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9 billion or 11 billion? The research behind new population projections

9 billion or 11 billion? The research behind new population projections thumbnail

Demographers from the United Nations Population Division and several universities published a paper in Science last week that argues the world population is unlikely to stop growing this century. They calculate that there is an 80% probability that world population, now 7.2 billion, will increase to between 9.6 and 12.3 billion in 2100, with the median at 10.9 billion.

Different projections for future fertility rates in countries such as China and Nigeria are one major reason for the difference in projections between IIASA and the UN. Photo Credit: Evgeni Zotov via Flickr

Next month, we will announce the results of our newest assessment at the launch of a new book entitled: “World Population and Global Human Capital in the 21st Century” (Lutz, Butz and KC, Oxford University Press 2014). Contrary to the UN projections, the IIASA medium (most likely) scenario indicates that world population will increase to 9.2 billion by 2050, peak at 9.4 billion around 2070 and start a slow decline to 9.0 billion by the end of the century.

The new UN paper uses a probabilistic approach to global population projections providing quantitative uncertainty ranges.  Such an approach was first developed at IIASA. In a 1997 Nature article, IIASA used probabilistic methods to indicate that the doubling of world population was unlikely. And in a 2001 Nature article, IIASA demographers projected that there was an 85% chance that the world’s population would stop growing this century.

The UN and IIASA population projections use very different approaches for defining the assumptions underlying future fertility and mortality trajectories. The new IIASA projections are based on the substantive input of more than 550 experts worldwide who were invited to evaluate in a peer review manner a set of alternative scientific arguments bearing directly on the future demographic trajectories. This was done through an online survey as well as a series of meetings on five continents. The resulting state of our knowledge and substantive reasoning is documented in over 500 pages in the OUP book.

Alternatively, the UN population projections have recently moved away from their earlier expert-based assumptions to the other extreme: Their new probabilistic population projections reflect expert judgment only in the design of a specific statistical model which then is applied to national time series of 60 years (1950-2010) to extrapolate 90 years (2010-2100) into the future. There is no room for country-specific expert knowledge or for substantive considerations.

There are two other factors explaining the difference: One is that IIASA now systematically adds a differentiation by level of education in addition to the conventional age and sex to its population projections, as education significantly influences fertility rates (Policy Brief: Rethinking population policies). Once this important source of population heterogeneity is explicitly taken into account the future looks different. In the example of Nigeria, the UN projects an increase from 160 million in 2010 to 914 million in 2100 while IIASA projects only 576 million. The IIASA projections do consider the fact that recently Nigeria has made significant progress in girls education, such that today half of the women aged 20-24 already have secondary education, while among women aged 40-44 the percentage is only 25 percent. And since more educated women consistently have lower fertility, future fertility is likely to decline as the more educated girls enter reproductive age. Disregarding this important structural change leads to higher projections of future fertility.

Another difference lies in the reading of the current fertility levels in Africa as well as in China. The UN assumes that fertility in Nigeria has been stagnant at 6 children per woman for the past decade and for this reason their purely statistical model results in very slow future decline. However, the most recent Demographic and Health Survey (DHS 2013) for Nigeria shows that fertility has already declined to 5.5—a level the UN assumes would only be reached by 2020-25.

The same is true for other African countries such as Mali where the DHS shows fertility has already fallen to 6.1 a value that according to the UN projections would only be reached in 2025-30. For China, currently still the world’s biggest country, the UN assumes that fertility stands at 1.66 and will not decline further but rather increase in the future. Based on expert reasoning the IIASA projections assume that fertility in 2010 was around 1.5 and will decline to 1.4 in the coming decades, following the patterns of Japan, South Korea, Taiwan and Hong Kong which are currently in the 1.0 – 1.4 range.



24 Comments on "9 billion or 11 billion? The research behind new population projections"

  1. Ron Patterson on Tue, 23rd Sep 2014 8:56 am 

    Of course all these population projections are based on the assumption that “business as usual” will continue into the next century. That is there will be no peak oil or at least no consequences from peak oil. Coal and natural gas consumption will continue to grow as the population expands and demands more energy.

    And of course there can be no disruption of the food supply. More and more food must be produced to feed more and more people.

    Of course none of this “continue growth in energy and food consumption” can possibly happen. What is far more likely is that things will soon start to fall apart. Chaos and resource wars will be the order of the day. The population, in 2100, will be a lot closer to 2 billion than 10 billion.

  2. noobtube on Tue, 23rd Sep 2014 9:40 am 

    These “too many people” articles never mention Europe or the United States, but ALWAYS mention Africa and China.

    If you didn’t know any better, you would think they didn’t like Africans or Chinese.

    Naw. Americans and Europeans have no history of violence or hatred or genocide towards Africans or Chinese.

  3. Makati1 on Tue, 23rd Sep 2014 10:07 am 

    noobtube, you are correct. Funny how it is always the other guy who is guilty. Not the over consumers in the West.

    What’s with all of these population guesses? No one knows what next year will bring let alone 2100. There may not be any humans in 2100.

  4. Ron Patterson on Tue, 23rd Sep 2014 10:39 am 

    Noob and Makati, there are no villains here, only victims. Strange how some people always seem to find good guys and bad guys in every story.

    Fertility rates are what they are, it is no one’s fault. People are only obeying their nature.

    When the population crashes it will crash in every nation on earth and it will be no one’s fault. It will be the result of an abundance of energy which brought on an abundance of food. Animal populations always expand to consume the available food supply.

    To divide the world into good guys and bad guys betrays a gross misunderstanding of the true nature of the human condition and how nature actually works.

  5. Northwest Resident on Tue, 23rd Sep 2014 11:21 am 

    Ron P: You are so correct. Posters like noob and Mak focus only on America and Americans, conveniently sidestepping the fact that there is nothing that America and Americans do that other nations past, present and future haven’t done or will not do in equal measure, and in many cases even worse. I guess it is also within the realm of “normal” human behavior to focus one’s animosity and frustration on the biggest target — in this case, America.

  6. Davy on Tue, 23rd Sep 2014 12:33 pm 

    When you do analysis if you feed poop in you get poop out. Poop stinks and you can’t get rid of the stink. These analysis use the BAU criteria more or less or they would not go up exponentially. They stink of BAU. They may have sophisticated statistical formulas organized by PHD’s but it is the starting point basic assumptions that matters. I would say the probability of hitting maybe 7.5BIL is a good bet in a few years. Then I think we should start discussing decline. Academia somewhere need to start discussing decline in a situation of lower economic activity with food and liquid fuel shortages. I have seen zero articles on this scenario. IMA a scenario that is legitimate. Where will population descend to in a contraction or collapse scenario? Is it an unwritten rule among academia they can’t discuss descent possibilities? Is it because it is so painful and ugly? I just don’t believe they don’t think about these dark doomer thoughts with the overwhelming data that is contrary to what is required for the continuation of BAU. Further, we need a picture of the degree and duration of a die off? Where is the worst likely to happen? What mitigation efforts can be taken now to head off the worst scenarios?

  7. Davy on Tue, 23rd Sep 2014 1:12 pm 

    Just heard on NPR that mathematical projections for new Ebola case will top 1.4MIL in west Africa by January 2015. We are at a critical point where this number can be drastically reduced if steps are taken NOW! This will be a test case in miniature for pandemic induced collapse. These are not terribly modern countries but they are still children of BAU. Much command and control depends on let’s say 10% of the population using modern resources. If that 10% is greatly reduced or flees the area I predict multiple failed west African states.

  8. JuanP on Tue, 23rd Sep 2014 1:24 pm 

    Davy, I believe TPTB have decided to let Ebola loose in Africa. The reaction to this epidemic has been so totally defficient since the very beginning that I can only explain it to myself as intentional. It could be simply another symptom of collapse, but compared to how fast and efficiently we reacted to ebola epidemics in the past, this seems like a decision was made to not fight it this time around. That is a disgusting idea, but TPTB could be thinning the herd on purpose, and Africa makes a good target.

  9. JuanP on Tue, 23rd Sep 2014 1:26 pm 

    I wonder whether this is pandemic induced collapse or collapse induced pandemic. Could be both, too.

  10. Davy on Tue, 23rd Sep 2014 1:48 pm 

    Well Juan, you can bet your ass that many cases will spread around the world unless free movement of people is restricted. Just another nail in BAU coffin. In today’s world this will put severe pressure on the financial markets, vital distribution networks, and critical global production. This could be the physical contagion that initiates the non-physical contagion of the end of BAU.

  11. JuanP on Tue, 23rd Sep 2014 2:05 pm 

    Davy, I don’t think it’s a smart idea, but I don’t think much of what we are doing as a species is smart, we are not smart enough, IMO. If this was allowed to happen on purpose, as I believe possible at this time, then it was a very foolish thing to do. I hope I am wrong and we make up for lost time and stop it, but I fear the will to do this may be lacking amongst TPTB.
    If it keeps growing at some point this epidemic would cause collapse in trade, travel, and tourism, and bring the whole house of cards down. With today’s JIT supply delivery we’d be screwed in no time. But the survivors may become resistant to it, if lucky, I don’t know how that works for Ebola.
    Maybe I am becoming paranoid? I hope not!

  12. Northwest Resident on Tue, 23rd Sep 2014 2:20 pm 

    JuanP — Just because you’re becoming paranoid doesn’t mean they aren’t out to get you.

    Whether it is this round of ebola or something else, we are most certainly heading for collapse in trade, travel, and tourism in any case.

    As shortonoil pointed out in one of his posts a while back, the ebola epidemic in Africa could also play a key role in lowering oil output from African nations, which would certainly be a kicker toward the final demise of BAU.

    Giant flocks of black swans are circling overhead — or are those buzzards — any one of which could be the catalyst that sets off the chain reaction. Until the collapse of BAU takes off in earnest, we can realistically expect that flock of circling black swans to grow larger and louder every day, probably becoming a deafening roar of squawks and mournful honks until that one day where they swoop down upon us en masse. Don’t look for a return to “normal” or anything close, expect more war and more famine and more disease, and you won’t be disappointed.

  13. Apneaman on Tue, 23rd Sep 2014 4:05 pm 

    “People are only obeying their nature.”
    Nobody’s responsible for anything. There is no blame and no credit.

  14. Davy on Tue, 23rd Sep 2014 5:47 pm 

    Apnea, in a certain sense I agree with you. This whole shit storm just self-organized like osmosis. None of us ask to be born. In another sense in those areas of human ethics I do think we have to hold some of these TPTB responsible. Of course little chance of physical justice but there is a relm of higher level justice found close to our heart and spirit. Some of these powerful folks are clearly allowing suffering for personal gain. That is evil in my book. Nature can give a rats ass about human duality of good and evil but I do find karma an interesting spiritual feeling. Who knows what’s in store for the evil doers maybe nature has a karma.

  15. Hiruit Nguyse on Tue, 23rd Sep 2014 6:16 pm 

    This article loaded with Al Bartletts picture in the little box to the left.

    Half a million sounds more reasonable than either figure.

  16. Hiruit Nguyse on Tue, 23rd Sep 2014 6:18 pm 

    As William S. Burroughs once said…A paranoid is someone who knows a little of what’s going on

  17. Plantagenet on Tue, 23rd Sep 2014 7:32 pm 

    I’m not worried about the Ebola epidemic getting out of control — Obama just sent 3000 US soldiers to Liberia to stop the outbreak. I’m confident that Obama and the CDC and the UN WHO will take the appropriate steps to bring this epidemic under control.

  18. noobtube on Tue, 23rd Sep 2014 7:48 pm 

    That’s what the Ebola scam was all about?

    The United States wanted to invade another country and needed a cover story.

    I wonder what the Americans are trying to steal, this time.

    Americans are low-grade garbage.

  19. Davy on Tue, 23rd Sep 2014 7:54 pm 

    Planter, not sure if you are being sarcastic or serious since you mentioned the “O” word. Anyway, I have been watching every documentary I can on Ebola. The problem is the people have a particular culture and they lack education. The cities are packed with people. If you could have all the wrong ingredients that is what West Africa has. 3000 US troops are a butt pimple. The real need is from the people themselves accepting the seriousness of this situation and accepting the steps to control it. The wider world must supply necessary manpower and equipment in addition. The documentaries show the horrible effects on these family oriented people. It is truly hart wrenching to see a 7 year old taken off to one of these camps probably to die alone. I only watch it because I know this is the type of problem that can go global. The disease is terrible but the systematic risk and destruction of the social fabric is just as bad. Many people are dying form treatable conditions because the doctors are dead or gone. We really need to take this seriously.

  20. Stercus Feri on Tue, 23rd Sep 2014 8:02 pm 

    Pop in 2100 c.e. will be closer to 1 billion

  21. Makati1 on Tue, 23rd Sep 2014 8:03 pm 

    There are ALWAYS bad guys in any scenario. That is the human way. Using the past to make the USSA seem average is BS. The past has not had a power that wants to take over the whole world and has(had) the power to succeed. Nor has any country/empire destroyed as many countries and killed as many people in obtaining it’s goal. NONE.

    The USSA deserves what is coming, as does the rest of the USSA wannabees. Citizens of a Democracy cannot blame the actions of their ‘elected’ government for the problems and not themselves. YOU and I are guilty, because we allow(ed) it to happen.

  22. SilentRunning on Tue, 23rd Sep 2014 9:27 pm 

    We need a crash program to get as contraceptives to everyone, educate people that they need to have fewer kids, reduce consumption, etc.

    But, if the usual business and religious idiots keep telling everybody to BREED, BREED, BREED!!! – then we need to let Ebola and other diseases take their natural course.

    So people –

    1) either listen to science – and use birth control and have modern medicine


    2) Have all the kids you want , and forget about vaccinations, and Ebola is a myth.

  23. Davy on Wed, 24th Sep 2014 7:34 am 

    Yea, silent, I think diminishing returns are here in population control. We are behind the eight ball now and will never get ahead until nature smacks us upside the head. When we enter the dark period coming there will be population measures both from the top and bottom. When food shortages and forced migration is the norm procreation will change. In other more stable areas the top will address the population issues. I imagine in desperation the big religions that promote population growth will back off at least in regards to contraceptives. It is amazing what crisis does. I believe it is one of the few effective tools with humans. Look at your personal life and look back to crisis periods. Many of us have made changes during these periods. During happy days and when all is well how many of us change or even prepare for hard times? few!

  24. Kenz300 on Wed, 24th Sep 2014 1:08 pm 

    Too many people….. finite resources………. not going to end well……

    Endless population growth is not sustainable.

    Around the world we can find a food crisis, a water crisis, a declining fish stocks crisis, a Climate Change crisis, an unemployment crisis and an OVER POPULATION crisis.

    Overpopulation facts – the problem no one will discuss: Alexandra Paul at TEDxTopanga – YouTube


    Wrap it up……. get it snipped……

    Birth Control Permanent Methods: Learn About Effectiveness

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