The United States. Though Americans make up just 4 percent of the world’s population, we produce 25 percent of the carbon dioxide pollution from fossil-fuel burning — by far the largest share of any country. In fact, the United States emits more carbon dioxide than China, India and Japan, combined. Clearly America ought to take a leadership role in solving the problem. And as the world’s top developer of new technologies, we are well positioned to do so — we already have the know-how.