This Article addresses the evolution of legal rules to govern the global environment. It traces the borrowing of legal ideas from national law into international law, in particular the borrowing of emissions trading and the comprehensive approach into the Rio and Kyoto climate change treaties. The Article argues that such vertical legal borrowing is related to, yet importantly different from, the pervasive horizontal legal borrowing across national legal systems that has been much studied by comparative law scholars. The Article develops both positive and normative assessments of vertical legal borrowing, arguing that it is often suppressed but increasingly essential to the success of global environmental law. Yet vertical legal bor...