Altimeter accuracy requirements for detecting changes in sea level rise

All nations with a maritime border are concerned about changes in sea level, with an increase in sea level leading to flooding of coastal areas, damage to property, salinification of fresh water aquifers and destruction of valuable agricultural land. Around the change of the millenium global sea level rise was estimated to be 2.7 mm/yr, but various climate models have suggested the rate of rise will increase markedly necessitating governments to take action more swiftly. This report looks briefly at the accuracy requirements and time series duration needed to be able to detect reliably a significant change in the rate of sea level rise. One constraint on detecting an increased trend is the natural interannual variability of the climate system, which implies that a minimum duration of around 10-20 years is required in order to detect a trend with confidence to within 1 mm/yr. Added to this will be the effect of efficiencies in the observing system. This is investigated through a series of simulations, mimicking the sampling of a long-time series by altimeters with random bias and drift terms. Whilst not directly addressing issues relating to the choice of orbit configuration, this preliminary work prototypes a methodology for assessing the design of a long-term altimetry observing system. Naturally the maintenance of multiple altimeter systems plus a representative global network of tide gauges provides the best basis for such monitoring. However, considering only a single system, one notes that the required dataset duration can be between 10 and 60 years depending upon the quality of the altimeter missions. Due to the difficulty of tying separate missions to a common datum, a single short interruption to precise monitoring may add more than a decade to the time required to detect an increased rate of sea level rise