# Why “Sea Level” isn’t as simple as you think.

You might be wondering how on earth sea level could be complicated. “It’s just the height of the sea!” I hear you say, and I wouldn’t necessarily disagree. However Mt. Everest is famously 8849m above sea level and I’ll ask you the question, how is that measured? Is it the height you have to climb to get to the nearest beach? Is it like a glass of water where all the water on the earth is at the same height? Is the Earth flat? Well I guess you’re going to have to read and find out!

# “Did you just ask if the Earth was flat?”

Well yes I did, what are you going to do about it? Now obviously the earth isn’t flat but let’s imagine it is for a second. Then the question of sea level is quite simple, you just measure where the top of the sea is you have yourself a perfectly accurate sea level! Now how can we apply this to our globe that we have? Well you might think that since the Earth is a sphere, you can just measure the height of the water from the center of the Earth. However something called “reality” wants to ruin our fun by making sure that the Earth is constantly rotating about an axis to ensure that the Earth isn’t a sphere but an “oblate spheroid”, a fancy name for a ball which has been squished a little bit. In reality the diameter of the Earth at the equator is 42km longer than the diameter from north to south. This means that if you were to use this method of finding sea level by measuring the height of the sea from the North Pole, at the Equator sea level would be 21km below where the surface of the ocean actually is. This is almost twice as deep as the Mariana Trench (the deepest point of the ocean), so it would be quite the disparity! This is actually the reason why despite Mt. Everest being the highest point above sea level on the planet, it is not the furthest point on the planet from the Earth’s core. This accolade goes to Mt. Chimborazo in Ecuador which is over 2000m further than Everest.

# The gravity of the situation.

Some people think Newton was a great guy and ever since he discovered gravity all of our problems have been solved. Well geodesists hate him (yes that is the name for people who measure sea level). Gravity is the sole reason this is such a difficult problem because even though in most school problems its quite easy to just assume that acceleration due to gravity is 9.8m/s² everywhere on Earth but in reality the gravity around Earth varies from place to place. this is due to the different densities of areas of the planet. One of the most obvious changes in density is where there are physical continents. These continents attract the water so the height of the where the tide on the land is noticeably above where the sea level is in the middle of the ocean. Ocean trenches have the opposite effect and lower the sea level comparably to other areas of the ocean. So with all these variables how do they measure sea level without measuring it at every single point on the planet.

# Measuring all of the sea levels.

So what they do is they measure it at every single point on the planet. I know, not the elegant solution you were hoping for but they have really clever techniques of doing so. The sea level is easily measurable at a point in the sea, where there is water to measure but what about our predicament at the start? How do you measure the height above sea level of Mt. Everest if there is no sea around it and the sea level is not constant around the globe? Think of it this way, since the sea level depends on gravity how would you know where the sea would be of the mountain wasn’t there? There are two ways of looking at it. In one you could say well if Mt. Everest wasn’t there, neither would its gravity so you can ignore its effects. But everywhere else in the ocean it is clear that gravity’s effects cannot be ignored so their results would be inconsistent with the rest of the world. However if you imagined that the mountains weren’t there but their gravity was you can have sea levels that are consistent with the rest of the planet! In order to do this geodesists have had to map out the gravity at every point on earth in order to predict the sea level as can be seen in this map.

Here it can be seen that in some places the sea level can be almost 180m higher than other places in the world! This is in stark contrast to what most people believe is a constant height around the planet.

# So why does this matter?

Well the ability to measure sea level gives us an ability to check how well we are doing with combatting global warming as we can tell how much the oceans are rising due to melting ice caps. Spoiler alert, it is not looking good! Apart from this, I guess it is just a fun fact you can share with your friends as they actively ignore you while you ramble on about physics…… again!

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