At the Planet Under Pressure conference in London, Diana Liverman and Will Steffen present something of a contrasting couple.

The two professors have been working together on a State of the Planet report, which has involved trawling through numerous reports and scientific papers. At the end of it all, the message of one appears somewhat optimistic, the other fundamentally pessimistic. They agree that changes to the world since about 1950 have been startling - rapid spread of the human population, accelerating exploitation of forests and marine resources, surging economic growth in successive waves across the world, and so on. This radical reshaping of the natural world by a single species is certainly unprecedented in Earth history, which a few years back led to scientists Paul Crutzen and Eugene Stoermer coining a special name for our epoch - the Anthropocene.