William Lee Steffen was an American-born Australian chemist. He was the executive director of the Australian National University (ANU) Climate Change Institute and a member of the Australian Climate Commission until its dissolution in September 2013. From 1998 to 2004, he was the executive director of the International Geosphere-Biosphere Programme, a coordinating body of national environmental change organisations based in Stockholm. Steffen was one of the founding climate councillors of the Climate Council, with whom he frequently co-authored reports, and spoke in the media on issues relating to climate change and renewable energy.

Planetary boundaries are a framework to describe limits to the impacts of human activities on the Earth system. Beyond these limits, the environment may not be able to self-regulate anymore. This would mean the Earth system would leave the period of stability of the Holocene, in which human society developed. The framework is based on scientific evidence that human actions, especially those of industrialized societies since the Industrial Revolution, have become the main driver of global environmental change. According to the framework, "transgressing one or more planetary boundaries may be deleterious or even catastrophic due to the risk of crossing thresholds that will trigger non-linear, abrupt environmental change within continental-scale to planetary-scale systems."