Rising sea levels have been highly topical for many years now, but exactly how are they worked out?
And more to the point, what is the initial reference?
Well, thanks mainly to variations in water temperature and salinity, sea level differs around the world... so it turns out there is no standard reference.
Currently, there are about 100 “points zero” in use around the world, in some cases differing by metres. Technical difficulties and a lack of political will have hampered attempts to do away with this confusing situation. However, modern technology and the increasingly pressing need to measure rising seas look set to force a change, with a hope of a global standard within five years.
The situation first came to light when European efforts to standardise latitude and longitude in the 1860s extended into the vertical. Several countries had already set up coastal tide gauges – essentially, a float attached to a pen that traced a line on a chart – and were calculating mean sea level, defined as the average of sea level measured at regular intervals between high and low tide. It turned out their levels weren’t the same.
Efforts nowadays to give one single standard of height could help us understand sea level rise and even remeasure mountains – (turns out that Everest is not the world’s tallest mountain).
The currently proposed standard for sea level traces back to the height of a dike measured by the Mayor of Amsterdam in 1675.