GLOBAL SEA-LEVEL CHANGES AND THEIR MEASUREMENT

Field sea-level data are generally only of local value, being affected by a complex of local, regional and global Processes which operate on different time and space scales. Averages or compilations of local sea-level data may lead therefore to misleading estimations of global sea-level changes and be biased towards predominant factors which are active in the study areas at the time scale considered. Approximate estimations of global sea-level changes can be attempted using proxy data, such as variations of oxygen isotope ratios in oceanic core sediments. These suggest an oscillatory pattern during the last 3 Ma, with glacial-interglacial cycles occurring with a periodicity of about 100 ka and sea-level fluctuations of the order of 100 m. Chronology in oceanic cores is calibrated with geomagnetic reversals, biostratigraphy and the identification of certain peaks in the isotopic curve, which allow a comparison with astronomical cycles. The amplitude of sea-level oscillations is refined through correlation with sequences of Quaternary marine terraces in uplifting areas. Holocene sea-level data show a great vertical dispersion, caused by the superposition of eustatic, isostatic, tectonic and other factors. Similar biases probably exist also for estimations of global sea-level change during longer (several Ma) and shorter (1-100 yr) geological time periods. This is true for tide-gauge data: only 13% of the stations with long enough records indicate a rise between 1.0 and 1.5 mm/yr (which corresponds to values often assumed as ''eustatic''), whereas interpretations of global sea-level rise deduced from tide-gauge records diverge appreciably, with estimates for the past century ranging from 0.5 to 2.4 mm/yr. It is hoped that data from new altimeter satellites (ERS-1, TOPEX/POSEIDON), combined with Global Positioning System geodesy data, will soon clarify this uncertain situation.