I think he means declining birth rates? They were low during the French Third Republic and didn’t recover until WW2 iirc

I feel the Wikipedia article is pretty clear. It refers to a repeated pattern of changes in births, deaths, technological change, and industrialization. The pattern can be seen in many countries, with various timing-offsets and rates.

It has nothing to do with any particular ethnicity. Insofar as "immigration" comes into play, it refers to economic demand for labor as the population-bump people exit the labor pool.

I'm trying to figure out what changes the OP claimed are being driven by this. Birth-rate-over-time changes happen everywhere and have throughout history, but apparently this is now driving major change?

I initially read the top-of-thread HN comment as:

1. Stross is trying to tie many events to a change from fossil fuels to solar power, but stronger drivers lie elsewhere.

2. It's better-explained by population dynamics, involving medical technology, mortality and longevity, contraceptives, the shifting balance of workers to retirees, etc.

3. [Charitable-reading effort increases here] These trends involved are old and multi-generational, arguably going back to the industrial revolution. As a casual way to show a very-long-ago datapoint, there are arguments/research about a secularizing France's odd population slump back in the 1700s, which predates the widespread use of fossil fuels.

> Birth-rate-over-time changes happen everywhere and have throughout history

If you look at a world population chart (logarithmic scale, naturally) [0] it becomes clear something in the last few hundred years caused a deviation from the old trend.

Stross might argue the trigger was fossil-fuels, others would argue the trigger was a change in human-capital from medicine/nutrition, perhaps a third group would argue both are inextricably intertwined.

[0] https://commons.wikimedia.org/wiki/File:World_population_gro...