Conference negotiators went behind closed doors in talks to pin down an elusive new pact on climate, talks in which the figure 350 looms as a goal for true believers, but one that appears impossible based on progress so far.
It refers to 350 parts per million of carbon dioxide in the atmosphere, the highest concentration that some leading scientists say the world can handle without sparking dangerous climate effects.
"It's the most important number in the world," said Bill McKibben, founder of the environmental activist group 350.org. "It's the line between habitability on this planet and a really, really desolate future."
Not everyone buys into that. But an entire environmental group has sprung up around the number, pushing 350 as a goal, sporting it on T-shirts and flags waved by throngs of protesters that marched to the conference center over the weekend. About 100 nations at the U.N. climate summit have signed on to the idea of heading for 350.
Text: APImage: A protestor has his mouth taped with a sticker that reads '350' as he takes part in a protest march towards the United Nations Climate Change Conference 2009 at the Bella Center during the 'Global Day of Climate Action' rally on December 12, 2009 in Copenhagen. (Photo copyright Getty Images)