Ann Carlson and Alex Wang are right to emphasize the domestic commitments of both the United States and China in the joint announcement on climate change, which was announced last week in Beijing. While not a formal agreement, this document reflects a meeting of the minds of the world's two largest emitters. The two countries have committed to quite different actions as befits their very different stages of development, but they have both advanced considerably even since Copenhagen in 2009 in their ability to commit to actions for the rest of the decade and for what their likely emissions trajectory will be for decades beyond that.
Concretely, this means both recognize the importance of the 2-degree limit for total warming, that the U.S. commits to a trajectory that reduces total emissions by 83% by 2050 from a 2005 baseline, and that China commits to peaking its emissions by around 2030 and then reducing from there (i.e. not just peaking and staying at a high level). Both also commit to increasing ambition over time.