Talks on a new climate pact have started in oil and gas-rich Qatar, where nearly 200 countries will discuss fighting global warming and helping poor nations adapt to it.
The 20-year-old series of conferences has not fulfilled its main purpose of reducing the greenhouse gas emissions that some scientists say are warming the planet.
Attempts to create a new climate treaty failed in Copenhagen three years ago but countries agreed last year to try again, giving themselves a deadline of 2015 to adopt a new treaty.
Several issues need to be resolved by then, including how to spread the burden of emissions cuts between rich and poor countries. That is unlikely to be decided in the Qatari capital of Doha, where negotiators will focus on extending the Kyoto Protocol, an emissions deal for industrialised countries, and trying to raise billions of pounds to help developing countries adapt to a shifting climate.
"We owe it to our people, the global citizenry. We owe it to our children to give them a safer future than what they are currently facing," said South African foreign minister Maite Nkoana-Mashabane, who led last year's talks in Durban.
The UN process is often criticised, even ridiculed, both by climate activists who say the talks are too slow, and by those who challenge the view that the global temperature rise is at least partly caused by human activity, primarily the burning of fossil fuels like coal and oil.
Environmentalists found the choice of Qatar as host of the two-week conference ironic. The tiny Persian Gulf emirate owes its wealth to large resources of gas and oil and emits more greenhouse gases per capita than any other nation.
Yet it has not announced any climate action, and former Qatari oil minister Abdullah Bin Hamad Al-Attiyah did not do so when he opened the conference.
"We should not concentrate on the per capita (emissions), we should concentrate on the amount from each country. I think Qatar is the right place to host" the conference, he said.
The concentration of greenhouse gases such as carbon dioxide has jumped 20% since 2000, according to a report released last week.
The goal of the talks is to keep the global temperature rise under 2C compared to pre-industrial times.
But efforts taken so far to rein in emissions, reduce deforestation and promote clean technology are not working. A recent projection by the World Bank showed temperatures are expected to increase by up to 4C by 2100.
"Climate change is no longer some distant threat for the future, but is with us today," said a Greenpeace spokesman. "At the end of a year that has seen the impacts of climate change devastate homes and families around the world, the need for action is obvious and urgent."
Scientists also say that extreme weather events such as Hurricane Sandy's onslaught on the US will become more frequent as the Earth warms, although it is impossible to attribute any individual event to climate change.
The Kyoto Protocol, adopted in 1997, is the most important climate agreement reached in the U.N. process so far. It expires this year, so negotiators in Doha will try to extend it as a stopgap measure until a wider deal can be reached.
The problem is that only the European Union and a handful of other countries - that together are behind less than 15% of global emissions - are willing to put down emissions targets for a second commitment period of Kyoto.