Atmospheric measurements show that the carbon dioxide (CO2) concentration in the atmosphere is currently '385 parts per million (ppm) and rising fast. But this value is a global average that tells us nothing about the regional distribution of greenhouse gas emissions. As the world embraces myriad mitigation strategies, it must gauge which strategies work and which do not. Gaining such understanding will require a greenhouse gas monitoring system with enough accuracy and precision to quantify objectively the progress in reducing emissions.