Changes in sea level are currently monitored by a network of tide gauges installed along the coast, and by altimetry satellites since the 1990s. Spatial coverage of tide gauges at the beginning of the 20thcentury was fairly limited, and only a few port stations can be traced back to the 19thcentury . What's more, the level observed locally is affected by a geophysical component first highlighted in the 18thcentury by Anders Celsius. This post-glacial isostatic readjustment continues to this day, as shown by current GPS measurements and physical modelling of the phenomenon. Correcting for this geophysical component leads to biases and uncertainties, as illustrated by the mean sea level record for the Baltic Sea. On a global scale, mean sea level has been rising steadily by around 25 cm since the middle of the 19thcentury .
To go further back, we need to rely on biological indicators such as micro-atolls in the intertropics, algal sidewalks on rocky coasts or fossils of flora and fauna adapted to the salinity gradients of coastal zones (e.g. salt marshes ). Several examples of sea-level series are presented for the coasts of Provence and Brittany, the Pacific atolls, the east coast of the USA and Greenland. As with current records, it is crucial to correct millennial series for the modelled effects of post-glacial readjustment, which often dominate the amplitude of observed variations (e.g. on the North American coast or the Pacific atolls).
The residual curve of sea level anomalies over the last millennium shows small fluctuations of around 10 cm in amplitude, with a maximum between 400 and 1000 followed by a minimum between 1200 and 1850. Details of the causes of sea-level variations are available for the period covered by satellite measurements. One third of the current 3 mm/year rise is due to thermal expansion of the ocean, and two thirds to freshwater flows from the continents, notably the Antarctic and Greenland ice caps and mountain glaciers. Both components are ultimately linked to global warming, which is spreading into the ocean and causing continental ice sheets to melt. Coupled climate models still have difficulty simulating these two components over the last millennium, but it is possible to use statistical relationships calibrated over the last century. The temperature anomalies observed over the last two millennia can be converted into sea-level terms, leading to variations of around 10 cm with a maximum corresponding to the CMO and a minimum for the PAG, in line with sea-level observations corrected for geophysical effects.
