Sea Level Change

Sea level has never been constant. Over geological time, global sea level has risen and fallen by over 200 metres in response to climate change, tectonic activity and gravitational variations. Understanding the causes and consequences of sea level change is critical for explaining present-day coastal landscapes and predicting future coastal change. This lesson addresses Edexcel A-Level Geography Enquiry Question 2: How do characteristic coastal landforms contribute to coastal landscapes? and provides essential context for the management topics that follow.

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Types of Sea Level Change

Sea level change can be absolute (a real change in the volume of water in the oceans or the volume of the ocean basins) or relative (the apparent change in sea level at a particular location, which depends on both absolute sea level and the elevation of the land). Two fundamental mechanisms operate:

Eustatic Change

Eustatic change is a global change in sea level caused by a change in the volume of water in the oceans. It affects all coastlines simultaneously (though regional variations exist due to ocean currents, atmospheric pressure and gravitational effects).

Causes of Eustatic Change

Cause	Mechanism	Timescale	Magnitude
Glaciation / deglaciation	During ice ages, vast quantities of water are locked up in ice sheets, lowering sea level. During interglacials, melting ice returns water to the oceans, raising sea level.	10,000–100,000 years	Up to ~130 m (last glacial maximum ~20,000 years ago, sea level was ~125 m lower than today)
Thermal expansion / contraction	Water expands as it warms and contracts as it cools. Global warming increases ocean temperatures, causing thermal expansion.	Decades to centuries	Current contribution: ~1.4 mm/year (IPCC AR6)
Tectonic changes to ocean basin volume	Mid-ocean ridge activity and continental drift can change the volume of ocean basins (more active ridges = shallower basins = higher sea level)	Millions of years	Up to ~300 m over geological time

During the Last Glacial Maximum (LGM, approximately 20,000 years ago), so much water was stored in ice sheets covering northern Europe, North America and Antarctica that global sea level was approximately 120–130 m lower than today. Britain was connected to continental Europe by a land bridge across what is now the North Sea (Doggerland). The English Channel was a dry river valley.

Since the LGM, the melting of ice sheets has caused a post-glacial marine transgression — a sustained rise in sea level that flooded low-lying coastal areas, creating many of the coastlines we see today. This rise was rapid between approximately 15,000 and 7,000 years ago (averaging 10–15 mm/year) and then slowed. Over the last 3,000 years, sea level has been relatively stable — until the acceleration caused by anthropogenic climate change.

Isostatic Change

Isostatic change is a local or regional change in the relative height of the land. It is caused by the loading and unloading of weight on the Earth's crust, which deforms the underlying ductile asthenosphere.

Causes of Isostatic Change

Cause	Mechanism	Location	Direction
Post-glacial rebound (isostatic uplift)	Ice sheets depressed the crust during glaciation; after melting, the land rises (rebounds) as the asthenosphere flows back beneath it	Scotland, Scandinavia, Canada	Land rising → relative sea level falling
Peripheral subsidence	Areas around the ice sheet were pushed up by displaced asthenosphere; now they are sinking as the asthenosphere flows back	Southern and eastern England, Netherlands	Land sinking → relative sea level rising
Sediment loading	Thick sediment deposits at river deltas depress the crust	Mississippi Delta, Ganges Delta	Land sinking
Tectonic uplift	Plate collision or volcanic activity raises coastal land	Pacific coast of Japan, Chile	Land rising

In the UK, isostatic adjustment is still occurring 12,000 years after the ice sheets melted:

• Scotland is rising at approximately 1–2 mm/year (post-glacial rebound), so relative sea level is falling — emergent features are developing.
• Southern and eastern England is sinking at approximately 1–2 mm/year (peripheral subsidence), so relative sea level is rising — exacerbating the effects of eustatic sea level rise from climate change.

This means that southern England faces a "double jeopardy" — eustatic rise from global warming PLUS isostatic subsidence. The combined effect in London is a relative sea level rise of approximately 3–4 mm/year.

graph TD
    A["Sea Level Change"] --> B["EUSTATIC<br/>(Global)"]
    A --> C["ISOSTATIC<br/>(Local/Regional)"]
    B --> B1["Ice melting →<br/>sea level RISES"]
    B --> B2["Thermal expansion →<br/>sea level RISES"]
    B --> B3["Ice formation →<br/>sea level FALLS"]
    C --> C1["Post-glacial rebound →<br/>land RISES<br/>(Scotland, Scandinavia)"]
    C --> C2["Peripheral subsidence →<br/>land SINKS<br/>(SE England, Netherlands)"]
    C --> C3["Tectonic uplift →<br/>land RISES<br/>(Chile, Japan)"]

Exam Tip: Distinguish clearly between eustatic (global, driven by water volume changes) and isostatic (local, driven by crustal adjustment). Many exam answers confuse the two. Remember: eustatic = water moves; isostatic = land moves.

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Emergent Coastal Landforms

Emergent coastlines form where the land has risen relative to the sea (through isostatic rebound or tectonic uplift) or where eustatic sea level has fallen. Former marine environments are now above sea level, preserved as relict features.

Raised Beaches

A raised beach is a former beach and wave-cut platform now elevated above the current sea level. Raised beaches consist of rounded pebbles, sand and marine shells sitting on a flat rock platform, often backed by a relict cliff (an old cliff face no longer reached by waves, now degraded by sub-aerial weathering).

In western Scotland, raised beaches at 3–8 metres above current sea level record the shoreline position during the early post-glacial period, when sea level was higher relative to the land (before isostatic rebound had fully compensated for the ice sheet melting). Further uplift has since lifted these beaches above the current shoreline.

Relict (Fossil) Cliffs

Relict cliffs are former sea cliffs now separated from the sea by a raised beach or coastal plain. They are no longer eroded by waves and are instead degraded by sub-aerial weathering and vegetation growth. Relict cliffs can often be identified by their rounded, vegetated profiles — in contrast to active, freshly eroded cliff faces.

Marine Terraces