Bloomberg: Progress toward adopting a treaty limiting climate change will depend on whether countries stick to goals outlined in the Copenhagen accord last year, and talks remain challenging, the chief U.S. negotiator said. “It’s been very difficult,” Todd Stern, U.S. special envoy for climate change, said today in a interview in Washington. “Countries have backtracked,” he said without identifying specific nations. U.S. President Barack Obama and Chinese Premier Wen Jiabao led nations in …