What is the Jomon transgression?

The Jomon transgression is the rise in sea level after the Ice Age that brought the sea deep into today’s plains. Reconstructions for the central Kanto Plain put sea level about 3 m (±1.5 m) above today 7,000–4,000 years ago (the time machine uses +3 m; the value of +4.5 m sometimes quoted is the spring high-tide level, not mean sea level). The sea reached as far as Omiya in Saitama, and Tokyo Bay extended far inland. This height is a relative value for Kanto; global mean sea level is estimated to have still been below today’s. Many shell mounds were formed near the coastline of the time.

How low was sea level in the Ice Age?

At the coldest time of the last glacial period, about 20,000 years ago, sea level was about 120 m lower than today. Hokkaido was joined to the continent via Sakhalin, and Honshu, Shikoku and Kyushu, with the Seto Inland Sea as dry land, formed one large island (Paleo-Honshu). Narrow sea channels are thought to have remained in the Tsugaru Strait and between the Korean Peninsula and Tsushima. People are thought to have arrived in the Japanese archipelago from about 38,000 years ago by routes through Tsushima, Okinawa and Hokkaido, crossing the sea by boat on the Tsushima and Okinawa routes. Mammoths crossed into Hokkaido from Sakhalin. The youngest fossils of Naumann’s elephant date to about 24,000 years ago, and some studies suggest it died out earlier. Because the map applies a water level to today’s terrain, the Korean Peninsula, Tsushima and Kyushu look joined by land at −120 m (a limit of the seabed data and the drawing method).

What can I learn with this time machine?

You can freely change sea level between −150 m and +50 m and see where the coastline would be at that level. It helps explain why Jomon shell mounds lie inland, which routes people took to reach Japan in the Ice Age, and how coastal areas could change with future sea-level rise.

How are the sea levels for each era chosen?

They are educational estimates compiled by this site from the literature. Before about 10,000 years ago they are global mean sea level (Fairbanks 1989; Lambeck et al. 2014; Deschamps et al. 2012; Smith et al. 2011); from about 8,000 to 2,000 years ago they are relative sea level on the central Kanto Plain (Tanabe 2020/2022; Tanabe et al. 2016); after that they follow the global mean (Kopp et al. 2016; IPCC AR6). Sea level differs by several metres to more than ten metres between regions, so coastlines outside Japan are approximate. The values, ranges and references for each era are on the academic background page.

Did sea level rise in the Medieval Warm Period?

A reconstruction of global mean sea level (Kopp et al. 2016) estimates variations of about 10 cm up or down from the start of the Common Era to the Industrial Revolution, with a possible slight fall in 1000–1400. The time machine uses +0.6 m for the Medieval Warm Period. This is a regional comparison hypothesis based on records from the Tokyo lowland (Katsushika City history, “Heian transgression and sea-level change”) and the Seto Inland Sea (Kimura’s coastal-terrace survey, +60–70 cm in high-tide level); the original study and method have not been confirmed, and it does not indicate a rise in global mean sea level. The Little Ice Age in the Edo period is a comparison setting of −0.3 m, the low end of the global range. The Yayoi & Kofun era assumes today’s level because the details are unconfirmed. For the sea-level rise called the “Heian transgression” in Japan, no peer-reviewed study supporting a rise of a metre or more has been confirmed, and views differ.

Where do the background map and elevation data come from?

Land, lakes, rivers and place names come from public-domain Natural Earth, simplified and served by this site; when you zoom in, coastlines switch to high-resolution GSHHG data (LGPL). Elevation is read on your device from the Terrain Tiles (Mapzen) on AWS Open Data, and the GSI hillshade and pale maps are overlaid in real time over Japan. Elevation tiles are neither stored nor redistributed.

Coastline Time Machine Ver.2 | Sea Level Simulation of Japanese Archipelago

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