By Alex Kirby BBC News Online environment correspondent

The Earth Summit, the World Summit on Sustainable Development (WSSD) in Johannesburg, faces a high risk of failure, according to a leading British environmental thinker.

He is Sir Crispin Tickell, former UK ambassador to the United Nations.

Humans are as qualified to be stewards as goats are to be gardeners

Professor James Lovelock

Sir Crispin says it is "hard to be optimistic" about what will happen in Johannesburg.

Little, he says, will change "unless and until we think differently".

Sir Crispin is now director of the Green College Centre for Environmental Policy and Understanding at the University of Oxford.

Speaking to the Society for Conservation Biology, he said the summit's agenda, sustainable development, meant "treating the Earth as if we intended to stay".

Affecting evolution

He said humans were changing the Earth in several ways: by increasing their numbers, through the loss of land quality and the build-up of wastes, by changing atmospheric chemistry, and by continuing to destroy other living species.

Coral reefs are being destroyed

He said our destruction of other species had reached "rates comparable to those caused by extraterrestrial impacts in the long-distant past. One in four mammal species, which are key indicators of ecosystem health, are facing a high risk of extinction in the near future.

"The future course of evolution will be substantially changed by current human activity.

"Bacteria and viruses learn how to react to almost any drug we may throw at them. Humans take 20 years to reproduce. Bacteria do the job in 20 minutes.

How we are changing the Earth

We are multiplying "at a giddy rate" 65% of all arable land may have already lost some biological and physical functions 60% or more of world fisheries are judged to be fully exploited or over-fished 27% of coral reefs are thought to have been lost, with another 32% at risk by 2032 Freshwater demand doubles every 21 years

"Nor can we yet assess the effects of the introduction of genetically modified organisms."

Sir Crispin said an occasional visitor from space would find more change in the Earth's surface in the last 200 years than in the preceding 2,000, and more in the last 20 years than in the preceding 200.

The need to conserve biodiversity, the Earth's wealth of life, was hard to get across to people.

There was an ethical reason to do so, but we seldom realised our vocation to be stewards of the Earth. Sir Crispin quoted the judgement of Professor James Lovelock, that "humans are as qualified to be stewards as goats are to be gardeners".

Stuck in a rut

There were strong economic arguments for conservation, from the range of drugs derived from plants to the need to cherish genetic diversity.

Ecologically, we relied on forests and vegetation to produce soil, regulate water supplies and recycle waste.

Sir Crispin: No optimist

But Sir Crispin said inertia was immensely strong, and that was why little would change until we learnt to think differently, and why he was not optimistic about the WSSD.

He said: "For change we need three factors: leadership from above, pressure from below, or some exemplary catastrophe.

"Do we know where we are going? Not yet: the juggernaut of conventional wisdom rolls on.

"Can we cope with the problems raised by the unstable and unsustainable society we have created for ourselves? My answer is also: not yet."