# How many humans can the world take?

I just came back from a lecture by Jared Diamond who talked about his latest book – Collapse: How Societies Choose to Fail or Succeed. It made me think about what the capacity of the world is if we relied entirely on energy arriving from the sun. The power density of solar radiation is about 1.4 KW/m^2 and the surface area of the earth is about 10^8 km^2. So in 12 hours of daylight we receive about 10^16 MJ. The energy consumption of the US is close to 10^12 MJ per day which amounts to about 10^4 MJ per person. An average person eats less than 10 MJ per day. If the entire world lived as Americans we would use almost 10^14 MJ per day. I may be off by a factor of 10 or so but it looks like we’re cutting it pretty close. If the world starts to live as Americans and we convert about 1% of the total sunlight into useable energy, then 6 billion people is at full capacity. I think this is a pretty good argument for us to conserve a little bit more (or we better get those fusion reactors up and running).

## 4 thoughts on “How many humans can the world take?”

1. Carson-OK, I’m thread-hijacking here, but 1 thing I’ve always wondered about is how those 140 lb Japanese dudes keep winning those mass-eating contests. Are there GI tracts more distensable than others?-David

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2. Hi Dave,I think it is because they’ve trained themselves to ignore all feedback regarding satiety. So, they’re eating until they’ve maxed out their physical capacity to hold food. I think this may be a factor in weight gain. Overeating may train you to handle more food at a given sitting and may even fool you into thinking you haven’t eaten enough compared to what you used to eat. But that is just speculation.

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3. Carson, interesting calculation, I agree with your suggestion that we ease up on the gas pedal, but I am not sure the calculation is the correct basis for this conclusion. Specifically, why do you impose the conservation of energy requirement? One could argue that we have a few hundred million years of plant life that has been banked, and we are just getting around to using “our” energy. Furthermore, the solar system (specifically the Jovian Planets) may hold some fantastic energy deposits in the methane and ammonia (when combined with the oxygen we have here on earth).I think a better argument for conservation and reducing environmental impact is that the earth, as a planet, will probably endure through a broad range of insults. However we humans are pretty delicate and we can easily render our home uninhabitable.-WJEV

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4. Hi Will,My posting was mostly a Fermi estimate of what sustainability would be if we relied entirely on solar energy. It wasn’t meant to be a pro-environmental posting per se although that is where my sentiments lie. I definitely agree with you on all points. The amount of stored energy in fossil fuels is an interesting question. I haven’t done the research to see how much is available and under what circumstances carbon will be preserved as fuel. In any case, that supply is finite and once we pass break even we’ll be dipping into the energy trust fund. After that we will need another source of energy whatever it may be.

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