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From problems with the moon, to the lumpiness of earth, sea levels aren't quite as exact as we have them figured out to be.

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Sea level might sound like a pretty simple concept.  It's the height of the sea, but coastal water levels are always shifting and even here in Missoula, Montana, I can confidently say that I'm 976 meters above sea level, which, what does that even mean?  There's no ocean for hundreds of kilometers.  Defining and measuring sea level is a surprisingly hard problem, and the reasons why are almost ridiculous.  I'm talking problems with the Moon, the lumpiness of the Earth, even basic laws of gravitation get in the way, but the impressive thing is that in spite of all that, we can measure it.

At first glance, measuring sea level doesn't seem like too complicated a problem.  Yeah, there's waves and stuff and weather and the tides go up and down, but you can average all that stuff out, right?  After all, storms pass and tides come in and go out every few hours, so you might think a few weeks' data should do the trick.  But the cycles that pull and push the water around the planet are way more complicated than that.  

For instance, you can watch tides change over a whole 29 day lunar orbit and factor out their effect to get a number that's sort of accurate, but that still doesn't account for the Sun, which creates tides about half the size of the Moon's.  To make matters worse, solar and lunar tides don't line up.  Sometimes they amplify each other.  Sometimes, they partly cancel each other out.  In fact, the Sun, Moon, and Earth only line up in the same way about every 19 years, so you have to wait at least that long to get a reliable average, and even then, one cycle isn't enough, because the Sun also drives Earth's weather, and weather alone can change sea levels way more than some of these tidal cycles, so it can take multiple decades to get a really solid average and this is the simple stuff.  The stuff that eventually averages out.  

A lot of things that affect sea level aren't so neat and cyclical.  For instance, the shape and location of coast lines constantly change, making sea levels sort of a moving target and in lots of places, land is subtly moving up and down.  Yeah, sea level is hard to measure partly because the land we're standing on keeps moving.  Tens of thousands of years ago, parts of Europe and North America were squashed under kilometers of ice and even though that ice is long gone, the Earth is still bouncing back.  Land that was crushed by glaciers is relaxing and in places like Scandinavia and Northern Canada, it's rising by about a centimeter a year.  In some places, like the East Coast of the United States, land buckled upward near all that ice, so today it's sinking down at a centimeter or so a year, and that might not sound like much, but centimeters here and there add up and each natural change makes it harder to separate out the human-driven changes in sea level as the planet warms.

It also means we can't just park instruments on land, because remember, the land isn't sitting still, so to get around that little detail, scientists use satellites with radar to measure sea level from space.  Radar works by timing how long light takes to bounce off a target, and the radar on these satellites is so sensitive that it could see a dime lying flat on the ground from a dozen kilometers in the air.  These satellites give us maps of global sea levels every couple weeks, letting us watch how sea levels change, which sounds great.  Throw a satellite around the Earth at a fixed orbit and measure how long the light takes to bounce back to the satellite, but there's another hitch.

Earth's gravity isn't perfectly even all the way around the globe.  Different regions have different rocks and different amounts of water making some parts of the planet denser than others.  These more or less dense regions are known as gravitational anomalies, and they affect how water spreads out across the surface.  Areas with stronger gravity pull more water toward them, so water bunches up in some places.  Meanwhile, where there's less gravity, it thins out, and I'm not just nitpicking here.  Some spots in the open ocean have water that's almost 100 meters higher or lower than you'd expect if gravity pulled evenly across the globe, so even if you could stick a number on global sea level, it wouldn't actually describe the whole globe.

Satellites have revealed that there are sea levels, plural, but that's still useful.  After all, one of the biggest reasons we're looking at sea level lately is to understand climate change, and that will affect sea level differently in different places anyway.  Except, the thing is, as the climate warms, even that pattern of ups and downs is changing in some surprising ways.  That's because two things happen as ice melts.  First, as glaciers melt into the ocean, the water level immediately around them drops.  The reason is, glaciers are so massive that they actually have a significant gravitational pull.  It's like having a bunch of mini Moons on the surface of the Earth.  They pull water toward them, so when that mass melts away, the surrounding water recedes.  For instance, sea level is rising three millimeters on average, but if all of Greenland's ice melted, sea level would actually drop by a couple dozen meters in Iceland, but on the flip side, it would rise more than the average in the Southern Hemisphere.

Second, all this melting redistributes mass on the planet's surface and that changes how the Earth itself spins in space.  That's right.  By heating our planet, we are melting glaciers, which shifts the Earth's mass, which changes how our planet moves in space, and that's not the end of the story.  Earth's rotation then affects how water and air spread out across its surface.  When its rotation changes, even a little, it alters how all that mass moves and as that mass redistributes itself, it changes the Earth's rotation again.  This changing spin can also lead to a drop in sea level in some places even as the average level rises.  

So if you haven't noticed yet, this whole thing makes sea level a really complicated thing to untangle or even define, for that matter.  Sea level means what we need it to mean.  Sometimes we need a global average that tells us something about the overall impacts of climate change, and sometimes we need a local estimate that tells us how ice melt will affect a certain city.  As long as we're consistent, however we define sea level can be meaningful and instructive, even if isn't simple.

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