What Is Sea Level, Anyway?

Welcome to Calipatria, a desert city near California's Salton Sea. This is the city with the lowest elevation in the Western Hemisphere, 55 meters below sea

level

. And here we also have a visual demonstration of

what

that means: because that thin white thing is a flagpole, and the American flag on top is flying above sea

level

. If it weren't for all the mountains and hills between here and the Pacific Ocean, then we would be wet, but that flag would remain dry. Maybe. We are dry because no river flows into this basin. The Salton Sea there only exists because some engineers made a mistake in 1905 and diverted a good portion of the Colorado River there for a couple of years.

That water is becoming increasingly saltier and more polluted as it slowly evaporates, and California will have to deal with it for the next two decades. But how do we know that we are 55 meters below sea level? What is sea level? In theory, it is the average height of the sea around the Earth. But there are some problems with that. Problem 1: The Earth is not a sphere. I mean, it's vaguely a sphere, don't get too excited, flat earthers. It's a little crushed. The force of its own spin causes it to bulge a few kilometers around the equator.

And when you define height above sea level in meters, that's a lot. So geographers solved this by defining the Earth as an oblate spheroid. Basically a soft, squashed sphere, only in fairly precise mathematical terms. But then there's problem number 2: the Earth is not a uniform piece of rock. Parts of the mantle are denser than others, so gravity is not the same everywhere on Earth's surface. Which means there's a little more water on those chunks. And by a little more I mean up to 80 meters more. These are not small amounts, that could be the difference between this city being above and below sea level.

And to complicate matters even more, the mountains are large enough that their gravity pulls on the ocean, even just a little. So during the 20th century, when scientists and Cold War governments began needing a global reference system for spacecraft and missiles, which were basically the same thing back then, they spent a lot of time measuring Earth's gravity. The result is the megabyte-sized Gravitational Model of the Earth, which asks, in purely mathematical terms: if this Earth wasn't here, but its gravity was, and if there were exactly enough water to fill the gaps... where would it be? Does all that water sit?

And how far away would it be from that theoretical smooth, flattened sphere? The model was finished in 1996. It is available to this day on a website that appears to have not changed since 1996, and is

what

your phone and GPS use. There are newer and more accurate models, but they are hundreds of megabytes in size and everyone agrees that now that we have a standard, let's not change it and confuse everyone. Additionally, we would also need to update sea level rise, either in centimeters at best or meters at worst. So saying that we are "55 meters below sea level" is mostly symbolic.

But so are many things in the world, and I'd like to think we still trust them.