On the uncertainty associated with validating the global mean sea level climate record

Satellite altimetry provides the ongoing sea level climate data record that provides evidence for one of the most significant manifestations of climate change on Earth. External and independent validation of satellite altimetry is a core component of mission design, providing confidence in such a seminal climate record. The global tide gauge network, corrected for the effects of vertical land motion, forms one of a suite of approaches used to validate satellite altimetry. Used as a tool to identify potential systematic error, the altimeter minus tide gauge approach ultimately leads to an improved geophysical data record through iterative diagnosis, correction and reprocessing of the mission record. A recent example includes the detection of a small but significant drift in the early part of the record associated with the TOPEX record. Here, we return to the approach that quantified the apparent drift in the TOPEX record and focus on further elucidating the uncertainty of that technique as a function of mission duration. We show that approximately 2.9 years is required to reach a validation uncertainty of ±1 mm/yr (1σ). This result appears optimistic by a factor of 1.5–2.2 in comparison to an error budget approach reported in the literature. Our results highlight the challenge of validating the altimeter record using a sparse and irregularly distributed network of tide gauges and points towards possible areas of future improvement in the validation approach.