A continuous watch on a changing sea
In the harbor of Bonifacio, on the southern tip of the French island of Corsica, a scientific instrument has been diligently recording the height of the sea since 1883. This device, a tide gauge or marégraphe, provides one of the longest and most complete sets of sea-level data in the world. Initially, these measurements were significant for maritime navigation, helping ships safely navigate the small tides and currents of the Bonifacio Strait. Today, its long, unbroken record has a different purpose: documenting the effects of global climate change.
The original instrument was likely a mechanical float gauge. This type of device operates on a simple principle: a float sits inside a vertical tube called a stilling well, which is connected to the sea. The well protects the float from waves, allowing it to measure the true water level. The float's vertical movement would be transferred by wires and pulleys to a pen, which would trace a continuous line on a paper chart on a rotating drum. While modern gauges now use digital encoders, radar, or pressure sensors to record data electronically, the continuous record at Bonifacio has been maintained, tying the old mechanical measurements to the new digital ones. The data collection is managed by SHOM (Service Hydrographique et Océanographique de la Marine), the French naval hydrographic and oceanographic office.