Eustatic and isostatic sea level changes and their causes

Sea level changes occur due to variations in the volume of water in the oceans or movements of the land relative to the sea. These changes are classified into two main types: eustatic and isostatic, each driven by distinct processes often linked to tectonic activity.

Eustatic sea level changes

Eustatic changes refer to global shifts in sea level caused by alterations in the volume of ocean water or the shape of ocean basins. These changes affect sea levels uniformly across the world.

Primary causes of eustatic changes:

• Climatic variations - Rising temperatures lead to the melting of ice sheets, increasing the volume of water in the oceans. Additionally, warmer water expands, further raising sea levels. Conversely, cooler temperatures cause more precipitation to fall as snow, locking water in glaciers and reducing sea volume.
• Tectonic alterations - Movements in the Earth's crust, such as sea floor spreading, can modify the shape of ocean basins. An increase in basin volume due to spreading typically results in a decrease in sea level.

Isostatic sea level changes

Isostatic changes are local adjustments in sea level resulting from vertical movements of the land relative to the sea. These changes are specific to particular regions rather than global in scope.

Primary causes of isostatic changes:

• Ice sheet dynamics - The accumulation of ice sheets can depress the Earth's crust, while their melting allows the land to rise (rebound). This rebound can persist for millennia after the ice has retreated.
• Sediment accumulation - Large deposits, often at river mouths, can weigh down the crust, causing local subsidence.
• Groundwater abstraction - Removing groundwater can lead to land subsidence due to soil shrinkage.
• Tectonic processes - Crustal movements, such as one tectonic plate being forced under another at plate boundaries, can cause local uplift or depression of land.

Historical and current trends in sea level variation

Sea levels fluctuate on various timescales, from daily tidal cycles to long-term changes spanning thousands of years. Historical data and recent observations reveal significant patterns influenced by both natural and human-induced factors.

Short-term and daily variations

• Tidal cycles - Sea levels rise and fall daily due to gravitational forces from the moon and sun.
• Weather influences - Onshore winds and low atmospheric pressure systems can temporarily elevate sea levels in specific areas.

Long-term historical changes

• Glacial period impacts - During the last glacial period (110,000 to 12,000 years ago), vast amounts of water were stored in ice sheets. At the peak of the last glacial maximum, around 21,000 years ago, sea levels were approximately 120 metres lower than today.
• Post-glacial rise - As temperatures warmed around 12,000 years ago, ice sheets melted, causing a rapid rise in sea levels. By about 5,000 years ago, sea levels stabilised close to their current position.
• Recent fluctuations - Over the past 5,000 years, sea levels have varied slightly around their present value. However, since around 1940, a noticeable upward trend has emerged.

Current trends and climate influence

• Global warming - Over the past century, global temperatures have risen sharply, with an increase of approximately 1.1°C between 1900 and 2020. This rise is largely attributed to human activities that elevate greenhouse gas concentrations, trapping more heat in the atmosphere.
• Sea level rise rate - Currently, global sea levels are increasing at a rate of about 2 mm per year. With continued high greenhouse gas emissions, this rate could accelerate to between 8 and 16 mm per year by the end of the 21st century.
• Mechanisms of rise - Rising temperatures contribute to sea level increase through the melting of ice sheets and thermal expansion of seawater.

Impacts of sea level rise on coastal areas

Rising sea levels pose significant challenges to coastal regions, affecting both human communities and natural environments. These impacts are becoming more pronounced as climate change intensifies.

Consequences of rising sea levels

• Increased storm frequency - Changes in ocean circulation and wind patterns result in more frequent and severe storms impacting coastal zones.
• Coastal flooding - Low-lying coastal areas experience more regular and severe flooding events. For instance, a coastal settlement in Australia has seen repeated flooding incidents with growing frequency in recent decades.
• Submergence of islands - Small, low-lying island nations face the risk of losing significant territory. A rise of just 0.5 metres could submerge large parts of a Pacific island state.
• Coastline alteration - Rising seas can create new islands while reducing overall land area. A densely populated delta region might lose around 8,000 km² of land with a 0.3-metre rise.
• Saltwater intrusion - Freshwater sources and agricultural lands become contaminated with saltwater, threatening water and food security.
• Coastal erosion - Higher sea levels accelerate erosion, endangering ecosystems, infrastructure, and local livelihoods.

Landforms associated with sea level rise and fall

Changes in sea level create distinct coastal landforms, depending on whether the sea level is rising or falling. These features provide evidence of past sea level movements and shape current coastal landscapes.

Landforms from sea level fall (coastlines of emergence)

• Raised beaches - Beaches that were once at sea level are left elevated above the current high tide mark as sea levels drop.
• Wave-cut platforms - These flat, exposed surfaces, previously at sea level, become raised above the current sea level.
• Relict cliffs - Cliffs no longer actively eroded by the sea become vegetated and inactive over time.
• Degraded wave-cut features - Notches, caves, arches, and stacks, originally formed by wave action, are gradually broken down by weathering processes when left above sea level.

Landforms from sea level rise (coastlines of submergence)

• Rias - Partially submerged river valleys with a gentle profile, wider and deeper at the mouth, becoming narrower and shallower inland.
• Fjords - Drowned glacial valleys, characterised by straight, narrow channels with steep sides, a shallow mouth due to glacial deposits, and depths often exceeding 1,000 metres inland.
• Dalmatian coastlines - Formed when valleys running parallel to the coast are flooded, creating a series of islands aligned with the shoreline.

Dynamic nature of coastal landscapes

Coastal landscapes are constantly evolving due to the interplay of various processes, including sea level changes, erosion, and deposition. These changes occur over different spatial and temporal scales, reflecting the dynamic nature of coastal environments.

Processes shaping coastal landscapes

• Combination of landforms - Coastal areas often feature a mix of landforms resulting from both erosional and depositional processes, creating complex landscapes.
• Temporal variability - Changes can be rapid and episodic, such as during storm events lasting mere hours, or slow and gradual, like tectonic uplift occurring over thousands of years.
• Spatial variability - The extent and impact of coastal changes vary across different regions, influenced by local geology, sea level trends, and human activities.

These dynamic interactions ensure that coastal landscapes are never static, continuously adapting to natural forces and human influences over time.