Translating the Legal Expectation of Highest Possible Ambition in Scenario Study Design

Under the Paris Agreement, states must communicate Nationally Determined Contributions (NDCs) to the collective achievement of the long-term temperature goal. The International Court of Justice’s (ICJ) Advisory Opinion clarified that the ambition communicated in NDCs is not discretionary to the state. NDCs reflecting highest possible ambition (HPA), as explicitly mandated under Article 4 of the Paris Agreement, must be an adequate contribution to the 1.5° C temperature goal. HPA forms part of states’ due diligence standard to the Paris Agreement, imposing a procedural obligation for the conduct of formulating the NDC targets. However, practical translation of how to operationalise states’ respective HPA in NDCs remains unexplored. This contribution bridges the gap between legal obligations and scenario modelling by proposing a framework that establishes a structured understanding of highest possible ambition across six elements, incorporating domestic, international and implementation considerations. These elements are translated into concrete operational terms through conduct and process expectations, including guidance for how scenario studies informing NDCs can be designed to inform the required faithful assessment the mitigation options that serves as a starting point for NDCs of HPA. By clarifying how such assessment should be situated within explicit considerations of respective capabilities, equity, and implementation pathways, this framework may shape future NDCs by informing modelling approaches and documentation choices. It supports the systematic and transparent exploration of higher ambition scenarios, strengthening the alignment between legal obligations, scenario modelling, and states’ national climate pledges.