Typically the only useful info you can get out of such long term studies is the sign of the change and the order of magnitude.
The fertility rate is about 2 kids per woman now. But the generation of 20 year olds is much larger than the generation of 40 year olds. So even though the generation of 0 year olds is the same size as that of 20 year olds, in 20 years time we will have replaced a small 40 year old generation by a large 20 year old generation.
There are only two factors here: Fertility rate per woman, and death rate. This prediction is not terribly sensitive to either, and both are easy to measure accurately and on quite stable long term trajectories:
Sorry for the poor choice of words, no chaotic model was implied or intended.
> Fertility rate per woman, and death rate.
No, there is also technological development, wars and so on. So any prediction that looks 80 years out that only looks at fertility rate and death rate is going to be found wanting. It's a bit long for a long bet, but I'd be more than happy to take one for a 30% or more deviation from the prediction.
The degree to which people are willing to believe simplistic models without further qualification is astounding to me, given the past 80 years in development of medical technology, electronics and our ability to wage war any predictions about the next 80 years should come with huge error bars. And that's before we get into the consequences of Climate change.
Which have an effect on fertility and death rate. There is literally nothing that matters to population dynamics other than fertility and death rate.
We have extensive experience on that now: Technological and social development drives fertility to at or below replacement levels rapidly. This is the main factor. Importantly though, _this is not a prediction_. This reduction has already happened. So the bulk of the prediction is just playing out the consequences of developments we've already seen.
Of course major events like climate change have the potential to invalidate this prediction. But that doesn't mean this prediction is useless. And this is would not be due to "sensitivity to initial inputs".
Yes, but given that they are unknowns you can't just extrapolate from the presently available data and call it a day.
> But that doesn't mean this prediction is useless.
It's an open door that the world population will stabilize at some point, it is also an open door that the speed with which the world population is increasing itself is reducing. It has to, simply because more affluent people will have fewer children because they are not looking at their children as their source of income when they are themselves unable to generate any. Children as a pension scheme is a very common thing still, but that is fortunately changing.
> And this is would not be due to "sensitivity to initial inputs".
To a degree it is: any error in the predictions for the next couple of years will result in a much larger error in the future, simply because of the fact that population increase is a non-linear affair to begin with, and that any med-tech development that causes people to live longer will also cause a large increase in the number of people alive at any one moment.
Compounded those two could have an enormous effect.
For reference, the world, 80 years ago, so just before WWII had only 2.2 billion people.
It is still the case, just on a larger scale.
Moreover, the major causes of birth rate decrease, like vaccination and refrigeration (which strongly impacts food supply), aren't going away. Look at a country with a high birth rate today, and you'll still find people walking for miles with buckets full of the day's water for their household. And anywhere that indoor plumbing and clean tap water is universal, the birth rate is down to replacement level. There are fundamental causes for birth rate decrease, and they're not going away.
Frankly, I think you're clinging emotionally to the Malthusian model, not rationally.
You cannot predict the direction that culture will change. Any predictions I make about this are extremely speculative, but it is easily imaginable that 20 years from now the zeitgeist reacts against the corpus of current culture to be more family-oriented. Think about how much you hear people complain against "the machine." Whether it's on r/latestagecapitalism, or r/the_donald, everyone hates the piss out of broad society, and may find solace in tight-knit families.
Atheist women choose not to have children, christian women choose not to, muslim women choose not to.
Give people the option not to have children, remove the stigma and maybe they have 1-2 children, but no more.
Given how many single people there are, how tight the fertility window is for women who have careers, how tight schedules are in big cities, how expensive it is to have children today, there's nothing we can reasonably do to increase birth rates.
I'm not even sure what you are trying to say.
I'm sure an alien species or a computer simulation having a few of the same basic drives as humans do (sex is very pleasurable, contraception exists, raising educated children is extremely expensive and hard, state provides pensions in old age, people are free to have or not to have children, child mortality is low, work provides money, money satisfies needs, quality work is abundant only in high density cities and a coupe of others) will arrive at same fertility problem we have arriven at.
My second point was that there is nothing we can reasonably do without a major overhaul of human traits or shutting down the technological advanced world we live in.
The implication being that the cultures of (some) people are unchanging and fixed. I think you'd agree with a statement like: Cultures and family values do change all the time, due to a variety of factors like better medical care, and economic empowerment, in reasonably predictable ways.
So it's not unchanging cultural differences between Africa and Europe that drive the different fertility rates, but rather different socio-economic factors that drive a different family culture. Changing the socio-economic factors will change the culture surrounding families, just as much in Africa as it did in Europe, the Middle East, Asia, etc... at various times in the last century and a half.
One extreme unknown is global life expectancy. New diseases, or new cures for existing diseases can make a huge difference. Add in wars, economic growth, and a host of other factors making long term estimates are hard. Just consider how accurate past population estimates have been.
https://population.un.org/wpp/Graphs/Probabilistic/FERT/TOT/...
I spend a bit of time looking for the publications underlying this model right now, but couldn't find any.
But accepting the error bars doesn't change the story. Which is that this isn't that much of a prediction. The main driver behind the population dynamics of the next century is the decline in fertility that has already happened.
It's not a highly uncertain prediction that can turn out to be vastly different based on minor changes in the initial conditions.
But if we underestimate the decline of fertility in Africa (where it is changing the most at the moment [2]) over the next decade or two it could easily end up being off by 10% or more.
It's all fairly predictable.
[1] For example the 95th percentile prediction sees a sudden bump in Asia, taking it up to fertility levels not seen since the early 2000s by the mid 2020s, despite a perfectly smooth decline since: https://population.un.org/wpp/Graphs/Probabilistic/FERT/TOT/...
[2] https://population.un.org/wpp/Graphs/Probabilistic/FERT/TOT/...
Long term estimates need to consider such black swans as a real possibility. Relative amounts of war and disease in Africa over the next 20 years will have a real impact.
As those countries get more powerful and develop nuclear, the consequences of conflict get nastier. Things like restricting water supply bring back old style 20th century slaughter.
> Just consider how accurate past population estimates have been.
Uh, fairly accurate as far as I know? There were always bad extrapolations and naive papers and articles, but the state of the art predictions didn't do badly, especially after data quality improved.
Also, again, the major driver for the predicted dynamics is the collapse in fertility rate that has already happened, and no longer needs to be predicted. This is a vastly simpler question now than it was in the 70s.
Look at this graph:
https://population.un.org/wpp/Graphs/Probabilistic/FERT/TOT/...
Of course in 1970 it's almost impossible to predict how quickly Asia will experience the cultural, economic and technological changes that reduced the fertility in Europe dramatically over the previous century. Today the transition towards small family sizes has happened. We still have drift (as we see in Europe too) but the important major driver of going from roughly 6 children per woman to 2 children per woman has happened. So the biggest uncertainty factor in old forecasts is gone.
They have gotten fairly close with long term estimates a few times, but a poor track record for predicting deviations from existing trends.