World governments are meeting for a key UN climate summit that will attempt to reach a deal on what should replace the Kyoto Protocol, which ends in 2012.
Talks will centre on whether binding targets are needed to cut emissions.
It is the first such meeting since the IPCC, a panel of leading scientists, concluded that climate change was "very likely" caused by human activity.
The two-week gathering in Bali, Indonesia, will also debate how to help poor nations cope in a warming world.
The annual high-level meeting, organised by the UN Framework Convention on Climate Change (UNFCCC), is under pressure to deliver a global agreement on how to cut rising greenhouse gas emissions.
UNFCCC Executive Director Yvo de Boer urged the international community to use the summit to take "concrete steps" towards curbing climate change.
"We urgently need to take increased action, given climate change predictions and the corresponding global adaptation needs," he said in his welcome message to delegates.
"In the context of climate change, projections of economic growth and increases in energy demand over the next 20 years, especially in developing countries, point to the urgent need to green these trends."
Earlier this year, the Intergovernmental Panel on Climate Change (IPCC) published its Fourth Assessment Report (A4R), in which it projected that the world would warm by 1.8-4.0C (3.2-7.2F) over the next century.
Mr de Boer added that the IPCC's conclusion that climate change was "very likely" the result of human activity ended any doubt over the need to act. ( BBC )