The 70-meter map, explained

Open Sea Level anywhere near a coast and move the slider all the way up. The headline number is 70 meters, and it deserves a paragraph of its own, because it is a thought experiment that got promoted into a number people argue about.

Here is what it actually is, where it comes from, and where it stops being a map.

The number, stated precisely

Seventy meters is the elevation that the global sea level would gain if every ice mass currently sitting on land melted and its water ran into the ocean. The qualification matters: floating ice, the ice shelves that already sit in the water, does not raise anything when it melts, for the same reason a melting ice cube does not overflow a glass. Only ice that is grounded on land, and therefore above the ocean it could enter, adds water.

The best-known estimate of that pool is often credited to a 2015 synthesis in Science of the World's Ice, and it is not a precise figure so much as a well-bounded one: roughly 65 to 70 meters of sea-level equivalent locked in Antarctica, Greenland and the mountain glaciers, with the ice sheets carrying the overwhelming share. Sea Level uses 70 meters as the round, defensible value, and states it as what it is: a scenario, not a forecast.

What the geometry requires

To draw the 70-meter map, the app takes your coordinates and walks the terrain until it finds every cell of the local elevation model that sits below 70 meters and is connected, through cells that are also below 70 meters, to open water. That last condition is what makes the coastline honest. A basin ringed by a 20-meter ridge does not fill on the map, because the water of a uniform 70-meter rise cannot get over the ridge to reach it. This is the difference between a flood fill and a naive low-water mask, and it is the difference between the map you see and the maps that famously drown inland salt lakes.

The elevation data comes from the public global model most services of this kind use: a 30-meter-resolution grid, sampled per request through the open elevation service. Its vertical accuracy is on the order of several meters in most places and worse in mountains, which sets the resolution of the whole exercise.

What the map is good for

Used honestly, the 70-meter map is the most intuitive climate graphic a person can hold in their hand, because it is local. It replaces "global sea level could rise by tens of meters on a full-melt timescale" with a street map that shows where the water would stop if that world arrived. It is the same move as a fire escape plan: you do not need the fire to arrive to want the plan.

It is also irreversible in an interesting sense. The melt of the great ice sheets, on the timescale of the full pool, is measured in millennia, not in your lifetime. The 70-meter world is not a projection for 2100. It is an envelope: an upper bound of what the planet's ice, if fully mobilized, could do.

What the map is not

It is not a forecast, and the app does not present it as one. It takes the whole ice reservoir and moves it into the ocean at once, with no timeline, no warming path, no thermal expansion (which adds perhaps half a meter over centuries, a rounding error against the pool), no groundwater, no subsidence, and no engineering. It also cannot know where the water would not reach because we would have built walls, which a city like Rotterdam already proves people will do.

None of that makes the map useless. It makes it a scenario, and scenarios are exactly what you should use an upper bound for: deciding where the line is that you cannot defend.

The honest footnote

Every time Sea Level shows you the 70-meter number, the map is telling you three assumptions it cannot do without: a uniform rise (in reality coastal distribution varies), a still ocean (no tides or surge), and a wet world with the full pool mobilized. Two of those never happen, and one of them is what raised the stakes in the first place. That asymmetry is the whole interest of the map, and it is why it stays in the app with the scenario label it has always carried.