William Rees, PhD, FRSC, is a population ecologist, ecological economist, Professor Emeritus and former Director of the University of British Columbia’s School of Community and Regional Planning. His research focuses on the biophysical requirements for sustainability and the implications of global ecological trends for the human prospect. He has special interests in cities as particularly vulnerable components of the human ecosystem and in psycho-cognitive barriers to rational ‘environmental’ behaviour, including sound public policy for sustainability. Prof Rees is perhaps best known as the originator and co-developer of ‘ecological footprint analysis’ which shows that the world is in far overshoot—we would need four Earth-like planets to support just the present world population at North American material standards. Dr Rees has authored hundreds of peer reviewed and popular articles on these topics.  He is a founding member and former President of the Canadian Society for Ecological Economics; a founding Director of the One Earth Initiative; a Fellow of the Post-Carbon Institute and Associate Fellow of the Great Transition Initiative. Internationally recognized, Prof Rees is a Fellow of Royal Society of Canada, recipient of a Trudeau Foundation Fellowship and both the international Boulding Prize in Ecological Economics and a Blue Planet Prize (jointly with his former student, Dr Mathis Wackernagel).  From 2014 to 2019 Dr Rees was a full member of the Club of Rome; in 2015 he received the 2015 Herman Daly award from the US Society for Ecological Economics; and, in 2016, was awarded a Dean’s Medal of Distinction from UBC’s Faculty of Applied Science.

Modern cities, particularly mega-cities, are arguably a product of fossil fuels (FF) and remain dependent on FF both for new construction and for continuous supplies of food, consumer goods, and the materials needed even for basic maintenance, imported from all over the world.  But FFs are the principal enabler of ecological overshoot and its many symptoms, including climate change. Burning FFs is a major source of carbon dioxide, the most important anthropogenic driver of global warming. Avoiding accelerating and potentially disastrous climate change therefore requires the decarbonisation of the economy. Problematically, there are, as yet, no quantitatively adequate substitutes for FF in many essential uses, including bulk transportation and agriculture. This has major implications for cities. Rapid decarbonisation (at least 8%/yr ) risks inadequate energy supplies, broken supply lines, and food and other resource shortages. In some cases this in turn implies local famines, civil unrest, abandoned cities, mass migrations, collapsed economies and political chaos.  On the other hand, if the world continues its intensive use of FF we risk more and longer heat waves/droughts, melting permafrost, methane releases, water shortages, failing agriculture, local famines, rising sea levels, increased flooding (and eventual loss) of certain coastal cities, mass migrations, collapsed economies and geopolitical chaos.  In these circumstances, what measures should the world be taking to limit climate change and other manifestations of overshoot?  What is the appropriate policy advice to politicians and other decision makers?