We live in an increasingly full world. During 2011 we pass the 7 billion mark. This means that world population has more than doubled during my lifetime. When J.F. Kennedy was President of the United States, humankind numbered a mere 3 billion. How has it been possible to sustain such burgeoning population growth? Answer: access to fossil fuel energy that has enabled huge increases in productivity. Back in 1800, before the industrial revolution took hold, production could only sustain 1 billion. Is the globe over-populated by the human species? On the face of it — yes, but not necessarily. The real problem is social justice (or the lack thereof) and failure to live within our ‘ecological budget constraint’. Based on the current business-as-usual model, we exceeded the carrying capacity of the Earth in the mid-1980s. In a non-polluting, renewable energy-fuelled world based on the principles of intra- and inter-generational equity, a population of 7 billion human beings may be sustainable.