Officials from 192 countries are gathering in Denmark's capital Copenhagen for talks aimed at reaching a new global deal on climate change.
Any agreement is intended to supplant the 1997 Kyoto Protocol, aimed at curbing greenhouse gas emissions.
The UN official panel on climate change says emissions must be limited to avoid dangerous global temperature rises.
Ahead of the talks, the panel hit back at claims that human influence on global warming has been exaggerated.
The Intergovernmental Panel on Climate Change (IPCC) said it was standing by its findings in response to a row over the reliability of data from a UK university.
Hacked e-mail exchanges from East Anglia's Climatic Research Unit have prompted climate change sceptics to claim that data has been manipulated.
Expectations lowered
The Copenhagen talks are being held in recognition of the fact that the Kyoto Protocol's targets are not sufficient to avoid impacts projected by the IPCC, and run out in 2012.
They are set to go on for nearly two weeks, with dozens of world leaders set to attend the later stages.
These include US President Barack Obama, UK Prime Minister Gordon Brown, French President Nicolas Sarkozy and Indian Prime Minister Manmohan Singh.
Almost all countries attending the meeting agree a deal must be reached.
The main areas for discussion include:
Environmental activists are planning demonstrations in Copenhagen and around the world on 12 December, in an attempt to encourage delegates to reach the strongest possible deal.
Tens of thousands marched in London and other UK and European cities on Saturday to urge action.
But expectations for the meeting have fallen, correspondents say.
Whatever is agreed will no longer have a legally binding basis. Instead, experts hope to produce a framework which could lead to the signing of binding final agreements by next year.
The European Union, which had sought a legally-binding agreement, has offered a 20% cut in its emissions from 1990 levels by 2020, rising to 30% in the event of a global agreement.
The US is pledging to cut its emissions in several stages, beginning with a 17% cut from 2005 levels by 2020.
India and China have both agreed to reduce their "carbon intensity", a measure of the amount of carbon dioxide emitted per unit of GDP.
Washington is currently unable to commit to its pledges for the talks, as a bill to cap its emissions is currently stuck in the Senate and will not be passed before the new year.
Australia is in a similar position, after its opposition-controlled Senate rejected a bill to curb emissions.