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Maximum population for 2000 Kcal per day

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beco
#1
Dec25-12, 10:44 PM
P: 5
Dear physics,

I'm new to the site. I found it reading this topic:
http://www.physicsforums.com/showthread.php?t=145043

I wonder if we could manage to answer, if some not critical precision, the question of:

How many people can live confortably, eating only potatos, in earth?

As of a first estimation, also used to set an example and the conditions, I'll try to answer. Please, help me correct the estimations, many of them I got from not so trusted sites.

1. How much power we need to function?
2. How much power the earth can give?
3. How much people can live with that?

---
1. From the link above, considering 2000 Kcal per day as a good consumption, we can reach that a human being is like a 100 Watt lamp.
(more details on the link)

---
2. Now, to estimate how much the earth can give, we need a lot of assumptions, each one must be checked and corrected if you find it wrong. Thanks.

First, lets talk about potatos. I found that a large potato (300g) has approximately 250 cal.
It takes 20 weeks to grow from seed. And some estimate that 6m x 6m can produce 200Kg of potatos.

That makes ~19 potatos per m^2. That makes 5.7Kg per grow per m^2, or 41g per day per m^2, or still 35 calories per m^2 per day.

Now, arable land. This google us to wikipedia, and they estimate 14000 km^2 of arable land, and 50000 km^2 of agricultural land. I'm not sure the difference, though I read the description. But I believe its safe to sum them. So we have 64 thousand km^2 to plant our potatos. That gives us 2600 gigacalories per day.

So the earth can produce 127 bilions Watts from potatos.

---

Now using 100 Watts per person, every one eating his 8 potatos per day at lunch and dinner, the maximum confortable population would be only 1 billion 300 millions.

And here, looking this number, I think there should be serious problems with this estimation!

Please, any lights to share?

Thanks,
Beco.
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Evo
#2
Dec25-12, 10:59 PM
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Potatoes aren't a complete nutritional source, so this thread is rather pointless based on your rationale of a potato only diet. Sorry if that sounds harsh, but why pursue an idea that isn't realistic to begin with?

Did you take crop rotation into consideration? Four or more years between planting potatoes in the same field are suggested. And it's not just land, climate has to be considered.

Potatoes require a lot fertilizer. Have you considered the costs involved in commercial potato farming?

Potatoes are an expensive crop to grow. In addition to a significant expenditure for variable inputs— seed, fertilizer, pesticides, labor, electricity and fuel—potato production requires a large investment in expensive, specialized machinery. The higher revenue potential for potatoes relative to most other crops also means higher land costs for potato growers because this higher revenue is capitalized into the price of land. For growers leasing land, it means paying higher cash rent for potato ground. Potato production costs under irrigation in the western U.S. range between $1,500 and $3,500 (U.S. dollars) per acre. Smaller-sized potato growers invest hundreds of thousands of dollars in potato production costs each year, while large growers spend millions.
http://potatoassociation.org/documen...nal-1.2012.pdf

There is a lot to consider besides how much land there is. I hope the link helps.
sirchick
#3
Dec26-12, 03:37 PM
P: 52
Potato has a very low protein level, and we need that to live... so you would probably die if you only eat potato.

beco
#4
Dec26-12, 04:07 PM
P: 5
Maximum population for 2000 Kcal per day

Hi guys,

Sorry if "potatos" sounded as a dogma. Potatos or no potatos, the question can be tunned by growing X, where X is some food, or a mix of food, in long term average, which renders irrelevant how many years to change from one food to another to keep the soil good, in a manner that we could establish the calories we can grow from one square metre of arable land.

Also, being X and "average" food, it does not matter if the climate is good for potatos or for grape fruits, as long as its arable (or agricultural) land.

I also read some very interesting arguments in this thread:
http://www.physicsforums.com/showthread.php?t=470256

But the efforts was toward sun light. I could quote a very nice calculation from Graal.

Earth absorbs ~122,000 TW from the Sun per second, thus a food energy potential of ~1220 TW.day/day. Assuming rapid harvest and processing, then the maximum human population sustainable is ~12.2 trillion people. A bit knife-edge because there's no reserve, but that's a design problem, not physics.

Doing away with the middleman entirely, powering humans directly via sunlight with 100% efficiency then the sustainable limit is ~1.22 quadrillion people.
Although easier to consider the sum energy, what about consider the food we have? If we get a sample of different growing food humans like to harvest, we can find an average of calories per grams that can grow.
Ryan_m_b
#5
Dec26-12, 04:25 PM
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Focusing on calories is the wrong approach. What you need to look at is what is required for crops that provide all the micro and macro nutrition a population needs.
beco
#6
Dec26-12, 04:50 PM
P: 5
Quote Quote by Ryan_m_b View Post
Focusing on calories is the wrong approach. What you need to look at is what is required for crops that provide all the micro and macro nutrition a population needs.
Actually, we need both. But considering we are growing a mix of food X, an average of a sample of human food usually eaten, and we are eating 2000 Kcal, we can safely assume we are eating all the nutrition we need.

For those familiar with programming with restriction, saying (1) a>3, (2) a<5, (3) a ≠ 7, you can safely let out restriction (3) for it adds nothing to help the problem.

Thats why I left all the nutrition outside my post in the first place, and choose "potatos" as an example of an average of everything. But of course, fixing only on potatos is not a good, because we can harvest better and all year using a mix of kinds.

What we need, to calculate that, is some guidance of how we are "now" cultivating the arable land (today). Of course, technology can change things, but lets not try to predict the future. Better focus on: with the current practice, how many will be enough?
sirchick
#7
Dec26-12, 05:15 PM
P: 52
You forget land that is not suitable can be changed to be suitable with the use of man made greenhouses...

Take Spain for example most of the land is dry semi - desert yet they converted ton of it to greenhouses so much infact you can see it from space its larger than a city:


phinds
#8
Dec26-12, 05:15 PM
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Beco, what is your POINT in all this? Do you have a plan to solve world hunger or something?

In practical terms, the whole thing seems pointless. Suppose you GET an answer. My question would be "so what?"

I just can't see where you're going
Ryan_m_b
#9
Dec26-12, 05:53 PM
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Quote Quote by beco View Post
Actually, we need both. But considering we are growing a mix of food X, an average of a sample of human food usually eaten, and we are eating 2000 Kcal, we can safely assume we are eating all the nutrition we need.
Assuming an average mix of foods and trying to work out what it takes to grow one will not give you an appropriate answer as that one food type will have very specific requirements not applicable to all.
willem2
#10
Dec28-12, 03:09 AM
P: 1,396
Arable and agricultural land are 14 and 50 million km^2

You also multiplied by 41 instead of 35 to get gigacalories/day, so you should
have gotten 2.24 * 10^15 kcalories/day or 1.08 * 10^14 Watt, allowing 1.08*10^12 people.


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