Reducing the amount of CO2 is - strictly speaking - not the immediate goal of carbon trading, the goal is to keep the annual emissions below a certain threshold, and that part is what carbon trading clearly is designed to do. Carbon trading is also called cap-and-trade and the 'cap' part refers to the limitation in emissions. Since, due to the artificially enforced limitations, emissions become just another scarce commodity like wheat or oil, the same trade mechanisms can be applied to it.
