Sign a treaty.
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Climate change is a global problem – it needs a global solution. In 2009, Obama tried and failed to broker a global agreement to cut carbon pollution; in 2015, the nations of the world are meeting in Paris to try again to forge a legally binding agreement. Unlike the last time, the U.S. will really have to lead the way. A deal is likely; the question is, will it be tough enough to make a difference?