This chapter debates the relevance of customary law to climate law. In climate-law discussions, most attention focuses on the treaties (the UNFCCC, Kyoto Protocol, and Paris Agreement) rather than on custom. Views differ as to the relevance or applicability of customary international law to state responsibility for responding to climate change. Sandrine Maljean-Dubois makes the case that norms of customary international law can meaningfully be applied to climate change mitigation, Christopher Campbell-Duruflé, by contrast, argues that those norms are too vague to address the problem in any meaningful way. This debate has important implications for determining the level of mitigation ambition that states must implement.

This chapter debates the relevance of customary law to climate law. In climate-law discussions, most attention focuses on the treaties (the UNFCCC, Kyoto Protocol, and Paris Agreement) rather than on custom. Views differ as to the relevance or applicability of customary international law to state responsibility for responding to climate change. Sandrine Maljean-Dubois makes the case that norms of customary international law can meaningfully be applied to climate change mitigation, Christopher Campbell-Duruflé, by contrast, argues that those norms are too vague to address the problem in any meaningful way. This debate has important implications for determining the level of mitigation ambition that states must implement.