One reason--maybe the biggest--it's important for the U.S. to cut its emissions is to give us moral authority in our dealings with other nations. As the world's largest consumer and one of the largest/wealthiest economies, we drive much of the worldwide consumptive behavior. And if we expect other, particularly poorer, nations to help with reducing emissions, we need to show real leadership by doing so ourselves. Do as we say, not as we do is a bad way to lead in negotiations.