A new study, for the first time, shows how the ‘highest possible ambition’ of countries' climate pledges can be assessed.

Collective progress towards meeting the Paris Agreement’s long-term temperature goal (LTTG) remains insufficient, a shortfall to be addressed by new Nationally Determined Contributions (NDCs). The Agreement stipulates that countries’ NDCs will represent a progression and reflect their highest possible ambition (HPA). However, the vague conceptualisation of this HPA norm challenges its translation into practice, allowing states to reference ambition in their NDC without clear evidence. Recently, the International Court of Justice’s (ICJ) advisory opinion on climate change provided clarification regarding this norm. The level of ambition communicated in an NDC is not entirely left to the discretion of states but must constitute an adequate contribution to the collective achievement of the global temperature goal. States are obliged to exercise due diligence to ensure their NDCs reflect their best efforts. Building on the ICJ’s interpretative guidance, we propose a structured framework to operationalise the HPA norm and reflect upon considerations of ambition in countries’ NDCs. The framework’s core pillars incorporate domestic, international, and implementation considerations in NDCs, elaborated upon in two elements per pillar. Under domestic considerations, NDCs should be based on a faithful assessment of mitigation options utilising national capabilities. International considerations require contributions that adequately support the LTTG while balancing equity. Implementation considerations demand that NDCs account for the co-impact of the measures considered and be actionable in the near term to ensure ambition is translated into action. The Paris Agreement-HPA Framework can support transparent communication of the conditions under which more ambitious targets can be pursued. We apply the framework to explore the engagement of current NDCs with the framework’s elements. Finally, the framework empowers societal stakeholders to interrogate, verify and, when necessary, challenge whether NDC preparations were carried out with due diligence, supporting accountability in global and national climate action.