Eustatic sea level change

Eustatic sea level change occurs when the volume of water in the oceans alters globally, or when the shape of ocean basins shifts. This type of change affects sea levels worldwide and can happen relatively quickly compared to other processes.

Climate shifts influence the amount of water in oceans

• Warming effects - Higher temperatures lead to the melting of ice sheets and glaciers, adding more water to the oceans and raising sea levels. Additionally, warmer water expands because its particles gain energy and occupy more space, contributing further to sea level rise.
• Cooling effects - Lower temperatures cause more precipitation to fall as snow, building up glaciers and ice sheets. This reduces the volume of water in the oceans, leading to a fall in sea levels.

An example of eustatic rise is seen in Solva, Pembrokeshire, South Wales, where this low-lying estuary experiences an annual sea level increase of around 2.5 mm due to such global processes.

Causes related to tectonic movements

Tectonic activity, which involves movements of the Earth's crust, can reshape ocean basins and affect sea levels. In areas like the Atlantic Ocean, the sea floor spreads apart, increasing the basin's volume. This allows more water to be held, which lowers global sea levels over time.

Isostatic sea level change

Isostatic sea level change results from vertical movements of the land relative to the sea, often due to changes in weight on the Earth's crust. These changes are local in scale and can unfold over long periods, sometimes thousands of years.

Key processes causing land to sink or rise, altering local sea levels

• Post-glacial adjustment:
  • During the last ice age, heavy ice sheets caused parts of the crust to sink, a process called isostatic subsidence.
  • As the ice melts, the reduced weight allows the land to slowly rise, known as isostatic rebound.
  • This rebound can continue long after the ice has gone, gradually lowering relative sea levels. For instance, some areas of Scotland are still uplifting at up to 1.2 mm per year.
• Subsidence from groundwater abstraction - Removing large amounts of groundwater causes the land to compact and sink, potentially leading to flooding by seawater as sea levels appear to rise relative to the land.
• Accretion of sediment or ice - The buildup of sediment at river mouths or coasts, or the accumulation of ice, adds weight to the crust. This causes the land to sink, resulting in a relative rise in sea levels.
• Tectonic processes - At plate margins, one tectonic plate may be forced beneath another, leading to uplift or subsidence of the crust. This can either lower or raise local sea levels depending on the direction of movement.

Emergent coastlines and falling sea levels

Emergent coastlines develop when sea levels fall relative to the land, exposing new areas and creating distinctive landforms. This often happens due to isostatic rebound or eustatic drops in water volume.

Formation of raised beaches

Raised beaches occur as sea levels drop, leaving former beach areas stranded above the current high tide mark. Over time, vegetation grows on the exposed sediment, and soil develops. This process also reveals wave-cut platforms, which are flat rock surfaces eroded by waves but now elevated. In the UK, these features are common along Scotland's west coast, including the Isle of Arran.

Development of fossil cliffs

As sea levels fall, cliffs that were once eroded by waves become inactive and are gradually covered by vegetation. These are called fossil cliffs. They often preserve erosional features like wave-cut notches, caves, arches, and stacks, though weathering slowly alters their shapes over time. Fossil cliffs are frequently found above raised beaches, as seen on the Isle of Iona in Scotland.

Submergent coastlines and rising sea levels

Submergent coastlines form when sea levels rise relative to the land, flooding existing features and creating new coastal landforms. This submergence can result from eustatic rises or isostatic subsidence.

Characteristics of rias

Rias are drowned river valleys created when rising seas flood low-lying areas. They have a gentle profile, being wide and deep at the mouth but narrowing and shallowing inland. An example is Milford Haven in South Wales, which shows this typical shape.

Features of fjords

Fjords are similar to rias but form from submerged glacial valleys rather than river valleys. They are straight and narrow with steep sides, and their depth is greatest inland, while the mouth is shallower due to a threshold — a raised area of deposited glacial material. Sognefjorden in Norway reaches depths of over 1,050 metres in places.

Formation of Dalmatian coastlines

Dalmatian coastlines feature long islands running parallel to the shore, formed when rising seas flood valleys between folded rock structures called anticlines (upfolds) and synclines (downfolds). The hilltops of anticlines become isolated islands, while the synclines fill with water. This creates a distinctive pattern, as seen along parts of the Croatian coast.

Current and predicted sea level changes

Sea levels are changing today due to a mix of human-influenced and natural factors, with implications for coastal areas worldwide.

Influences from climate change

Over the past century, global temperatures have risen rapidly due to increased greenhouse gas emissions from human activities, contributing to climate change. This warming melts the cryosphere — the Earth's frozen regions including ice and snow — adding water to the oceans. It also causes thermal expansion, where ocean water increases in volume as it heats up. Currently, global sea levels are rising at around 3.5 mm per year, threatening low-lying coasts and settlements. If emissions stay high, predictions suggest rises of 7 mm to 14 mm annually by 2100.

Role of tectonic processes

Tectonic activity continues to influence sea levels, though less frequently than climate factors. Large earthquakes or volcanic eruptions can reshape land, causing sudden local changes. Shifts in tectonic plate positions may alter ocean basin capacities, leading to global rises or falls in sea levels.

Challenges in predicting sea level rise

Improved instruments now allow better monitoring and modelling of sea level changes. However, multiple variables — such as varying emission levels, unpredictable tectonic events, and regional differences in isostatic adjustment — make accurate long-term predictions difficult.