Sea level

What it does

Sea level shows where the sea reaches at a chosen level, from −134 m — the lowest sea level of the last glacial maximum, about 21,000 years ago — to +70 m, when all land ice has melted.

Below today the dry sea floor is drawn as land: Doggerland joins Britain to the continent, and the Bering Strait closes. Above today the sea floods the coasts.

The sea only goes where it can reach. A depression floods once the sea spills over its lowest saddle, not at its own depth: the Black Sea connects at −28 m over the Bosporus, the Caspian at +25 m, the Dead Sea at +58 m.

Using it

1. Open the tool and move the slider, or use the step buttons and ▶ to play. Today goes back to today's level.
2. Switch to Time to move through years instead of metres. The level then follows a sea level curve (Lambeck et al. 2014), and the panel names the period — meltwater pulse 1A, the Younger Dryas, Doggerland drowning.

Closing the panel leaves the map at the level you chose, so you can measure, query or print it. The legend follows the slider: "Sea", "Dry sea floor" and "As today".

Not for navigation: coastal land heights are only indicative.

In your own map

Add sealevel to a toolbar. Nothing else is required. The tool adds its own layer from the coastal zones archive (tiles) when the map does not already show one — or from the same zones as GeoJSON (geojson) on an engine that cannot read the archive — and reads its curve from data. All three paths are config assets, relative to the config file.

The archive is coastal_zones.pmtiles (~15 MB; the GeoJSON is 10 MB, 0.8 MB gzipped), built from the GEBCO_2026 grid and published as a release of edugis-org/coastal_zones. It is read with HTTP range requests, so any static host serves it — but copy it to your own host: GitHub release downloads are not readable by a browser. The data are CC BY 4.0; credit EduGIS and GEBCO.

A coastal-zones layer already on the map is adopted rather than added again. The tool never takes it from the catalog: which format to load is the engine's to decide, not the config's.

Extending it

The layer is split once into one sublayer per flood_level class, each with a constant colour, and a move rewrites only the classes that change role. A single data-driven fill-color expression was re-evaluated for every feature of every loaded tile on each move: about 335 ms per step against 32 ms now.

The tool is the same on every engine; only the data differs, and the adapter decides which (canDrawSource). MapLibre reads the pmtiles:// archive. OpenLayers cannot, and gets the same zones as one GeoJSON file at 16′ (geojson), read once into the view's projection — so the animation also runs in Equal Earth or Mollweide, where a world map shows areas truthfully.