICJ was asked to address two key questions. First, what are states’ legal obligations under international law to ensure the protection of the climate? Second, what are the legal consequences when states cause climate harm by their acts or omissions, particularly to vulnerable states and affected present and future generations?

The ICJ received over a hundred written and oral submissions, reflecting an unprecedented global engagement – but also differing views – over the content and reach of states’ obligations in the climate crisis.

The ICJ Opinion follows two earlier Advisory Opinions by the Inter-American Court of Human Rights (June 2025) and the International Tribunal for the Law of the Sea (May 2024), which focused respectively on states’ obligations within the framework of the American Convention on Human Rights and United Nations Convention on the Law of the Sea.

Legal framework governing states’ obligations

The Court confirmed that the United Nations Framework Convention on Climate Change (the “UNFCCC”), the Kyoto Protocol, and the Paris Agreement are the key international legal instruments governing the global community’s response to climate change. They are “mutually supportive” and can apply concurrently.

For the first time, the Court confirmed that climate treaties do not displace, and should work in tandem with, existing international law principles. This means the customary duty to prevent significant harm to the environment also applies with respect to the climate system. States are required to adopt appropriate rules and measures to combat climate change, and to exercise a heightened degree of vigilance in their enforcement and the exercise of administrative control. The role of customary international law – which reflects the general international practice of states, accepted as law – is of particular importance as it applies regardless of whether a given state has made political commitments under the climate treaties.

Building on earlier jurisprudence – including the ECtHR’s judgment in Verein KlimaSeniorinnen Schweiz and Others v Switzerland (on which our team commented here) – the Court affirmed the interdependence of the protection of human rights and the protection of the environment, confirming international human rights law as forming part of the “most directly relevant applicable law” in this context.

Substantive legal obligations of states

The Court made several important observations about states’ obligations under the climate treaties, customary international law, and human rights law:

• It recognised the findings of the Intergovernmental Panel on Climate Change (IPCC) as the best available science and accepted that “anthropogenic emissions of greenhouses gases are unequivocally the dominant cause of the global warming observed since the mid-20th century”. States have binding legal obligations to protect the environment from anthropogenic GHGs.
• Under the Paris Agreement, states have a legal obligation to maintain their progressive nationally determined contributions, which must be capable of achieving the temperature goal of limiting global warming to 1.5°C above pre-industrial levels. 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.
• States must act with due diligence and take appropriate rules and measures, including regulatory mitigation and adaptation mechanisms “designed to achieve the deep, rapid, and sustained reductions of GHG emissions that are necessary for the prevention of significant harm to the climate system” and which “reduce the risk of significant harm occurring”. Such mechanisms are relevant for assessing whether a state is fulfilling its customary obligations with due diligence.
• Climate change may impact the enjoyment of the right to life. In that regard, certain conditions resulting from climate change are “likely to endanger the lives of individuals” and may lead them to seek safety in another country or prevent them from returning to their own. States are legally obliged to respect the principle of non-refoulement where there are substantial grounds for believing that there is a real risk of irreparable harm to the right to life. This is of particular importance to those who are from low lying island states which are some of the most vulnerable to sea level rise and the effects of climate change.
• Climate change can further affect the right to health, adequate standard of living (including the rights to food, water, and housing), the right to privacy, and the rights of children, women, and indigenous peoples.

Legal consequences for states for causing harm

The Court confirmed that a breach by a state of its legal obligations constitutes an internationally wrongful act and will invoke the international responsibility of that state. It may be required to cease the wrongful act or omission, provide assurances and guarantees of non-repetition of wrongful actions or omissions, and give full reparation to injured states in the form of restitution, compensation, and satisfaction.

On causation, the Court recognised that establishing causal links in climate cases is complex, but emphasised that it is “not impossible” to establish. This provides crucial judicial validation for the scientific methodologies underpinning climate litigation causation arguments. Significantly, the Court noted that individual state contributions can be determined through analysis of their emissions.

Each state injured by the breach of another state’s international obligations may separately invoke the responsibility of every state that has caused climate harm through its actions or omissions. Therefore, concurrent causes of climate harm are not sufficient to exempt states from the obligation to make reparation in respect of an internationally wrongful act.

The Court also restated the basic principle that states are responsible for the conduct of their organs, including both their acts and omissions. Moreover, while private conduct is not attributable to the state, a state may nevertheless be responsible for its failures to exercise due diligence in the regulation of private conduct, including omitting to limit emissions by private actors under their jurisdiction.

Impact on climate litigation

While lacking direct enforceability, Advisory Opinions by the ICJ are among the most persuasive sources of international legal interpretation and help crystallise emerging international customs. They carry legal weight when regional and domestic courts around the world answer these same questions, which, in turn, impact domestic legal regimes. For practitioners in this area, the Opinion significantly strengthens the legal framework for pursuing climate accountability claims.

The Opinion can be expected to inform and bolster both ongoing and future strategic climate litigation. Three key ways in which the Opinion is likely to expand / strengthen claimants’ arguments are (i) the validation of attribution science and its role in assessing responsibility and proving harm; (ii) the confirmation that states can be held liable for their failure to regulate the actions of private companies, especially those related to fossil fuel production, consumption, and subsidies. This could broaden the scope and prospects of litigation relating to indirect / ‘scope 3’ emissions; and (iii) the recognition that climate change imposes disproportionate burdens on vulnerable countries and populations, and that any state, even if not directly injured, can raise legal challenges when other states fail to meet their climate duties. This opens avenues for new legal strategies, particularly in securing compensation and remediation.

With thanks to Trainee Solicitor Sarah Dent Cullen for her assistance with this piece.