The Legal Power of Highest Possible Ambition: Setting Legal and Scientific Indicators to Assess Highest Possible Ambition under Article 4(3) of the Paris Agreement

Abstract The Paris Agreement requires each state party to submit a nationally determined contribution ( ndc ) every five years. Current ndc s fall short of limiting global warming to 1.5°C, making the 2025 submissions crucial. Against commonly held belief, the level of climate change mitigation ambition in ndc s does not entirely lie within the discretion of Parties. Article 4(3) of the Paris Agreement requires that each party’s successive ndc will reflect its ‘highest possible ambition’ ( hpa ); an important normative requirement crucial for raising mitigation ambition. This article examines hpa and provides legal and scientific indicators for assessing whether parties’ 2025 ndc s and subsequent submissions align with the hpa requirement in Article 4(3). In doing so, a comprehensive framework essential for ensuring that ndc s are ambitious, credible, and effective in reducing emissions and mitigating climate impacts is offered. The success of the global effort against climate change depends on parties reflecting their hpa in ndc s and implementing effective mitigation measures, highlighting the urgency of hpa in each party’s climate efforts.