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Not sure if this is the right stack for this but:

I have been arguing with friends on overpopulation, several are convinced that Earth cannot sustain the current population. Without getting into all of the ways that we are destroying various ecosystems and polluting, I believe that if we were actively working to optimize the environment and how we use resources, Earth could easily handle many, many more humans.

Such as renewable energy, reducing waste, committing to reversing desertification, and so much more. But regardless of the behaviors we specifically change, looking at the raw chemistry of the planet, i.e. how much water, oxygen, carbon, plant life, and so on, there is some maximum amount of humans that can live healthy lives.

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The FAO estimate that we use 11% of the Earth's surface area for agriculture, and that that represents approximately a third of the land that 'could' be used (http://www.fao.org/3/y4252e/y4252e06.htm). If we accept that current agricultural production is at a level that allows the current population to live healthy lives that implies that we could triple our population and not starve.

In this very simplistic view I am ignoring the very real questions of inequity in distribution, what constitutes 'healthy' and the potential of changing efficiency in agriculture or transitioning populations away from expensive meat based diets.

However, and it's a really big however, you couldn't make that sort of increase in the area of globe used for agriculture without destroying natural ecosystems, and it's arguable that already much of global agriculture is unsustainable in it's present form, as aquifers are overexploited and soil quality degrades.

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  • $\begingroup$ I've seen a lot of great research into hydroponics and urban farming where you convert a floor of a building into an efficient indoor farm, so this might be a way to avoid just turning every square inch of land into farming, if that helps with this thought experiment. $\endgroup$ Feb 1 at 22:40
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Welcome to Earth Science SE! I actually think this belongs on the Sustainability SE more than the Earth Science SE. But, in the spirit of giving an answer, I'll try to answer it (probably more my opinion, but I have sources and reasoning to back it up).

I think it really depends on what you mean by capacity. The discredited Malthusian ideas still have many proponents (which may make my answer a bit unpopular). Even if you think that it is not a discredited idea, it is still a bit overstated. However, idealists from all over the political spectrum (from Mises to Marx) disagree with Malthus. Still, a lot of ideas stem from the idea of overpopulation.

Let's think about what would cause a drastic population collapse? The dystopians, when they talk about capacity, often think about the amount of food. But the reality is that most famines are human generated (by economics, war, etc.), though climate does have an impact. A recent example of a modern famine is Venezuela, where inflation and price controls have made food unpurchaseable. Barring desertification (caused by changes in the Hadley cell), war and political/economic factors, the food supply generally doesn't collapse all at once. Even when it does (such as the Dust Bowl, where few died directly from starvation, but from the dust), people often resort to other food sources.

One of the common assumptions for Malthusianism is that humans are a peaceful lot. We are not. This creates a negative feedback mechanism. Until 1945, the number of deaths from wars was increasing, which I think is part of the reason the population has exploded (granted, I will admit I am not an expert on such matters).

But let's for a second, consider 'what if they are right?' Then it becomes a utilitarian problem. How would they choose to solve it? A movie pondered the solution to be effective genocide. But such thinking could justify actual genocide, and it has been entwined as a a motivation for eugenics. Even if the genocide isn't intentional, the force required for collective action to convert to an agrarian society can be catastrophic in its own right. Perhaps the biggest danger to the hypothesized 'capacity' is the risks associated with nuclear technology, since it has been shown to render areas uninhabitable for millennia, though some argue that it is necessary in order to prevent Malthusian collapse.

And perhaps the biggest piece of evidence (for me) is the increase in urbanization and the decrease in agriculture, despite claims of excess food demand.

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  • $\begingroup$ A lot of this sounds like arguing whether humans can manage co-existing and not an objective measure of whether the planet has the raw materials to support the caloric and shelter needs of the current much less larger population. $\endgroup$ Feb 8 at 19:01
  • $\begingroup$ Doesn't matter if humans are peaceful or not -- no war has ever caused a net decrease in the world population. For example, the world population grew by 8% between 1940 and 1950. $\endgroup$
    – Mark
    Feb 11 at 0:27
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The population is degradatively and unsustainably overconsuming

Everyone truthful agrees the above statement.

The keywords are degradatively and destructively for people and crowds. Few people deny that.

The seas are already being overfished with many species collapsed to low volume stocks, the land is intensely overfarmed with cocktails of chemicals and pesticides, the rivers near industrial farms also absorb nitrogen and pesticides, the sky is destabilized, deforestation is impissible to control. Dolphins tigers rhinos birds insects etc are now extinct, and 80 percent of the world population want to increase resource consumption to 500% of current levels, i.e. young Africans want playstations, SUVs... in a reasonable compromise we cant responsibly manage current overpopulation, its very poor on average, so from a responsible point of view, having children are being born in poor megacities, uneducated and thirsty to have a house garden and car. Its destructively overpopulated without and end to the degradation predicted.

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