Eustatic sea level change is a change in the level of the whole world's ocean, caused by a change in the amount of water in it or the size of the ocean basins. It differs from local change, where land moves up or down.
Key takeaways
- Eustatic change affects the ocean everywhere at once, because the volume of water changes.
- Local (relative) sea level also depends on whether the land is rising or sinking.
- The Ice Age fall of roughly 120 m is the kind of global change our slider imitates.
- The slider lowers the sea by one amount everywhere, so it is a eustatic-style model only.
How eustatic change works
The word means the whole ocean going up or down together. A eustatic change happens when something alters the total volume of ocean water or the capacity of the basins that hold it.
The classic cause is ice. When ice sheets grow, water is taken out of the sea and stored on land, and the global level falls. When they melt, the level rises. That is what happened around the Last Glacial Maximum, when sea level stood roughly 120 m lower than it does now [2].
Not every change in sea level is eustatic. At a single coast, the measured level depends on the global average and on what the land is doing. NOAA points out that global and relative sea level trends are different measurements, and that local factors include land subsidence and land still rebounding from Ice Age glaciers [1].
So there are two layers to keep apart:
- Eustatic (global): the ocean's volume changes, so the level moves everywhere.
- Relative (local): the level compared with the land at one place, which also depends on the land rising or sinking.
Scientists separate the two to work out how much of a change comes from the water and how much from the ground.
An example from our data
Our drain slider is a eustatic-style model. It lowers the sea by the same amount everywhere. Choose 50 m and 11.8 million km² of seabed becomes land. Choose 120 m and that grows to 21.7 million km², which is 14.8% of today's land area.
Because the amount is the same everywhere, the shape of what is exposed depends only on today's seabed depths. That is exactly why shallow shelves dry out first.
The thing people get wrong
A common mistake is to treat any change in local sea level as a change in the volume of the oceans. A coast can see the sea rise relative to the land because the land is sinking, with no change in the global ocean at all [1]. The reverse also happens.
The slider also cannot show local differences. It applies one global drop to a fixed seabed, so it leaves out the local effects described under isostasy.
Sources
- Is sea level rising?, NOAA Ocean Service.
- Last Glacial Maximum, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).