China is so big—and is growing so fast—that in 2006 it passed the United States to become the world’s largest producer of greenhouse gases. If China’s emissions keep climbing as they have for the past thirty years, the country will emit more of those gases in the next thirty years than the United States has in its entire history. So the question is no longer whether China is equipped to play a role in combatting climate change but how that role will affect other countries. David Sandalow, the U.S. Assistant Secretary of Energy for Policy and International Affairs, has been to China five times in five months. He told me, “China’s investment in clean energy is extraordinary.” For America, he added, the implication is clear: “Unless the U.S. makes investments, we are not competitive in the clean-tech sector in the years and decades to come.”