"This is our chance. If we miss it, it could take years before we get a new and better one, if we ever do," said Connie Hedegaard, president of the climate conference and Denmark's former environment minister.
Further momentum came from the U.S. Environmental Protection Agency's declaration that greenhouse gases endanger human health, clearing the way for regulation of emissions without new laws passed by Congress, and signaling that President Barack Obama's administration is prepared to push for significant controls in the U.S. if Congress doesn't act on its own.
Protesters staged demonstrations in Canada and Denmark, dangling from the roofs of the Parliament Buildings and unfurling climate-change banners in Ottawa, and immersing themselves in frigid water in Copenhagen to highlight flooding risks. But negotiators say countries are still far apart on many of the meaty issues that will make up a legally binding treaty to lower emissions through to 2020.
Yvo de Boer, the United Nations climate chief, said those issues will only be overcome with a new push from all countries that re-examine talks with an eye to signing a deal.
"The time for formal statements is over. The time for restating well-known positions is past," he said. "Copenhagen will only be a success if it delivers significant and immediate action."
Even environmentalists, normally malcontents when discussing the intentions of countries in the UN forum, reported optimism, buoyed by commitments in recent days from the United States, China, India, South Africa and other major emitters to cut greenhouse gases, blamed for global warming.
Indian Prime Minister Manmohan Singh, whose country is the fourth-largest greenhouse-gas emitter, said he would attend a closing summit in Copenhagen, joining 104 other leaders, a move hailed by Danish Prime Minister Lars Lokke Rasmussen.
"Together these 105 leaders represent 82 per cent of mankind, 89 per cent of the world's GDP and 80 per cent of the world's current emissions," he said. "If this group of assembled leaders can agree, then their decisions can change the course of the planet."
There is a "good energy" at the conference, said Keya Chatterjee, the U.S. director of World Wildlife Fund's climate program.
The meeting is also chock-a-block with debate over contentious issues such as financing for poor countries, how to account for emissions and how to share technology between nations. Some of those problems, which are contained in an unworkable thousand-page negotiating text, are what make it unlikely a legally binding treaty will be signed in Copenhagen by December 18.
Perhaps the most sensitive debate is over the science underpinning climate change itself.
A number of countries may have been thinking it, but Saudi Arabia was the only country to say that it had doubts about working toward a deal with multiple investigations ongoing into the work of climate scientists in Britain to see if they suppressed evidence that refutes the global warming theory.
It has been whispered that the sentiment is echoed in Alberta, though politicians there are certainly more media savvy than to share any misgivings and put Canada further offside than it already is.
The UN's lead climate scientist took the occasion of the opening day of the Copenhagen conference to shoot back at his critics and pull on-the-fence skeptics to what he said was the side of hard science.
"The recent incident of stealing the emails of scientists at the University of East Anglia shows that some people would go to the extent of carrying out illegal acts, perhaps in an attempt to discredit the (Intergovernmental Panel on Climate Change)," said Rajendra Pachauri.
The stolen emails show climate researchers discussed keeping some scientific papers out of the IPCC's report, which has formed the basis for two years of UN-led climate-treaty talks.
In Ottawa, 20 Greenpeace protesters scaled the Parliament buildings and unfurled banners accusing Prime Minister Stephen Harper and Liberal Leader Michael Ignatieff of failing to fight global warming. They were brought down and arrested after three hours.
Shortly after, Canada was awarded one of three "Fossil of the Day" prizes by environmental groups in Copenhagen. Canada was criticized for vowing not to shift off its emission reduction targets by the "hype" around the summit. It came in third behind "all industrialized countries," the inevitable villain in a climate change debate, and the trio of Sweden, Austria and Finland. Those countries are seeking loopholes in a new deal that would allow them not to count emissions from forests so they can continue cutting down trees for profit.
Canada's chief climate negotiator, Michael Martin, brushed off the dubious distinction. "I've never been sneered at here," he said. "I don't think we can get a fair and accurate perception of the conference of the parties based on the press release that is issued by the non-governmental organizations."