Oh, I agree with you that China's demographic problems are quite interesting. By some estimates its population is already decreasing and obviously this poses severe challenges in terms of living standards since this will be a drag on economic growth, but unlike the West or Korea/Japan they are not wealthy enough yet to sustain the increased social spending that is likely to be required with an aging population.
Not sure why this is necessarily bad in terms of emissions though. Assuming emissions per capita remain constant a decline in China's population would result in a decrease in emissions.