The Parties to the Paris Agreement have committed to communicate successive ‘nationally determined contributions’ (NDCs) to the global response to climate change. Each NDC is expected to reflect the Party's ‘highest possible ambition’ (HPA) on the mitigation of climate change. This article envisages the possibility of taking HPA seriously: that is, of approaching it as an effective legal standard. It shows that, in some circumstances, the HPA standard can help to assess whether a State has complied with due diligence obligations on climate change mitigation.

Courts and scholars have interpreted open-ended legal norms as imposing due diligence obligations on States and other entities to mitigate climate change. These obligations can be applied in two alternative ways: through holistic decisions, where courts determine the level of mitigation action required of defendants; or through atomistic decisions, where courts identify some of the measures that the defendant must take. This article shows that, whilst most holistic cases fail on jurisdictional grounds, atomistic cases frequently succeed. Overall, it is argued that atomistic litigation strategies provide more realistic and effective ways for plaintiffs to prompt enhanced mitigation action.

In 2017, France established a due diligence statutory obligation for French parent companies to monitor extraterritorial human rights and environmental abuses committed by their off-shore affiliates. Switzerland is also considering adopting a similar law for Swiss parent companies. These obligations are comparable to the duty of care that, according to recent case law, British parent companies owe towards their subsidiaries’ neighbours. This article compares and contrasts the newly introduced French due diligence statutory obligation, the UK precedents, and two alternative Swiss legislative proposals on the due diligence and duty of care of parent companies.