I'm not asking what will happen if everyone in the world suddenly turned to vegetarian. In an ideal world with current population, can everyone follow a vegetarian diet (except for people who may need to eat meat if they have allergies to some other vegetarian alternative)?

I've tried to search for answers but all I found was things like what would happen if everyone suddenly turned vegetarian or things like that. But I want to know can everyone turn into vegetarian in some time? If yes/no, how much population can Earth sustain if everyone is always vegetarian. And animals are only breed in zoological parks or for medical or some other science related needs (or for exceptional cases)

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    The answer is yes, it not only can but it does this right now,we humans eat meat from plant eaters so if we start eating only plant matter and skip eating meat there still will be plenty of food. – trond hansen Jul 16 '18 at 6:16
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    You realize that a lot of those cows eat grass, right? When was the last time you enjoyed a heaping plate of grass? – Jean-Paul Calderone Jul 17 '18 at 13:13
  • And there are places where the ground is so poor that only grass (and other stuff not edible by humans) grows, so we can use that land only for livestock. – RedSonja Jul 18 '18 at 11:04
  • @trond hansen, doesn't it create an imbalance in food chain? I wanted to ask if we can do it without endangering any other species (except for the animals that are raised for food, in which case I don't know if we have any other choice) – user41965 Jul 19 '18 at 1:26
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The carrying capacity of planet Earth is about 1 billion Humans (with lifestyles similar to the USA in 1999). There are over 7.6 billion people on the planet right now. We're in overshoot. Our current population is only possible because we are exploiting non-renewable energy sources. As they run out the population will crash and ultimately stabilise at an equilibrium level — a small fraction of what it is now.

At this point it really doesn't matter what you eat. The best thing you can do to sustain life on Earth is to avoid having children.

Trivia: Each Joule of energy you get from corn requires the same amount in oil to produce. Apples and chicken require about 7x as much oil as they yield in nutritional energy. Milk is ~2x. Pork is ~12x. Beef is ~23x.

The average US diet a few years ago required 9 Joules of fossil fuels for every 1 Joule of food consumed. It is tempting to use this as a mathematical basis for saying "the planet can support 11x as many vegetarians as is can normal people" — but that would be a mistake.

The depletion of non-renewable energy sources (oil, specifically) will force the Human species to find more economical food sources... but that won't lift the carrying capacity of the planet because solar panels displace photosynthesis.

Overshoot is overshoot. Unless someone manages to defy the laws of physics and invent Free and Unlimited Energy™ we're in for some rude shocks as the crude runs out.

tl;dr: The Earth cannot sustainably support the current population, regardless of what they eat. Switching from an omnivorous to a vegetarian diet won't change that. The Earth can, however, sustainably support about 1 billion people. When natural forces reduce our population back down to that level folks will have more pressing issues on their minds and no-one will care what people eat. Diet is not the problem — (over)population and extravagant (energy-demanding) lifestyles are.

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    Could you please provide a reference or explanation for the 1 billion humans carrying capacity? – THelper Jul 18 '18 at 14:31
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    Countless ways to work it out. I used the planetary energy budget filtered through trophic layers. Perhaps the easiest way is to simply look at the USA's Earth Overshoot Day for 2018: March 15. That means if the whole planet lived like Americans, all of the planet's renewable resources would be consumed in only 73 days. That's 1/5th of a year. Thus if the world wants the USA's standard of living, there can only be 1/5th as many people. 7.6 billion / 5 = 1.52 billion. And that number is getting smaller as resources run out. It converges on 869 million by my math. – Tim Jul 18 '18 at 14:48
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    Many countries enjoy a similar standard of living as the U.S. without the same level of resource depletion. – LShaver Jul 18 '18 at 15:05
  • True, but pretty much all comparable countries have overshoot days in March/April/May. Whether the carrying capacity of Earth (calculated by that method) is 1.5 billion or 2.5 billion doesn't really matter. Overshoot is overshoot, and eating lentils instead of pork isn't going to make a lick of difference. Besides, a later overshoot day only means that things like oil will run out slower — it doesn't stop the oil from running out. Global commodities are global — what the Europeans save the Americans will squander. – Tim Jul 18 '18 at 15:39
  • @Tim thank you for your answer, sorry if my question was misleading, but I only wanted to know about the things we consume as food, is there enough arable land for us to lead a healthy life eating only vegetation. My question was not related to fossil fuels or energy we consume for other purposes(as far as I understand your question), but if they are related, I would be happy if you can explain a bit more. Sorry if I didn't understand any part of your answer – user41965 Jul 19 '18 at 1:47

Earth's total land area is 196.9 million square miles.

The proportion of this which is farmable is 37.7%.

Therefore the total farmable land area is 74 million square miles.

Intensively cultivated corn yields 15 million kcal per acre.

There are 640 acres per square mile.

Therefore intensively cultivated corn yields 9.6 billion kcal per square mile.

Therefore intensively cultivating corn on all farmable land on earth would produce 7.1 * 10^17 kcal (9.6 * 10^9 * 74 * 10^6).

Each human needs 7.3 * 10^5 kcal per year (based on 2000 kcal/day).

Therefore, the number of humans that can be fed if all farmable land on earth grows corn is 970 billion ((7.1 * 10^17) / (7.3 * 10^5)).

If you like eating (field!) corn all day, every day, that is. Also, it's a good thing there's an unlimited supply of natural gas because this is a (practically) required feedstock in producing all that corn.

  • From the article you cited: "That means corn averages roughly 15 million calories per acre." — not 15 million kCal. Your numbers are thus off by a factor of 1,000 and the sustainable population reduces to 0.97 billion — a figure in line with my own research, as well as that of the United Nations. It doesn't matter how you cut the cake, there are already too many people on the planet. – Tim Jul 18 '18 at 13:20
  • yay, the ambiguity of the word "calories". Thanks. I'll double check the article and adjust the answer accordingly. – Jean-Paul Calderone Jul 18 '18 at 13:47
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    @Tim the confusion is likely due to the fact that in the U.S., the layperson's term for energy in food is "calorie," which is more accurately a large or kilogram calorie (Cal or kcal). The same word is also used to describe the small calorie or gram calorie (cal). "In spite of its non-official status, the large calorie is still widely used as a unit of food energy." – LShaver Jul 18 '18 at 15:01
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    Good answer, except for the last sentence: natural gas is not a required feedstock. It's the feedstock most commonly used, but it's not required. – EnergyNumbers Jul 18 '18 at 20:05
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    @Tim You can safely assume that in any non-scientific publication calories actually means kilocalories. – Jan Doggen Aug 7 '18 at 14:55

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