Relative sea level and subsidence in Sydney
Description
The relative sea levels are growing not only because of the thermosteric effect, aka the increase of volume of the ocean waters by thermal expansion and melting of ices on land, but often also because the land is sinking, as anthropogenic subsidence by water withdrawals, mining, infrastructure extension, and similar effects is by no way negligible compared to the thermosteric effect. Both relative sea level and subsidence have in recent times (end of the 1800s/beginning of the 1900s to present) a parabolic profile. The subsidence rate in 1886, when the tide gauge of Sydney Fort Denison started recording, is suggested to have been 0 mm/yr (Peltier Glacial Isostatic Adjustment (GIA) Model VM4). The current subsidence measured by Global Positioning System (GPS) indicates a subsidence rate of -1.640 mm/yr since 2012 (figure in png file NGL FTDN). This permits to define a parabolic subsidence profile 1886 to present (analysis in excel file SYD). The average subsidence throughout observation is– 0.82 mm/yr. The subsidence acceleration can be approximated as -0.01223 mm/yr². Since 1886, the relative sea level at Syndey Fort Denison has risen at a rate of 0.74 mm/yr subjected to an acceleration of 0.01810 mm/yr² (data in txt file PSMSL SYD, analysis in excel file SYD). Based on the above figures, the thermosteric effect accounts for 0.74 – 1.64 = -0.90 mm/yr of relative sea level rise (which is negative), and 0.01810 - 0.01223 = 0.00587 mm/yr² of relative sea level acceleration. In Sydney, subsidence completely explains the relative sea level rise and about 66% of the relative sea level acceleration.