Progression requirements applicable to state action on climate change mitigation under Nationally Determined Contributions

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.