Method

This is not a forecast

The slider is a question, not a timeline: if the sea stood this much higher, what would be under water? It says nothing about when — or whether — any given level is reached. Nothing on this site is a projection of future sea level.

For scale: melting all land ice on Earth would raise the sea by roughly 65–70 m, over many thousands of years. Levels above that on the slider are there to explore the terrain, not to describe anything that could happen.

How the water spreads

Water only reaches ground that is actually connected to the sea at the level shown. A basin lying below the water line but ringed by higher ground stays dry, because the sea cannot get there.

Deep basins are the exception. Where the terrain drops below sea level into a closed basin — the Dead Sea (−430 m), the Qattara Depression in Egypt (−133 m), the Danakil Depression in Ethiopia (−125 m), Death Valley (−85 m) — the map fills the basin from its own lowest point as the water line rises through it, and the slider starts at that floor instead of at sea level. Treating those basins like the rest would leave them dry until the sea broke over the rim: Death Valley would sit unchanged until 594 m and then fill in one step.

Landlocked regions have no coast for the sea to enter from at all. There the slider starts at the lowest point of the terrain rather than at sea level, and shows how far the surrounding land rises above it.

What is left out

• Time. No rate, no scenario, no year.
• Land movement. Coasts rise and sink on their own — post-glacial rebound in Scandinavia, subsidence in river deltas. Not included.
• Tides, storm surge and waves. The water surface is treated as flat and still.
• Buildings. The elevation model describes terrain, not what stands on it.
• Everything outside the border. Neighbouring countries and regions are drawn flat, for orientation only. They are not part of the calculation and never flood, however far the slider is pushed — that is the map ending, not the water stopping.

How precise is it

Most maps rest on elevation data with a grid spacing of about 90 m, some on 30 m, and a few remote areas on roughly 450 m. Vertical accuracy is on the order of a few metres — so the difference between a 1 m and a 2 m step is close to the noise floor of the underlying data, especially on flat coastal plains. Read the low end as indicative, not exact.

Elevation minima and maxima shown on each page come from published figures where these exist, because values derived from the raw grid are unreliable: measurement artefacts over water regularly produce spurious readings tens of metres below sea level.

Population figures

“People affected” counts residents inside the flooded area according to a global population grid, scaled so the total matches published population for that country or region. It is an estimate of who lives on that ground today — not a forecast of who would be displaced, and not an account of where anyone would go.

The datasets behind all of this are listed under Sources.