20,000 years ago sea levels were 120m lower than today yet civilisation has thrived during that period and we've hardly noticed the rise. People move house every decade or so and the change in a decade has averaged 6 cm so it's possible to outrun it by moving.

20 000 years ago we didn’t have closed borders, nation states, global markets, etc. Heck, the crops we grow today and depend on were mostly still in their way less nutritious and harder to grow wild variants.

20 000 years ago we also had a massive number of big game animals which grazed in gigantic herds on vast prairies. These prairies don’t exist anymore, and neither do the herds.

On top of that, our historic sources don’t reach this far back, and our archeological records gets really blurry this far back, so what do you know. Perhaps there were multiple time periods where societies did indeed collapse, where a governing structure spanning an entire region was suddenly and violently disrupted and never returned, with cultures and languages lost with no descendants. Surely this happened multiple times between 30 and 20 thousand years ago.

Around 3000 years ago society in the Middle East, North Africa and Balkans certainly did collapse because of some volcanic eruptions and the subsequent migration crisis and the market failure of a single industry.

I googled the 3000 year ago thing and the Wikipedia for Late Bronze Age collapse says competing theories for the collapse have been proposed including volcanoes and also "disease, invasions by the Sea Peoples or migrations of the Dorians, economic disruptions due to increased ironworking, and changes in military technology and methods that brought the decline of chariot warfare."

So a lot of stuff going on really. I think similarly through our lives there will be all sorts of stuff going on such that the sea rising a foot or so will be not that noticable.

I think most historians agree that the Sea People invasions was a result of the migration crisis from Europe into the area. Whether the initial cause for this migration crisis was a volcano or some other disaster is kind of irrelevant to our discussion. What matter is something changed, causing people to need to migrate, causing wars and destruction, and an eventual collapse.

The climate crisis isn’t just the sea-level rise, I think civilization would survive Florida and Bangladesh going under (but it will suck). But our climate crisis is this sea-level rise at the same time as a bunch of wild fires, more powerful hurricanes, crop failures, droughts, ocean acidification, etc. All of this in tangent will result in mass migration, just like the Sea Peoples, just in the opposite direction. And unless our governments change their border policies (unlikely) there will be wars and destruction in our remaining cities.

I think the current thinking is that the sea people were a symptom more than a cause of the collapse. The few surviving records talk about crop failures and widespread unrest even before the sea people appeared. You had a number of feudal kingdoms teetering on the brink of collapse and the sea people provided a convenient excuse for the rulers so they didn't have to blame themselves.

Exactly. As I understand it the invasions came as a result of a migration crisis, this migration crisis was also a symptom of the collapse, i.e. something else (most likely devastating volcanic eruptions followed by nuclear winder, crop failures, etc.) which made life in Europe really hard, forcing the migration. I imagine these migrants weren’t super welcomed into the more rich Bronze age cultures, otherwise perhaps the migration might have been less violent, and collapse could have been avoided.