Asked by: Oliver Neely, Folkestone
Mean sea level (MSL) is widely used as the standard reference for the altitude of towns, mountains and aircraft. That’s because once the effect of tides and waves has been averaged out, sea level depends on just two forces: the strength of gravity and the effect of the Earth’s spin – and these depend on the distance from that ultimate reference point, Earth’s centre.
But while their surface provides a handy reference point, the oceans themselves are not all at the same height above the Earth’s centre. As the strength of the force generated by the Earth’s spin is strongest at the equator, the MSL bulges outward there, putting it further from the centre of the Earth than at the poles.
Differences in the Earth’s density also affect the strength of gravity, causing MSL variations of as much as 100 metres. MSL is also changing over time, largely through global warming causing seawater to expand and land ice to melt.
Read more:
- Do rising sea levels mean mountain elevations will need to be adjusted?
- What would happen if the sea level rose by two metres?
Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.
Authors
Robert is a science writer and visiting professor of science at Aston University. He likes maths, West End musicals and hamsters.
