GCOS has defined a list of the Essential Climate Variables (ECVs) that are both currently feasible for global implementation and have a high impact on the requirements of the UNFCCC and IPCC. There are additional climate variables that are important to a full understanding of the climate system and many of these are the subjects of current on-going research, although they are not currently ready for global implementation on a systematic basis.

As our knowledge and capabilities develop, it is expected that some of these variables will be added to the list of ECVs.

The Global Climate Observing System requires observations from land-based and airborne in situ and remote sensing platforms, in addition to satellites. Since no single technology or source can provide all the necessary data, there will be instruments at ground stations, as well as on ships, buoys, floats, ocean profilers, balloons, samplers, aircraft and satellites. This information is then transformed into products through analysis and integration in both time and space.

Information on where and how the observations are taken (meta-data) is absolutely essential, as are historical and palaeoclimatic records that set the context for the interpretation of trends and variability. GCOS stresses that its Plan is both technically feasible and cost-effective. While its implementation is fully dependent on national efforts, success will be achieved only through internationally-coordinated action.