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I'm not asking what will happen if everyone in the world suddenly turned to vegetarianism. In an ideal world with the 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 vegetarianism in some time? If yes/no, how much population can Earth sustain if everyone is always a vegetarian. Also, animals would only be breed in zoological parks or for medical or some other science related needs (or for exceptional cases)

Fred
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user41965
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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. –  Jul 16 '18 at 06:16
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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
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    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 01:26
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    @user41965 Your question basically asks "Can humanity switch to an ~100% vegetarian diet?" The answer is a qualified "Yes", but I wonder if you could explain the reason why you are asking this question? Why would humans want to go vegetarian? What problem are you trying to solve? – Tim Jul 19 '18 at 02:51
  • Food intake is ~3.2GJ/year for adults, compared to 290GJ/year total energy consumption per capita (for USA in 2014). At a mere 1.1% of total energy, food is almost irrelevant. Switching to a vegetarian diet thus has 'negligible' sustainability value. I'm not sure what the purpose of the question is. – Tim Jul 19 '18 at 04:07
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    Related: What are the greenhouse gas benefits of vegetarianism? – LShaver Jul 24 '18 at 13:54
  • I started to write an answer, but I see you adding additional questions (health) and clarifications (arable land) as comments to answers. Together with ambiguous wording in your question I think any answers should be postponed until you have cleaned up your text and ask one question. And as others comment, adding the purpose of your question gives us some context. –  Aug 07 '18 at 14:52

3 Answers3

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The answer by Jean-Paul Calderone correctly specified how much population the food can support. I'm not focusing on that at all in my answer; I'm only focusing on energy.

This answer, however, did not take into account energy aspects, only mentioning than an unlimited supply of natural gas is needed. That is most certainly false: methane can be created with only energy, carbon dioxide and water as the source materials. As we all know, there is no shortage of carbon dioxide.

Carbon dioxide can be recycled in industrial processes, and natural gas can be synthetically created using water electrolysis and Sabatier reaction.

Thus, the only problem is how much input energy we can harness. Solar radiation is 1000 W / square meter on non-cloudy days, and Earth receives 1.27*10^14 square meters of radiation minus clouds. If we could harness only 1% of the radiation before clouds (seems doable), we have 1275 terawatts of radiation. This is 11166905 terawatt hours per year.

US primary energy use is 88000 kWh / year / capita (source). Thus, it is possible to support 127 billion people having US lifestyle.

More likely, the answer would be larger, as 30% of Earth is land, cloudy days are perhaps 50% of the days and we could perfectly well use 15% of landmass for solar power and the rest 85% for other purposes. Thus, 0.30 * 0.50 * 0.15 = 0.0225, or 2.25%, larger than 1%.

So, even with solar radiation as the only energy source, a US lifestyle is possible for hundreds of billions of people.

If we want to support a larger number of people, e.g. thousands of billions of people all having a western lifestyle, we cannot do it with solar power only, but there is always the nuclear option. Once-through fission cycle will quickly lead to exhaustion of cheap uranium. However, it is possible to build breeder reactors that use uranium 50-100 times more efficiently than current reactors.

With such breeder reactors, at the current consumption level, uranium will last for billions of years (source).

Multiply the population having a western lifestyle by 1000, and uranium is still enough for millions of years. Some could consider it unsustainable if the human population runs out of energy in millions years. Some other could consider it sustainable enough.

I'm not saying these breeder reactors are the optimal solution. Growth to population of 127 billion seems unlikely, so it is likely that exponentially reducing solar power production costs mean solar power will be enough and needs not be supplemented by nuclear.

Also, with a population of 1000 billion, all using nuclear power to have a western lifestyle, the waste heat would be enormous and could somewhat heat up the planet directly.

Some might protest that solar can be generated only when sun shines and there are not enough battery minerals for storing that electricity for cloudy days. The most likely solution to this problem is power-to-gas, where solar power is stored as synthetic hydrogen or synthetic methane (along with carbon dioxide recycling, of course, if methane is selected instead of hydrogen). Enormous amounts of energy can be stored as gas, with no need for battery minerals.

juhist
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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.

Jean-Paul Calderone
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  • 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
  • http://mathscinotes.com/2017/01/calorie-per-acre-improvements-in-staple-crops-over-time/ suggests that corn is indeed close to 15 M kcal/acre. http://www.waldeneffect.org/blog/Calories_per_acre_for_various_foods/ suggests it may be closer to 3 M kcal/acre (comment section). Neither of these suggests an overestimate of 1000x in this answer though. Do you have a source for the correct value being 15 million calories instead of 15 million kilocalories? – Jean-Paul Calderone Jul 18 '18 at 13:54
  • I just quoted the text in the body of the article you linked to "Intensively cultivated corn yields 15 million kcal per acre." It may be that the author just made a typo in the body. Doesn't change the reality that Earth cannot support what we have now, let alone 129x as many. – Tim Jul 18 '18 at 13:58
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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
  • That Wikipedia link says it all: "Although these units relate to the metric system, all of them have been considered obsolete in science since the adoption of the SI system." – Tim Jul 18 '18 at 15:10
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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. – 410 gone Jul 18 '18 at 20:05
  • @Jean-PaulCalderone thank you very much for your answer, but one of the main thing I wanted to know is if meat alternatives (which gives us protein, fat, etc) can be sufficient for humans. One more thing is can we actually grow corn in all kinds of land(maybe we can, maybe we can't) and also when we consider other things too (which provide us important nutrients apart from energy) is there enough cultivable land for those vegetation(and also it should be feasible and sustainable)? – user41965 Jul 19 '18 at 01:40
  • @user41965 I don't believe there is any science-based medical consensus about whether vegetarian diets are safe and healthy for humans (particularly children). So it's not clear to me that even one person can be sustained on a vegetarian diet from a health perspective. – Jean-Paul Calderone Jul 19 '18 at 14:37
  • whether vegetarian diets are safe and healthy for humans ... do you mean vegan, a diet which consists of no animal products? There are millions of vegetarians who eat dairy products (and in some cases, chicken eggs), but no meat. Some 40% of Indians fall into this category. – LShaver Jul 19 '18 at 18:41
  • I guess I'm not sure what I meant there. The context is confused as the original question says "vegeterian/vegan" as though these are interchangeable. The tone of the question makes me think the question is about veganism (eg "And animals are only breed in zoological parks ...") so I probably mistakenly started using "vegetarian" to mean "vegan" myself. – Jean-Paul Calderone Jul 19 '18 at 19:03
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    @Tim You can safely assume that in any non-scientific publication calories actually means kilocalories. –  Aug 07 '18 at 14:55
  • @Jean-PaulCalderone sorry for the delayed reply, if it makes difference I would like to know the answer to both the cases vegetarian and vegan. Thank you – user41965 Sep 25 '18 at 04:59
  • This answer is probably out by at least an order of magnitude. A 2004 meta-study (https://academic.oup.com/bioscience/article/54/3/195/223056#126051279) suggests human carrying capacity between 0.65 and 98 billion, median 7.7 billion. – aucuparia Oct 16 '19 at 11:27
  • Table 5 in that study suggests that whether or not a study takes into account regional variations in diet - which can be taken as a very crude proxy for assuming vegetarian diets - makes a difference of a factor of 1.4 in ln(carrying capacity) estimates. Plugging 7.7bn in there gives 17.7bn as the vegetarian carrying capacity: a difference much smaller than the error in the estimates! – aucuparia Oct 16 '19 at 11:34
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    It seems quite likely to me that calorie production is not the limiting factor for Earth's carrying capacity, indeed, and my answer only addresses calorie production. – Jean-Paul Calderone Oct 16 '19 at 13:09
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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.

Tim
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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 01:47
  • The point I was trying to make was that both vegetarian and omnivorous diets are dependant on non-renewable fossil fuels (primarily oil). Once the oil runs out, neither will be able to be sustained. If the species switched to a 100% vegetarian diet today, and the oil ran out tomorrow, ~6.6 bn people would starve to death within a year. A vegetarian diet not only requires arable land — it requires oil (for all the machinery that prepares land, plants seed, fertilises soil, harvest crops, processes the food, distributes it, etc.). Without oil, no diet is viable for > 1 bn humans. – Tim Jul 19 '18 at 02:00
  • The short answer is that planet Earth can support ~7.6 billion vegetarian humans only as long as we have oil. Once that runs out planet Earth can only support ~1 billion vegetarians. Vegetarian or omnivore actually doesn't make any meaningful difference to the population level that can be supported — planet Earth can only sustain ~1 billion humans with American lifestyles, period. – Tim Jul 19 '18 at 02:21
  • Could you please back up your claims in your answer instead of in the comments? Also, not all people on Earth have an American lifestyle so if you use the calculations of Earth Overshoot Day for calculating the carrying capacity then why not say that given our current lifestyle the Earth's carrying capacity is 7.5 billion * 213/365 (august 1 = day 213), but it's declining? That's more accurate than stating it's 1 billion. – THelper Jul 23 '18 at 06:59
  • Since carrying capacity is almost entirely about lifestyle, there is no point using a 'global average lifestyle', because that's not something the OP can relate to. By their use of the English language, it's highly likely that the OP is a westerner and thus an "answer in western units" is most appropriate. No-one here can relate to the lifestyle of someone in an impoverished part of the world. Since vegetarianism can't save the planet, and seems (to me) to merely be a lifestyle choice, I think that was the right call. – Tim Jul 23 '18 at 09:32
  • Further, with August 1 being 2018's Overshoot Day, even if you went down the 'global average lifestyle' route, doing the math correctly would mean that the carrying capacity would by 7.6213/365= 4.4 billion. A 3.2 billion overshoot cannot be fixed by dietary changes — so that's pointless as well. The reality is that people like to think* that becoming a vegetarian is more sustainable. It isn't. People also don't like to hear that population control is the only answer, or that the USA is the biggest threat to life on Earth. That's why this answer is at -2. The truth is painful. – Tim Jul 23 '18 at 09:37
  • There's a discussion of the validity of the "Earth Overshoot Day" concept over on Skeptics.SE. – LShaver Jul 24 '18 at 13:56
  • There are dozens of different ways to calculate how overpopulated the planet is. Overshoot Day is just one of them. Overpopulation Denial is a thing now. – Tim Jul 24 '18 at 14:01
  • The core of the question revolves around the (apparent) assumption that switching to a vegetarian diet is sustainable. As mentioned in a comment on the original question, food — being a mere 1.1% of a westerner's total energy consumption — proves that diet is irrelevant to sustainability of a population with western lifestyles. Even if the West suddenly had no need for food whatsoever, 98.9% of the consumption would still occur and the planet's resources would be depleted. Western lifestyles are not sustainable. Basic math. There's nothing you can do to 1.1% to offset 98.9%. – Tim Jul 24 '18 at 14:28
  • If 98.9% of your body is riddled with cancer, it doesn't matter how healthy your little finger is — you're still going to die. Taking better care of that finger won't change the outcome. – Tim Jul 24 '18 at 14:34
  • Let us continue this discussion in chat. – LShaver Jul 24 '18 at 14:54