By Alex Kirby BBC News Online environment correspondent

The curtain is about to rise on the World Development Summit in South Africa.

Yet another international conference is being billed by some idealists as "the last chance to save the planet".

The hard-nosed realists, though, are already writing off Johannesburg as a waste of time and energy.

Probably both are wrong, and there will be progress. But it may be depressingly slight for all the effort.

The Johannesburg conference - properly the World Summit on Sustainable Development (WSSD) - will run from 26 August to 4 September.

Its job is to review the progress made since the Earth Summit in Rio de Janeiro in 1992, and to give it new impetus. Some fear it will be "a conference to celebrate a conference".

Rio was the UN Conference on Environment and Development. At this meeting the emphasis is squarely on development, on reducing poverty.

Hoping for results

The organisers do not want it even to be called the Earth Summit, and there will be no environmental agreements like Rio's on climate and conserving wild species.

So some narrowly-focused campaign groups may be disappointed at the focus on the world's poor, forgetting how extreme poverty (and extreme wealth) destroy the world.

But anti-poverty groups will be just as disillusioned, because there are no great treaties in prospect for them either.

Nitin Desai: WSSD architect

The conference will probably agree the Millennium Development Goals, which boil down to saying the world will halve poverty by 2015.

But that agreement is likely to be so short on detailed timetables and targets that it will be simply an aspiration.

Kofi Annan, the United Nations secretary-general, has urged Johannesburg to concentrate on water, energy, health, agriculture, and biodiversity (Wehab, for acronym lovers).

Summit facts

65,000 delegates 27,000 police on duty Estimated production of carbon dioxide: 500,000 tonnes Estimated cost: $55m

There will be about 6,000 delegates to the official summit, and more than 15,000 for the parallel civil society conference.

More than 100 heads of state or government are expected for the final two days, though probably not President Bush. The US will have a large delegation at the WSSD, but is accused of obstructing preparations and resisting moves to tackle poverty.

Different agendas

Another built-in tension stems from the wish of much of the developed world to involve business and industry in forging voluntary deals (known in the WSSD jargon as "Type II agreements") with developing countries.

The poor world itself often feels that getting rich governments to sign up to cast-iron ("Type I") agreements offers a better prospect of something actually happening.

A boy and his pet bird: Poverty and nature are WSSD concerns

It also sees Johannesburg as an opportunity to obtain badly-needed money to modernise its industries, and feels less able to worry about the longer-term environmental concerns of northern countries.

But there are more fundamental problems for Johannesburg to acknowledge. Real help to the poor of this world would involve radical changes to trade and aid policies.

It would mean the developed countries, for instance, agreeing finally to give 0.7% of their gross domestic product in development aid. The UK, one of the better performers, has now reached 0.4%.

Nothing changes

It would mean allowing poor countries fair access to rich markets. But neither trade nor aid will be up for discussion in Johannesburg.

It would mean an end to subsidies to US and European Union farmers, and to other industries protected from competition from the south. Subsidies are not on the summit agenda.

And perhaps Johannesburg's biggest danger is that it will prove blandly irrelevant.

"Sustainable development" sounds neat. It is not the easy option it sounds.

A sustainable world could not tolerate poverty. This world shows little sign yet of finding it intolerable.

After Rio the then UN head, Boutro Boutros-Ghali, said: "One day we will have to do better." It is a tall order for Johannesburg.