This July 28 letter from the National Academy of Sciences says that there is no existing way to monitor CO2 emissions around the world — meaning that there would be no way to verify that countries around the world are complying with emissions limits that may be set by international treaty.
Of course, monitoring and verification are just the first steps of enforcement. How would anyone actually compel China, India, Mexico, Russia and the rest of the developing world to comply with limits on emitting greenhouse gases?
Would the UN invade? Would Wal-Mart be forced to stop buying Chinese? What about the sort of international sanctions that have worked so well with Iran and North Korea? A beer summit? What?