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. Beca use 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 ti de 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 a nd absolute gravity have now achieved the equivalent accuracy of a few centimetres that is compatible with the above variability in annual m ean sea levels. Measurements of vertical crustal movements at tide gau ges using these techniques are now being carried out in various parts of the world. A summary is given of the recommendations of an internat ional working group on the geodetic fixing of tide gauge bench marks a nd some of the measurement errors that are now being investigated are discussed. These measurements are of interest to oceanographers workin g on climate change and to geophysicists working on vertical crustal m ovements.