Sea-level Change

Y1

What is fetch and why does it matter for wave energy?

Fetch is the area of open water over which wind blows to generate waves. Greater fetch, wind speed and duration produce larger, more energetic waves. The Southern Hemisphere has a virtually uninterrupted fetch — producing the largest waves ('Roaring Forties')

What is wave shoaling?

As waves move into shallower water, orbital motion transitions from circular (deep water) to elliptical (intermediate) to to-and-fro (shallow). Waves build up height (shoal) until they break, creating wave bores

What is a longshore current and when does it occur?

A current running parallel to the shoreline that forms when waves approach at an oblique angle. It drives longshore sediment transport — rates are greatest when waves approach at ~45°

What is the difference in sediment transport zones between gravel and sandy beaches?

Gravel beaches: transport occurs primarily in the swash zone (uprush of wave). Sandy beaches: transport occurs across the entire surf zone

What are coastal barrier systems and where do they commonly form?

Elongated ridges of unconsolidated sand and gravel built by waves, tides and wind, protecting the mainland from open-water processes. They cover ~15% of world coastlines and are common on low-gradient passive margins with abundant sediment supply

What are the differences between wave-dominated and tide-dominated barrier coasts?

Wave-dominated: long linear islands, few tidal inlets, open water lagoon behind. Tide-dominated: low wave energy, short barriers, more tidal inlets, back barrier filled with sediment. Mixed energy: intermediate, with more inlets than wave-dominated

What are the four main estuary types based on water mixing?

Salt wedge (river-dominated, strong halocline, little mixing)

Partially mixed (moderate tidal range, weak halocline — e.g. Thames)

Well mixed (broad/shallow, strong tides overcome freshwater — e.g. Severn

Fjord-type (sill limits mixing, freshwater lid, deep anoxic water — e.g. Nuuk, Greenland)

What drives flocculation in estuaries and why does it matter for sedimentation?

In freshwater, clay minerals carry a negative charge; in marine water, positive charge. Where they mix, attraction between opposite charges causes clay particles to clump (flocculate) and settle out — promoting estuarine sedimentation

What are tidal flats and where do they form?

Gently sloping intertidal surfaces of unconsolidated silt, clay and organic matter, formed where slack currents allow deposition. Coarser sediment near the estuary mouth, finer higher up. They dissipate wave energy and recycle nutrients

What are saltmarshes and where do they occur?

Coastal wetlands dominated by short grasses, herbs and shrubs, restricted to mid-to-high latitude coastlines. They show vertical species zonation parallel to the shoreline, reflecting tidal inundation frequency, and are highly productive ecosystems

What determines whether a saltmarsh drowns, emerges or remains stable?

Balance between sediment accretion rate and sea-level rise. If accretion < sea-level rise: marsh drowns. If accretion > sea-level rise: marsh emerges. If equal: equilibrium

What is mangrove vegetation and where does it occur?

Trees or shrubs occupying low-energy tropical and subtropical coastlines, forming the low-latitude equivalent of saltmarshes

What is relative sea level (RSL) and why is it used instead of 'global sea level'?

RSL accounts for both changes in ocean water height and vertical land movement at a specific location. There is no single global sea level — every site has experienced a different RSL history due to variations in water distribution and local land movement

What are the four main causes of eustatic sea-level change?

Tectono-eustasy (changes in ocean basin volume via seafloor spreading rates)

Glacio-eustasy/barystasy (transfer of mass between ice sheets and oceans)

Geoidal eustasy (redistribution of ocean mass due to gravity changes)

Steric changes (thermal expansion/contraction of seawater — volume change without mass change)

What is glacio-eustasy and how large were sea-level changes during glacial cycles?

Sea-level change caused by the transfer of water mass between land-based ice sheets and the oceans. During glacial cycles this produced changes of ~120–130m globally. 1mm of sea-level change = ~360 km³ of continental ice

What is geoidal eustasy and how does Antarctic melting affect nearby sea level?

The ocean surface is not flat — large ice sheets attract ocean water gravitationally. When the Antarctic Ice Sheet melts, its gravitational pull decreases, water is redistributed, and sea level near Antarctica actually falls while rising elsewhere

What is glacial isostatic adjustment (GIA)?

The slow vertical movement of the Earth's crust in response to the loading and unloading of ice sheets. Ice loading causes the crust to sink and pushes mantle material outward, creating a forebulge at the ice margin. After melting, the crust rebounds isostatically

What is a forebulge and what happens to it after deglaciation?

A bulge in the crust around the margin of an ice sheet caused by displaced mantle material during ice loading. After the ice melts, the forebulge collapses, causing RSL rise in areas beyond the former ice margin

What are the three components of hydro-isostasy?

Loading of ocean basins by meltwater; movement of water to fill collapsing proglacial forebulges; loading of continental shelves by water causing crustal warping

What are near-field, intermediate-field and far-field sea-level zones?

Near-field: directly beneath or adjacent to former ice sheets (strong isostatic uplift after melting). Far-field: oceanic islands and continental margins far from ice sheets. Intermediate-field: beyond former ice margins but still indirectly influenced — locations that experienced forebulge then collapse

What drove UK relative sea-level change since the last glaciation?

Two competing factors: eustatic rise from melting global ice sheets (raising ocean volume) and isostatic rebound of Scotland (which was under the British-Irish Ice Sheet) vs. subsidence in southern England (forebulge collapse from the Fennoscandinavian Ice Sheet). Their relative influence changed through time

What is a storm surge and how does it differ from storm tide?

A storm surge is the abnormal rise of water above predicted astronomical tide levels, driven primarily by wind. A storm tide is the combined total water level from storm surge plus the normal astronomical tide. Surges can raise coastal water levels by metres for hours to days

What were the major European storm surge disasters?

1703: England, Belgium, Netherlands, Germany — thousands of deaths. 1953: Netherlands/UK — 2,533 deaths in Netherlands, 307 in UK. 2013: Major surge threat in UK with significant flood risk but 0 deaths due to improved defences (Thames Barrier)

How do subduction zone earthquakes drive relative sea-level change?

Through the earthquake deformation cycle: (1) rapid coseismic subsidence (RSL rise); (2) centuries of interseismic uplift (RSL fall); (3) pre-seismic submergence (RSL rise); (4) rapid coseismic change. Magnitudes can be metres over seconds to centuries

What is the Cascadia subduction zone earthquake of AD 1700 and how do we know about it?

A Mw9 earthquake on 26 January 1700 at ~9pm local time, evidenced by drowned coastal forests (ghost forests), tsunami deposits in Japan, and stratigraphic records of sudden subsidence in coastal sediments

What is Mean Sea Level (MSL)?

The arithmetic mean of hourly water heights — approximately the level halfway between mean high and mean low tide. It forms the baseline reference for measuring sea-level change

What are the two astronomical drivers of tides and what patterns do they create?

Gravitational pull of the moon and sun. When aligned (syzygy): spring tides (higher highs, lower lows). When at right angles: neap tides (lower highs, higher lows). Tidal patterns can be diurnal (one cycle/day) or semi-diurnal (two cycles/day)

What is a sea-level indicator and what does it require?

A physical, biological, geochemical or archaeological proxy with a systematic and quantifiable relationship to a tidal level. Each indicator has an 'indicative meaning' — the range of elevations over which it forms — which introduces uncertainty into RSL reconstructions

What are the main categories of sea-level proxy indicators?

Physical (surveyed palaeoshorelines, raised beaches); Biological (stratigraphic sediment cores with microfossils, pollen, foraminifera, diatoms); Geochemical (bulk sediment and compound-specific analysis); Archaeological (harbours, fish tanks, tide marks on structures)

What is radiocarbon (¹⁴C) dating and what are its limitations for sea-level work?

Developed by Libby (1949). Organisms incorporate ¹⁴C while alive; after death it decays (half-life 5,730 years). Age is determined from remaining ¹⁴C. Limitations: maximum age ~50,000 years; requires calibration (production rate not constant); errors from contamination, old carbon reservoir effects, and radiocarbon plateaux

What is luminescence dating and when is it used in sea-level research?

Measures light emitted from mineral crystals (quartz or feldspar) when exposed to heat or light, proportional to radiation received since burial. The 'clock' is reset to zero by sunlight or heating. Useful for dating wind- or water-transported sediments — including coastal and aeolian deposits

What are coral microatolls and why are they useful for sea-level reconstruction?

Flat-topped coral heads that grow upward until they are limited by low tide exposure — their upper surface records the position of mean low water. Growth bands can be dated and their elevation measured, providing a precise, datable sea-level indicator

What does the indicative meaning of a sea-level indicator mean in practice?

It is the relationship between where an indicator forms and a specific tidal level (e.g. high marsh forms between mean high water and highest astronomical tide). It defines the indicative range — the vertical uncertainty in reconstructed RSL from that indicator

What are the main contributors to 20th century sea-level rise?

Thermal expansion of seawater and melting of mountain glaciers dominate, accounting for ~75% of observed rise since 1971. Contributions from the Greenland and Antarctic ice sheets have increased since the early 1990s. Land-water storage has made a smaller measurable contribution

What does the IPCC AR5 project for sea-level rise by 2100?

A likely range of 0.26–0.82m by 2100, though some studies suggest this may be too conservative, particularly given uncertainty about ice sheet dynamics. Global mean projections must be regionalised to RSL to be locally useful

Why will global mean sea level continue to rise even after CO₂ and temperature stabilise?

Because of thermal inertia — the deep ocean takes centuries to warm fully, so steric expansion continues. Ice sheet mass loss also responds on long timescales. The climate system has committed warming and sea-level rise already 'in the pipeline'