The Global Centre for Risk and Innovation (GCRI)
Sea level is rising as oceans warm and land ice melts, and coastal floods that were once rare are becoming frequent. Extreme sea levels combine mean sea level, tides, storm surge and waves, and local land subsidence can add as much as the global rise. This page sets out how coastal flooding develops, how extreme sea levels and projections are estimated, and what evidence serves the authorities planning coasts for decades ahead.
GCRI · In development · service level R0
Sea-level rise is the increase in mean sea level caused mainly by warming seawater expanding and land ice melting. Locally it combines with land subsidence, tides, surge and waves to drive coastal flooding, so modest rises make today's rare floods frequent. Projections differ by scenario, with ice sheets the largest uncertainty.
For sea level and coastal flood, in GCRI's spatial-intelligence reading, the map opens on marine heat, true-colour imagery and sea temperature with relief in 3D and a 15-day forecast chart at the centre, framed on Pacific Island states; every other NASA layer, live feed and forecast waits in the layer list.
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NASA GIBS / EOSDIS · ASTER GDEM (METI, NASA) · Blue Marble (NASA) · © OpenStreetMap contributors (ODbL) · Natural Earth
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For sea level and coastal flood, in GCRI's spatial-intelligence reading, the map opens on marine heat, true-colour imagery and sea temperature with relief in 3D and a 15-day forecast chart at the centre, framed on Pacific Island states; every other NASA layer, live feed and forecast waits in the layer list.Satellite and model products as NASA publishes them, each with its own date; indicators, not assessments of any place; claim ceiling INTEGRATED.
Layers and dates
The working screen behind this subject: its structure, terms and states. Names are placeholders and values illustrative.
Sea level and coastal flood · Coastal zone A · Sea level scenario set
placeholder coastline
| Dune nourishment | 0.4 | 2 | Raise seawall | Overtopping twice in 5 yr |
| Raise seawall | 0.5 | 10 | Managed realignment | Seawall at design limit |
| Managed realignment | 1.0 | 14 | Relocate assets | Community decision |
Shoreline exposure and changing sea levels. The dossier below sets out how the risk develops, what is measured, where the data come from, how it is modelled and who decides.
Thermal expansion of seawater and melting glaciers and ice sheets raise global mean sea level. The rise continues for centuries after warming stops, and ice sheet loss is the largest uncertainty.
Subsidence from groundwater extraction, sediment compaction and tectonics makes the sea rise faster relative to the land in many deltas and coastal cities.
Storm surge, high tides and waves come on top of mean sea level. A modest rise in the mean makes today's rare extreme levels occur far more often.
Beaches erode, wetlands may migrate or drown, and saltwater intrudes into aquifers and rivers. Defences shift flooding elsewhere and can fail when overtopped.
Each term as this page uses it. Where a standard-setting body defines the term, the wording follows that body.
| Term | Definition |
|---|---|
| Thermal expansion | The increase in seawater volume as it warms, a major contributor to sea-level rise. |
| Low-elevation coastal zone | Land contiguous with the coast below a stated elevation, often ten metres, used to estimate exposure. |
| Vertical land motion | Upward or downward movement of the land surface that changes relative sea level. |
Each indicator carries its unit, its cadence, the kind of source behind it and the evidence label a value would carry. No values are shown: a programme supplies them under its own authority.
| Indicator | Unit | Cadence | Source type | Label |
|---|---|---|---|---|
| Relative sea-level trend | millimetres per year | annual | Tide gauges | OBSERVED |
| Global mean sea level | millimetres relative to a reference | monthly | Satellite altimetry | OBSERVED |
| Extreme sea level at a return period | metres above a datum | per analysis | Extreme value analysis of tide gauge records | ESTIMATED |
| High-tide flood days | days per year | annual | Tide gauges and flood thresholds | OBSERVED |
| Vertical land motion | millimetres per year | per analysis | GNSS and radar interferometry | OBSERVED |
| Projected sea-level rise | metres by a stated year and scenario | per assessment | IPCC AR6 projections | MODELLED |
| Population in the low-elevation coastal zone | people | per assessment | Elevation and population grids | ESTIMATED |
The topic as a computational graph: named sources feed the methods, an evidence review gates the outputs, and the decisions stay with the authorities that hold them.
Fig. 02Nodes carry their type, two metric rows and a state; connectors show what feeds what; dashed frames name the owners; the timeline is the cadence. Design of the pipeline, not a record of a run.
Named public datasets and institutional records, each with its licence class. Sources by agreement are used only under their terms and never republished.
| Source | What it gives | Licence class |
|---|---|---|
| Permanent Service for Mean Sea Level (PSMSL) | Monthly and annual mean sea level from tide gauges worldwide | Free with citation, PSMSL terms |
| GESLA global extreme sea level archive | High-frequency tide gauge records for extremes | Free with citation, licence per contributor (to check) |
| Copernicus Marine Service sea level products | Satellite altimetry sea level and anomalies | Free, Copernicus Marine licence |
| IPCC AR6 sea level projection tool (NASA) | Regional sea-level projections by scenario | Open, NASA data policy |
| NOAA tides and currents data | Water levels, high-tide flooding outlooks and datums for US stations | Public domain, US government data |
| Copernicus DEM and other global elevation models | Coastal elevation for exposure analysis | Free, licence per product (to check) |
Oceans and Coasts prepares evidence. The decisions below are taken by the authorities named, under their own rules, and never by the platform.
| Who decides | The question | Evidence this page serves |
|---|---|---|
| Coastal and flood defence authorities | What standard of protection to build and when to raise it | Extreme sea levels; projections; land motion |
| Spatial planning authorities | Where to restrict development or plan retreat | Exposure maps by scenario |
| Port and infrastructure owners | Which design sea levels to adopt for long-lived assets | Projections with ranges stated |
| Water utilities | How to protect aquifers and intakes from saltwater | Sea-level and intrusion evidence |
Global mean sea level rises mainly through thermal expansion of warming water and through the loss of land ice from glaciers and the Greenland and Antarctic ice sheets. Regional sea level departs from the global mean because of ocean circulation, the gravitational and rotational effects of where ice is lost, and vertical land motion from tectonics, glacial rebound and subsidence. In many deltas and coastal cities, subsidence from groundwater and hydrocarbon extraction and sediment compaction is a large part of relative sea-level rise.
Extreme water levels arise when storm surge, driven by wind and low pressure, coincides with high tide and large waves, and when rivers carry high flows to the coast. As the baseline rises, a flood height that was once rare is reached more often, and the shoreline itself retreats where sediment supply cannot keep pace.
Long tide-gauge records and satellite altimetry measure sea level; GNSS measures land motion. Gauges are sparse in many regions.
Return levels are estimated from observed maxima; short records and changing baselines widen the uncertainty.
Surge and wave models drive inundation maps; results depend strongly on the accuracy of coastal elevation data.
Limits: ice-sheet processes add deep uncertainty to late-century projections, and coastal elevation models can misstate flood extent where vegetation or buildings bias the terrain. Flood maps should state their scenario, time horizon, datum and resolution.
The current service level is R0: information and enquiries. An enquiry receives a reference and names the next step; applications, payments, compute allocations and registrations open only when their team, terms and review capacity exist.