Japan isn't an example of a possible future, because Japan is still in its glory days as far as population goes. Here [1] is their population pyramid. You might notice it still looks kind of vaguely pyramidish. In about 30 years Japan will hit the full-on inverted pyramid, which is when their collapse will "really" begin. And this makes perfect sense. They only started having very poor fertility rates in the 80s. [2] And you only really start feeling demographic collapse once the first generation with poor fertility starts dying. So about 80 years after the 80s - ~2060, or ~30 years from now.
I can explain the math, but the result is pretty simple. If we imagine an equilibrium point where everybody has the same fertility rate (the typical case being when the final healthy fertility rate generation dies off - just leaving offspring upon offspring of comparably low fertility rate generations), then at the point your population will start changing by a factor of ~fertility_rate/2 every 20 years. So for Japan, with a current fertility rate of ~1.4, they'll be losing 30% of their population every 20 years, for an annual decline of around 1.5%.
Right now they're only losing about 0.5% per year. So they're still in their glory days relative to what's to come starting around 2060. And that future which they're headed towards is the one most Western economies are also headed towards if we don't fix this problem.
Comments
Japan isn't an example of a possible future, because Japan is still in its glory days as far as population goes. Here [1] is their population pyramid. You might notice it still looks kind of vaguely pyramidish. In about 30 years Japan will hit the full-on inverted pyramid, which is when their collapse will "really" begin. And this makes perfect sense. They only started having very poor fertility rates in the 80s. [2] And you only really start feeling demographic collapse once the first generation with poor fertility starts dying. So about 80 years after the 80s - ~2060, or ~30 years from now.
I can explain the math, but the result is pretty simple. If we imagine an equilibrium point where everybody has the same fertility rate (the typical case being when the final healthy fertility rate generation dies off - just leaving offspring upon offspring of comparably low fertility rate generations), then at the point your population will start changing by a factor of ~fertility_rate/2 every 20 years. So for Japan, with a current fertility rate of ~1.4, they'll be losing 30% of their population every 20 years, for an annual decline of around 1.5%.
Right now they're only losing about 0.5% per year. So they're still in their glory days relative to what's to come starting around 2060. And that future which they're headed towards is the one most Western economies are also headed towards if we don't fix this problem.
[1] - https://en.wikipedia.org/wiki/File:Japan_Population_Pyramid....
[2] - https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...