From ‘well below 2°C’ to 1.5°C: The ICJ Advisory Opinion on Obligations of States in respect of Climate Change

Abstract On 23 July 2025, the International Court of Justice issued its Advisory Opinion on the Obligations of States in respect of Climate Change, wherein it designated the 1.5°C threshold as the ‘primary temperature goal’ of the Paris Agreement. This article situates that interpretive move within its broader scientific, political and judicial context, tracing how the 1.5°C threshold has gained prominence despite the textual primacy of the ‘well below 2°C’ standard and the aspirational formulation of 1.5°C in Article 2(1)(a). It demonstrates that, while the 1.5°C limit has become deeply embedded in global climate discourse, State practice remains uneven and does not provide a clear legal basis for prioritising the 1.5°C threshold over the ‘well below 2°C’ limit. The article critically examines the Court's reasoning, arguing that neither the doctrine of subsequent agreement nor recourse to ‘best available science’ justifies overriding the ordinary meaning of Article 2(1)(a). The article argues that this doctrinal overreach is of limited normative consequence, since the ICJ invoked the temperature goal primarily when formulating a ‘stringent’ and objectively assessed due diligence obligation governing the preparation of nationally determined contributions (NDCs). However, the 1.5°C–2°C range, when read against current emissions trajectories and extensive scientific evidence, together with existing case law and custom could itself have sustained a ‘stringent’ and objectively measured standard of due diligence. The article concludes that, by anchoring this obligation firmly in States' duty to prepare sufficiently ambitious NDCs, the Court arguably strengthened, rather than displaced, the operational relevance of the Paris Agreement's dual temperature goal.