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When discussing with a colleague about ecology yesterday, we had agreed on the following premise :

  • Before industrial age, world population was low, and the environment was not significantly damaged
  • Today, world population is too high, and the environment is awfully damaged

What we didn't agree on is the reason why environment is so much damaged. In his opinion, it is simply that there is too many human people. If the world population quantity was like the middle-ages but living with today's modern lifestyle, the environment would not be damaged, despite people burning petrol, using cars and throwing plastic and chemicals in nature, because there would be a small quantity.

According to myself, the problem is today's lifestyle, of people burning petrol and using plastic when this could be avoided, moving themselves in heavy individual vehicles, among other similar absurdities. If people would live a pre-industrial lifestyle, in simplicity and poverty, and travelling mostly by foot (and horse for the riches), no matter how much people there would be around, the environment would not be damaged.

Both of those views are just opinions. Has there been serious studies comparing the theoretical environmental impact of:

  1. Modern lifestyle society with 350 million people (estimated world population of year 1400 according to Wolfram Alpha )
  2. Middle-ages lifestyle with a world population of 7.5 billion people (estimaged world population today)
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There are a couple of errors in your question.

Firstly regarding environmental damage in pre-industrial times, in the Middle East they ran out of trees over two thousand years ago and had to import timber. Easter Island had similar problems. Over-use of resources has existed since time immemorial, not just due to industrial revolution.

Secondly, regarding the world population today being too high. Its doubled since the 1970s, and quadrupled since the 1910s. In fact the world population is eight times larger now than when Thomas Malthus wrote about the population trap. Its very difficult to point at a graph of the human population over time and select the year when the there was an ideal human population.

Lifestyle or the use of sustainable resources is more significant than the size of the population. The world could support a larger population of humans if we made a more sustainable use of resources.

Halving the population would be futile as the population would bounce back in less than forty years.

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You don't need a "scientific consensus", you just need "math".

Consumerism (the exploitation and consumption of natural resources to support modern/unsustainable lifestyles) is what produces pollution and environmental/ecological degradation. The magnitude of the damage done (D) is thus the product of the lifestyle (L) and the number of people living it (P).

L * P = D

Thanks to "math" it also holds true that:

P * L = D

Neither P nor L are solely responsible for D — they both contribute. For the purposes of calculating D they are inseparable. Neither is "the biggest" because it is the product that is the problem. Thus no-one should be overly fixated on the issue of P or L. You can achieve a 10% reduction in D by reducing either by 10%.

The world is far too complicated and interconnected to ever support the simplistic notion that X — and only X — is the cause of Y. There are always multiple factors in every equation.

As any student of history knows, the inefficiencies of medieval agricultural practices (read: amount of land required to support a peasant family engaged in subsistence agriculture) means that it would simply be impossible to support 7.5 billion people on Earth using them, so scenario 2 is not viable at all — the farmland required would far exceed the arable area available.

Crunch the numbers yourself. Figure 30 acres of arable English farming land per family of 5–6. Only 35% of the country was arable. Snap productivity to a diminishing sine wave as you leave the ideal temperate climate zone and head towards the sub-tropic or the sub-polar zones. Integrate over the planet.

A modern lifestyle with 350 million people, on the other hand, is doable. I crunched the numbers back near the turn of the millennium (initially following energy flows via trophic layers) and 869 million people with a lifestyle equivalent to those in the USA during 1999 were possible. Allow for per-capita increases in energy consumption over the last 20 years and that number is now probably around the 600 million mark. Scenario 1 is totally sustainable.

A note: It seems that you may not be aware that, in order to live a medieval lifestyle, people needed to burn a lot of wood. The primary constraint to population growth (in temperate climates) was the availability of trees. Trees were the energy source that fuelled growth. That why, wherever a town or city existed, the land for miles and miles around would get increasingly stripped bare of trees that got burnt for wood. Once the trees were gone, then the springs dried up, and soil erosion kicked in, crop failures would increase in frequency and severity, and peasants in the urban areas would freeze to death in winter. It was only the 'miracle of coal' that allowed the population to grow above 600 million circa 1700. People dug up coal instead of cutting down forests. The forests returned, agriculture prospered once more, and populations boomed.

The reality is that the overwhelming majority of the current global population is addicted to a convenient, portable and highly-dense form of energy. It's addicted to oil. It cannot be sustained without oil. We currently burn 9 Joules of oil to produce every Joule of food. As the oil runs out the planet will be forced to ratchet down to less convenient, less portable, and less-dense forms of energy. That will impact the food supply. The population will track down accordingly.

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    I'm not the person who downvoted - but I am dubious that people froze to death because they ran out of trees in the middle-ages. Maybe that happened, but if it did you definitely need to back this claim up with sources. – Bregalad Mar 24 at 15:08
  • You are dubious that people without access to heating die in winter when it's very cold, freezing or snowing? Seriously? 50,100 "excess winter deaths" occurred in England and Wales alone over the 2017/18 winter. ons.gov.uk/peoplepopulationandcommunity/… Do a quick search on "excess winter mortality" and/or "fuel poverty crisis". Nothing has changed — if you can't get fuel for heating the risk of death skyrockets. In the middle ages, wood was the only fuel. – Tim Mar 25 at 2:14
  • This doesn't change the conclusion about which scenario is sustainable and which isn't, but: How do you know that D = f (L, P) is a simple multiplication? I'd suspect that for many aspects of pollution D (L, P) increases non-linearly (in particular for the question here: more than proportionally for high population density) – cbeleites May 25 at 1:03
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The core problem is the scale of pollution overall.

Let's take excess anthropogenic greenhouse gas releases as being our single worst problem, which, broadly speaking, it is.

We know we need to get to zero net emissions, (and, for a period, net negative emissions), quickly - within 25 years or so - to avoid even worse catastrophic consequences than those we've already put in process.

That means that the size of the human population, multiplied by average emissions per human, has to be zero.

And that means that the average emissions per human has to be zero. And at that point, it doesn't actually matter what the size of the population is, for this particular group of pollutants.

People did indeed damage their environment in pre-industrial societies. Open sewers in the streets were common. Mass deforestation happened. Destruction of the fertility of land happened.

We have pretty much all the technology we need to get to zero net emissions. It's not clear which is the most cost-effective, but it is clear that we can do it, and support our projected population growth. And it is clear that the cost of delay is far higher than choosing the wrong basket of technologies to get to net zero, for almost all technologies (with the exception of global nuclear proliferation, which increases existential risk, rather than decreasing it).

  • You have no real evidence or plan for the future. Considering that in the developed world about 75% of people don't give a shit about the environment, even more so in the developing world, I doubt you can get to zero emissions in 25 years, yet alone at all. Also, greenhouse gas is by far not the only destruction of the environment man does. You seem to look at technology as the solution and not the problem. As I said in my question, I am very dubious of that - technology is the problem and why environment is damaged - not the solution. – Bregalad Mar 24 at 15:07
  • I don't see how you can conclude from the need to reach 0 average emission that population size does not matter: that holds only if there is no interaction between population (density) and emissions. (I'm thinking, e.g. along the lines: at population densities of 10^-x people / km² in central/western Europe we would need hardly any wastewater treatment: natural self-cleaning capacity would cover everyone s****ing in the woods. At a population density of 200/km², we build, maintain and run massive infrastructure and technology to clean up after ourselves. – cbeleites May 25 at 0:19
  • (just to be clear, I totally agree that modern technology rather than middle age or stone age lifestyle/technology is the solution we should seek) – cbeleites May 25 at 0:22
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Whether serious studies have occurred I do not know, but you surely can answer your question yourself without much effort.

For the effect of over-population of an area go visit a beef-stockyard, where the beeves are fattened before slaughter. Cattle practice a completely pre-technological lifestyle.

For the effects of technology, consider the currently fashionable suggestion that we can substitute wind-power for coal and oil and thereby escape the adverse effects of carbon dioxide pollution. The question is, what monster will we raise up in place of global warming.

Consider why the wind blows where it does reliably blow. This is not a facetious question. There are regions on earth where the wind rarely or never blows, in the great age of sail collectively known as the 'doldrums'. In contrast places like Cape Horn exist in a perpetual gale.

Obviously, one would site windmills where the wind reliably blows, and their cumulative effect when providing power for 8 billion people would seriously impede the air drainage of the entire planet. Would you not expect that to have profound effects on planetary weather patterns?

Any technological system to achieve any effect whatever can be effective only over a certain range of application. Outside that range it will be ineffective or harmful.

If there were 8,000 of us it would not matter what technology we used, because it would be operating within its effective range of application. But as there are almost 8,000 million of us it makes no difference what technology we apply, because all of them would exceed their effective ranges of application, and be harmful.

Global warming is not the problem, it is a symptom of a disease. The disease is us, the overwhelming number of us.

  • To quote from Cradle to Cradle: "As long as human beings are regarded as 'bad,' zero is a good goal. But to be less bad is to accept things as they are, to believe that poorly designed, dishonorable, destructive systems are the best humans can do. This is the ultimate failure of the 'be less bad' approach: a failure of the imagination. [...] This is a depressing vision of our species' role in the world. What about an entirely different model? What would it mean to be 100 percent good?" – LShaver May 6 at 20:12
  • OK so basically, triggering world war 3 would be much more efficient than cycling and tax fluel in order to save the environment, Sad but true. – Bregalad May 7 at 7:15
  • Not that humans per se are bad, but that 8 billion of us, whatever we do, is beyond the carrying capacity of the earth. – user985675 May 8 at 16:40

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