Although a few important functional modalities of the Paris Agreement (PA) are still under negotiation, this book, authored by Benoit Mayer (a renowned scholar of climate change law), provides a critical reflection on the identification and application of general and specific climate change mitigation obligations as grounded in international treaties (that is, both climate change and human rights treaties) and customary international law (CIL). The book fills the doctrinal gap by proposing an alternative concept of “corollary duty” when concepts like substantive and procedural obligations are insufficient to assess states’ compliance with their “general” mitigation obligations of prevention and cooperation.

Chapter 1 lays down the methodologies (ascending and descending reasoning), objectives, and the need for doctrinal research on the identification and application of “general” mitigation obligations. Chapters 2, 3, and 4 identify, respectively, mitigation commitments (that is, general commitments laid down in CIL and the United Nations Framework Convention on Climate Change and specific commitments/measures required in the PA), customary obligations (for example, obligations of due diligence and cooperation), and obligations implied from human rights treaties requiring states to mitigate climate change to protect human rights. Since the identification process primarily relies on treaties and CIL, it hardly touches another important source of identification, that is, the general principles of law recognized by nations. While Chapter 5 characterizes the nature of “general” mitigation obligations, Chapters 6 and 7 propose two alternative ways of assessing compliance with these obligations. In the literature, the nature of “general” mitigation obligations is understood in two ways: obligations of conduct and result and substantive and procedural obligations. The author, while contesting the relevance of any such categorization of obligations into substantive and procedural, identifies normative difficulties in assessing compliance with the obligations of result and conduct.

Chapter 6 attempts to assess the requisite level of mitigation action a state must take to meet its “general” mitigation obligation. The book argues that such an assessment could be expressed in terms of the “result” a state is expected to achieve in a prescribed time. However, the author highlights that such an assessment would face two normative difficulties. First, though states have agreed to achieve collective objectives (for example, a temperature goal of 1.5/2 degree Celsius), there is no obligation to act “consistently” with these objectives, nor is it clear what they mean for individual action on climate change. Second, the climate change regime does not prescribe any criteria for objectively assessing “burden-sharing” among states. Therefore, Chapter 7 proposes an alternative concept of “corollary duty” in the form of the “appropriate measures” a state is expected to implement to comply with “general” mitigation obligations. To this end, the author identifies three corollary duties of “general” mitigation obligations. These are the duty of cooperation, the duty of vigilance, and the duty of consistency. The application of the concept of “corollary duties” appears reasonable, but its effectiveness and successful implementation rest on a proper assessment of “consistent state practice” and “good climate change mitigation practice”, which states are yet to establish.

Overall, the book provides a new doctrinal perspective, the relative concept of “corollary duty”, to assess compliance by states with “general” mitigation obligations emerging from CIL and climate change treaties. Judiciaries (both international and domestic) can use this concept to assess the cases of compliance and fix the liability/responsibility of a state in case of non-compliance with, or breach of, any specific duty. The Implementation and Compliance Committee set up in the PA may also use this concept to assess individual cases of compliance/non-compliance with the mitigation provisions of the PA. The book's contribution could be seen in a larger context where the precise “identification and application” of mitigation obligations would not only influence the behaviour of states to implement their respective mitigation obligations but also help to pursue a successful domestic/regional/international climate litigation.