The Global Atmosphere Watch (GAW) programme of the World Meteorological Organization (WMO) promotes systematic and reliable observations of the global atmospheric environment. Of the long-lived greenhouse gases that are directly affected by anthropogenic activities, carbon dioxide (CO2) has the largest total radiative effect, followed by methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs). Reliable long-term estimates of sources and sinks appropriate to particular emission management scenarios require very high accuracy and precision observations of the abundance and the vertical distribution of CO2 and CH4 as well as their isotopes and related reactive (O3, CO, NOx, SO2, VOC, etc.) gases.