Coastal Ice Age Civilization- Dealing With Sea Level Changes

Question

I have some isolated ice age civilizations in a setting I'm working on and there's one thing I'm not quite sure about; rapid sea level changes.

Sea level during the last 10,000 years or so of the ice age was absolutely not stable, unlike recorded history, changing quite a bit even before the big Meltwater Pulse. Civilizations tend to stick to rivers and coastlines for all sorts of good reasons, so this is not something to be ignored. Any civilizations that existed back then would likely be on the continental shelves or on rivers leading to them (which would also make them not evident in modern day). Prior to the Meltwater Pulse, the rate of sea level increase looks to be several cm/year. Year-to-year not so bad, but it adds up over time.

On the one hand, that's kinda gradual, but continental shelves have gentle slopes, so an increase would cover land area faster than, say a steep volcanic island. Not to mention storm surges reaching even higher. How manageable is this? Cities, villages and farms could be built higher up over centuries, but I've mostly I'm wondering about sea walls. I've seen those depicted in numerous sci-fi franchises around coastal cities. Is a "Great Sea Wall of China" doable with stone/concrete? Such a wall would need to also run up any rivers a ways up too, I imagine, turning them into something like The Los Angeles River. I know there are dams holding back some pretty big lakes, but this the ocean we're talking about.

Am I missing something? Underestimating the problem? Or do I have the basics down and I'm just second-guessing myself?

Answer 1

Nomadic <-> settled is a sliding scale.

There are, shall we say, "levels of settlement". Moving a modern city 10m higher up the coastline is a massive undertaking, but moving a few tents is pretty easy. Moving a town of mud huts with thatched roofs and preparing new farmland is somewhere in between on the logistics scale. I'm guessing your civilisation has wood or mud brick houses, thatched roofs, and hunts for food with a mostly meat diet (not a lot of farming in an ice age). Dismantling and rebuilding that architecture is annoying, but not impossible.

There will be warning the seas are coming, depending on how frozen the water is at that particular location.

If it's mostly water: king tides, storm surges reaching the town, and then submerging it. (Kinda like what's happening to us now). Thered be some rule of thumb devised - eg When the water touches the town twice in a year, time to move.

If it's mostly ice, frozen very thick, the ice will literally approach the villages, knocking trees down as it comes like a slow motion villain. They'll have plenty of warning to move when the ice is hundreds of metres away.

So your society will need to move, say 5m up every few generations. After a few repeats and it gets into the culture they'll become pretty good at moving, dismantling, and rebuilding.

Answer 2

You are not overestimating the problem. There have been humans around for 300 000 years.

But only after the coastal regions became usable and reliable, we were able to start our civilization, some 30 000 years ago.

This is a huge one.

• Walls don't work eternally. The sea water will come into the city through the ground and inject salt into the drinking water reservoir. That's a problem northern Europe will have to face in the coming decades, Jakarta already today.

Answer 3

There were very few cities more than 10,000 years ago and those that did exist were located on rivers such as the Tigris and Euphrates. Up until 10,000 years ago most of humanity lived as hunter gathers so would not have noticed the sea level rise - their lands would simply have shifted over time.

Even 8000 years ago the number of cities was still small and by then most of the flooding had already taken place so the issue is limited.

A Great sea wall of China might well have been possible. Looking at what the Egyptians did with the pyramids, the technology would have been available to build a very considerable wall. However the difficulty is why? How valuable is the city and a bit of farmland? It would almost certainly have been easier to abandon the city and move further inland.

Or more likely some sea walls would have been built, but eventually the effort would have drained the local economy. People (especially the younger ones) would start to leave to make a fresh start elsewhere. In the end salination would have started to contaminate the ground water and that would rapidly accelerate the process.

Answer 4

Look to New Orleans for a Reference

The city sits on the mouth of the Mississippi River feeding into the Gulf of Mexico. Only the old city is above sea level. The parts of the city below sea level form part of the brackish swamps in most of the region. You might rightly call the old city a high spot in the swamp, located on the river. Expansion of the city has been with fill dirt or building on top of old buildings, sea walls, and pumping out isolated lowlands.

Or look at Venice

You could choose not to fight the ocean. There may be enough (118) high spots in the new lagoon created by water incursion, and they may be close enough together, that personal boats can conduct a person from place to place. Most of the city however will grow along the new coastline.

Answer 5

Let's get some numbers here. The sea level change was 120 meters over 20,000 years. Now, the bulk of that was in the middle 8 thousand years of that range. So the peak rate was something like 2 meter per century.

Tides range from 0.6 meters per day (twice a day) up to several meters (16.3 m at largest) in unusual places like interesting shaped bays and such.

So each century, the semi-nomadic folks would need to move five times as far inland as the tides moved up the beach each day. Not counting the distance from the water they would keep due to storms, ice, etc.

So the result is very obvious. /sarcasm They would of course be taken entirely surprise by the alarming inundations of monstrous flood waters and be utterly destroyed. /end_sarcasm

But seriously, most of the folk living by the ocean at this time would be building structures that would be very unusual to last as long as a century. Even as recently as 2K years ago, a structure built to last that long would be highly unusual. Especially within a few meters of the edge of the ocean. They would simply build new buildings a few meters further inland about once every 4 or 5 generations. It might be interesting to have a look to see if there are settlements that this has happened to.

It would be complicated in some places by rebound of the land after the ice melts. So, probably the best place to look for such settlements would be somewhere not too far from the equator, with lots of ocean front. Central America possibly.