Asked by: Damien Butler, Worthing
It’s not that simple. Although average height of the sea is rising, this does not affect Mean Sea Level (MSL). MSL is a reference level that refers to historical measurements. In the UK, MSL is defined by data that was collected from tide gauges in Newlyn in Cornwall between 1915 and 1921.
But mountain elevation isn’t measured relative to sea level anyway. Now, map makers use a geoid, which is a mathematical representation of the Earth, to define the reference height. The geoid is the hypothetical shape that all the oceans of the world would take if they were only affected by gravity and the Earth’s rotation. In other words, it excludes the effects of weather and tide. The geoid is determined by measuring the gravity fluctuations over the Earth’s surface, due to the varying thickness of the crust and densities of magma below.
The reference geoid does get adjusted from time to time, and in 2016, Calf Top in the Yorkshire Dales was promoted from a hill to a mountain because changes to the geoid meant it was now 2cm taller than previously recorded. But this was because of more accurate modelling, not changing sea levels.