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Okay, let's say that we have a starting population of 7.7 billion, and a brutal, month-long war wipes out 1.3 billion people. 1% of Earth is rendered uninhabitable by the war. This would put carrying capacity at 9.9 billion, and there is an population explosion afterwards. The last time the human population was at 6.4 billion was in 2004, where the growth rate was 1.25%. Let's assume that due to environmental damage, it is in fact closer to 1.2%, which is larger than what it is now (thanks, COVID). With that, I can estimate how long until the population reaches 7.7 billion people again, assuming a fixed rate. 7.7=6.4*1.012^t 1.20=1.012^t log[1.012]1.20=t t=15.5 yrs

The real question is, is my 15.5-year estimate for how long it takes for a population-reducing event like the one I described realistic? Why or why not?

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    $\begingroup$ A month long war is probably not long enough to trigger a baby boom. A baby boom is just shifting all the births that should occur during a given time to single smaller point in time. It is also not a particularly deadly war, for scale WW2 killed about 3% of the global population. $\endgroup$ – John Feb 22 at 16:54
  • $\begingroup$ "for how long it [would take to repopulate after said] event" or "how long it takes [to reverse] a population-reducing event" - or is the question: killing 1.3B people in a month feasible? $\endgroup$ – Mazura Feb 22 at 17:09
  • $\begingroup$ Mazura It's sci-fi, with extremely dangerous weapons. $\endgroup$ – TysonDennis Feb 22 at 21:08
  • $\begingroup$ "is my 15.5-year estimate for how long it takes for a population-reducing event like the one I described realistic?" - you said it took a month. (I'm saying the last sentence is missing a word; one that negates the reduction, i.e., undo or recover from... "for how long it takes [to undo] a population-reducing event) $\endgroup$ – Mazura Feb 22 at 23:39
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is my 13.9-year estimate for how long it takes for a population-reducing event like the one I described realistic? Why or why not?

First, It all depends on this statement of yours: 1% of Earth is rendered uninhabitable by the war.. 71% of earth is water, and 29% is surface. Roughly 57% of surface area is uninhabitable, which means you are left with 29%*43% = 12.47% of area as habitable. Even within this area, you will find clusters of populations centered around major cities and major cities around major rivers.

Assuming you meant 1% of earth's area, sure, there is already a lot of area that is not habitable, so your rate is achievable.

But the moment you meant to subtract 1% out of the 12.47% above, you will find effective habitable area available has decreased by ~8%. If this decrease happened in areas where major cities are located, or major centers of forests are located, you have seriously hurt the economy and the environment respectively, which will make a recovery seem more distant.

Second, you are assuming people still want to breed like rabbits after that brutal war. But why should things normalise so soon? Statistically, roughly 1 in 7 persons died. Assuming an average person knows 150 people, an average person will know 21 people who died during the war. You haven't noted that only males die off in majority so the average sex ratio has been impacted, or any other such hypothesis (which happened during the world wars). So, the psychological effects of such a drastic war will be many fold, and a chunk of people may actually become disinterested from starting families, re-enforcing a trend that is already seen in current era.

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Since we are dealing with humans, the likelyhood of the survivors loosing interest in having sex is very low. On the other hand, after a devastating global war, production capacities and infrastructure necessary to supply contraceptives will be severely impacted. And reproduction generally correlate negatively with and economic prosperity, which took a severe hit as well.

On the gripping hand, i would expect far larger reproduction rates at least in the first few years after the war, unless ABC weapons impacted fertility, so your number is probably even too high.

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