In Rio de Janeiro a decade ago, leaders took the landmark decision to try to stop rising emissions of the greenhouse gases which trap heat in the atmosphere, and created the United Nations Framework Convention on Climate Change.

U.N. scientists said the build-up of carbon dioxide in the atmosphere since the Industrial Revolution was trapping heat in the atmosphere. They predicted major climate disruption if emissions were not cut.

Five years later, with emissions still rising, countries beefed up the convention with the Kyoto Protocol which contained binding targets on emissions reduction for industrialised countries.

But the pact has yet to come into force and the United States put its future in doubt when it pulled out last year.

"If you look at the record since Rio, climate change is the most glaring failure," said Rob Bradley of the campaign group Climate Action Network.

"Countries took a commitment to stabilise emissions and then promptly didn't do it. That gave the lie to the idea that countries were there because they realised how serious it all was."

U.S. CLOUD OVER SUMMIT

Kyoto can still survive without the world's biggest producer of greenhouse gases, but not until Russia ratifies, supplying the required number of developed countries for it to take effect. That is not expected for another several months.

While Kyoto's supporters are disappointed it will not be in force before the summit, they blame U.S. influence for the fact that climate change is barely mentioned on the agenda.

"EPA (U.S. Environmental Protection Agency) officials told me the American administration preferred to have climate change not at all on the agenda at Johannesburg, to instead focus on water," said European Parliament member Alex de Roo.

"What do you see? The first item on the agenda is water. The second is energy, which has some climate implications, but the word climate isn't mentioned. That's the cloud of the Bush administration hanging over the Johannesburg summit."

But other Kyoto supporters are happy that the treaty will not be the centre of attention at Johannesburg.

"We more or less have solved the negotiations. To have major discussions again in Johannesburg would perhaps give the impression that something more has to be done," said Jan Pronk, the former Dutch environment minister who chaired the key climate negotiations before and after the U.S. withdrawal.

Pronk, U.N. Secretary-General Kofi Annan's special envoy to help prepare the summit, wants to see Washington return to the treaty, but said any such discussions at Johannesburg "would not be very useful" because they would be unlikely to succeed.

ENVIRONMENT VS DEVELOPMENT?

The summit's focus on fighting poverty reflects the overriding concern of developing countries where scourges such as water-borne diseases, malaria and AIDS, which kill millions every year, appear far more menacing than global warming.

Many scientists say climate change will exacerbate those problems. Research over the past 10 years has given scientists a better idea of what effects global warming could have on water supplies, agriculture and population migrations.

While some scientists are sceptical about climate change and its effects, a broad-based U.N. scientific panel has predicted that unchecked emissions could raise global temperatures by between 1.4 and 5.8 degrees Celsius this century.

Reports of coral bleaching and melting ice sheets have indicated that global warming may be well under way.

Mick Kelly, an atmospheric scientist at Britain's University of East Anglia, said policymakers would have to take on board detailed forecasts of the impact of climate change on populations to enable countries to cope.