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.