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 se a-level data may lead therefore to misleading estimations of global se a-level changes and be biased towards predominant factors which are ac tive in the study areas at the time scale considered. Approximate esti mations 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 glac ial-interglacial cycles occurring with a periodicity of about 100 ka a nd sea-level fluctuations of the order of 100 m. Chronology in oceanic cores is calibrated with geomagnetic reversals, biostratigraphy and t he identification of certain peaks in the isotopic curve, which allow a comparison with astronomical cycles. The amplitude of sea-level osci llations is refined through correlation with sequences of Quaternary m arine terraces in uplifting areas. Holocene sea-level data show a grea t vertical dispersion, caused by the superposition of eustatic, isosta tic, tectonic and other factors. Similar biases probably exist also fo r estimations of global sea-level change during longer (several Ma) an d shorter (1-100 yr) geological time periods. This is true for tide-ga uge 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 ass umed as ''eustatic''), whereas interpretations of global sea-level ris e 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 d ata from new altimeter satellites (ERS-1, TOPEX/POSEIDON), combined wi th Global Positioning System geodesy data, will soon clarify this unce rtain situation.