Sea Level Map

About

SeaLevelMap.io is an interactive map of the world's coastlines and ice sheets over the past 500,000 years: five ice age cycles, from today back through the Last Glacial Maximum to Marine Isotope Stage 13.

World map at lowered ice-age sea level, with continental shelves exposed and ice sheets over the northern continents

Why this exists

I grew up near the shores of Lake Huron, and I have always been fascinated by the Great Lakes, particularly that they are only around 14,000 years old and were not fully formed until as recently as 4,000 years ago. Before the five Great Lakes, the whole area lay under the southern edge of the Laurentide Ice Sheet, with some places covered by up to 10,000 ft (3,000 m) of ice. That fascination led me to research the large-scale changes since the Last Glacial Maximum, roughly 25,000 years ago, to better understand how the climate has reshaped the Earth's surface.

The map grew out of that research and out of correspondence, since 2022, with sixteen scientists who shared their data and answered my questions along the way. They are thanked on the Data page.

How the map is made

Coastlines. For each time on the slider, the map takes today's bedrock and sea floor from GEBCO's global grid and moves it up or down by the change in relative sea level since then. That sea level comes from a model computed by Dr. Jacky Austermann from the ICE-6G_C ice history (Peltier et al. 2015); from 50,000 to 80,000 years ago it comes from the model of Gowan et al. (2021), built on the same ICESHEET 2.0 ice the map draws, which matches the dated sea level evidence better at those times. Because each model includes the solid Earth's response to the ice, the sea is not one global number: near the ice sheets the land was pressed down and the sea reached places that are dry today. Low ground that the sea could not have reached, such as the Dead Sea or the Caspian depression, stays land.

Ice sheets. Up to 80,000 years ago the ice comes from ICESHEET 2.0 (Gowan et al. 2021), a reconstruction built from geological evidence of where the ice margins stood. Before that, no such reconstruction exists, so the map shows the ICE-6G ice history that the sea level model was run with. At every step the ice is shaded by its ICE-6G thickness, so the ice sheets read as domes. Around the Last Glacial Maximum the map also shows the glaciers of the world's mountain ranges, from GLACIMONTIS (Lima et al. 2025). On the older glacial stages, with the Evidence button on, a dashed red line marks how far the ice reached according to the geological evidence, compiled by Batchelor et al. (2019).

Proglacial lakes. From 20,000 to 5,000 years ago the map also shows the lakes that pooled along the retreating ice in North America, among them Lake Agassiz and the larger, earlier forms of the Great Lakes. They come from the Geological Survey of Canada's maps of the ice's retreat (Dyke et al. 2003). From 100,000 to 24,000 years ago, before those maps begin, the lakes come from a model of where meltwater pooled against the ice (Austermann et al. 2022).

Sea level evidence. The Evidence button shows the dated records that sea level reconstructions are tested against: shells, peat, corals and raised beaches, each placed on the time step nearest its age and coloured by whether the sea stood at, above or below it. They come from GAPSLIP, Evan Gowan's global archive of paleo sea level indicators, and are densest for the last 15,000 years.

Beyond 400,000 years. Where the sea level model stops, the map carries on to 500,000 years by borrowing the coastlines and ice of an earlier time with about the same global sea level, at the same point in the ice age cycle. The sidebar names the time each step borrows.

Relief. Land is shaded by height from the same GEBCO bedrock the coastlines are cut from, so the continental shelves exposed by lower seas show their real relief. The 3D button raises that relief and tilts the map.

Basemap. Today's coastline, lakes, borders and place names come from Natural Earth.

What is measured and what is schematic

None of it is exact. Sea level was never one number: near the ice sheets the land sank under the weight of the ice and rose again after it melted, and the ice's own gravity pulled the ocean toward it, so at the same moment the sea could be rising in one place and falling in another. The map shows our best understanding of what the Earth looked like, built from the evidence we have.

The coastlines at every step, the ice up to 80,000 years ago and the proglacial lakes come from published reconstructions and models. The ice before 80,000 years and the steps beyond 400,000 years are schematic: they repeat the shapes of one glacial cycle in another, so they show the pattern of the ice ages rather than a record of any one of them. The map says which is which wherever it matters, in the sidebar for each time step.

All of it works on a grid of about 3.7 km, so it shows the shape of past coastlines, not local detail. The full sources and citations are on the Data page.

Supporting the map

SeaLevelMap.io is free and has no ads. If it's useful to you, you can support it on Buy Me a Coffee.

Acknowledgements

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