Mankind must make deep, dramatic cuts in carbon dioxide and other heat-trapping gases it spews into the atmosphere, and even then there's only an "even-money" chance it can head off catastrophic global warming, leading climate scientists warned Thursday. More than 200 experts, in a rare foray into global politics, issued a declaration at the four-day-old UN climate conference on Bali Island calling for a 50 percent reduction in such emissions by 2050. "We have to start reducing greenhouse gas emissions as soon as we possibly can," said Australian climatologist Matthew England, a group spokesman. "It needs action. We're talking about now."