The cornerstone of climate science is provided by the “climate data records (CDRs)” that provide the observed record of climate and its past changes. CDRs provide the basis for assessing our current understanding and the attribution of past changes in climate (e.g. the activity of IPCC and UNFCC), and are used, in part, to validate the models used for climate prediction. The most essential characteristic of a CDR is the stability of its uncertainty characteristics over time. While grounded in observations, CDRs are thus much more than the raw observations: rather, they are a synthesis of related observations, often from multiple instruments, taking account of anything that disturbs the homogeneity of the time series: e.g. variations in spatial coverage, orbital drifts, instrument degradation, and inconsistencies between the calibration of different versions of the same instrument.