Agreed, China’s population has stabilized, but their carbon emissions are the largest in the world since 2006 and growing at rates far exceeding the U.S., EU and most the developing countries. Their one child policy has worked fairly well, but someday soon they will have an enormous aging population problem to deal with.
As far as carbon leakage, this is a major gap in nearly all national/international climate treaties and agreements. As far as how much of this issue contributes to developed countries ‘true carbon footprints’, I have yet to find a reasonable study on the subject. The bottom line is that carbon emissions directly from developing countries are growing at rates much greater than developed countries (re. recent IEA or EIA/IEO recent studies-forecasts). Developing countries coal and petroleum consumption is rapidly increasing while developed countries, such as the U.S. is in significant decline.
Perhaps to control the apparently uncontrollable world carbon leakage, this issue should become a future carbon tax focus. But, would not such an action be discriminating against the growing populations in developing countries.