Computations of Absolute Sea Levels at Tide Gauge Locations Accounting for Variable Subsidence

Correcting the relative trends for sea level rise with the absolute value of the subsidence of the instrument, despite its popularity, is mistaken. This is because the supporting data refer to a much shorter time window for the global positioning system (GPS) position than the mean sea level from the tide gauge. In many worldwide locations, recent subsidence has grown for many reasons, from groundwater withdrawal to mining. This is producing unrealistic values for the absolute rates of rise and acceleration of the sea levels. The absolute rates of rise of the sea levels may become even negative in case of recent significant subsidence. Also, non-existing positive accelerations can be shown in the absolute sea levels. The proposed mathematical approach uses a linear profile for the subsidence rate of the tide gauge instrument from an unperturbed value of zero (or whatever the Glacial isostatic adjustment (GIA) value suggests, if the area is subjected to GIA movement) at the time the tide gauge record starts, to the latest GPS subsidence rate which applies to approach the end of the tide gauge record. In the specific cases considered of Sydney and Fremantle, the produced absolute sea levels are shown to be much more realistic than with previous assumptions.