AbstractThe 2017 Advisory Opinion of the Inter‐American Court of Human Rights broke new ground in finding that States' failure to regulate with due diligence the activities taking place within its territory could constitute a breach of the human rights of persons located elsewhere. How might this finding inform the law on prevention of transboundary harm? Similarly, how might it inform an identification of the standard of care required of States in respect of climate change? This article argues that the law on transboundary harm should now be understood as requiring States to regulate the reduction of greenhouse gas emissions in accordance with stabilizing global warming at a temperature, which, at a minimum, prevents widespread threat to life around the globe. States' differing capacities to invest in mitigation strategies, their varying historic contributions to climate change and the urgency of reducing greenhouse gas emissions are also factors affecting a reasonable standard of care.