Abstract/Summary

Climate treaties impose few substantive obligations with respect to climate change mitigation. This Article explores customary international law as an alternative source of such obligations. Such a task faces considerable methodological difficulties due to the tension between ascending and descending reasoning in the identification of customary international law. This Article argues that the methodology typically followed by international courts would likely lead to the identification of a customary obligation of climate change mitigation, though only one which requires states to comply with the standard of care that most of them generally follow-rather than the ambition suggested by global mitigation objectives. Although it could be difficult to assess a state's requisite level of mitigation action, compliance with customary law could be tested by breaking down the customary mitigation obligation into implied duties that reflect the measures that states would generally be expected to take when exercising due diligence.