Is there a scientific consensus whether lifestyle or amount of human population the biggest problem

Question

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)

Answer 1

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).

Answer 2

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.