An introduction

The International Court of Justice (ICJ) characterised climate change as “an existential threat” in its advisory opinion of 23 July 2025 — the most significant development in international climate law since the adoption of the Paris Agreement. The opinion was adopted unanimously in its operative part, a rare outcome that underscores broad global consensus. The opinion was requested by the UN General Assembly following a campaign initiated by law students at the University of South Pacific, then led by Vanuatu and supported by over 130 states (p. 19). The ICJ received over a hundred written and oral submissions, with nearly 100 states and multiple international organisations participating in the proceedings (p. 19).

Scope of state obligations and legal framework clarified

The ICJ identified a comprehensive legal framework encompassing the UN Charter, climate change treaties (UNFCCC, Kyoto Protocol, Paris Agreement), UNCLOS, and other environmental treaties. Customary international law duties — including the duty to prevent significant environmental harm and the duty to cooperate — also apply, as does international human rights law, including the rights to life, health, food, water, and housing. Principles such as CBDR-RC, equity, intergenerational equity, and the precautionary principle are applicable (p. 20–21). The ICJ rejected the lex specialis argument made by some states — including the United States — finding no inconsistency between climate treaties and broader international law (p. 21–22).

Due diligence over discretion

The ICJ affirmed a stringent due diligence standard, holding that states must act urgently based on scientific consensus as established by the IPCC. States’ discretion in preparing Nationally Determined Contributions (NDCs) is limited; parties are obliged to exercise due diligence to ensure that, taken together, NDCs achieve the 1.5°C temperature goal (para. 245, p. 22–23). Inaction or failure to act decisively may constitute an internationally wrongful act.

Customary international law and the climate system

The customary duty to prevent significant environmental harm applies to the climate system and binds all states, including those not party to climate treaties (para. 273, p. 23). Significant harm can arise from cumulative impacts by states and non-state actors (para. 276, p. 23). Key elements of due diligence include: adopting appropriate legal and regulatory measures; acting on best available science; applying the CBDR-RC principle; taking preventive action amid uncertainty; conducting risk and impact assessments; and notifying and consulting other states (p. 23–24).

Protection of the marine environment, sea level rise, and statehood

The ICJ reaffirmed that anthropogenic greenhouse gas emissions constitute marine pollution under UNCLOS, making Part XII applicable to climate governance (paras. 339–340, p. 25). States have both positive and negative obligations under UNCLOS. On statehood, the ICJ found that “once a State is established, the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood” (para. 363, p. 26), offering cautious reassurance to small island states.

International human rights law and the climate

The ICJ recognised a clean, healthy, and sustainable environment as “a precondition” for the enjoyment of human rights (para. 393, p. 26–27), and highlighted the particular vulnerability of children, women, and indigenous peoples. Human rights treaties may apply extraterritorially where a state exercises jurisdiction outside its borders (paras. 394ff, p. 27).

State responsibility and reparations

The ICJ affirmed that failure to take appropriate action to protect the climate system — including through fossil fuel production, consumption, granting of licences, or provision of subsidies — may constitute an internationally wrongful act (para. 427, p. 27–28). Legal consequences include cessation, non-repetition, and full reparation (restitution, compensation, and/or satisfaction). These obligations are erga omnes — any state may invoke responsibility (paras. 439ff, p. 28).

The court’s restraint: Silences, gaps, and self-reflection

Contributors in Part II examine what the ICJ did not address, including: differentiation of responsibilities and historical emissions; an African perspective on the opinion; the role of colonialism in structuring the climate crisis; military emissions (estimated at 5.5% of global emissions as of 2022, p. 347); and how the ICJ’s own interpretive method could be used to fill remaining gaps on fossil-fuel licences, reparations, and human rights. The ICJ’s restraint is read both as deliberate judicial caution and as a set of elastic openings for future litigation.