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Humans currently consume the ecological resources of 1.75 Earths (https://www.footprintnetwork.org/).
Assuming that global average consumption remains stable (less in the global North and more in the global South) this would imply that the human population would have to be 43% lower and that a fertility rate of 2.1 is unsustainable. But at the same time fertility rates under 1.0 seem to lead to socio-economic dislocations including potentially unsustainable dependency ratios as larger older generations have to be supported by smaller younger generations. Which fertility rate would allow the human population to adjust to Earth's carrying capacity as soon as possible while minimising socio-economic dislocations?

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    Wealth leads to a decline in reproduction rates, visible e.g. in the demographic pyramids of e.g. Germany or the UK. So the question should rather be "how much do we need to reduce our consumption rate". – Erik May 6 at 8:50
  • @Erik: I’m trying to understand what the basic global aggregate fertility rate would need to be primarily to get a sense of magnitude. I think the dynamics of how to get to this rate would be a separate question, probably centred on the problem of decoupling: how to increase (or maintain) well-being sufficiently for birth rates to decrease (or remain low) while simultaneously lowering the consumption of natural resources. – Steve222 May 7 at 12:14
  • You need to population to drop to about 60 %, so when you know the time frame, you can calculate this quite easily. Within one generation it'd be a fertility rate of 0.6, the longer you stretch you time frame, the higher the rate may be. Still, it'd couldn't be 1.0 or above. – Erik May 7 at 12:30
  • If consumption is held stable I understand that the global fertility rate would have to drop to 1 / 1.75 (EF) = 0.57 x 2 (the fertility rate measures number of children per woman) = 1.14 for the population to shrink into the ecological footprint within a generation. Only a few industrialised territories are at or below this level but it is generally not considered sustainable socio-economically and it is usually compensated with immigration of working age people to increase the dependency ratio. On a global aggregate level immigration would of course not be possible. – Steve222 May 7 at 14:42
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I think this is question is difficult to answer for several reasons:

  1. There is no agreed upon carrying capacity of the Earth. The Global Footprint Network organization does a nice job estimating the Earth's capacity in terms of land usage (biocapacity), but their ecological footprint is a very rough estimate and is not suitable for decision-making. For example, in their calculations the biocapacity of farmland is higher than that of forests, so according to this method you can increase the Earth's biocapacity by chopping down all forests and turning them into farmland!

  2. The question assumes that there is an optimum where we can decrease human population and minimize socio-economic dislocations. I think it's far more likely that the quicker you want to achieve this, the bigger the socio-economic effects will be, so to me it seems more of a question how much problems are you willing to accept.

  3. The question assumes that the impact of consumption is constant, but this is very unrealistic. Many countries signed the Paris climate agreement and started implementing measures to reduce their greenhouse gas emissions. Also many countries are looking to change their environmental impact when it comes to plastic pollution and use of finite resources.

Perhaps the best is to strive for a global fertility rate of just under 2.1. This will reduce the global population slowly (and thus also the footprint of mankind) and at the same time have little other side-effects. This is assuming all countries take their equal share in this reduction compared to current fertility rates.

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