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This year's UN climate summit has opened in Mexico, with expectations of significant progress generally low.

In contrast to last year's summit in Copenhagen, there is a general belief that no new global deal will emerge.

The basic divisions are as stark as ever, and some nations and observers are arguing that smaller, less formal pacts are the way forward.

However, countries appear to want the UN process to survive, so major public disputes are unlikely.

Another contrast to Copenhagen is that very few heads of state or government are expected to attend.

The summit takes place against the backdrop of forecasts that carbon emissions are set to start rising again after a brief interlude from the recession, and analyses showing that countries' current pledges are not big enough to keep the global average temperature rise within bounds that most nations say they want.

On Friday, former UK deputy prime minister Lord Prescott - who led the UK delegation to the 1997 climate summit that agreed the Kyoto Protocol, and played a key role in negotiations there - called for governments to acknowledge that the UN process is failing.

"The legal framework is falling apart. Let us be practical, recognise that it has happened, and go for an alternative," he said.

"Each country is still trying to cut carbon emissions in its own way - the US has a programme, the EU has a programme, China has a programme.

"Let each of them go ahead voluntarily... but let's at least agree on a few basics of fairness and transparency."

However, this approach is diametrically opposed to the stance taken by some developing countries, which insist that Western nations must live up to promises they have made at key points along the UN route that began in 1992 at the Rio de Janeiro Earth Summit.

This stance is echoed by most environmental organisations, which see the UN framework as the only way of providing a comprehensive raft of solutions to the issues posed by climate change.

"Only an equitable, comprehensive and legally binding agreement will bring the much needed international commitment to manage the climate crisis," said Stewart Maginnis, director of environment and development with the International Union for the Conservation of Nature (IUCN).

"What governments should focus on in Cancun is ensuring that confidence in the UNFCCC (climate convention) process is rebuilt, which will bring us a step closer to that final deal."

For some governments, Cancun will be a success if it generates a constructive atmosphere of constructive dialogue, rather than descending into the rancour that characterised the Copenhagen gathering.

Others, however, are looking for concrete progress on issues such as transfer of clean technologies from the industrialised world to developing countries, the provision of funds from the west to help poorer nations adapt to climate impacts, and reducing deforestation.

But even these matters are unlikely to prove straightforward.

Developed countries have put forward "fast start" finance, as they pledged to do in Copenhagen.

But not all of it is transparently new money, as it is supposed to be.

Movement towards an agreement on Reducing Emissions from Deforestation and forest Degradation (REDD) has aroused the ire of some indigenous peoples, who regard it as an excuse for not cutting carbon emissions from fossil fuel burning.

"REDD and other false solutions like agrofuels and carbon sink plantations are ways to grab productive land in the South so that more cheap resources and food can be accessed by industrialised countries," said Blessing Karumbidza from the South African group Timberwatch.

They are also calling on any REDD deal to respect fully the rights of indigenous peoples who live in and off forests.

The main issues, though, are economic and geopolitical in nature.

Many governments are worried about losing their economic competitiveness through curbing emissions, particularly against key global or regional rivals.

Main players in the summit itself are optimistic that something worthwhile can emerge, despite these obstacles.

Christiana Figueres, executive secretary of the UN climate convention, recently said: "At this point, everything I see tells me that there is a deal to be done.

"Cancun will be a success, if parties compromise."

The lesson of Copenhagen, though, is that this is likely to prove a very big "if".