An historic deal. We'll have to monitor closely to see what actually happens. Here is the gist as reported by Jeff Spross at ThinkProgress:
The pledge commits the U.S. to cut its emissions 26 to 28 percent below their 2005 levels by 2025. This builds on the current target of a 17 percent reduction below that baseline by 2020, and could actually double the pace of emission cuts set by that initial goal — from 1.2 percent a year to as high as 2.8 percent per year. The White House has actually been looking into the possibility of expanding beyond the 2020 target since 2013, and has been involved in occasional interagency meetings to that effect.
For its part, China is committing to get 20 percent of its energy from non-fossil-fuel sources by 2030, and to peak its overall carbon dioxide emissions that same year. China’s construction of renewable energy capacity is already proceeding at a furious pace, and this deal will require the country to deploy an additional 800 to 1,000 gigawatts of zero-carbon energy by 2030. For comparison, 800 to 1,000 gigawatts is close to the amount of electricity the U.S. current generates from all sources combined.
In the US, with virtually no solid Congressional support, it will be interesting to see how such a commitment will fair in the short-term, let alone spanning multiple Congressional and Presidential terms. China, on the other hand, with a centralized one-party rule might be able to enable a more consistent and coherent energy transition strategy. It's also important to remember that most of the carbon in the atmosphere now was put there by Europe and the United States over the past 150 years or so.