Reuters: President Barack Obama’s top climate change negotiator on Thursday said international talks to cut carbon emissions were “difficult,” adding that the U.S. Senate must pass a domestic bill to fight global warming to give the talks a boost. “Let me say bluntly that the tenor of negotiations in the formal UN track has been difficult,” said Todd Stern, Obama’s special envoy for climate change. In prepared testimony to the House energy independence and global warming panel, Stern …