Reuters: The biggest climate talks in history must deliver an ambitious, sweeping agreement to capitalise on pledges by countries to fight global warming, the United Nations said on Sunday. A day before the two-week talks in the Danish capital formally begin, the U.N. climate chief said time was up to agree on the outlines of a tougher climate deal after troubled negotiations have deepened splits between rich and poor nations. “I believe that negotiators now have the clearest signal …