Sea Level Change

The difference between eustatic and isostatic sea level changes

Sea level changes are influenced by various factors, and it's important to understand the difference between eustatic and isostatic sea level change. Let's take a look.

Sea Level Change: Tectonic factors

A coastal region experiencing seismicity (earthquakes) can encounter land being shifted upwards (uplift) or downwards (downthrust) as geological pressures are released, and the adjacent rocks adjust at fault lines. Consequently, there can be a rapid change in relative sea level depending on whether the land has been shifted up or down. Some notable land/sea-level changes are:

• On Boxing Day 2004, an earthquake caused a tsunami in the Indian Ocean. Close to the epicentre, the Indonesian island of Sumatra was the worst hit. The city of Banda Aceh was flooded after it was hit by a 15m wave, which was later followed by an earthquake that caused the crust to sink, flooding areas of the city permanently.
• In 2011, the earthquake in Tohoku caused a drop of 0.6m in parts of the Japanese coastline, as well as causing the Earth to shift its axis.

Sea Level Change: Submergent coasts

On coastlines not affected by glacial ice cover, the post-glacial sea-level rise caused drowned, submergent coasts. Examples of these can be found along the south coast of England and the east coast of America. The most common feature is a ria.

Fig. 1 - example of a submergent coastline. The ria of the Georges River

• Ria. This is a river valley which has been flooded by the eustatic rise in sea level. They're like a typical river valley, but they have even more water in them. The cross-section of a ria is similar to the one you'd expect to find in the lower course of a river. Rias can be used as ports as they offer good shelter. The Kingsbridge Estuary in Devon is an example of a ria.
• Another feature is a fjord. These are similar to rias but are steeper and more profound, with a shallow mouth known as a threshold; this is where the glacier deposited its load. They have a u-shaped cross-section and can be found in icy parts of the world. Fjords can be found in Canada, New Zealand, Chile, and Norway – e.g. the Sogne Fjord in Norway.
• A dalmatian coastline is composed of long, narrow islands running parallel to the coastline and separated by narrow sea channels. These sea channels are also called sounds. They are produced by rising sea-level flooding the shoreline with the geological structure of folds aligned parallel to the coast. The best example of a dalmatian coastline is the Dalmatian coast in Croatia (this gave them their name).

Sea Level Change: Emerging land

Emergent landforms were produced towards the end of an ice age when isostatic rebound occurred faster than the eustatic rise in sea level. In other words, the land's height rose more quickly than the sea's. An example of this is the eastern Scottish coastline, near Earlsferry in Fife.

Previous sea levels are reflected in their landforms:

• Raised beaches – these are wave-cut platforms and beaches above the current sea level. They have a flat surface covered by sand or rounded pebbles/boulders. They are usually vegetated by plant succession. They no longer experience coastal erosion, but they are still weathered biologically, chemically, and via freeze-thaw weathering. An example is Lendalfoot in Ayrshire, western Scotland. Here, a flat raised beach surface is used as the main road, A77.
• Fossil features – features such as wave-cut notches, caves, and arches are left higher up from the coast on raised marine platforms.
• Relic cliffs – these are found at the back of a raised beach, a steep slope, displaying evidence that they were formed through marine erosion but are now above high tide level.

Fig. 2 - A raised beach, Towards Lendalfoot

Isostatic sea level change

Isostasy is where one layer of the earth's crust floats on top of another more pliable layer beneath. As a result, the natural buoyancy of crustal rocks gets depressed when land-based ice sheets are formed due to the weight of the ice sheets. However, when the ice sheet melts, the weight reduces, resulting in the land surface being released and slowly rebounding upwards over thousands of years. This is known as post-glacial isostatic adjustment. Isostatic sea-level change can also be caused by tectonic uplift or depression; however, as this only takes place along plate boundaries, this isostatic change only occurs in particular areas of the world.

Two different isostatic changes have occurred in the UK since the last ice age.

• Land in the north and west, covered by ice sheets during the last ice age, is still rising due to the recent isostatic recovery.
• Land in the south and east, which was not covered by the ice sheet, is sinking. As a result, rivers pour water and sediment into the Thames estuary. This causes the crust to drop there and relative sea levels to rise, which means there is an increased flooding risk due to isostatic change and rising sea levels caused by global warming.

Eustatic sea level change

When the ice age began, the temperature fell. Water froze into glaciers, meaning that it was removed from the sea but not put back in. This led to a fall in sea level. Conversely, when the ice age ended, the temperature rise and the melting ice returned the water to the sea, causing it to increase globally. Global temperature increases cause the volume of ocean water to increase (thermal expansion), leading to sea level changes.

Key points:

• Isostatic sea level change commonly arises from the movement of the land relative to the sea, they are a local rise or fall of the ground, and the changes take time.
• Eustatic involves a rise or fall in water levels caused by a change in the volume of water. It's a global change and occurs relatively quickly.

The scale of isostatic and eustatic changes during and after the last glacial period are linked but are happening at very different rates.

• Postglacial sea level rose rapidly, drowning many coastlines – submergent coasts.
• The isostatic adjustment was prolonged so that in previously ice-covered areas, newly drowned coastlines slowly emerged from the sea – emergent coasts.

As a result, some coastlines may be emerging while at the same time some are submerging, and others will be achieving a state of dynamic equilibrium with the changing sea level.

Reasons for sea-level changes

We constantly hear that sea levels are still rising. The reasons are widely debated and two of the most common answers are:

• We are still coming out of our last ice age. Therefore isostatic rebound is ongoing and melting from the last ice age is still occurring.
• The planet is getting hotter due to global warming. Average global temperatures rose by 0.85c from 1880–2012. During a similar period, 1870–2010, average sea levels rose by 21cm.

We know that sea levels rise due to climate change, it causes the melting of ice sheets, and because of thermal expansions, the sea expands as it gets warmer.

The tectonic activity that affected Aceh province's coastline in Sumatra also created tectonic subsidence resulting in a ria. At the same time, in some locations, coral reefs were raised and then subsequently eroded by wave action. This type of change is rare, localised, and limited to tectonically active zones, but it causes significant instant changes to the coastline when it happens.

The effects of sea-level rise

The effects of changing sea levels are already being felt globally. Millions of euros have already been invested in the Delta works in the Netherlands, and substantial public funds are earmarked for their continuing maintenance and improvements to safeguard this densely populated region of northern Europe. The Sundarbans region of West Bengal and the low-lying delta of Bangladesh are amongst the fastest eroding coastlines globally, with some places experiencing more than 20 meters of erosion each year. In addition, there appears to be an increasing frequency of cyclones with their associated tidal surges. In the Pacific Ocean, low-lying island states are already suffering the worst effect of sea-level rise, with increased rates of salinisation, coastal erosion, and storm surges forcing many to consider migration, and environmental refugees.

In the UK, as mentioned earlier, the east coast has a high risk of being flooded and destroyed. Lots of people inhabit the east coast. As a result, their homes and livelihoods are at risk. There is also some critical infrastructure present in the area, including several power stations. Six of them are nuclear power plants, two of which have been deactivated.

Changes in sea level are part of ongoing concerns regarding coastal risks. Other coastal risks include coastal retreat and coastal flooding. There are management approaches that we can take to prevent coastal risk and be prepared for any natural hazards that may occur. It is also important to be aware of the risks worsened by human activities.