ABSOLUTE SEA-LEVEL MEASUREMENTS, CLIMATE-CHANGE AND VERTICAL CRUSTAL MOVEMENTS

Annual mean sea level observations from tide gauges around the world, usually show interannual and decadal variations of order 5-10 cm. Because of these variations, several decades of data are normally required for a reliable determination of the secular trend in mean sea levels. Tide gauges only give relative sea level trends, since a rise in sea level cannot be distinguished from a subsidence of the crust at the tide gauge and thus estimates of the ''global'' rise of mean sea levels must be corrected for these vertical crustal movements. A brief review is given of recent work on relative mean sea levels, which shows the importance of these land movements. Modern space geodetic techniques and absolute gravity have now achieved the equivalent accuracy of a few centimetres that is compatible with the above variability in annual mean sea levels. Measurements of vertical crustal movements at tide gauges using these techniques are now being carried out in various parts of the world. A summary is given of the recommendations of an international working group on the geodetic fixing of tide gauge bench marks and some of the measurement errors that are now being investigated are discussed. These measurements are of interest to oceanographers working on climate change and to geophysicists working on vertical crustal movements.