There is scant sign of the dramatic cuts in emissions of greenhouse gases needed to stop global warming exceeding 2C and devastating vast areas of the planet.
Failure to achieve meaningful progress could seal the fate of hundreds of millions of people living in some of the poorest parts of the world and in greatest danger from rising sea levels, drought and famine.
Delegates to the conference were urged by climate experts, NGOs and by the people whose lives are most likely to be destroyed not to miss this opportunity for progress.
The fudge of pledges to reduce emissions that came out of last year's discredited Copenhagen climate change conference are not enough to give even a 50/50 chance of keeping warming to within 2C. Leading climate experts warned last night that the world risks the potentially catastrophic effects of changes to the climate triggered by a global average temperature increase expected to reach 3C or even 4C by the end of the century. The Arctic ice cap would disappear, and the west Antarctic ice sheet could collapse. Entire island nations would sink under rising sea levels. Summer temperatures in southern England could reach 45C.
Delegates to Cancun will try to hammer out a deal to restrict warming to 2C. Other issues to be discussed include funding for climate adaptation, sharing of green technologies and detail of what will or won't count towards meeting emissions targets. A key item on the agenda will be the extension of the Kyoto protocol, which is due to expire in 2012.
Expectations are low 11 months on from the Copenhagen summit which descended into farce, with a globally binding deal abandoned in favour of a vague statement of intent – the Copenhagen accord. The lack of a result was exacerbated by a backlash against climate science by sceptics, some funded by fossil fuel companies. The net effect has been yet more delay in the international response to the need to cut emissions.
Despite this, British government ministers admitted there is virtually no chance of the deal that scientists and campaigners say is needed. Chris Huhne, the Energy Secretary, said: "We want to see progress, which means we get within shouting distance of a serious deal that we can rely on to tackle this massive problem." Writing in today's Independent on Sunday, Huhne describes the talks as the "best chance" of keeping global temperature rises within 2C. But he cautions: "We should not expect an 'instant coffee' deal – just add water and stir. It takes time to get negotiations right."
This will be little comfort to people already suffering from dramatic climate changes – the 1.7 billion of the world's poor living on what Christian Aid calls "the climate change frontline". Seidou Samba Guindo, 67, chief of Anakila village in drought-stricken Mali, is one of a growing number of climate change victims calling on politicians to act. "We need help – emergency help. My message is there have been many meetings but no agreement. They must find a solution."
Lord Professor Julian Hunt, former director-general of the UK Met Office, said: "Global warming is the greatest danger to humanity in the 21st century. We simply cannot afford to see the shambles of Copenhagen repeated in Cancun." Professor Kevin Anderson, director of the Tyndall Centre for Climate Change Research, warned: "Our commitments to reduce emissions are in line with a 4C future but we are planning for the impacts of only 2C. This is the worst of all worlds."
The evidence for man-made global warming has become "even stronger" over the past year, according to the Met Office. It revealed new data showing that global temperatures had risen over the past decade.
The chances of keeping global warming within a 2C threshold are "increasingly implausible" said Professor Michael Grubb, a member of the Committee on Climate Change. He added: "We will be damned lucky, given what things look like at the moment, if we manage to keep the atmosphere below 500 parts per million and most scientists would translate that to be probably in the region of 3C."
To have even a reasonable chance of keeping an increase to within 2C, global emissions would need to peak by 2020, with deep cuts in that decade and a halving of emissions by 2050, according to Lord Adair Turner, chair of the Committee on Climate Change.
Cancun is a "make or break opportunity to save the lives of millions of children whose lives will be made even worse because of climate change", said Lydia Baker, Save the Children's humanitarian policy adviser.
Tim Gore, Oxfam's senior climate change policy adviser, added: "Climate funding holds the key to unlocking the talks and steering the world to a global solution that tackles the threat and the reality of climate change." But continuing tensions between rich and poor countries, along with the failure of the US to lead by example, make the chances of an agreement remote. Developing countries, including China and India, are anxious to avoid measures that would curtail their economies.
Funding is a key stumbling block, with countries yet to agree on how they will achieve the "goal" of $100bn a year in funding by 2020 for developing countries. Christiana Figueres, executive secretary of the UN Framework Convention on Climate Change, has described funding for climate change adaptation as "the golden key to an outcome in Cancun. Without it, there is little to discuss".
Attempts to introduce new US energy legislation earlier this year were defeated in the Senate. And the resurgence of the Republican Party could set a global deal back by years, with a "witch-hunt against climate scientists and officials", warned Joan Walley MP, chair of Parliament's Environmental Audit Committee. She added: "Unmatched by almost any other major party across the developed world, the Republican right has rejected climate science absolutely."
Additional reporting by Pavan Amara
What the experts say
"It is almost two decades since the Rio Earth Summit... the world is awash with well-meaning words but little to no sign of courageous leadership. It is such leadership that needs urgently to be forthcoming from negotiations in Cancun."
Professor Kevin Anderson, Director, Tyndall Centre for Climate Change Research
"The international system has been built on a principle of the developing world following the industrialised world... if the US is unable to deliver, and if India and China and others stick to the view that they won't do much until the US does, we really are stuffed."
Professor Michael Grubb, Member, Committee on Climate Change
"Cutting CO2 in the short term is necessary for avoiding a 2C rise. This means having policies in place that deliver global emission reductions by 2015. The rate of emission growth driven by globalisation makes this highly unlikely."
Dr Alice Bows, Sustainable Consumption Institute, University of Manchester
"Ideally we would see agreement on this pathway [halving emissions by 2050]. But in practice this is unlikely, particularly given the political situation in the US. However, this should not stop countries from continuing to reduce their own emissions unilaterally."
Lord Adair Turner, Chair, Committee on Climate Change
"The most probable outcome if you look at the negotiations is 3C or 4C, and that would be terrible. This would be very damaging to the countries of the world and therefore we've got to do all we can to bring it back to more like 2C."
Sir John Houghton, Co-collector, Nobel peace prize 2007 on behalf of the IPCC
"In war, people make inventions at a very rapid rate because you are in a very stressed situation, and I think the world is in that situation. Therefore, it is very important to have meetings like Cancun where people share information."
Lord Professor Julian Hunt, Former director-general of the UK Met Office
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