Sea Level Change

By Alex Jackson

The sea level has and continues to fluctuate greatly throughout time. On a day to day basis, the sea level changes according to the tide but the sea level also changes on a much grander time scale too. These changes in sea level are normally caused by ice ages or other major global events.

The sea level changes for a variety of reasons. These reasons can be put into two categories, eustatic and isostatic change, depending on if they have a global effect on sea level or a local effect on the sea level.

Eustatic Change

Eustatic change is when the sea level changes due to an alteration in the volume of water in the oceans or, alternatively, a change in the shape of an ocean basin and hence a change in the amount of water the sea can hold. Eustatic change is always a global effect.

During and after an ice age, eustatic change takes place. At the beginning of an ice age, the temperature falls and water is frozen and stored in glaciers inland, suspending the hydrological cycle. This results in water being taken out of the sea but not being put back in leading to an overall fall in sea level. Conversely, as an ice age ends, the temperature begins to rise and so the water stored in the glaciers will reenter the hydrological cycle and the sea will be replenished, increasing the sea levels.

Increases in temperature outside of an ice age will also effect the sea level since an increasing temperature will cause the ice sheets to melt, putting more water in the sea.

The shape of the ocean basins can change due to tectonic movement. If the ocean basins become larger, the volume of the oceans becomes larger but the overall sea level will fall since there’s the same amount of water in the ocean. Conversely, if the ocean basins get smaller, the volume of the oceans decreases and the sea level rises accordingly.

Isostatic Change

Isostatic sea level change is the result of an increase or decrease in the height of the land. When the height of the land increases, the sea level falls and when the height of the land decreases the sea level rises. Isostatic change is a local sea level change whereas eustatic change is a global sea level change.

During an ice age, isostatic change is caused by the build up of ice on the land. As water is stored on the land in glaciers, the weight of the land increases and the land sinks slightly, causing the sea level to rise slightly. This is referred to as compression. When the ice melts at the end of an ice age, the land begins to rise up again and the sea level falls. This is referred to decompression or isostatic rebound. Isostatic rebound takes place incredibly slowly and to this day, isostatic rebounding is still taking place from the last ice age.

Isostatic sea level change can also be caused by tectonic uplift or depression. As this only takes place along plate boundaries, this sort of isostatic change only takes place in certain areas of the world.

Features of Sea Level Change

Sea level change can produce many features along coastlines. Again, we can categorise these features based on how they’re formed.

Emergent Landforms

Emergent landforms begin to appear towards the end of an ice age and they occur when isostatic rebound takes place faster than a eustatic rise in sea level. Put more simply, the land’s height rises faster than the sea’s. Emergent features are features of coastal erosion that appear to have developed well above the current sea level. Really, they developed when the sea was at that level and then the sea level changed during and ice age and now they’re above sea level.

One such emergent landform is a raised beach. Raised beaches are wave-cut platforms & beaches that are above the current sea level. You can normally find some old cliffs (relic cliffs) too behind these raised beaches with wave-cut notches, arches, stacks etc. along them.

These emergent features no longer experience coastal erosion but they are still weathered, often being weathered biologically, chemically and via freeze-thaw weathering.

Submergent Landforms

Submergent landforms are the opposite of emergent landforms. They form when the eustatic rise in sea level takes place faster than the isostatic rebound after an ice age. Basically, the water starts to flood the land and fills up landforms on the land.

One submergent feature is a Ria. This is a river valley that’s been flooded by the eustatic rise in sea level. They’re almost exactly like a typical river valley but they have even more water in them. The cross section of a ria is really similar to the one you’d find for a river in the lower course. One thing to note, the floodplain of the river also gets flooded, altering the cross profile of a ria ever so slightly so that it includes the floodplain.

Another submergent feature is a Fjord. These are steeper and deeper variants of riases that are relatively narrow for their size. They have a u-shaped cross profile and are often found in particularly icy sections of the world. Any guess what they could be? That’s right, they’re flooded glacial valleys (I’d only expect you to know that if you did “Ice on the land” for GCSE geography). In general, fjords are really deep however they have a shallow mouth (known as a threshold) as this is where the glacier deposited its load. Fjords are pretty stunning pieces of scenery, an example of one is Sogne Fjord in Norway which is really big.

The final submergent feature is a dalmatian coastline. These form in areas of the world where valleys (especially glacial valleys) lie parallel to each other. When the valleys are flooded by the rise in sea level, the tops of the valleys remain above the surface of the sea and appear to be a series of islands that run parallel to the coastline. The best example of a dalmatian coastline is the one from which they get their name, the Dalmatian coast in Croatia.

The Future

As we are constantly hearing, sea levels are still rising. The reasons are pretty widely debated. We know that one of the reasons is because we’re still coming out of our last ice age (amazingly, isostatic rebound is still taking place) and ice from the last ice age is still melting. Most of us also think that it’s because the planet’s getting hotter (probably because of us, but not for certain) which is melting even more ice on top of the ice that was already defrosting from the last ice age. Whatever the reason, the sea is rising and it’s a bit of a problem.

In the UK, the east coast is at a particularly high risk of a) being flooded and b) being destroyed. In fact, we know that sections of the east coast have already been destroyed and we’re fairly sure it’s because the sea is rising and the land is sinking. While the northern parts of the UK are experiencing an isostatic rebound and are rising above sea level, the east coast is sinking and the water along the east coast is rising. This is resulting in more coastal flooding and erosion along the east coast which is destroying it at a concerning rate. Obviously there’s a lot people living along the east coast but what’s more concerning is the fact that there’s a lot of power plants situated along the east coast and four of them are nuclear power plants (there’s also two deactivated nuclear plants). Even if we know that those plants are going to be destroyed by coastal erosion, there’s not a lot we can do to prevent an accident since these things stay dangerous for many thousands of years after they’re deactivated.