At the same time, thousands of protestors across Europe Saturday: time to stop talking and make a deal.
Reaching that deal has now been complicated by what's being called "Climate Gate," a string of hacked private e-mails between global climate change scientists in the U.S. and Europe, casting doubts on the very science of which this summit is based, reports CBS News Correspondent Kimberly Dozier.
The e-mails show some of the world's top experts decided to exclude or manipulate some research that didn't help prove global warming exists.
1998 was the hottest year since record-keeping began...but the temperature went down the next year, and it's only spiked a couple times since.
An e-mail exchange in 1999 shows scientists worked hard to demonstrate an upward trend. They talk of using a "trick" to "hide the decline" in global temperatures.
It worked like this: when temperature readings extrapolated from tree rings showed what looked like a decline in temperatures from the 1980s to the present, the scientists added in measurements taken later by more modern instruments, which gave them the answer they wanted.
The scientists say the e-mails are being misinterpreted.
"A lot of charges have been made that I think are quiet unjustified, cherry-picking information and taking it our of context and misrepresenting what it is actually saying," Kevin Trenberth of National Center for Atmospheric Research told Dozier.
Oklahoma Republican Sen. James Inhofe - a climate change skeptic - says the e-mails prove the global warming threat is exaggerated. He wants a congressional investigation.
"We are relying upon that science for a bunch of stuff that we would be doing that would lose a lot of jobs and really affect Americans," Inhofe told Dozier. "I do believe an investigation would show that they clearly have manipulated the data."
The official U.S. position hasn't changed: that climate change is real and needs to be addressed.
"Climate change is happening," White House Press Secretary Robert Gibbs told reporters. "I don't think that's anything that is, quite frankly among most people, in dispute anymore."
Two major carbon producers - India and China - agree, and just announced they will set goals to lower their carbon emissions.
Though Congress hasn't agreed on an American plan to lower emissons, President Obama's visit to Copenhagen at the end of the meeting, instead of the beginning as originally planned, is being taken as a positive sign.
"President Obama said he will come to Copenhagen if a deal is feasible, and I have always said, and we have in the United Nations said, that while nothing is guaranteed, the world is ready for a deal," Achim Steiner, director of the United Nations Environment Programme, said.
But the leaked memos have given ammunition to critics of climate change, so climate change supporters say Copenhagen may only produce a framework for an agreement for an agreement that could be finalized next year.