From the first members of the human species wandering the African savannahs down to the present day we have witnessed ice ages, extreme events of various kinds and a plethora of cultural, political, historical and other changes. We live in a changing world and it was ever thus. But things are different now. At the beginning of the third millennium we have a nexus of social, economic, technological and environmental trends the like of which has not been seen before. Population growth has brought us to the point where we are the dominant species on this planet, and there is growing evidence of our power to modify the global climate. Human activity is affecting the global biosphere in ever more complex ways as a result of technological development, resource use and industrialisation. We are approaching global constraints on our activities, particularly through our modification of global cycles of energy, water and nutrients. We have unprecedented global connectivity through advanced transportation and telecommunications systems. Our social organisation and our economic activity have grown to the point where we have reached and exploited just about every corner of the globe; so we are now the dominant planetary engineers. In the past thirty years there has therefore been a sea change in our relationship with the planet on which we live. We now have a much more complex and recursive relationship with ourselves and with nature.