Britain and the world at every sea level, from the depth of the last Ice Age, when you could walk from Yorkshire to Denmark, to a world with no ice left at all. Move the sea and watch the map, the land and the numbers change.
The coast is not a fixed line. Twenty thousand years ago, at the height of the last Ice Age, so much water was locked up in ice sheets that the sea stood about 120 m lower than today. The southern North Sea was a country of hills, rivers and marshes that archaeologists call Doggerland, and Britain was the far corner of Europe. If the ice that is left melted, the sea would rise by tens of metres and draw a new coast through London, the Fens and half the world's great cities.
This page is a simple instrument for thinking about that. It takes a global model of the height of the land and the depth of the sea, sets the water to any level you choose, and floods every place below that level that the sea can reach. It is what geographers call a bathtub model: crude, honest about being crude, and good for seeing the shape of things. It is not a flood map, and it cannot tell you about your street.
Drag the slider, press play, or jump to a moment. The map redraws as you go, and so do the numbers under it. Switch between the whole world and Britain with the North Sea, and zoom in for sharper terrain where your browser can fetch it.
Map: my bathtub model on AWS Terrain Tiles (zoom 4 for the world, about 10 km cells at the equator; zoom 7 for Britain and the North Sea, about 700 m). Zoomed in, the page fetches finer tiles from the same source and checks them against the coarse model; if it cannot, it shows the coarse cells. Antarctica is drawn as it is today. Coast: Natural Earth 1:10m. Numbers: worldwide figures leave out Antarctica; UK figures use a 190 m grid.
The model raises or lowers a flat sea and floods whatever is lower and joined to the ocean. Real coasts have sea walls, tides, storms, rivers, sinking and rising land, and an ocean whose surface is not flat. The elevation is coarse: about 10 km for the world, 700 m around Britain on the map and 190 m for the UK numbers, with errors of metres. Results for one town are indicative only, and nothing here tells you whether a particular house will flood. For that, use your national flood maps (in England, Check your long term flood risk).
Drop the sea to the Ice Age low and the map fills with sand colour: the floor of today's shallow seas, dry. Worldwide, outside Antarctica, the model exposes 19.4 million km² of sea floor at −125 m, an area 14% the size of today's land. Among the gains are the great continental shelves: the plain between Siberia and Alaska, the shelf joining Borneo, Sumatra and Java to Asia, and the one joining New Guinea to Australia.
Around Britain, the whole of the southern North Sea comes up. In my box from 51.0° to 57.5° N and 3.0° W to 9.0° E, the model has 340,713 km² of dry land at the Ice Age low, almost all of the box. At −40 m it still has 180,256 km², and at −20 m only 42,627 km², mostly in strips along today's coasts. The Dogger Bank, the high ground in the middle, goes under in the model at −22 m; Wikipedia says it remained an island until at least 5000 BCE.
The moment that matters for British history is when the last land link to the continent goes under. Using today's seabed, that happens in the model when the sea reaches −30 m (on the coarse world grid, −29 m). Ireland parts from Britain earlier, at −38 m. Wikipedia's account of Doggerland says it was inundated between 10,000 and 7,000 years ago, and that the tsunami from the Storegga underwater landslide off Norway, about 8,200 years ago, may have flooded much of what was left. I do not turn my levels into dates. Today's seabed is not the seabed of 8,000 years ago: rivers, tides and storms have moved sediment since, and the land itself has risen in the north of Britain and sunk in the south since the ice melted. The order of events is the robust part; the exact metres are not.
The same calculation for other famous land bridges, all on the coarse world grid and today's seabed, is under the map: light up a chip and the sea drops to that level.
Now raise the sea. The first metre matters most to people alive today. The Intergovernmental Panel on Climate Change's Sixth Assessment Report (AR6) gives likely ranges for global sea level in 2100, relative to 1995 to 2014, of 0.28 to 0.55 m under very low emissions and 0.63 to 1.01 m under very high emissions. Those are the slider's thin coral band. Over the next 2,000 years it expects about 2 to 3 m if warming is limited to 1.5 °C, 2 to 6 m at 2 °C and 19 to 22 m with 5 °C of warming.
A bathtub model is at its weakest exactly there. The world grid only resolves whole metres and 10 km cells, so at +1 m it mostly measures how many cells sit right on the shore. Around Britain, at +1 m the model drowns 275 km² of land that it puts above today's sea level, plus 574 km² that it already puts below sea level (land kept dry today by banks, pumps and sea walls, which the model does not have). Real flood risk in 2100 will be set by storms and defences far more than by this arithmetic.
Higher up, the shapes become clear. At +7 m, about what the Greenland ice sheet would add if it all melted, the model drowns 10,509 km² of the UK (4.3% of its land), puts the centre points of 61 towns and cities under water, and reaches neighbourhoods home to 2,645,000 people in England and Wales (4.4%). Worldwide, 1,877 cities of more than 15,000 people have their centre below the line, home to 234 million.
At +70 m, about what you get if every ice sheet and glacier melted, 33% of the UK's land is gone and 60% of the people of England and Wales live in neighbourhoods whose centre is under water. The world loses 14.0 million km² (10.2% of its land outside Antarctica) and 12,857 cities with 1.91 billion people. The Netherlands loses 97% of its land in the finer grid, and among large countries Denmark and Bangladesh lose about nine tenths. That would take many centuries to millennia, but it is the size of what the ice holds.
These tables follow the slider. Countries are ranked by land drowned (model height at or above today's sea level), first by area and then as a share of the country, among countries with at least 20,000 km² of land in the grid. Cities are GeoNames places of more than 15,000 people; their heights are GeoNames' own point heights, which are rough, and a city counts once the modelled sea comes within one grid cell of it.
Search for a British town or city (OS Open Names, 1,426 places) or any city in the world over 15,000 people (GeoNames). You will see the height the data give for its centre point and the level at which the modelled sea first reaches it. A town is a place, not a point: its centre may stay dry while its harbour floods, or the other way round.
Elevation. AWS Terrain Tiles combine several open elevation and bathymetry datasets into one global model of land height and sea depth in metres. I used the zoom-4 tiles for the world (4,096 by 4,096 web-mercator cells, about 9.8 km wide at the equator and 6 km at 52° N), the zoom-7 tiles for Britain, Ireland and the North Sea (about 700 m), and a zoom-10 extract for Britain and Ireland, averaged to about 190 m, for the UK numbers.
Today's coast. Level 0 is today's coast from Natural Earth's 1:10m land, not from the elevation model, so the map at 0 m looks like the world. At or below 0 m, today's land stays land; at or above 0 m, today's sea stays sea. The Caspian Sea is treated as a lake, apart from the ocean.
Flooding. For each of 216 levels from −130 m to +70 m (1 m apart, and 0.25 m apart between −2 and +3 m), a cell is sea if it is below the level and joined to the ocean through other cells below the level (cells touching at edges or corners). A falling sea leaves enclosed basins as water. Land that is already below sea level today floods at the first rise if the ocean can reach it, because the model has no sea walls; I count it separately.
Places. A place is under water when its point height is below the level and the modelled sea at that level reaches within one grid cell (about 10 km for the world, about 700 m around Britain on the coarse grid, about 190 m for UK places). Point heights: GeoNames' own height for world cities; the zoom-10 elevation for UK places. People in England and Wales are counted by Census 2021 Middle Layer Super Output Areas (neighbourhoods of about 8,000 people) whose population-weighted centre is under water. Scotland and Northern Ireland are in the town counts and land areas but not in the population figure.
Land bridges. For each pair of places I find the highest sea level at which they are joined by land (cells touching at edges) on today's seabed.
No tides, storms, waves, defences, rivers or groundwater. No change in the shape of the land: since the Ice Age the ground has risen where the ice was thick (Scotland) and sagged around it, and melting ice today would change the Earth's gravity and crust so that the sea would not rise evenly. No erosion or sediment: the seabed of 8,000 years ago was not today's. The elevation model mixes sources of different ages and accuracy, sea-floor depths are interpolated in many places, coarse cells average land and sea at the coast, and lakes are mostly recorded at their surface. The world grid has whole-metre heights, so its numbers move in steps. Worldwide figures leave out Antarctica, whose land surface is mostly the ice sheet itself. GeoNames point heights are averages over 90 m or 900 m cells. All of this makes the model good for broad shapes and comparisons and poor for any single place.