Sea level: 300 years of observation

In fact, as the astronomer, Pierre Simon de Laplace, described in his Mémoire sur le flux et le reflux de la mer (1789), Brest is an ideal location to observe the sea level, and systematic observations of the sea level have been conducted there since 1679. Brest, thanks to its harbor extending out into the sea that is yet protected, gives a better visibility of the sea level. By not facing interannual variations due to world or regional weather processes, the evolution of the sea level in Brest is constant and regular with time.

Observations were initially conducted with tide gauges. These measurement tools have developed and become extremely precise and accurate. Tide gauges operated by SHOM (French Navy National Hydrographic and oceanographic Services) since the middle of the 19th century in several urban harbors have evolved and their automatic and continual measurements are now, more than ever, irreplaceable.

The structure of these long series of sea level observations gives a global view to better understand the processes linked to climate change and its evolution. Over three centuries of measurements in Brest have shown a local sea level rise of 30cm, as observed on this graph, with a more significant rise during recent decades. The various measurements taken around the world show that sea level variations are not identical everywhere.

In France, as part of SONEL (long term sea level variations observation system), the evolution of the sea level is calculated for all of the French ports. SONEL provides high quality data collected from tide gauges, to calculate the relative sea levels based on modern geodetic technics to determine the vertical movements and absolute levels of the sea.

Throughout the world, sea level observation represents a quantifiable data of climate change effects because it is one of the physical-chemical indicators the most used by the Intergovernmental Panel on Climate Change (IPCC).

Regular observation of water levels, including during storm episodes, is irreplaceable information for the analysis of extreme levels and wave submersions, which are further proof of current changes. In reality, forecasting these phenomena is essential for tsunami alert centers as well as for submersion wave weather watch. It is also possible to calculate the recurrence of extreme events for prevention policies such as rapid submersions plans, which covers flood risks of submersion waves, sudden floods or confined streaming and dyke bursting.

Consequently, the measures implemented for observation and dissemination of information on water levels is crucial to understand, analyse and prevent climate change manifestations that are potentially amongst the most devastating for lower zones in direct connection with the sea, in the medium term with the slow process of elevation of the mean level, or with violent weather phenomena.

Observations of water level in real time, generated by tide gauges operated by SHOM and its partners (coordination by REFMAR) are available online at: data.shom.fr.