The United States and China are the world’s two largest economies, two largest consumers of energy, and two largest emitters of greenhouse gases. Together we account for about 40 percent of the world’s emissions. We need to solve this problem together because neither one of us can solve it alone. Even if the United States somehow eliminated all of our domestic greenhouse gas emissions, it still wouldn’t be enough to counteract the carbon pollution coming from China and the rest of the world. Likewise, even if China went down to zero emissions, it wouldn’t make enough of a difference if...