Question of the Week Podcast

Question of the Week episode

Tue, 1st Oct 2013

How many people can Earth support?

earth (c) NASA

We examine whether Earth can sustainably support this many humans.

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In this edition of Question of the Week


  • earth (c) NASA

    How many people can the earth support?

    I frequently wonder where we are heading as a species and see the one big issue we continue to ignore is sustainable human population. I've heard that the Earth can support about 2 to 3 billion people comfortably and sustainably but I don't have a source other than the interne...



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I have long thought that a 5% tax refund for parenting a first child sounds good, and 10% for the second - parents are great!
Of course, you lose the lot if you have a third; responsible parents are even better!

Not gonna win a lot of votes with that one. Responsibility?

My own gut feeling is that we could probably support the current 7 billion sustainably if the resources were distributed responsibly. That isn't going to happen any time soon and the poor old girl shows signs of creaking at the seams. Academically I'd be fascinated to hear learned opinions discuss the question and look forward to the show. Still, a more pressing question, from my point of view, is what happens when we  reach the limit? Maybe we have already but, either way, I see no chance now of implementing any reasonable solution before we do. The alternatives are unpleasant.

Granted, neither tax nor sociology have a real place in a scientific discussion on global "capacity", but a bit of ominous is always good for the mix :). Skyli, Thu, 26th Sep 2013

One of the problems is that the population, as well as the population growth is not distributed evenly around the globe, and some of the highest growth regions are also the least sustainable.

I agree that a major component of the current climate debate SHOULD be the growing population.  Certainly I think the population growth should be halted at the current 7 billion, and a slow decline should be targeted.

There is also the major concern that the world would have major problems if the general economic wealth of all countries would match the USA and Europe.  In which case, 7 billion Americans and Europeans may not be sustainable.  3 billion may still be high.  Perhaps the ultimate target should be closer to 1 billion, or 700 million (10%) (which is still a lot of people).

It is quite possible that the general happiness and contentment of the people would be higher with a lower population density.  And, it would help share the globe with the other ten million or so eukaryote species.

In the USA, we have a similar method of giving "tax deductions" for children and dependents.  However, I also agree that one should give negative deductions for 3 or more children for a family, or more than one child per person.  One could easily justify it as the burden on society grows with the number of children including making new schools as well as road construction and infrastructure improvements.

It is hard to change the rules for preexisting children, but tax incentives could be changed for future children added to families after a certain cutoff date. 

Immigration, of course, is also a major component of population growth.  It is complex, but it could be determined somewhat by sustainable population policies in the country of origin. CliffordK, Thu, 26th Sep 2013

You can calculate the optimum population if you start with a quantifed and reasonably universal measure of standard of living. The simplest is, I think, access to "artificial" energy - gas, electricity, vehicle fuels, etc.

Current world average power consumption is about 1.5 kW per capita. To live at a reasonable European standard of comfort and convenience, we need to increase that to about 4 or 5 kW, quite a bit more in hot or cold countries and less in temperate areas (Mediterranean, California...)

Now you can look at indefinitely sustainable sources of that energy. Biofuel is best because it is easy to store and switch on when you need it. In order not to impact on our food availability or turn the remaining rainforest into farmland, we need to reduce the population to the point where we can grow all the fuel and all the food we need on existing farmland. This works out at somewhere around 1 - 2 billion humans.

Easy to achieve in the UK by simply not paying any benefits or tax deductions for children and offering women 500 every 6 months if they are not pregnant. If you can thus reduce the birthrate to half the replacement rate, i.e. about 1 child per female, the population will decrease smoothly to any number you want. Better yet, the "workiing fraction", i.e. the proportion  of the population who are paying taxes to support the rest, increases smoothly from 50% to about 64%, so everyone is better off in all ways.      alancalverd, Thu, 26th Sep 2013

Of course, not all food stuff can be cultivated with the same amount of calories or gallons of water per acre.

Here is a chart of million calories per acre
On this list, taters seem to top the list with 18 million calories per acre.
Wheat is in the middle at 6.4 million calories per acre.
And beef is at the bottom at 1.1 million.
The page lists wheat as irrigated, but around here it is a non-irrigated crop, and in many places, corn is not irrigated.

So, if we all ate a tater rich and beef poor diet, we could potentially grow more food in less space.

Undoubtedly crop yields will increase over time, but I also don't wish to have a future where our dietary selection is severely restricted by population demands.  And, of course, it is nice to have a bit of a buffer in case of a calamity. CliffordK, Thu, 26th Sep 2013

Eee! Did a shiver of Soylent Green run up anybody elses spine just then?

How many mouths can an average corpse satisfy and how many corpses can we expect per day? A very "unpleasant alternative" from a dystopian sci-fi novel right now. I wonder if it will stay that way. Skyli, Fri, 27th Sep 2013

I like that proposal, but can't see it being implemented. Better to stick to the policy of making anyone who already contributes to society pay even more for the people who don't. Then they're doing their "fair share" as David Cameron and Nick Clegg would say. In other words, the workers do their "fair share" of paying tax, and the non-working - with 15 kids by 5 different fathers - posse do their fair share of spending it... Everyone's a winner... erm...I think... chris, Sun, 29th Sep 2013

Most people eat roughly their own weight of "live" meat (i.e. including skin, bones etc) each year. You can expect only one sixtieth of that weight of human corpse to occur naturally, so "long pig" is a treat rather than a dietary staple.  alancalverd, Sun, 29th Sep 2013

I personally think that the stresses the world is under due to over population will be resolved through technology..

Against popular belief the world is not 'over populated' there is enough food in the world NOW to feed everyone.. the problem is distribution. And believe it or not food is 100% sustainable. It grows from the ground or is bred in a sustainable manner.. (okay i dont eat fish so you piscitareans are the ones depopulating the seas).

The idea that we cannot produce enough food to feed the world is laughable.. with current techonology the deserts can be irrigated, the forest chopped down to make way for fields, even the sea bed reclaimed for more land. The idea theres not enough energy is pretty amusing too.. theres enough solar, wind and geothermal to go around.. plus if we get desperate chuck a few people on bikes wired up to the national grid and bingo!.. Seriously though.. we will find a way.

What IS impeding the sustainability of the world. and limiting peoples imaginations.. is the finance.. it would cost immeasurable amount of money to make the world sustainable in the right ways.. why would it cost money? as we are all greedy and want paying to grow food, or to build things, or to dig a ditch.. WE have attached a value to things we get from the earth and we squabble between ourselves about who is more important and who should have more of this thing or more of that thing.

Greed is what limits sustainability, Greed is what 'over populates' the planet, 'Greed' is what stops us developing new, lifesaving technology, Greed is what stops us being able to cure diseases, Greed is what stops us exploring the stars. Think about it.. why are things 'too expensive'? SimpleEngineer, Tue, 1st Oct 2013

Alas, not so. About 30% of your body weight comes from food produced thanks to the artificial fixation of nitrogen, probably the most energy-hungry single industrial product of all.

And you can't solve the problem of distribution without a lot of primary energy going into processing, preservation and transport.

As I stated earlier, a western lifestyle (which is presumably what everybody considers to be their entitlement) demands at least 5 kW per capita, but there is no sustainable means of producing that for 9,000,000,000 people. 

No. it's physics and chemistry multipled by numbers.

No. In my case it was the sheer joy of having four children. In less developed economies it is considered a necessity to guarantee support in your old age - but nowadays all four are likely to survive and you only need two.

No! I work in medical engineering. I start companies and develop products that save lives in order to fund my lavish lifestyle! I employ greedy doctors and nurses to treat the patients with my inventions! My greed saves lives. 

No. Physics and chemistry set the limits. Astronaut pay is not excessive (it's an ordinary military rank) and neither is that of the guys who build the rockets. In the words of Alan Shepherd "I think about a million moving parts, each made by the lowest bidder".

Because they are not indefinitely obtainable or sustainable.
alancalverd, Tue, 1st Oct 2013

You missed the point.. Greed makes doctors ask for money to save peoples lives, above and beyond what they need to live and bring up a family, making the poorest poorer and (because they are poor) more likely to need to go back to the doctor in the future. (yes not all doctors but are you getting my drift?)

In this world the rich get richer as they have what the poorest need or just plain want. The poorest get poorer as the rich have the finances to decide what they give and dont give to the poorest. they have the money to decide what happens and what doesnt happen.

The conspiracy theory of the "cure for cancer that exists but the pharma companies wont give it as they make more money out of treating the disease than they would for curing it" is all too believable.

Greed for all to live as you say a 'western' lifestyle, rather than live as a sustainable community where everyone puts in and everyone gets what they need and want.

Nitrogen fixing for being 'artificial' is an application of technology and as i stated technology is what will bring the planet to sustainability.. how can energy coming out of plastic holes in our houses be not considered artificial? so how can this energy be sustainable if you consider artifical to be unsustainable? be this from burning a tree or by a fan blowing in the wind.

You state physics and chemistry limit us.. do they? the technology to populate Mars exists and is even being put into practice.. but we are not doing it large scale even though popular belief is we cant all live on this planet.. physics and chemistry have shown how this is possible.. physics and chemistry show what is possible.. it does not show what is impossible.. impossible just says we dont know how to do it yet ;) and if we stop trying.. then the human race is doomed.

Something that is rare or hard to get may be considered expensive to be fair, but what about digging a mile long ditch, this is considered expensive.. but its not hard to get or in limited quantities.. irrigating a desert.. not particularly hard, not particularly expensive, but too expensive to provide desalination plants to ensure water gets there in sufficient quantities.. this would save thousands if not millions of lives over the average life of a plant.. but its not done.. as there is no payback to the greed of the people of the world. 

SimpleEngineer, Tue, 1st Oct 2013

Regrettably, burning trees or subsidising windmills will not provide 45,000,000 MW indefinitely without severely impacting (i.e. annihilating) most of the arable land on the planet. 

Even if I believed this, it just makes more of a problem. What do you want to die from? Infectious disease and starvation are no longer an option, but if you like making babies (and most of us do) you have to make room for them somehow. And don't blame the pharma companies! This week I'm refereeing a pharmaceutical project trying to keep lung cancer patients alive just a few more weeks than at present. The clinical trial is set to run for 18 years and cost zillions before anything reaches the market - greed or humane speculation? The problem could be solved at a stroke by banning the sale of tobacco.  But if we did that, we wouldn't get enough taxes to keep the NHS working for everyone else, and income tax would have to rise to pay for the pensions of people who would otherwise have died early from lung cancer. 

The problem is that most people need at least 3 kW to live in anything you might consider a decent lifestyle, and most aspire to at least 5 kW - what you need to sustain a healthy western diet, keep the lights on (especially in the hospitals!), and keep the trains running.  alancalverd, Tue, 1st Oct 2013

Mars is not the answer to the population problem.

I do believe that a colony on our moon will have some scientific and perhaps even commercial benefits.  The thin atmosphere, and protection from Earth's light and radio pollution would make the moon ideal for astronomy.  Satellite launches from the moon would be cheaper than from Earth.

Colonizing Mars or Venus may be more for curiosity than other reasons. 

However, the cost of colonizing the Moon, Mars, or Venus will preclude them from being a solution to our population problem.  The resources required to send one person to Mars is likely more than that person would use in their entire lifetime.  Perhaps the energy and expense could be made up in a couple of generations, but simple population control is a much better solution.

I don't foresee more than perhaps a few thousand, or perhaps in the low millions of people as the initial colonists for the moon, Venus, Mars, and perhaps Jupiter's moons.  And that won't make much of a dent in the 7 billion population on Earth.  And the cost of transporting a million people to Mars will be truly astronomical. 

In past centuries there were huge ditching and irrigation projects.  There is no reason it couldn't be done again.

I did some experiments this summer with irrigation from a spring.  It became obvious that I can be happy with < 20 gallons a day for personal use.  But irrigating took far more resources, and I quickly used up all the water I could produce.  Desalination plants are EXPENSIVE both in capital cost, as well as operating cost.  It just doesn't make sense to use desalinated water to irrigate millions of square miles of desert.  If energy is the issue, then where are we getting the energy to run the desalination plants?

Farming the desert would also require reducing the porosity of the soil.  Shipping in clay?  Certainly one could select dryland crops such as dryland wheat which grows with minimal rainfal so water supplements could be minimized.  It is possible that if one could increase the desert vegetation, it would also increase the humidity and rainfall in the area making maintaining the desert crops easier.  Would it lead to a negative effect on Earth's Albedo and more heat?

There are proposals to desalinate deep brackish water.  The vertical pumping would be expensive, but the horizontal pumping would be less, and of course the desalination. 

Anyway, the obvious solution is to simply reduce the population growth to slightly below replacement population.  1 child per person, 2 per couple, and design the socioeconomic environment to enforce it.  As not all couples would have their 2 children, if the maximum was strictly enforced, then the population would slowly decline to a reasonable level. 

I have no doubt that if the governments decided the population was decreasing too quickly, a government campaign suggesting that having more children was one's patriotic duty, and a declining population could be reversed. CliffordK, Tue, 1st Oct 2013

By what means? Sterilisation, infanticide, or simply killing the mothers of 3 to balance the books? None of these looks like being very popular. alancalverd, Wed, 2nd Oct 2013

Good thoughts, and by no means do I say my examples are the answer to anything.. I dont have the mental capacity to point to the specific solution (otherwise I would be a millionaire..).. I just point out options that exist..

The world per say could support more and more people as, quite correctly, we all like making babies.. however the impact on the world will be significant..

What my thoughts are that, the answer to how many people the world can support is relative to how much impact and resource usage we are willing to tolerate.

Power is not an issue.. there are a multitude of solutions with current technology to provide it.. be it nuclear or renewable. the issue is how much impact can we tolerate from generating this power.

Food is not an issue.. as I said there is enough food in the world now to feed a lot more, however the issue is how much impact can we tolerate from the production of the food. (thinking soylent green, is it so bad if there is no alternative?)

Land is not an issue.. We can colonize planets, build space stations, burrow into the earth and build into the sky, just how much resources and impact are we willing to tolerate from this..

Be it greed for resources or greed for less impact on your favorite part of the world, greed and selfishness is what limits us to not doing the right things for our specific priorities..

It seems in one breath we mourn the losses of children due to starvation, and in the other we fervently campaign against the development of technology to solve the issue. Many complain the cost of energy, yet any plans to reduce it meet with rejection due to 'spoiling' the landscape. SimpleEngineer, Wed, 2nd Oct 2013

Although I agree with some of your points, SimpleEngineer, I have to take issue with this one. The laws of thermodynamics set some pretty harsh boundaries on what is possible. They show many dreams to be impossible.

I myself do not know how many people the world could support. Energy is probably not the limiting factor. The sun supplies us with almost 10,000 TW of energy, if we could only harness it! The main issues will all be recycling and distribution of matter. Conservation of matter assures us a fixed amount of each element (more or less). Really it is entropy we have to worry about... Resources aren't lost, they are merely diluted or contaminated.

I'm sure there will be some mind-boggling technology to come, but as long as we live on one planet, there will always be a trade-off between how many people there are and how well off they can be. chiralSPO, Mon, 7th Oct 2013

The policy seems to be to strip as many resources now, and leave the next generation to fend for themselves.

No doubt, over the next few centuries, we'll use up much of the easy to access petroleum deposits, and other minerals too.  And, many predict that we'll also substantially change our climate in ways that is hard to predict now.

Hopefully we'll have time to prepare before the next major crisis comes.  However, if all fossil fuel usage ceased tomorrow, the world would be plunged into chaos. CliffordK, Mon, 7th Oct 2013

We keep burning our boats and relying on future technological advance to get us out of a tight fix, but there is no guarantee that it will. Nuclear fusion may save the day if we're lucky - we'll then be able to grow vast amounts of food indoors. We may also be able to grow artificial meat in vats using as little energy as growing vegetables and fruit, but we're not there yet. Many proposed solutions will not work - irrigating deserts tends to bring salt to the surface and that kills the crops, though genetic engeneering may produce new varieties of crops that can tolerate this. Creating useful amounts of land from the seabed would be a nightmare involving enormous amounts of concrete, and it would be at threat all the time from sea-level rise. Cutting down forests to grow crops is also not wise - we've done too much of that already. However, there is enough food for us all at the moment - all we need to do is stop wealthy people destroying their health by stuffing themselves with unhealthy food like beef so that it can be distributed more fairly, though it would be even better if people switched over to meats like chicken which can be produced for 1/6 of the energy.

I think it would be worth acting to try to reduce population growth rather more actively than just leaving it to settle down by itself. The current trend is for it to stop at 9 to 11 billion (depending on whose calculations/assertions you believe), but there's no guarantee that people in the developed world won't suddenly start having bigger families again once they're liberated from pointless toil by intelligent machines in the next decade (or possibly sooner). What we need is a system that discourages them from going beyond three children - many will have none or stop at one, so allowing up to three without penalty may be fine for maintaining a stable population. If there is to be a penalty for breeding to excess, it should be the parents that pay it, and in such a way that their children are not made poorer (because they are not to blame). Perhaps they could progressively lose the right to free healthcare, and/or have their future income reduced in some way (which will be easy to arrange, because most income will soon have to come in the form of a basic income paid to all - there will be precious little work available, and a lot of work that is currently done will be banned outright as it is of negative value to society). David Cooper, Mon, 7th Oct 2013

In 1955 (ZETA), free fusion power was 5 years away. By 1985 (JET), it was not free, and 10 years away. Today (ITER) it is possibly affordable and 20 years away. Somehow I think the maths is diverging from "lucky". alancalverd, Thu, 10th Oct 2013

One of the problem with "Fusion" power is that while everyone wishes that it could use simple hydrogen 1H of which we have an abundance in the oceans.  However, I believe all current tests use much rarer isotopes such as 2H, 3H, 3He, & Lithium. 

Even if it does work, finding the right isotopes to fuse may be quite difficult. CliffordK, Fri, 11th Oct 2013

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