The need for observations is formally addressed through the United Nations Framework Convention on Climate Change (UNFCCC), which has charged the Global Climate Observing System (GCOS) with the responsibility for defining requirements for observations relevant to climate change, both in situ and space-based. GCOS works with partners to establish requirements and to ensure the sustained provision of reliable physical, chemical and biological observations, building on relevant observing systems.
GCOS has identified a set of geophysical variables, called Essential Climate Variables (ECVs), which need to be observed to obtain evidence of climate change and to support climate research and emerging climate information services. ECVs are grouped into three categories: Atmospheric, Terrestrial and Oceanic.
In view of the relatively slow dynamics of climate change, observations of ECVs have to be accurate, well-calibrated and homogeneous, to generate time-series than span decades; these data records are referred to as Climate Data Records (CDRs). The longest CDRs are used to distinguish climate trends from shorter-term climate variability. CDRs can be used alone to analyse climate variability and change, or can be ingested by numerical Earth system models that are used to reconstruct (“reanalyse”) consistent climate records that utilise a broader range of ECVs.