In July 2025, the International Court of Justice adopted its first advisory opinion on the climate obligations of states, settling a decade-old interpretive dispute over the Paris Agreement and confirming that nationally determined contributions (NDCs) are subject to a due diligence standard rather than unfettered state discretion.

One year on, analysis of the Court's jurisprudence shows the ICJ consolidating a communitarian conception of climate obligations, grounded in due diligence and the global public interest. The Court (2025, par. 440) defined these obligations as a "common interest" and an "essential interest of all states," language that parallels the International Tribunal for the Law of the Sea's 2024 characterisation of mitigation obligations as bearing on "the interests of mankind as a whole." Four leading international courts — the ICJ, ITLOS, the European Court of Human Rights and the Inter-American Court of Human Rights — have now ruled on climate obligations, and all define climate change as a "common concern of humankind."

The Paris Agreement question

The central question before the ICJ concerned Article 4 of the 2015 Paris Agreement. The provision's "constructive ambiguity" left unclear whether the obligation was simply one of result — communicating an intended reduction — or one of due diligence, under which courts could assess whether a submitted reduction met the minimum ambition expected of that state.

The divide played out in written submissions to the Court. The United States, Kuwait, Ecuador, Saudi Arabia and the Russian Federation backed the discretion model. Kuwait (2024, par. 2.6) argued that Article 4.2 does "not require anything in terms of the content of each state's NDC." The United States, then under the Biden administration (2024, par. 3.15), stated that "neither Article 4.3 nor any other provision of the Paris Agreement provides any legal standard against which to judge the sufficiency of a Party's NDC."

On the other side, the European Union, the African Union, Switzerland, Colombia, Mexico and the Commission of Small Island States argued for a due diligence reading. France (2024, par. 8) summarised the position: states enjoy a margin of appreciation, but "this margin of appreciation is not, however, unlimited."

The Court sided with the European and African perspective. According to the ICJ (2025, par. 245), "in the exercise of their discretion, parties are obliged to exercise due diligence and ensure that their NDCs fulfil their obligations under the Paris Agreement." The level of ambition required of each state depends on objective factors, such as capacity to achieve the 1.5°C target, and subjective ones, including historical emissions and level of development.

Developing countries and China

The opinion also breaks ground on what can be required of developing states. By 2023, when the UN General Assembly requested the advisory opinion, China was emitting 12.89 billion tonnes of CO₂ — 31% of global emissions. The Court held that the UNFCCC's 1992 binary classification of developed and developing states is no longer sufficient, noting "clear differences among developing countries" that carry legal relevance (par. 149–150).

Crucially, the Court accepted (par. 292) that "as states develop economically and their capacity increases, so too are the requirements of diligence heightened" — a gradual, dynamic reading of differentiation with clear implications for China's future obligations.

Judge Xue, while voting with the majority, called the gradual distinction "misleading and confusing" in her separate opinion (pars. 62–65), arguing it distorted the climate regime's fundamental structure. The US Ambassador to the United Nations, who considered broadly that the Court had "misinterpreted the treaties," nevertheless welcomed "the Court's important caveat" that states which have developed economically "should not be given a free pass."

Sovereignty and non-parties

The Court further restricted states' ability to opt out. According to the opinion (2025, par. 315), even states that have not ratified the Paris Agreement — a category that includes the United States following its second withdrawal — bear a customary duty to cooperate with the rest of the world to mitigate the causes and consequences of global warming. The United States has since declared the opinion unfounded, containing what it calls erroneous interpretations and incorrect legal conclusions.

The General Assembly response

On 20 May 2026, the UN General Assembly adopted resolution A/80/L.65, proposed by Vanuatu, welcoming the advisory opinion as "an authoritative contribution to the clarification of existing international law" and calling on states to fulfil Paris Agreement obligations as the ICJ identified them — namely as due diligence obligations. The vote was 141 in favour, 8 against and 24 abstentions.

Major fossil fuel exporters — Norway, Australia, Canada and the United Arab Emirates — voted in favour alongside EU member states and island nations including Vanuatu and Tuvalu. China, Ecuador and Egypt also supported the resolution despite having opposed the due diligence reading during proceedings. India, South Africa and Nigeria abstained. The eight votes against came from the United States, the Russian Federation, Saudi Arabia, Iran, Israel, Liberia, Belarus and Yemen.

Practical consequences for practitioners

For practitioners, the opinion resolves legal uncertainty that had produced contradictory rulings among national courts over the legal nature of the Paris Agreement. Litigators and counsel advising governments can now anchor arguments in a binding clarification — endorsed by a supermajority of the General Assembly — that NDC adequacy is justiciable and measured against a multifactorial, evolutive due diligence standard. States with significant resources and rising emissions face heightened expectations as their development advances, and even non-parties to the Paris Agreement must contend with customary cooperation duties. The opinion aligns the ICJ with ITLOS and regional human rights courts in subordinating unilateral sovereignty claims to the collective interest in climate protection, giving claimants in domestic and international fora a substantially stronger doctrinal foundation than existed before July 2025.