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While discussing global warming and climate change, I have often seen people say that overpopulation is the real problem and that the biggest thing anyone can do is have one fewer child. Some people extend that a bit further and aim to never have children.

How effective would this be at reducing human impact on climate if everyone decided to live this way? Would it be enough to put the brakes on climate change and limit warming to 1.5º C? Or maybe 2.0º C? Assume everyone alive maintains the same behaviour and standard of living.

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I'm not a demographer, but I attempted a first order approximation based on what I could find. World-O-Meter has a page with world age structure grouped into buckets of 5 years (eg. 20-24). I roughly calculated the total population over the next 100 years by assuming everyone survives until about age 100, so I just zero out one bucket every 5 years.

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Annual emissions are 35.9 Gt CO2, so divided evenly among 7.7 billion people that would be 4.7 tonnes CO2e per person, per year. Assigning those and adding them up gives a total emissions curve like this.

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The estimated carbon budget to stay below 2º C is somewhere between 565 Gt CO2 and 1550 Gt CO2. In a no-birth scenario with everything else held steady, we would cross that threshold around 2035 or 2063, respectively.

Even if everyone in the world stopped having children entirely, we would still cruise past the targets to limit global warming to 1.5º or 2.0º. A control on population growth would not, on its own, be a sufficient emissions control measure.

Of course, there is plenty of ways to refine this estimate. More granular age buckets could be used. Populations and emissions factors for different countries could be incorporated. Mortality rates for all age groups could be accounted for. But I'm not a demographer, so I haven't done that.

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  • It might be possible to come up with a better estimate by using this tool. tool.globalcalculator.org/globcalc.html – Nic Oct 16 '19 at 0:02
  • If everyone in the world stopped having children entirely, humanity would go extinct and nature would rebound strongly in a post-human world. – gerrit Oct 16 '19 at 11:07
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Well if everyone stopped having children the Earth would return to "normal" pretty quickly. But we don't want quite this much of a reaction, we just want to stop the population increasing any further, for our own sakes, as much as for the purpose of avoiding Armageddon. I realise that this overcrowding of the human population is not very obvious in some places, so I will speak of the global impact. There is two much free carbon dioxide in the air and increasingly less free oxygen. Now apart from all industry even the population itself is largely responsible for this because we now number more animal weight on land, than the planet has ever had on it before. And we all breathe out carbon dioxide. Therefore it should be obvious that a little reduction in the human population is necessary, and it is achievable simply by taking the simple step of not having more than two children.

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  • "the Earth would return to "normal" within twenty years", how do you define normal? Does this include 'normal' CO2 levels? Btw human breathing hardly influences atmospheric CO2 levels because it does not add new CO2 to the atmosphere. – THelper Oct 20 '19 at 17:30
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    Can you explain your reasoning about how you picked 20 years as the time period under which the Earth would return to normal? Adding references or citations would be ideal. – Nic Oct 21 '19 at 4:34
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    I suggest editing your answer to remove all except the first two sentences. The rest of the post, although interesting, is not relevant to the question. As written right now, this answer could be considered for deletion because most of it is unrelated to the question. – Nic Oct 21 '19 at 4:37
  • Human breathing does influence the atmosphere because it is a direct result of human farming which is a direct result of human industry. If we were not farming on an industrial scale the population could never have grown to its present proportions. The air we breathe out is greater than the air breathed out by all animal life on land in past ages. Remember that only man maintains full activity throughout the year, and therefore is breathing out more than animals most of whom have a much lower metabolic rate for most of the year. – Janet Rooke Oct 22 '19 at 9:04
  • @JanetRooke The CO2 we breathe out comes from our food. For plant-based food it's CO2 that was taken from the atmosphere by those plants, so it's a closed loop, no new CO2 is added. For animal food, the loop is more complicated but is basically still the same. No new CO2 is created. The exception is if we harvest more plants and trees than there originally were and don't replace them, but then it's more fair to say that deforestation is adding CO2 to the atmosphere. – THelper Oct 22 '19 at 12:09
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The CO2 we breathe out comes from our food. Exactly, and we eat richer food than any other animals, with a higher calorific value. Moreover our brains use more calories than those of other animals. I am going to avoid figures but I seem to remember that the sugar our brains use is a surprisingly high proportion of our total sugar intake. Also bear in mind that this is mostly refined sugar, and no other animals eat refined sugar or indeed anything like as much as we do. So, it isn't exactly a closed loop. There is a lot of unnatural chemistry in the mix, and some of it is inorganic. In other words it is likely that humans are freeing CO2 into the air at a higher rate than would happen in nature. I totally agree about deforestation, and would also point out that rotting things release CO2 and mankind is responsible for a much higher rate of rotting vegetation than would ever happen naturally. It is our large brain which sets us apart from nature, and from the natural cycles of the planet, because it has enabled us to alter the natural cycles of all materials in a way which the Earth can no longer sustain.

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    This doesn’t really answer the question. Please read the FAQ on how to write a good answer. – Nic Oct 26 '19 at 17:54
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    This is your second answer to this question. Did you mean to write a comment in reply to THelper? – Nic Oct 26 '19 at 17:56
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    Regardless of the animal that eats the food, basically all of it is turned into greenhouse gases. Even if you just let the food rot, the same thing happens. And refined sugar is nothing special. It is just sugar that isn't mixed with other plant matter. – Joonazan Nov 10 '19 at 19:34

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