Climate change talks in December will seek a successor to the Kyoto Protocol, which expires in 2012, and Secretary-General Ban Ki-moon is being lobbied to call a crisis summit of world leaders to set ground rules for that meeting.

A study last week by leading scientists blamed human activities such as burning fossil fuels for accelerated warming and UN officials and campaigners hope the findings will spur governments to curb greenhouse gas emissions.

While Ban assesses how much political support he has for calling a summit, de Boer said he had seen more encouraging signs -- including from the United States, the world's biggest emitter of carbon dioxide.

"The US energy secretary, while rejecting the notion of mandatory emission caps, has said he feels there should be a global response to climate change," de Boer said.

"I interpret that to mean we need a global discussion on how we move forward."

In its starkest warning yet, the United Nations on Friday said warming may trigger more storms, floods, droughts, heatwaves and rising seas. It also said it was at least 90 percent certain humans were to blame for most warming over the last 50 years.

That was up from a 66 percent probability in the last report by the authoritative group of 2,500 scientists in 2001.

Kyoto obliged 35 developed nations to cut carbon dioxide emissions by at least 5 percent below 1990 levels, and the challenge for any successor treaty is to bring on board the United States and rapidly growing economies like China and India which were not bound by the original treaty.

Achim Steiner, executive director of the UN Environment Programme, said national politicians must take the lead.