LE BOURGET, France (AP) -- Addressing the twin threats of global warming and extremist violence, the largest group of world leaders ever to stand together kicked off two weeks of high-stakes climate talks outside Paris on Monday, saying that by striking an ambitious deal to cut emissions they can show terrorists what countries can achieve when they are united. "What greater rejection of those who would tear down our world than marshaling our best efforts to save it?" President Barack Obama asked his fellow world leaders.