COPENHAGEN — The largest and most important U.N. climate change conference in history opened today, with organizers warning diplomats from 192 nations that this could be the best last chance for a deal to protect the world from calamitous global warming.

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, but with major issues yet to be resolved.

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 might 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.

Denmark's prime minister said 110 heads of state and government will attend the final days of the conference. President Barack Obama's decision to attend the end of the conference, not the middle, was taken as a signal that an agreement was getting closer.

By The Associated Press