No one knows for sure. A lot of reporters want to “clutch at straws”, but a dose of realism is required too.
You will find a brief survey of some estimates here. Let’s start with a few simpler points, however.
First, China is notorious for making announcements about air pollution and then not implementing them. This is only partially a matter of lying, in part the government literally does not have the ability to keep its word. They have a great deal of coal capacity coming on-line and they can’t just turn that switch off. They’re also driving more cars, too.
Second, China falsifies estimates of the current level of air pollution, so as to make it look like the problem is improving when it is not. Worse yet, during the APEC summit the Chinese government blocked the more or less correct estimates coming from U.S. Embassy data, which are usually transmitted through an app. A nice first step to the “deal” with the United States would have been to allow publication (through the app) of the correct numbers. But they didn’t. What does that say about what one might call…”the monitoring end”…of this new deal?
Third, a lot of the relevant Chinese regulatory apparatus is at the local not federal level (in fact it should be more centrally done, even if not fully federalized in every case). There are plenty of current local laws against air pollution which are simply not enforced, often because of corruption, and often that pollution is emanating from locally well-connected, job-creating state-owned enterprises. Often the pollution comes from one locality and victimizes another, especially in the north of the country. Those are not good local regulatory incentives and it will take a long time to correct them. Right now for instance Beijing imports a lot of its pollution from nearby, poorer regions which simply wish to keep churning the stuff out. The Chinese also do not have anything close to a consistently well-staffed environmental bureaucracy.
Fourth, if you look at the history of air pollution, countries clean up the most visible and also the most domestically dangerous problems first, and often decades before solving the tougher issues. For China that highly visible, deadly pollutant would be Total Particulate Matter, which kills people in a rather direct way, and in large numbers, and is also relatively easy to take care of. (Mexico for instance has been getting that one under control for some time now.) The Chinese people (and government) are much more worried about TPM than about carbon emissions, which is seen as something foreigners complain about. Yet TPM is still getting worse in China, and if it is (possibly) flat-lining this year that is only because of the economic slowdown, not because of better policy.
When will China cap carbon emissions? “Fix TPM and get back to me in twenty years” is still probably an underestimate. Don’t forget that by best estimates CO2 emissions were up last year in China by more than four percent. How many wealthier countries have made real progress on carbon emissions? Even Denmark has simply flattened them out, not pulled them back.
The Chinese really are making a big and genuine effort when it comes to renewables, it is just that such an effort is dwarfed by the problems mentioned above.