You probably think the ocean is flat. Or at least, that "zero" on a topographic map is a fixed, simple thing. It isn't. Not even close. If you’ve ever wondered how is sea level determined, you’ve stepped into a world of complex physics, wobbling tectonic plates, and satellites that can measure the height of the ocean to within the width of a fingernail from 800 miles up in space.
The ocean has hills. It has valleys. Because the Earth isn't a perfect sphere—it’s more like a lumpy potato that’s been squashed at the poles—gravity pulls harder in some places than others. This means "sea level" is actually a moving target.
The Old Way: Sticking a Ruler in the Mud
For centuries, we did things the manual way. We used tide gauges. Basically, these are long tubes or wells stuck into the coastline that filter out the "noise" of waves but let the slow rise and fall of the tide in.
Coastal cities like London, New York, and San Francisco have records going back well over a hundred years. This is "Relative Sea Level." It tells you how high the water is compared to the land right next to it. But there’s a massive catch. The land moves. Sometimes the ground is sinking because we’re pumping out groundwater (look at Norfolk, Virginia). Sometimes the land is still "rebounding" or rising because the massive glaciers from the last Ice Age melted and the weight is gone.
Why the "Average" is a Mess
To get a single number, scientists have to take the average of all those high and low tides over a 19-year period. Why 19 years? It’s because of the Metonic cycle—a specific period related to the orbits of the Earth, Moon, and Sun that accounts for every possible tidal variation.
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But if the land is sinking and the water is rising, your measurement is biased. You aren't just measuring the ocean; you’re measuring the earth's crust. Honestly, it’s a bit of a nightmare for geoscientists who want a "global" number.
The Modern Tech: Bouncing Lasers Off the Waves
Since the early 1990s, specifically starting with the TOPEX/Poseidon mission, we stopped relying solely on the coast. We went to space. This is how we get the "Global Mean Sea Level" (GMSL).
Satellites like the Jason-3 or the newer Sentinel-6 Michael Freilich use radar altimeters. They send a microwave pulse down to the ocean surface and time how long it takes to bounce back. By knowing exactly where the satellite is in space (using GPS and laser ranging), they can calculate the height of the ocean surface with incredible precision.
It’s not just one measurement. It's millions. Every day.
The Gravity Problem: Geoids and Ellipsoids
Here is where it gets weird. To truly understand how is sea level determined, you have to understand the Geoid. Imagine if the ocean were completely still—no wind, no tides, no currents. The shape it would take is the Geoid. It’s an "equipotential surface" where gravity is the same everywhere.
Because the Earth’s interior is uneven (some spots have denser rocks than others), the Geoid is lumpy. If you sailed your boat along the Geoid, you’d never feel like you were going "uphill," even though your distance from the center of the Earth might change by 100 meters.
- The Ellipsoid: A smooth, mathematical approximation of Earth.
- The Geoid: The true "level" based on gravity.
- Sea Surface Topography: The difference between the actual ocean surface and the Geoid, caused by currents and temperature.
NASA’s GRACE-FO mission actually measures these gravity shifts by using two satellites that "chase" each other. When the lead satellite passes over a dense area (like a mountain range on the seafloor), it speeds up slightly due to stronger gravity. The distance between the two satellites changes, and we map the Earth’s gravity field. This is fundamental to correcting our sea-level data.
What’s Actually Making the Water Rise?
When we talk about how we determine these levels, we are also looking at why they change. It’s not just melting ice.
About a third of the sea level rise we see today is actually "Thermal Expansion." You remember high school physics? Things expand when they get hot. The ocean is absorbing more than 90% of the excess heat from global warming. As that water warms up, the molecules move more and take up more space. The ocean literally grows.
Then you have the "Cryosphere" contribution. This is the big stuff. Greenland and Antarctica.
The Greenland Factor
Greenland is losing roughly 270 billion tons of ice per year. That water has to go somewhere. When we use satellites like ICESat-2, which uses green lasers to measure the height of ice sheets, we can see the literal thinning of the glaciers. This isn't theoretical. We are measuring the height of the ice decreasing and the height of the ocean increasing simultaneously.
Why Your GPS Might Lie to You
Ever looked at your phone and seen your altitude? It might say you're 50 feet above sea level when you're standing on a beach. That’s because your phone is often referencing the "Ellipsoid"—that smooth mathematical shape—rather than the "Geoid" or the actual local sea level.
Engineers and surveyors have to use "Vertical Datums." In the U.S., the big one was the North American Vertical Datum of 1988 (NAVD 88). But even that is being replaced. The National Geodetic Survey (NGS) has been working on GRAV-D, a project to create a new gravity-based vertical datum because we realized the old one was off by up to a meter in some places.
The Complexity of Local Variation
If you’re in Stockholm, sea level is actually "falling." Not because there’s less water, but because Scandinavia is rising so fast after the weight of the last ice age disappeared that it’s outpacing the rising ocean.
In contrast, if you’re in the Mississippi Delta, the land is "subsiding" or compacting. The combination of the land going down and the water going up makes it look like sea level is rising at a terrifying rate.
We determine these differences by combining:
- GNSS (GPS) Stations: To see how the land moves up/down.
- Tide Gauges: To see how the water moves relative to that land.
- Satellite Altimetry: To see the absolute "bulge" of the ocean.
How to Check This Yourself
You don't need a PhD to see the data. The University of Colorado’s Sea Level Group and NASA’s "Vital Signs" website provide near real-time updates of the Global Mean Sea Level.
You’ll see a graph that looks like a jagged staircase. The "jags" are the seasons—water evaporates from the ocean and falls as snow on land in the winter, then melts back in the summer. But the "staircase" is undeniably heading up. Since 1993, the average rise has been about 3.4 millimeters per year. That sounds tiny. It’s the thickness of two pennies. But over decades, across the entire globe, it’s a staggering volume of water.
Actionable Steps for the Curious
If you really want to understand how this affects your local area or your data projects, do the following:
- Consult the NOAA Tides & Currents Map: Look for "Relative Sea Level Trends." This will show you exactly which coastal areas are sinking versus where the water is actually rising.
- Use VLM (Vertical Land Motion) Corrections: If you are doing any construction or surveying, never rely on a simple GPS altitude. You must check which "Vertical Datum" your software is using.
- Follow the ARGO Float Data: There are thousands of robotic floats in the ocean right now. They dive down 2,000 meters, measure temperature and saltiness (salinity), and pop back up to beam data to satellites. This tells us the "steric" component—how much the ocean is expanding due to heat.
- Download the "Sea Level Rise" App: There are several visualization tools (like those from Climate Central) that overlay this scientific data onto Google Maps. It’s a sobering way to see how the "determined" sea level of the future will reshape your local geography.
Determining sea level isn't a "one and done" measurement. It’s a constant, high-tech surveillance of a planet in flux. We use gravity, light, and old-fashioned rulers to make sense of a world that is literally shifting under our feet.