Reuters: U.S. President Barack Obama’s administration promised to push for a new global treaty to fight global warming at United Nations climate talks on Sunday but cautioned it had no magic wand. “The United States is going to be powerfully and fully engaged,” U.S. special envoy for climate change Todd Stern said at the opening of 175-nation U.N. talks in Bonn. “But we are all going to have to do this together, we don’t have a magic wand,” he told a news conference at the March …