As a breakup of the eurozone -- a once seemingly impossible scenario -- becomes increasingly likely, economists are starting to sketch out what a post-euro world would look like. Many are warning that if political leaders don't change course, a breakup of the eurozone would plunge the United States and the rest of the world into a slowdown and possibly another recession. "If Europe turns out badly, it's much more likely we'll go into recession," said Michael Spence, a Nobel Prize-winning economist at the New York University Stern School of Business.