The two-week conference, the climax of two years of contentious negotiations, convened in an upbeat mood after a series of promises by rich and emerging economies to curb their greenhouse gases. Still, major issues have yet to be resolved.
At stake is a deal that aims to wean the world away from fossil fuels and other pollutants to greener sources of energy, and to transfer hundreds of billions of dollars from rich to poor countries every year over decades to help them adapt to climate change.
Scientists say without such an agreement, the Earth will face the consequences of ever-rising temperatures, leading to the extinction of plant and animal species, the flooding of coastal cities, more extreme weather events, drought and the spread of diseases.
Conference president Connie Hedegaard said the key to an agreement is finding a way to raise and channel public and private financing to poor countries for years to come to help them fight the effects of climate change.
Hedegaard — Denmark's former climate minister — said if governments miss their chance at the Copenhagen summit, a better opportunity may never come.
"This is our chance. If we miss it, it could take years before we got a new and better one. If we ever do," she said.
Negotiations have dragged on for two years, only recently showing signs of breakthroughs with new commitments from The United States, China and India to control greenhouse gas emissions.
But the commitments remained short of scientists' demands, and the pressure was on those major emitters for bigger cuts. Swedish Environment Minister Anders Carlgren, speaking for the European Union, said it would be "astonishing" if President Barack Obama came for the final negotiation session "to deliver just what was announced in last week's press release."
The conference opened with video clips of children from around the globe urging delegates to help them grow up without facing catastrophic warming. On the sidelines, climate activists competed for attention to their campaigns on deforestation, clean energy and low-carbon growth.
Mohammad Shinaz, an activist from the Maldives, plunged feet-first into a tank with nearly 750 liters of frigid water to illustrate what rising sea levels were doing to his island nation.
"I want people to know that this is happening," Shinaz said as the water reached up to his chest. "We have to stop global warming."
Leah Wickham from Fiji, broke down in tears as she handed a petition from 10 million people asking the negotiators at Copenhagen to come up with a deal to save islands like hers.
"I'm on the front lines of climate change," she said.
At stake at the largest and most important UN climate change conference — the climax of two years of contentious negotiations — is a deal that aims to wean the world away from fossil fuels and other pollutants to greener sources of energy, and to transfer hundreds of billions of dollars from rich to poor countries every year over decades to help them adapt to climate change