GCOS is reporting under UNFCCC Workstream “Systematic Observations”:
https://unfccc.int/topics/science/workstreams/systematic-observation
This information paper provides an easy-access informal compilation of relevant mandates on systematic observation. It provides, in chronological order, all relevant decisions of the conference of the Parties (COP); and conclusions adopted by the COP, the Subsidiary Body for Scientific and Technological Advice (SBSTA) and the Subsidiary Body for Implementation (SBI):
GCOS' Mandate for Systematic Observations under the UNFCCC