So how many Planet Earths do you think we will need by 2050 to keep humankind in the style to which we have become accustomed? Two? Three? Half a dozen? It's an absurd question, of course, and there's an absurd answer: two Planet Earths would apparently suffice, according to a Worldwide Fund for Nature (WWF) report published recently. But it's nothing like as absurd as the fact that there's not a single world leader prepared to give more than the most spurious consideration to our imminent collision with ecological reality.

WWF's Living Planet report provides an annual snapshot of the state of the critical ecosystems we depend on, and humankind's total "ecological footprint" - a measure of our collective use of renewable natural resources such as crop land, grazing land, forests, fishing grounds and so on. The "footprint" is a neat if simplistic way of getting a handle on the degree to which we can claim to be using these renewable resources sustainably.

WWF's "ready reckoner" sets the total of productive land and sea at about 11.4 billion hectares; divide by 6 billion (the current world population) and you get the magic number 1.9 hectares per person. Having crunched a huge amount of data from around the world, the average rate of use for 1999 emerges at 2.3 hectares per person - already 20% above the Earth's basic biological capacity of 1.9 hectares per person. Fast forward to 2050 (with a projected population of around 9 billion), and that average use rises to around 4 hectares per person - an ecological deficit equivalent to one whole Planet Earth.