To get a sense of how hard it is to measure greenhouse gas emissions in China, it pays to visit the Deqingyuan poultry farm on the outskirts of Beijing, where streams of chicken manure are piped from wooden sheds to an industrial gas digester that rises above the ground like a tethered balloon.   Turning waste into kilowatts qualifies Deqingyuan for valuable carbon credits under a UN-backed scheme known as the Clean Development Mechanism. The digester turns all that chicken slurry into na...