5 planets to support us
From Sustainable Cities, a clear definition of what sustainability is. It is tied to per capita energy consumption; the idea is that there is only so much earth and resource to go around. If everyone in the world consumed at the current level of Americans consumption, we would need 5 planets to support us.
From the article, Ecological Footprint: Humanities killing Nature:
Ecological footprints may be used to argue that many current lifestyles are not sustainable. In 2003, the average person’s ecological footprint was 2.2 global hectares, while there is only 1.8 global hectares of biological productive area per person available on the planet. Such a global comparison also clearly shows the inequalities of resource used. The smallest footprint is 0.1 global hectares per capita in Afghanistan, whereas the largest are 11.9 global hectares per capita in the United Arab Emirates. The US footprint in 2003 was 9.6 global hectares per capita. This means that if we were all Americans, we would need 5 planets to support us and Americans would, on average, need a reduction of five-sixths (83%) to be sustainable.