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Pretty much what it says on the tin:

It's the beginning of 2016. Due to a fast-spreading virus, humans have lost the ability to reproduce themselves. The virus makes current artifical fertilization techniques useless as well. As a result, the population starts decreasing, due to the complete lack of newborns.

Now, how much time would be needed to have a 10% decrease of the population, assuming 7 billion people living on Earth before the crisis?

A bit of backstory on the question

I wrote a story which started with this premise and tried - without much success - to figure out how much time would have humans to "react" and actively find a cure before the population drop becomes so severe that:

  • strategic facilities (like power plants, food production chains...) starts being abandoned due to lack of personnel;
  • only medical laboratories are mantained active in order to try to find a cure, while the rest of the research is stopped or diverted to solve the problem.

Since if I had asked this, it would have been probably too broad/opinion based (correct me if not), I have resorted to the question in italics, in order to reduce the scope.

I hope the question is both answerable and on topic. If not, feel free to point it out.

Andrea Jens
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Based on some handy data from 2011, we can estimate worldwide deaths at around 55 million per year. At that rate, for 700 million people to die, you’d only have about 12.65 years.

Now, this annual death toll is definitely going to see some yearly variance based on current world conflicts, epidemics, natural disasters, etc. In general, I would not expect this to have a major effect on that estimate. However, you’re talking about a world where the existence of our species is now in serious jeopardy. Societies don’t handle existential threats very well. With almost half a million births every day, the medical community will notice this number plummet to zero very quickly. It is quite likely that there will be global unrest within a few months, if not weeks. Since we don’t have any historical precedents (fortunately) for “imminent” global existential threats, it’s difficult to say just how bad this would get. I think it is safe to assume that the worldwide death toll is going to increase, probably on the order of millions and then tens of millions. This will start to shrink that 12 year timeframe fairly quickly.

On a broader scale, how long would it take for humanity to be doomed? According to census.gov, there are over 600 million children in the 0-4 age bracket. Fertility in women peaks around the age of 31 and then steadily declines for the next decade. Women can continue to give birth in their forties, though this tends to require access to better medical care and has much higher risk to both mother and child. You could then reasonably suggest that we would have about 40 years, plus or minus a couple of years, to cure the virus and still be able to save the species.

Bear in mind, however, that this is going to be 40 years of lots and lots of unrest. Children will be particularly vulnerable during that period, so you may see a smaller percentage of those 600 million children surviving that long. Worse still, population distribution means a great many of those children are going to be in poorer nations with less access to the kind of healthcare that could help them continue to give birth into their forties.

If it takes you decades to find a cure, there’s going to be a major population decline. If it takes more than 40 years, humanity might not recover.

Avernium
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  • Excellent answer, it adresses properly all my points :D – Andrea Jens Sep 25 '15 at 17:42
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    One more point, the intersection between the 10%-ish of people who are going to die in the next 12.65 years, and the people operating strategic facilities, is fairly small. Especially in developed countries people expect a substantial retirement, and the death rate among people with jobs is rather lower than the death rate of the whole population including retirees. And whoever replaces those who die in post will be older than 12.65 years! So if it weren't for the general panic, despair and collapse of civilisation as we know it, 10% natural deaths in itself wouldn't hit key personnel. – Steve Jessop Sep 25 '15 at 19:28
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    I think the number of deaths would be a little less than 55 million per year because the 55 million probably also includes newborns who die every year – Ovi Sep 25 '15 at 19:53
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    @Ovi That's a fantastic observation. It looks like there were 4.5 million infant deaths in 2015... that's a big difference. – Avernium Sep 25 '15 at 19:58
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    It would take at least three months for the medical community to reach consensus that there is no more pregnant women in the world. If there is no fertilization starting today, there will still be births for the nine months ahead, so you have to add nine months to your 12.65 year figure. – Mindwin Remember Monica Sep 25 '15 at 20:43
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    Also, humanity won't be doomed if 40 years or more passes by. If female eggs are still functional, they can be frozen. You can add 20 more years (since there are several successful cases of births for mothers 60 years old). – Mindwin Remember Monica Sep 25 '15 at 20:45
  • "Children will be particularly vulnerable during that period". Surely if no more children can be born, the general consensus would be that keeping existing children alive is the #1 priority? –  Sep 26 '15 at 10:05
  • @Stacey you seem to be expecting the government to somehow work under a crisis - history tends to show otherwise, especially considering the slim "population" weight of "1st-world" countries - that said, for humanity to survive, only a small fraction of those kids would need to be safe anyways, so I suppose we could only focus on "1st-world" kids for the purposes of the question – user2813274 Sep 26 '15 at 13:05
  • "55 million deaths per year" assumes there are 7 billion people living on Earth. As the population goes down, the total number of deaths will necessarily go down as well (assuming no other changes). Also after only 12 years you'd still have the same number of children entering the workforce as you would otherwise... – BlueRaja - Danny Pflughoeft Sep 27 '15 at 03:15
  • I agree with BlueRajaDanny, the amount of deaths within a given timespan is proportional to the population count, so it's more like approximately 0.78% of the remaining population dies every year. – Patrick Roberts Sep 27 '15 at 04:18
  • The effect of "no new babies" on the working population would only be seen when last existing children reach working age (i.e. around 18-25 years from now). Of course, when the cure is found, you need the same time until the then newly born ones are able to support. – Paŭlo Ebermann Sep 27 '15 at 21:12
  • This answer is the best one in tie with @Francine DeGrood Taylor's! :D I really would like to mark both as an accepted answer :D I think I will have to choose one :/ – Andrea Jens Sep 28 '15 at 20:01
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Well, to begin with, the negative economic impact wouldn't even begin until, say 20 years later, which is when the missing newborns would have become seriously productive in society. Prior to that, the economic impact might have been positive, as all the money that would have gone into feeding and educating the missing newborns is, instead, available for other things.

It's the psychological impact that would hit the world like a meteor. Suddenly, our species has lost its future. Newlyweds ask themselves; what is my marriage for, if there will be no family to raise? All older-than-newborn children currently in adoptive care will be snatched up by parents who no longer have the option of producing their own children.

To answer your question about the workforce, according to these statistics (http://www.dop.wa.gov/SiteCollectionDocuments/Reports/2009_State_Workforce_Report_Pages/WorkforceAge.html) they would probably start really feeling the pinch in about 40 years, IMO. Prior to that there would be adjustments (the average worker age would be rising rapidly). Workers who would previously have been retired are instead, persuaded to continue for as long as they can do their jobs.

However, the bigger issue is not whether there are enough workers to man the power stations but whether there are enough women of child bearing age to be able to repopulate the world.

If they discover a cure after 20 years, the world will have suffered a blow but it can recover, albeit with many changes to the way society functions. We've lost a huge chuck of population but the lack has not yet cut into our childbearing population.

After 30 years we've lost ten of our prime years (physically speaking) but the world could recover, especially if very young women bear children to be raised by older parents.

After 40 years we've probably only got 15% of the population who is young enough to reproduce. That's down from maybe 50%. And even worse, women over 40 have a much harder time giving birth and there is a much higher rate of birth defects (https://en.wikipedia.org/wiki/Advanced_maternal_age). At this point there will be a general collapse of some sort, though it may not happen for another decade or two.

If they discover a cure after 50 years, they will have passed the point of no return for civilization as we know it. Most women are too old to conceive. Even if they do, by the time these children are 30 most of the older generation will be dead or senile. Or society depends greatly on the information which is passed down through the education system, but even more on the experience of coworkers and peers, gained through work experience.

By the time sixty year have passed, all the workers will be old. Many of them will be suffering from physical and mental disabilities but may be continuing to work because there are no replacements. Society will have given up. There may be scientists still working on a cure even though they know it will do us no good. Mass suicide and other effects of hysteria and depression will further disrupt what is left of our civilization.

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    "the negative economic impact wouldn't even begin until, say 20 years later..." What about the daycare and education industries? Manufacturers of diapers, car seats, toys, etc? – djs Sep 25 '15 at 21:51
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    @djs For macroeconomic purposes they would be canceled out by increased demand on other industries. – Random832 Sep 26 '15 at 02:17
  • This answer is the best one in tie with @Avernum's! :D I really would like to mark both as an accepted answer :D I think I will have to choose one :/ – Andrea Jens Sep 28 '15 at 20:00
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A couple decades. The scientists would be searching for these things, simultaneously:

  1. Immune people. There are sure to be people somewhere that are immune to your retrovirus infection. (There are people immune to AIDS). If you find one young immune women (and probably track down her relatives), you can collect her eggs and start repopulating Earth with immune people. You could also use cloning techniques to produce a immune female zygote from a immune male cell.

  2. A way to remove retrovirus DNA insertions. This is not easy and there are people researching it for 30 years-ish already (this would cure AIDS). Maybe the breakthrough is near, since it is already being researched.

  3. You could use gene therapy to lock down the viral DNA inside the cells, converting it into an inert endogenous retrovirus (so it becomes inert and no longer activates - human genome has hundreds of these strands of DNA). A crafted virus that hunts the infertility virus.

  4. Uninfected people. Depending on your infection vectors, these can be more or less rare, but it is almost impossible for the virus to reach everyone. If you can secure uninfected people and make sure they do not get infected, humanity is saved. Some isolated community (like the Easter Island natives) might not have contact with the virus.

The world human population would take a huge drop, maybe half or more. Also the cure would have to be protected since every force able to mount an attack would try to seize it.

Mindwin Remember Monica
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  • Point n°2 was the solution in the story, though following a "non-conventional" path ;) By the way, I agree on most of your points XD Strangely enough, in my story the virus mutated in several varieties which could pass to humans from several different animals, so point 4 would have some issues... but since this was not specified in the scenario, this is still a really good answer ;) – Andrea Jens Sep 25 '15 at 21:39
  • Your point 1 reminded me: of this https://en.wikipedia.org/wiki/The_Handmaid's_Tale_%28film%29 (actually I've never seen the film, only heard about it). – celtschk Sep 26 '15 at 09:44
  • Suspended animation.
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