The Paris Agreement reflects an expectation that each party’s action on climate change mitigation should progress with the adoption of successive Nationally Determined Contributions (NDCs). This article assesses the legal force and effectiveness of such requirements on progression in a state’s overall ambition on climate change mitigation. First, it identifies legal requirements through a legal analysis of the sources of international law and a review of national legislation. The article introduces a distinction between requirements on active progression in new NDCs and on non-regression in NDC updates. Second, the article reveals obstacles to assessing compliance with any of these requirements. A review of the Paris Agreement’s first ambition cycle sheds light on frequent issues relating to changes in the type of mitigation objective included in the NDC, in the normative nature of an NDC, and in the underlying data. Breaches of progression requirements appear extremely difficult to demonstrate, not only due to practical difficulties in gathering evidence, but also to the lack of a clear definition of these requirements. In particular, one can approach progression alternatively in light of the efforts implemented by a state or the results promised by that state, and one could assess these efforts or outcomes on the touchstone of either national or global emissions. This analysis thus shows that progression requirements are at present rather ineffective, although they could become more effective with the development of new mechanisms and methodologies.

《巴黎协定》反映了这样一种期望，即各方减缓气候变化的行动应随着国家自主贡献（NDC）的相继通过而取得进展。本文评估了此类要求对一个州减缓气候变化总体目标的进展的法律效力和有效性。首先，它通过对国际法渊源的法律分析和对国家立法的审查来确定法律要求。本文介绍了新 NDC 中主动进展要求和 NDC 更新中非回归要求之间的区别。其次，本文揭示了评估对这些要求的遵守情况的障碍。对《巴黎协定》第一个雄心周期的审查揭示了与国家自主贡献中包含的缓解目标类型、国家自主贡献的规范性质以及基础数据的变化有关的常见问题。违反进展要求似乎极难证明，不仅因为收集证据的实际困难，而且还因为这些要求缺乏明确的定义。特别是，人们可以根据一个国家实施的努力或该国家承诺的结果来选择进展，并且可以根据国家或全球排放量的试金石来评估这些努力或结果。因此，该分析表明，进展要求目前相当无效，