Last week, the leaders of the United States and China, the world’s two largest emitters of carbon dioxide, announced a historic deal to reduce their greenhouse gas emissions. President Obama committed the US to cutting its emissions 26 to 28 percent below 2005 levels by 2025, and President Xi committed China to peaking its overall carbon dioxide emissions no later than 2030, the first time it has agreed to such a peak. According to the White House, the US will reduce its emissions by 1.2 percent per year from 2005 to 2020, and then double that rate to 2.5 percent from 2020 to 2025. The two countries also agreed to strengthen cooperation on clean energy research and development, institutionalize the sharing of best practices in city-level leadership, and promote the trade in green goods. Although the deal alone will not be sufficient to stabilize the world’s climate at safe levels, its primary importance is political: creating much needed momentum on the road to the big climate negotiations in Paris next year.