Humanity's choices are getting harder and fewer. The Earth's population has doubled since 1950 and consumption has risen even faster. There has to be a reckoning. For many people, it is here already. The few first-class passengers on the planet that is our Noah’s Ark are safe for now on the upper deck. It’s a very different story down below. How much longer can the rich keep their feet dry?

Oil consumption has increased seven-fold in the last 50 years and meat production, marine fish catches and carbon emissions from fossil fuel burning have all at least quadrupled. And freshwater use increased six-fold last century. According to one recent study, the human race is consuming the Earth's resources at a rate that is 20% faster than it can replenish itself, with the result that we would need 1.2 Earths to sustain this lifestyle. The gap between rich and poor is becoming wider and more visible. Nearly 30% of the world's population suffers some form of malnutrition and almost two thirds of humanity lives on less than $2 a day. And sustainable development is critical for the world's poorest. The family that has to level a forest to grow its food and find the fuel to cook it does not have the choice of living sustainably. The poorest have least power to protect themselves from the effects of global environmental problems such as climate change. And, with the richest 1% of the world's population consuming as much as its poorest 44%, we would have to use massively more resources if the poor were to live as the rich world does.

What is sustainable development?

In principle, sustainable development means not using up resources faster than the Earth can replenish them - "treating the Earth as if we intended to stay".In practice, everything from biotechnology and nuclear power to vegetarianism and rail travel is promoted in the name of sustainable development – to the point where some campaigners say the term has become meaningless. The optimists say disaster will never strike, as development knows no limits. They argue that human ingenuity will always find a way. Society will find new raw materials, develop cleaner technologies and manage water scarcity so resource depletion and pollution cease to be problems. One argument runs that a better world needs us all to spend and consume more to generate wealth for all – and that industrialisation slows population growth and raises environmental standards. But if "development" means every person on the planet aspiring to own a car, fly half way round the world on holiday and get a new mobile phone every year, we may as well forget it, the sceptics say. We don't have enough Earths for this sort of consumer capitalism.

Talking shops

And this clash of perspectives is at the heart of the often rancorous and divided international negotiations on conservation and development – from the Kyoto climate chan ge treaty and the World Food Summit to the Johannesburg World Summit on Sustainable Development. Industry wants open markets and voluntary agreements. Campaigners fear the globalised economy has outrun the capacity of political institutions to control it. Both talk of sustainable development as the way to square the circle, though it does not always provide easy answers, or any at all. And the crisis is for this generation as well as the future. As government officials, industry representatives, scientists, policy makers and campaigners continue to thrash out the future of the planet, more than 30,000 under-fives – the equivalent of about 10 World Trade Center attacks - die every day from hunger or from easily preventable diseases. Our record for managing to think ahead is poor. Professor James Lovelock has said humans are as qualified to be stewards of the Earth as goats are to be gardeners. And we do not have 1.2 Earths.