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> i think we need to define limits in terms of energy Ok. Let's do that. The earth receives about 10,000 exajoule per day (3850000 per year) [1]. Daily human c

by iofj 11y ago

> i think we need to define limits in terms of energy Ok. Let's do that. The earth receives about 10,000 exajoule per day (3850000 per year) [1]. Daily human caloric intake is 2500 calories [2]. Let's say inefficiency and the fact that you actually need food, not pure energy, let's say that means 20% efficiency, so 12500 calories per human daily. 12500 calories is about 50k joules. Things like running a tv/computer/... would be peanuts on top of that so they "come free". So divide one by the other. The earth, without space based power, can keep about 1e18 humans alive on this planet using just earth-based solar power. Plus you have to keep into account what humans actually do : they convert chemical energy into heat. If we cover the planet in solar panels, why couldn't we recover most of that human-generated heat ? Let's say we can recover 99% of energy that we put into humans back out, that's another factor of 100 right there. We could use nuclear to increase that number to 1e30 or so for a millenium and if you count fusion based power, the numbers become completely ridiculous. To put that number in perspective, take it like this : when every human alive today breeds into a population the size of today's entire human population, then we'd be near the "pure solar" carrying capacity of just the planet itself. If we were to beam power from space to the planet, we wouldn't run into the next limit until 1e50 at least. So we are very, very, very far away, even on an exponential population curve, from the carrying capacity of the planet. When fitting an exponential curve to some sample data (very inaccurate, but good enough for a ballpark figure), I get that if the growth since WWII (1.36% per year, starting at 2000 with 6 billion people), then we'd get to 1e18 a little after the year 3400. > we broke those limits by displacing a lot of other life Only for a few measurements. There are a lot of problematic measurements of life that are inaccurate and therefore full of noise, and can be outright deceptive. Some of those measurements look bad : number of species, for instance. But how relevant is that number ? Sure, if it keeps dropping our zoos will become boring, but what will actually happen ? The truth is nobody knows, and there doesn't seem to be a reason, aside from nostalgia, to assume it's bad. One "real metric" that is at least measurable and I believe is meaningful : the total amount of life is total biomass, and it is pretty well known that humans have increased that by a lot. > at some point however that fact will effect us unpredictably It seems to me to be pretty much a given that lots of things can affect the human race unpredictably, and given how many such factors there are, it seems a safe bet that a few are indeed going to affect us (though the vast majority won't). Malthusianism and global warming are such factors, malthusianism has been proven false 3-4 times at least, and many limits to growth theories both before and after have been (I like the economic "debt limit" ones, stating that we can never have an economy larger than the value of all gold, then we passed that, then money became "loaned gold", and the next theory was that there was some factor that limits how big the economy can be n * total_gold, n was to be 10, then 100, now it's at least 100 million, and they're still at it), I wonder what the fate of global warming will be. If you believe in Darwin and evolution, this is a positive. We will adapt to whatever effect, predictable or unpredictable, starts affecting us. [1] https://en.wikipedia.org/wiki/Solar_energy https://en.wikipedia.org/wiki/Solar_energy [2] http://www.webmd.com/diet/estimated-calorie-requirement http://www.webmd.com/diet/estimated-calorie-requirement

eru 11y ago
> Let's say we can recover 99% of energy that we put into humans back out, that's another factor of 100 right there. Thermodynamics doesn't work that way.
- dalke 11y ago
  I'll be more mathematical. By Carnot's theorem, the maximum possible efficiency of a heat engine = 1 - (T_cold/T_hot). Using very rough figures, T_hot = body temperature = 37 C = 310 K and T_cold = average temperature of the earth = 16 C = 289 K. Thus, the maximum possible efficiency is 1-289/310 = 6.8%. To get 99% energy recovery, assuming human body temperature remains the same, would require that the cold side of the heat engine be at 3.1K, or only slightly warmer than the background temperature of space. The only possible way to achieve that difference is by radiative heat transfer to space. However, we can use the temperature of the buried lunar regolith as an estimate of what the the "average" temperature of the earth could be without an insulating atmosphere. http://www.asi.org/adb/m/03/05/average-temperatures.html http://www.asi.org/adb/m/03/05/average-temperatures.html says it's -35 C = 238K. Even if we could use -35C as the cold side of our heat engine, that would still only be 23% efficient, assuming maximal thermal efficiency and, you know, no air.
  - eru 11y ago
    Thanks for actually doing the calculations!
schoen 11y ago
Nutritional "calories" are actually kilocalories (kcal), which leads to occasional confusion when trying to do calculations that relate food energetic content to other things. For example, I had a book relating someone's calculation that you should be able to diet by drinking scotch and soda because "the body must furnish 7400 calories" to raise the drink from 0°C to body temperature, which would presumably dwarf the energy input from the liquor and indeed from whatever else the drinker consumed all day. (The author agreed that this "didn't work" but didn't understand why not.) The problem turned out to be that, while the body does indeed furnish around 7000 calories to heat up an ice-cold beverage, the drinker is consuming 2000 or more kilocalories daily, including around 100 from the drink itself. So, you should lose a factor of 1000 for the definition of nutritional calories. But the numbers in your calculation are so staggering that this barely affects your point, which is that human population isn't close to being fundamentally physically limited by energy inputs from the Sun. I think you're basically right conceptually, but should give more weight to the difficulty of adapting to some kinds of environmental change. There's no guarantee that human beings can invent everything we need when we need it, and some changes are difficult or impossible to reverse. So even if some limits aren't at all theoretically insurmountable, humanity could fail to surmount them because things that would need to be done can't be done in practice in the required timeframe.

by iofj 11y ago

eru 11y ago

> Let's say we can recover 99% of energy that we put into humans back out, that's another factor of 100 right there. Thermodynamics doesn't work that way.

dalke 11y ago
I'll be more mathematical. By Carnot's theorem, the maximum possible efficiency of a heat engine = 1 - (T_cold/T_hot). Using very rough figures, T_hot = body temperature = 37 C = 310 K and T_cold = average temperature of the earth = 16 C = 289 K. Thus, the maximum possible efficiency is 1-289/310 = 6.8%. To get 99% energy recovery, assuming human body temperature remains the same, would require that the cold side of the heat engine be at 3.1K, or only slightly warmer than the background temperature of space. The only possible way to achieve that difference is by radiative heat transfer to space. However, we can use the temperature of the buried lunar regolith as an estimate of what the the "average" temperature of the earth could be without an insulating atmosphere. http://www.asi.org/adb/m/03/05/average-temperatures.html http://www.asi.org/adb/m/03/05/average-temperatures.html says it's -35 C = 238K. Even if we could use -35C as the cold side of our heat engine, that would still only be 23% efficient, assuming maximal thermal efficiency and, you know, no air.
- eru 11y ago
  Thanks for actually doing the calculations!

schoen 11y ago

Nutritional "calories" are actually kilocalories (kcal), which leads to occasional confusion when trying to do calculations that relate food energetic content to other things. For example, I had a book relating someone's calculation that you should be able to diet by drinking scotch and soda because "the body must furnish 7400 calories" to raise the drink from 0°C to body temperature, which would presumably dwarf the energy input from the liquor and indeed from whatever else the drinker consumed all day. (The author agreed that this "didn't work" but didn't understand why not.) The problem turned out to be that, while the body does indeed furnish around 7000 calories to heat up an ice-cold beverage, the drinker is consuming 2000 or more kilocalories daily, including around 100 from the drink itself. So, you should lose a factor of 1000 for the definition of nutritional calories. But the numbers in your calculation are so staggering that this barely affects your point, which is that human population isn't close to being fundamentally physically limited by energy inputs from the Sun. I think you're basically right conceptually, but should give more weight to the difficulty of adapting to some kinds of environmental change. There's no guarantee that human beings can invent everything we need when we need it, and some changes are difficult or impossible to reverse. So even if some limits aren't at all theoretically insurmountable, humanity could fail to surmount them because things that would need to be done can't be done in practice in the required timeframe.