Maximum Sustainable Earth Population

[Kracatoan]

Climate change and environmentalism are some of my favourite areas of science and from both reading around and going to lectures I've got two figures for the maximum sustainable population of the earth, those being:

1 billion - from James Lovelock (one of his Gaia books)
1-2 billion - from the head of the Geoengineering department at Bristol

and I was wondering if anyone else knew of any other estimates, just so I can get a better idea on this subject.

I know all estimates have huge uncertainties, but an "in-the-ballpark-of" sort of figure would be appreciated,

 

[Borek]

Define "sustainable". And a lot depends on the level/way of life. See this post:

https://www.physicsforums.com/showpost.php?p=2652522&postcount=10

from this thread:

https://www.physicsforums.com/showthread.php?t=391197

 

[Kracatoan]

Sustainable, as in, has no net effect upon world resources and can be maintained indefinately, using current technology. I think Lovelock's estimate was based upon average, developed country lifestyles.

 

[socean]

I do not think the carrying capacity of the Earth can be accurately predicted. One must take into account the effect of accelerating returns.

We don't even know what humans will be like 30 years hence. How much of our physiology will be enhanced? My mother sees better than I do now thanks to lens implants. What happens when we can all wear contacts that create virtual realities that mix with our physical ones? What if we cure heart diseases, cancers, HIV, and other maladies?

As our brain scanning technologies improve, we'll understand better how to improve our minds. Perhaps we'll find some way to enhance our intellectual and ethical being that enables some profound shift in our ability to socialize.

Unless we experience some catastrophic losses as a species, we'll need to find ways to support many times our current population as we continue to enjoy increasing lifespans.

I think it is most likely that our species will continue to learn to improve its understanding of the universe, and that we will use this knowledge to better ourselves and our circumstances. We do not have to stretch our imaginations very much to believe that we can inhabit the sea, the air, and even space as needed. Eventually, our social structures and infrastructures will evolve to support a nearly infinite number of people in completely sustainable ways. The entire Earth will come to be regarded as a universally shared resource of our species, wherever we might reside.

Assuming of course, we don't screw things up too badly in the interim by doing something stupid like nuking ourselves.

 

[Evo]

I do not think the carrying capacity of the Earth can be accurately predicted. One must take into account the effect of accelerating returns.

We don't even know what humans will be like 30 years hence. How much of our physiology will be enhanced? My mother sees better than I do now thanks to lens implants. What happens when we can all wear contacts that create virtual realities that mix with our physical ones? What if we cure heart diseases, cancers, HIV, and other maladies?

As our brain scanning technologies improve, we'll understand better how to improve our minds. Perhaps we'll find some way to enhance our intellectual and ethical being that enables some profound shift in our ability to socialize.

Unless we experience some catastrophic losses as a species, we'll need to find ways to support many times our current population as we continue to enjoy increasing lifespans.

I think it is most likely that our species will continue to learn to improve its understanding of the universe, and that we will use this knowledge to better ourselves and our circumstances. We do not have to stretch our imaginations very much to believe that we can inhabit the sea, the air, and even space as needed. Eventually, our social structures and infrastructures will evolve to support a nearly infinite number of people in completely sustainable ways. The entire Earth will come to be regarded as a universally shared resource of our species, wherever we might reside.

Assuming of course, we don't screw things up too badly in the interim by doing something stupid like nuking ourselves.

Let's keep this scientific, and not science fiction.

We've already had dozens of threads identical to this, so unless there is some new peer reviewed research posted today that makes this an improvement to past discussion, this will also be closed. The science forum is not for idle speculation.

 

[Kracatoan]

I know there have been many threads discussing the ethics of population control, but none seem to have any actual estimates, apart from 1 a couple of years ago. I started this thread just to see if anyone had any good, referenced numbers, rather than opinions.

 

[Dug]

Until you critically define "sustainability" you can't get an estimate. However, basic biomass and or life cycle analysis calculations can give us some rough estimates if we define productivity of the respective natural system.

In my work we have found that max. biomass for natural systems is relatively uniform - whether you talk about fish, birds, pigs, cows or humans - as it relates to the total available organics in the respective system. However, when you start having nutrient and energy inputs into the system - all things "natural" go by the way side. Even in natural systems you can get more of one species - if that species can occupy the niches of other species and displace them and or if that species can make use of waste nutrients before the algae and bacteria get to them - or use the algae and bacteria. We need to remember that biomass in natural systems is limited by nutrient, energy, and for us oxygen.

Unfortunately, for the case of humans our entire existence since the early 1800s has been increasing by the use of fossil fuels, petro-chemical fertilizers, fossil-chemical energy inputs and of course our displacing other species. We are so far beyond "natural" systems in terms of human population today it's - well bizarre. Bizarre when you consider that 85% of human food is produced using petro-chemical fertilizers and 95% of our food require petro-fuels to get from the field to your mouth. Bizarre when you consider that all commercial human food production uses mined phosphates that are now estimated to peak in 30 years and be essentially gone in 50 years. So, it's bizarre that we are knowingly letting our population get far, far, beyond our food production capacity without petro-chemicals which we know are available only in finite quantities which have apparently already peaked in availability.

Perhaps the simplest answer is to say that the demonstrable natural maximum sustainable human population was reached just before industrial revolution - when first coal and then oil started providing energy and nutrient inputs far above natural levels. One of the most dramatic examples of human population growth as a response to unnatural energy and nutrient inputs can be found here:
- or you can visit Bangladesh during the rainy season.

So the bottom line from a practical standpoint regarding human population sustainability is really about our ability to produce food for humans. And as food production technology stands today - it is limited by the amount of petro-chemical fertilizers available. If nothing changes dramatically technologically in food production in the next 50 years - well let's just say that we in the US we will have finally licked our obesity epidemic.

 

[DrClapeyron]

You would have to consider the lag time it takes to regenerate a resource after using it, and factor in consumption rates. It takes like millions of years for ore bodies, oil, gas, top soil and such to form. Seems like an ungodly amount of guess work. I would maximum sustainablity can't happen because all the usefull energy on Earth will be used up eventually.

 

[TechnoPagan]

You would have to consider the lag time it takes to regenerate a resource after using it, and factor in consumption rates. It takes like millions of years for ore bodies, oil, gas, top soil and such to form.

Well, yes, it does take a long time for these resources to form, but it doesn't take that long to recycle them. Using mulching and composting we can create new topsoil. We don't have to wait for a car to rust away to iron ore, then re-refine it. We can just recycle the metal and use it over and over.

I would maximum sustainablity can't happen because all the usefull energy on Earth will be used up eventually.

So basically, to be sustainable, we would need an energy source beaming down to us from outer space. I'll work on it.

 

[Dug]

So basically, to be sustainable, we would need an energy source beaming down to us from outer space. I'll work on it.[/QUOTE]

That is the exact point. Solar does just that - beams down from space. Relatedly we get tide, wind, and wave - all off planet energy sources available to us in forms that don't diminish our on planet critical resources. We can't on one had scare hell out of people over peak oil - and then on the other promote the development of biofuels that will require the use of petroleum based fertilizers to have any significant impact on our energy needs. Our inability to produce sufficient food once we use up readily available petroleum and phosphate resources - is a far more serious problem than just running out of cheap fuel. By prioritizing the development of off-planet energy sources like solar- we can extend critical finite resources on the planet and hopefully develop technologies to manage our species population such that it doesn't self destruct and or until we can achieve successful off planet colonizations such that the species can keep growing. What most people don't realize is that our current economic system (capitalism) doesn't work in declining population models = declining market size and ultimately declining demand. Allowing politics to mis-direct technological alternative energy development toward petro-chemically fertilized biofuels is dangerous, self-destructive and a willfully ignorant mistake for all of us.

 

[Kracatoan]

What most people don't realize is that our current economic system (capitalism) doesn't work in declining population models = declining market size and ultimately declining demand.

A most interesting point, I'd never thought of that perspetive before.

Allowing politics to mis-direct technological alternative energy development toward petro-chemically fertilized biofuels is dangerous, self-destructive and a willfully ignorant mistake for all of us.

Indeed, it is most irritating that politicians are butting much emphasis on biofuels, when that land could be put to much better use growing food.

 

[mugaliens]

Indeed, it is most irritating that politicians are butting much emphasis on biofuels, when that land could be put to much better use growing food.

Agreed. Arable land isn't exactly in plentiful supply. It's stupid using arable land to gather energy from the sun in the form of biofuel here in the U.S. when there's more than enough desert which can be used for that purpose to gather energy in the form of electricity.

Question: Given plentiful electricity, can fuel be synthesized from raw, non-biological materials?

 

[PRDan4th]

Agreed. Arable land isn't exactly in plentiful supply. It's stupid using arable land to gather energy from the sun in the form of biofuel here in the U.S. when there's more than enough desert which can be used for that purpose to gather energy in the form of electricity.

Question: Given plentiful electricity, can fuel be synthesized from raw, non-biological materials?

Yes, H2 from H2O.

 

[qraal]

Climate change and environmentalism are some of my favourite areas of science and from both reading around and going to lectures I've got two figures for the maximum sustainable population of the earth, those being:

1 billion - from James Lovelock (one of his Gaia books)
1-2 billion - from the head of the Geoengineering department at Bristol

and I was wondering if anyone else knew of any other estimates, just so I can get a better idea on this subject.

I know all estimates have huge uncertainties, but an "in-the-ballpark-of" sort of figure would be appreciated,

Depends on what's meant by sustainable. The thermodynamic limits are considerably higher, but the end state is perhaps unpalatable. If we assume half the biomass is in humans and the other half is in food species - probably a highly efficient algae - then we can equate the two and derive a limit. Active humans need ~8,700 KJ per day (~100 J/s bioenergy released) and we might engineer food plants to be ~1% efficient at energy storage. 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.

 

[Evo]

Doing away with the middleman entirely, powering humans directly via sunlight with 100% efficiency then the sustainable limit is ~1.22 quadrillion people.

Post the peer reviewed scientific study that shows that food, clean water, suitable housing, waste control, schooling, jobs, medical care and transportation could be physically and economically viable to sustain that many people.

This is getting absolutely ridiculous.

 

[Phrak]

Gas, coal and crude sustain world population. Gasoline is condensed, liquid sunlight with an energy density of 34 billion Joules per cubic meter. Take a google world tour and you will see crude oil and coal put to use, visible as green circles that have turned deserts green with corn. Usually, at the center of each is a well and pump. These are not fueled by punny solar cells nor windmills.

In all of history, an unprecedented bounty of energy per capita fuels population and the lifestyles to which we have come to think of as 'normal'.

current world population stands at about 7 billion, by the way. It seems James Lovelock, et al were a bit inconsiderate of our ingenuity in clawing wealth out of the ground if they gave it a thought at all.

 

[qraal]

Post the peer reviewed scientific study that shows that food, clean water, suitable housing, waste control, schooling, jobs, medical care and transportation could be physically and economically viable to sustain that many people.

This is getting absolutely ridiculous.

You clearly missed my point - if it was just a matter of physics then the limit is huge. Clearly there's a lot of other dissipative processes involved in sustaining human life and the question was kind of vague in it's terms.

 

[qraal]

You clearly missed my point - if it was just a matter of physics then the limit is huge. Clearly there's a lot of other dissipative processes involved in sustaining human life and the question was kind of vague in it's terms.

A more critical point is that a lot of people seem to be talking about "sustainable" based on agricultural practices that seem heavily labour intensive, yet don't seem inclined to be the ones doing the hard work. Clearly they have no idea of what agricultural, pre-industrial, societies were like.

 

[Kracatoan]

Current world population stands at about 7 billion, by the way. It seems James Lovelock, et al were a bit inconsiderate of our ingenuity in clawing wealth out of the ground if they gave it a thought at all.

There is a difference between maximum possible population and maximum sustainable population. Naturally a population can exceed the limit of its sustainiblity for a short while, however the length of time such a large population can last for is definitely finite.

 

[Phrak]

Can you clearly state a definition of "sustainable" including a nominal span of years somewhere less the remaining life of the sun? Also, when considering environmental values, which do you wish to be unconsidered such as sunspot activity and asteroid impact perhaps.

 

[Algr]

Levels of technology are very important to this question. If we all suddenly had to use only roman technology, the Earth's population would probably be cut in half within 2 years.

So a major problem with this calculation is that even if you could define "Sustainable", it is possible that technological advancement might increase the sustainable population faster then the actual population is growing. Advancements in solar and wind have certainly increased the hypothetical sustainable population. What if we discovered cheap fusion? Or seaweed that makes oil?

 

[yuiop]

First we have to define "sustainable". One obvious limitation is the lifespan of the Sun, which has already been mentioned, so we cannot mean indefinitely sustainable, and the only really long term hope for mankind is to get off the Earth, but for now let's assume "sustainable" means for as long as the Sun lasts. See http://en.wikipedia.org/wiki/Earth#Future

Secondly, by "sustainable population" do we mean just the human population or do we wish to sustain the diversity of life on Earth? If we do, then we should be aware that even the current human population of 7 to 8 billion is already threatening the survival of many other species on Earth.

Next, to obtain a ball park figure for sustainable population we should ignore all fossil fuels as a source of energy, as they cannot be replaced at any reasonable pace, so the base survival figure should be based on solar energy. The average number of Joules of sunlight per square meter of the Earth's surface is approximately 14.2 MJ/day. For the entire Earth's surface the potentially available energy (PE) is 14.2*5.1*10^14 = 7.24*10^15 MJ/day.

If we care about diversity of life on Earth then obviously we cannot cover the entire Earth with solar panels and some areas will have to be left natural. Even if we do not care about diversity, if the Earth is entirely covered by solar energy panels there will no room for plants or algae and so there will be no CO2 recycling or oxygen production. Additionally if we use a very large fraction of the Earth's surface for solar energy production there will be artificial climate change implications. Thus there is limit to the Earth's surface that can be used for solar energy production and bearing in mind that 2/3 of the surface is ocean, a figure for the fraction of surface area that is exploitable (EA) might be somewhere in the region of say 2% of the total surface area, so that gives a figure of PE*EA = 1.44*10^14 MJ/day.

Now we need to take into consideration the conversion efficiency (CE) rate that we can convert exploitable solar energy into usable energy. Photo voltaic cells have an average conversion efficiency of solar power into electricity of around 20%. Systems that covert solar energy directly into home or water heating can be more efficient, but we have to bear in mind the energy costs of constructing solar power systems. When we take into account the energy to manufacture solar power systems and deduct this from the energy captured from the devices, the effective efficiencies are much lower. In fact some calculations show that more energy is used in manufacturing some of these devices, than is ever recovered and they only exist because of government subsidies. Taking into account manufacturing energy costs we might guess that the conversion efficiency is somewhere in the region of 5% and that might be generous. This makes the total usable energy PE*EA*CE = 7.24*10^12 MJ/day.

Now that we have a rough total potentially usable solar energy figure, we can in principle estimate the maximum sustainable population, if we know the average energy consumption per person. An average person consumes 1800 calories of food per day. This equates to 0.0075 MJ per day which is almost negligible even if takes ten times that in raw solar energy to produce the food. I say it almost negligible, because In 2008, total worldwide energy consumption was 474×10^12 MJ/year. This equates to an average energy consumption (EC) per person worldwide of about 200 MJ/day. Compare this to an average consumption of 895 MJ/day/person in the USA. This is energy used for home heating, air conditioning, cooking, refrigeration, entertainment devices, hot water, lighting, manufacturing energy costs, waste and water treatment, and vehicle fuel and they make food energy requirement pale into insignificance. There are countries that consume more than USA, but if we assume in the future that the worst users cut back their consumption a bit and that third world countries aspire to a living standard similar to that of the USA and other first world countries, then EC= 800 MJ/day per person might be a conservative estimate of usage in the future.

Now we can do a very rough estimate of a solar sustainable world population (SP) using SP = PE*EA*CE/EC = 7.24*10^12/800 = approx 9 billion people.

Of course you are free to come up with your own future projections for EA, CE and EC (and I invite you do so), but you should be aware that even with the current population we are causing climate change, fighting over resources like oil, water, land and threatening the survival of various species, so bear in mind that there is more to sustainability, than just theoretical availability of energy.

Note: I have only considered solar energy which includes indirectly wind power. Wave power might be considered as lunar power and independent of solar power. If we extracted 100% off all available wave power, would this have any consequences on the rotation rate of the Moon or the Earth? It certainly would have an effect on sea life affecting migration routes etc. Fusion power might be another option, but the means to reliably produce usable power from fusion has yet to be demonstrated and perhaps more resources should be put into fusion research.

 

[qraal]

Dietary calories are 1,000 thermal calories. Thus 1,800 cal/day = (1800 x 4184 J)/day = 7.53 MJ/day.

On average the Sun supplies ~(1360/4)*0.7 ~240 W/sq.metre of energy per second to the Earth - 20.36 MJ/day. Autotrophs store a tiny fraction of the incident photon-stream:

(1)the total productivity is ~275-300 billion tonnes of "sugars" (storage molecules) per year - as 1 mole (mu = 0.03 kg) is energetically ~480 KJ, that equates to ~10^16 moles @ 480 KJ/mole = 4,800 exajoules of primary production per year.

(2) Total surface area is 510 x 10^12 sq.metres thus total supply of Solar energy is ~10,575 exajoules/day. There's 365.25 days per calendar year, so total captured by primary producers is ~0.124%.

(3) What happens then depends on how we assume the future of primary production utilization relates to the population future. Wandering away from physics into social engineering and quasi-Eugenics/Nazism...

 

[VS63]

I guess another question is, who would want to live on a planet that is sitting at the maximum level of sustainability. Is there any room in this situation for any other species, or would we only allow species to exist that directly contribute to the food chain, or other necessary production process?

And as others have said, that level of sustainability would change as technology finds new ways to reap more from the same piece of dirt, or whatever.

Technically, I guess it would be possible for a person to be maintained in an artificial environment that nutritionally sustains them and could stimulate the muscles artificially, provide the person stimulation through virtual reality. Ever seen The Matrix? These environments could even be stacked up in multi levels, further reducing the amount of planet surface area that each person uses.

In a situation like that you could live in an area of say 2 cubic metres, leaving the rest of the space you occupy now for other people, or for food production. But like I said, would you really like to live like that?

 

[Delong]

Maybe you misunderstand sustainability. There aren't levels of sustainability. Something is either sustainable or it isn't. How you live under sustainability is irrelevant. The example you could is possibly sustainable but so could something else that is more comfortable, natural, and less science fiction. It doesn't matter about the differences what matters is that it is sustainable. If it's sustainable it's sustainable there aren't degrees. Trying to achieve sustainability seems to me to be rather a hit or miss endeavor.

 

[Czcibor]

Without getting into the politics of things, I would say that market driven economy is ill prepared to handle big global problems. Global problems need to rest on planing, because markets are always shortsighted.

May I use my country (Poland) as example to illustrate what you just said here?
My country can proudly claim that it fulfils requirements of Kyoto protocol and there are even emission rights left. Why? Well, because the period which was used as base period it were the last days of communism. Actually during that period we had a magnificent heavy industry and chemical industry. (and empty shelves in shops selling consumer goods) During the transition it turned out that no one really demands all those steel and chemicals. Moreover, it turned out that factories that in communism produced them very wastefully, were actually unprofitable in market economy and went bankrupt on mass scale (or in some case the agony was postponed by gov only because of political reasons). Now we're both richer and more ecological - thanks to a market.

Yes, that was a low hanging fruit. Further emission reduction imposed by the EU are harder.


Concerning the sustainability there is one implied assumption in the whole discussion that I find problematic - constant technological level. We wonder how to maintain some population indefinitely with early XXIst century tech. There is enough fissionable material (for breeder reactors - uranium and thorium) for millennia and the whole point is to be able to research in the meantime price effective tech for nuclear fusion (or mining the Moon / asteroids).

 

[VS63]

Maybe you misunderstand sustainability. There aren't levels of sustainability.

I should clarify my sentence, I meant "The maximum human population that can be sustained on the planet." There certainly are different levels of sustainability. I seriously doubt the planet would have sustained a population of 6 Billion people 1,000 years ago. To me, the level of sustainability now is higher than the level back then. Perhaps "level" is the wrong word.

How you live under sustainability is irrelevant.

How I live is certainly relevant to me. I prefer living on the outskirts of suburbia over eating Soylent Green, or living in a Metropolis and never seeing a tree or a green patch of grass, or a wild bird, or a dog.

I would guess we can't use much less space per person than if we were all brains in jars connected to the grid. Point still remains, would you like to live like that? If I were born as a brain in a jar, I would probably be quite content, never knowing anything different, but looking forward from now, I would be horrified.

It's nice to know that technology can extend the population boundaries, as long as it isn't me that is asked to reduce my space to make way for someone else. I think we would all agree on that point. Who among you would volunteer to be placed in a jar forever so another 100 or 1,000 people can move into your living space in jars of their own? What if it were 100,000 people? That were going to die if you didn't give up your space. What if you could trade your body for immortality? This is a decision that someone in our future is going to have to make, if we keep to our current path.

I think a much better solution is to not have those extra people in the first place. Population limiting. Nothing sinister. Financial incentives to families with one or two kids. Free birth control. Worldwide. No sense limiting my family to one child per adult when people from other poorer nations are having 10 or 15. Because then the 5 or 6 survivors end up taking space from my offspring, reducing my incentive to limit my family. To me, it makes no sense to have 6 kids when you can only feed 2 of them. Unless your neighbour has 6 and those will take the resources from yours, unless you also have 6 to compete. That's why population limiting needs to be globally available, and implemented universally. When global population stabilises at a sensible level, whatever that is based on comfortable sustainability, the restrictions can be tuned to suit, allow 3 kids per family to compensate for the people who choose to have none.

I remember reading somewhere that no civilisation with a birth rate less than 2 has ever survived. But surely that is in the face of competition from other more prolific neighbours, and given sufficient resources. If a village can sustain 1,000 people, and those 1,000 people have 1,200 offspring, then 200 people are going to starve to death. If that village has 500 residents and they have 600 offspring, the neighbouring village might just send in another 400 people to fill the void, or they might send in 1,000 troops to kick the original inhabitants out. One dead civilisation.

Wow, sort of got off the topic there. Sorry. Is there a better thread to put those comments in?

 

[Dug]

As a biologist who has spent a 40+ year career in food production technology development - when I think sustainability, I look for natural limits to sustainability. Resources limit most natural populations, usually by food, or nutrient, or both. Before the industrial revolution the human population was limited by agriculture techniques which were limited by the natural phosphorus replenishment cycle in soils. While many think Thomas Malthus was wrong about human population limits - they are wrong. He was quite correct within the technological context of his time. Of course he failed to foresee the advent of fossil fuels and the industrial revolution. He failed to see their extension in the invention of modern NPK fertilizers that artificially replenished the phosphorus in soils and allowed previously unknown agricultural production levels. Today the world's food supply is 85% dependent on NPK and as Asia moves from manure to NPK - it is heading to 100% NPK dependency. NPK component mining and processing is 100% dependent on petroleum. Taken together the global human population's food supply is 95% dependent on the petroleum/phosphate complex. Consider that every living organism on the planet is absolutely phosphorus dependent.

While the world has the same amount of phosphorus that it has always had, humans went through 100% of the global guano phosphate deposits in just a few decades, and now much of the easily and economically accessible rock phosphates are gone in a little over a 100 years. In 2011 according to the USDA Fertilizer import/Export Summary the US imported 52% of its fertilizer components and 15% of it's phosphates came from Morocco (85% of remaining world reserves - maybe). A decade ago the US was the largest NPK exporter.

We have relied on the USGS to establish world fertilizer component reserves which they have estimated at about 300 years of current usage levels. Unfortunately, the USGS has no quantified or qualified basis for these estimates, but instead use host country or fertilizer company asset balance sheets to establish it's reserve estimates. More unfortunately, its those same bankable assets that countries and companies are motivated to exaggerate for obvious financial reasons. Considering the highly variable concentration and contamination levels of potential phosphate reserves - without a high level of analytical surveys - no one really knows how much rock phosphate reserves remain. Some experts are saying less than 30 years - not 300. The remaining petroleum reserves are also expected to run out in about... 30 years.

While the planet has the same amount of phosphorus that his has through out man's history, it doesn't have the same amounts of economically exploitable - highly concentrated phosphorus deposits. Those deposits have been steadily mined and diluted into soils and water sheds to dilution levels economically impossible to recover. That's based on an intensive effort over the past 40 years to discover such methods - which have failed to date. Recycling wastes helps, but if we recycled 100% of our wastes (not near possible) - it has been estimated that it would offset about 3% of the current rock phosphate fertilizer demand. A couple of companies have been looking at the economics of UW mining of phosphates - also yet to be economically successful and even if they were - what happens when the UW phosphates are used diluted and lost to food production.

Before the industrial revolution we demonstrated that the natural phosphorus replenishment cycle in soils could support food production for less than 2 billion people. With today's technology the number could be higher. However, even if we develop economically competitive alternative energy sources to mine fertilizers - they still won't supply the petrochemicals or the petroleum industry economies of scale that we take for granted economically today - and necessary to equate to our current NPK economic/food production paradigms.

My long winded point here is that if there is going to be a human population sustainability limitation - it is very likely to be phosphorus and the related economics associated with turning it into food production. While technology will advance as long as food is cheap and plentiful, it would be a very risky assumption to believe that as human populations continue to grow (albeit and lower rates), while critical non-renewable resources such and petroluem and rock phosphates decline - that technology can necessarily rescue us - or likely that it will do it in time to prevent a population crash - just like any other species that exceeds its critical resources.

 

[divedeep129]

Define "sustainable". And a lot depends on the level/way of life. See this post:

https://www.physicsforums.com/showpost.php?p=2652522&postcount=10

from this thread:

https://www.physicsforums.com/showthread.php?t=391197

Sustainable being not more than you need. It's pretty much a cancer if you acquire wealth beyond your needs. If everyone had only what they need, that's the sustainability that's being talked about @ population levels of 10 billion...

 

[divedeep129]

oh hey, so the reason i showed up... I'm looking for where i heard that discussed... i thought it was on a TED talks... about the Earth sustaining 10 billion... i need to cite it for a paper... anyone know if that was on TED? or where if not?... thanks. :)

 

[jim hardy]

"Dug" , two posts above, gets it.

I recommend reading "Life's Bottleneck " by Isaac Asimov.

 

[johnbbahm]

To put it in simple terms, we eat oil!
Without the benefits of the high energy density hydrocarbon fuels provide,
many people will starve.
The problem is energy storage, We could make enough photovoltaic panels to
achieve the energy currently used, and much more,
but the energy is not where, when, and in the physical form it needs to be into be utilized.
Storing the energy as hydrocarbons, allows a path forward for humanity,
and buys us time to find other alternatives.
http://www.nrl.navy.mil/media/news-releases/2012/fueling-the-fleet-navy-looks-to-the-seas
http://www.pv-magazine.com/news/det...ower-to-gas-facility_100011859/#axzz2uvraLciF
The US Navy and Audi, are developing technology from Fraunhofer University,
to store gathered energy as hydrocarbon fuel.
http://www.fraunhofer.de/en/press/research-news/2010/04/green-electricity-storage-gas.html
Once stored, the fuel can be distributed as needed through the existing infrastructure.

 

[jim hardy]

Isaac Asimov wrote about 1949:

... Asimov "One billion well-fed, creative human beings arc a far happier and worthwhile load for our good planet than six billion starving, half-mad wretches.

Since then genetic modified crops and a booming phosphate industry have enabled feeding us all.

That Navy article is really something, johnbbalm !

 

[.Scott]

Doing away with the middleman entirely, powering humans directly via sunlight with 100% efficiency then the sustainable limit is ~1.22 quadrillion people.

Post the peer reviewed scientific study that shows that food, clean water, suitable housing, waste control, schooling, jobs, medical care and transportation could be physically and economically viable to sustain that many people.
This is getting absolutely ridiculous.

I must say I'm kind of stunned by this moderator response. Finding upper or lower bounds in a discussion like this, even ridiculous ones, assists in directing the community's investigation.
In this case, the point is that it is "ridiculous" to attempt to address the OP without setting some restrictions on "maximum sustainable population". From an engineering point of view "maximum sustainable population" is not a sufficient statement of the requirements. We have a "dream" of what would be a valuable society and what would be a only a bizarre science experiment. One way of furthering this conversation (and driving it to greater productivity) would be to try to explicitly identify what those requirements might be. Without doing that, it would be difficult to determine whether a particular scientific study even addressed an issue pertinent to the OP.