COPENHAGEN - As police cracked down on climate protesters, church bells tolled 350 times yesterday to impress on the U.N. global warming conference a number that is gaining a following, but is also awash in contradictions.

Conference negotiators, meanwhile, 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 "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.

Actually, the world has lived with more than 350 for a while.

The last time the Earth had 350 ppm of carbon dioxide in the air was a generation ago, in the fall of 1989. This year CO2 pushed over the 390 level. When scientists started measuring carbon dioxide in 1958 it was 315.

Since the atmosphere passed the 350 level, ice sheets have been melting and other dramatic changes have been happening. Prominent scientists - notably NASA's James Hansen, one of the earliest to warn about global warming - have said 350 is the only safe level of carbon dioxide in the air.

Still, many economists, political leaders, and even some scientists believe that the worst effects of global warming can be avoided even with less stringent actions.

For a second day, police cracked down on climate activists marching through the Danish capital. More than 200 were detained as police stopped an unauthorized demonstration. Meanwhile, nearly all of the 1,000 detained on Saturday - from Europe, Africa, Asia, and the U.S - were freed without charges.