COPENHAGEN (AFP) - UN chief Ban Ki-moon on Tuesday said he expected the Copenhagen climate summit to seal a historic deal on cutting carbon emissions as scientists reported that the first decade of the 21st century had bust records for global warming.

"I am encouraged and I am optimistic," Ban said, reflecting the weight of expectation resting on the 12-day negotiations.

"I expect a robust agreement in Copenhagen that will be effective immediately and include specific recommendations."

Prospects of a breakthrough were bolstered late Monday when the United States declared it would start to regulate carbon dioxide, the principal greenhouse gas, as a dangerous pollutant.

But the cost of failure in the Danish capital was highlighted when the World Meteorological Organisation (WMO) said the Noughties were shaping up to be the hottest since records began.

"The decade 2000-2009 is very likely to be the warmest on record, warmer than the 1990s, which were in turn warmer than the 1980s," Michel Jarraud, the WMO's secretary general, told a press conference.

Jarraud also said the year 2009 would probably rank as the fifth warmest since accurate records began in 1850.

The December 7-18 talks, under the banner of the UN Framework Convention on Climate Change (UNFCCC), are the boldest attempt in a 17-year odyssey to turn back the threat of climate change through consensus.

If all goes well, the conference will yield an outline agreement that sets down pledges by major emitters of greenhouse gases to curb pollution.

It will also set down principles of long-term financing to help wean poor countries off high-carbon technology and beef up their defences against climate change.