Sea level changes: the data available at the PSMSL and SONEL and the results of satellite altimetry

Abstract

Abstract Sea level changes are a complex and quite discussed topic in the media, mostly because of their relationship with “climate change”. The first instruments to evaluate sea level changes are the Tide Gauges (TG) that began to register data in 1700, at Amsterdam. The TG registrations are validated and homogenized, to be comparable, by PSMSL (Permanent Service for Mean Sea Level). The longest record is the one in Brest (France) that begins in 1807. They register the Relative Sea Level (RSL). In fact, the data show that the vertical movements on the continent strongly affect the RSL curves. The recent implementation of GPS near the TG allows to evaluate the “absolute” sea level changes and calculate the respective trends. The average values, including some “suspect” outliers, is about 2 mm/yr. However, due to the lack of existence of GPS near all the TG and the completeness/duration of TG curves, only 164 stations are registered as defining a probable eustatic, “absolute” sea level trend. Since 1993 it is implemented a satellite altimetry system that used several satellites and is improving the techniques and models to allow the definition of a global sea level change. However, their results (3,55 mm/yr) are much different from the painstaking TG trends, constructed through more than 2 centuries and complemented by the recent GPS information. It is a good technique to show the marine currents, their development, and the influence of warm water in the middle of the oceans, affecting local sea level. But it seems inappropriate, right now, to give the “absolute” sea level trend near the coastline, where this data is most needed.