365 Days of Climate Awareness 14 - Relative Sea Level

 

365 Days of Climate Awareness

14 - Relative Sea Level

Relative sea level is the elevation of water compared to a datum (established baseline). Historically the datum would be local, a function of the natural tide range at any given location. Averaged over a long period of time--at the bare minimum, one lunar cycle of new moon to new moon, but ideally more than the 19-year metonic cycle, after which the moon's phases repeat on the same days of the year (important because the earth’s orbit around the sun is elliptical, and therefore the sun’s gravitational effect on tides is not uniform throughout the year).

From this data mean high and low tides can be computed and correlated to a benchmark, a nearby fixed object whose elevation is known geodetically. In this way local water level measurements can be correlated to others around the world. (The National Geodetic survey keeps a database of thousands of tidal benchmarks and geological monuments around the country as control points, so local work can be tied into the international geodetic system.)

It is local tidal gauge measurements, typically every six minutes, which provide NOAA, the National Oceanographic and Atmospheric Administration, with the high-resolution data needed for safety-of-navigation and research purposes. Satellite altimetry provides global extent of observations not possible with coastal instruments, but a typical spatial resolution is 0.25 degrees, or 15 nautical miles horizontally, and slightly over 1 cm vertically.

Vertically the accuracy of satellite altimetry is comparable in to tidal gauges. But local tidal stations are an important quality check on the satellite data and also ensure data at important coastal locations for shipping and research. Furthermore, tidal gauge data extends back as far as 1700 (Amsterdam), and where the data quality can be reliably estimated, forms an important data set for tracking sea level rise.

Tomorrow: estimating paleo sea levels.

Be well!