Human population size: speeding cars can’t stop quickly

28 10 2014

Stop breeding cartoon-Steve Bell 1994Here at, I write about pretty much anything that has anything remotely to do with biodiversity’s prospects. Whether it is something to do with ancient processes, community dynamics or the wider effects of human endeavour, anything is fair game. It’s a little strange then that despite cutting my teeth in population biology, I have never before tackled human demography. Well as of today, I have.

The press embargo has just lifted on our (Barry Brook and my) new paper in PNAS where we examine various future scenarios of the human population trajectory over the coming century. Why is this important? Simple – I’ve argued before that we could essentially stop all conservation research tomorrow and still know enough to deal with most biodiversity problems. If we could only get a handle on the socio-economic components of the threats, then we might be able to make some real progress. In other words, we need to find out how to manage humans much more than we need to know about the particulars of subtle and complex ecological processes to do the most benefit for biodiversity. Ecologists tend to navel-gaze in this arena far too much.

So I called my own bluff and turned my attention to humans. Our question was simple – how quickly could the human population be reduced to a more ‘sustainable’ size (i.e., something substantially smaller than now)? The main reason we posed that simple, yet deceptively loaded question was that both of us have at various times been faced with the question by someone in the audience that we were “ignoring the elephant in the room” of human over-population.

Human population size has always been a sticky issue, for it evokes all sorts of emotions and sentiments that have nothing to do with science. Everything from religious freedoms to human rights are brought to the fore, with horrible examples of where some societies have pulled demographic levers to and beyond their ethical limits (think eugenics, enforced one-child policies and genocide). I want to make it clear that it was not our intention to advocate any of these extremes – we merely wanted to know what even the unfathomable could do to the human population trajectory over the coming century.

As any conservation biologist will attest, if we can’t get the human population to more sustainable size, most of our efforts to conserve biodiversity will be futile. But population size is only one part of the famous ‘I=PAT‘ equation of Holdren & Ehrlich (1974): Impact = Population size × Affluence (per capita consumption) × Technological and sociological choices. Arguing whether P or A is more important is like arguing that length is more important than width for determining the area of a rectangle – the two are inseparable.

So back to that central question – how fast can we get the P side of the equation down? As it turns out, not very fast at all. We investigated 10 different scenarios where the fertility and/or survival probabilities changed gradually or in a stepped function, mimicking everything from gradual fertility reductions to all-out catastrophic mortality scenarios (emulating world wars or global pandemics). The take-home message is that even extreme scenarios will not deliver a major reduction in human population size this century. It is a process that will take centuries to occur, rather than decades. Unfortunately, we only have decades to act.

We also examined where in the world biodiversity will most likely be affected by human population increase. We did this very simply be examining regional population trajectories within the existing 35 Biodiversity Hotspots. Few prizes for guessing where the most damage to endemic species will likely occur – most of Africa and the subcontinent. Will my daughter ever get to see rhinos and African elephants in the wild? Unless I take her there soon, it’s increasingly unlikely.

As such, our immediate sustainability gains will be better served by tackling the A & T parts of the equation, but this does not mean we should ignore the P. On the contrary, we should implement global-scale family planning to reduce P as much as possible. Hundreds of millions fewer people stressing our planet’s resources and biodiversity could result if we do, and if we don’t start somewhere, slowing the speeding car in the future will be even more difficult than it already is. Had we seriously addressed the issue before it became so intractable (say, 50 years ago), we might not now find ourselves in such a mess. What can I say? Humanity isn’t notorious for forward thinking.

CJA Bradshaw



35 responses

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23 09 2015

Interesting article. It reminds me of a class in college where we were given a graph of human population growth over ~200-500 years (I don’t recall). What I recall is seeing the near-exponential curve of humans. This was right after a history class where we’d discussed how many millions had died in the 20th century from WW1, WW2, 1918 influenza pandemic, cultural upheaval (including Stalin and Mao). I was shocked to see that removing over 50 million people didn’t even show as a blip on the growth curve.


23 09 2015
Earl Babbie

If you look at age/sex pyramids for Germany and others right after WWI, for example, you’ll see a decided notch where young men should have been. And yet, as you observed, it doesn’t amount to even a blip in the overall pattern of overbreeding and population growth.


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31 12 2014
Earl Babbie

While the egalitarian approach to PAT is politically correct, it isn’t really accurate. P is a more fundamental problem than A & T. If the world’s human population were a couple of dozen hardy souls, they could all drive Hummers and live unbelievably affluent lives–and the rest of the planet would do just fine. On the other hand, there is no level of A & T low enough to support, say, 100 billion of us sustainably. Unfortunately there is more support for improving A & T (which is vital) than controlling the bloated P in the corner of the room. Indeed, attempts to responsibly control overbreeding is attacked from both the Right and the Left, though for different reasons. It is time for us to tell the truth about the centrality of overpopulation in dealing with all the environmental issues that rightly concern us.


29 11 2014
Josie McLean

I/we have focussed research on how to shift individual and organisational thinking and repriortise values to shape a sustainable world.
This fits in the ‘social innovation’ part of the T.
Appreciate your bringing it to people’s attention. Summarised here


23 11 2014
Karin Eckhard

Thank you for touching on this subject. I become very annoyed when I read articles that state that the decline in fertility (Some European countries) is a bad thing and it is going to be coming economic disaster. I’m not an economist but I have read the arguements and still I do not agree. How can less people be a bad thing, less crowded cities, smaller classrooms sizes, more jobs,etc.


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4 11 2014
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[…] Les Bradshaws eige blogginnlegg om studien […]


31 10 2014
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[…] Les Bradshaws eige blogginnlegg om studien […]


29 10 2014

Most of the commentary on this subject and others such as climate change revolves around economic and social change proposals enacted at the highest political levels. As a retired civil/structural engineer and IT practitioner, I am more interested in what can be done at a lower, more practical level which, if it catches on, might produce significant reductions in these pressures.
My laboratory is Australia, which encompasses a huge variety of physical detail in its landmass, significant variations in weather conditions, together with all sorts of vegetable and animal life. If we can develop modestly scaled ideas for remote human settlement, food production, biodiversity management and so forth, we may be able to increase the sustainable population of Australia from the currently proposed maximum of 5 persons/ (up from todays 3 persons/ to perhaps 10 persons/sq. km. More to the point, the technologies and organisational ideas could be exported to those parts of other countries whose topography mimics those parts of Australia where the research proved successful. Feel free to comment on my website at Criticism and denigration is quite acceptable so long as it is delivered in a polite and argumentative format!


1 11 2014

But why would you want to increase human carrying capacity? It will, no matter how it is done, come at increased pressure on biodiversity. Why not stop breeding instead? Its such a basic and simple solution, and the only argument against it that floats is that the guys at the top make less money…


4 11 2014

Hi, Walter, I do agree with your sentiments here (well, sort of) but my focus is on setting up processes to use the world’s landmasses as efficiently and as carefully as we can, to house the burgeoning world population and offer the best living opportunities that we can. How to control the increases in population is another, but related matter. In the poorest communities in the world, children are a form of insurance. More people to work and look after the old after they can no longer work and (sadly) to provide more muscle power to defend their communities and nations. In India, for example, birth rates have fallen rapidly among those lucky enough to experience higher incomes and better living conditions. Arbitrary rules to limit birth rates in China failed because boys were considered more valuable than girls, to the point that many female children were sold for overseas surrogacy or even murdered.
On another matter, could the site manager please remove my oldest posting. My connection dropped out and when I resumed, the posting (probably undergoing moderation) was not there, so I reposted.


29 10 2014

Nearly all of the proposals on what to do about climate change, sustainable population and so forth are focussed on the large-scale political and economic forces in the world with ideas on how to modify or control them. As a retired civil/structural engineer and IT practitioner, I am more interested in relatively small-scale things to do which might expand to have greater impact in times to come. My laboratory is Australia, which currently has a population density of 3 per sq. km with estimates of a future maximum set at about 5 per sq. km. I believe that we could accommodate perhaps 10 per sq. km. If the technologies I am in the course of proposing (see could be applied to (say) arid parts of Asia, Africa and South America. We could possibly give a reasonable (though modest) lifestyle to many people who are currently very poor and breed children as a source of income and old age insurance.


29 10 2014
Colin Megson

I suspect it’s beyond correlation and is indeed causation:

Energy use (availability) is inversely proportional to family size!

So the solution is then very simple: family size will plummet, if every person on the planet had energy availability at the level of the developed world.

We’ll need to make available 3 to 4 times more energy than we use now, but again, the solution is very simple: the rapid word-wide deployment of Gen IV breeder reactors (be they IFRs or MSRs), which can provide all of the energy humanity can ever use, for all of eternity.

After the likely 9½ billion peak in 2050, the population would decrease rapidly – for all the right reasons.

Otherwise, we just wait for the sick race between water wars, energy-security wars and/or climate change to do it for us.


28 10 2014
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[…] commented on Corey Bradshaw’s blog,  Conservation Bytes, after his recent […]


28 10 2014
Geoff Beacon (@GeoffBeacon)


With regard to your PNAS paper “Human population reduction is not a quick fix for environmental problems”.


I heard you on the BBC World Service early this morning (28th October) and noted there was no mention of reducing consumption or of the uneven distribution of relative consumption between the rich and the poor. Having paid for access to your PNAS paper I find…

“A corollary of this finding is that society’s efforts toward sustainability would be directed more productively toward adapting to the large and increasing human population by rapidly reducing our footprint as much as possible through technological and social innovation”,

So you have an emphasis on “rapidly reducing our footprint”.


Before the potato famine in Ireland, human population reached 1000 people per square kilometer, on simple arithmetic the land surface of the Earth now supports about 50 per square kilometer. It seems that the Earth could support many more people (if absolutely necessary) were their lifestyles less resource intensive and polluting.

The carrying capacity of the Earth clearly depends on the lifestyles of the inhabitants.

Is “limited change in per capita consumption” an important assumption? You say “technological improvements .. allow for decoupling of impacts … and so can vastly reduce consumption rates of primary resources.” Does this imply we can get by without radically changing the nature of “per capita consumption”?

Do you assume we can keep the current levels of the most polluting activities like flying, driving cars and eating beef?


The affluent of the world are increasing their footprints at a rapid rate both by becoming more numerous and consuming more per capita. In “Growth and Inequality: Understanding Recent Trends and Political Choices”, Thomas Pogge points to the increasing inequality in developing countries – in other countries too. I note that this goes alongside their increasing footprints.

Technological fixes, like feeding the world with GM foods mentioned on the BBC, may help but this is a solution from the globalisation model – global companies solving problems with technologies to make profits which feed back to the affluent thus fuelling high-footprint affluent life-styles.


I have been recently struck by Richard Wilkinson’s TED TALK , “How economic inequality harms societies.” Among developed countries unequal societies are the least happy. The bike-riding inhabitants of Copenhagen are happier than the inhabitants of the unequal car cities of the USA. They eat less beef too!

You might like to look at, where I discuss some of these issues.


28 10 2014
Ruth Sponsler

I am 50 years old.

My father and mother were early, University of Chicago-educated believers in ZPG.

Largely for that reason, I was an only kid.

Fifty years ago, the reduced fertility message haardly disseminated beyond academia.

Deliberate efforts have been made during the 1985-present time period to discourage education of women and birth control.


28 10 2014
Michael Lardelli

The PNAS paper is interesting and especially useful in order to appreaciate the effects of demographic momentum. However, some might argue (including myself) that the most catastrophic scenario depicted could be an under-representation of a possible reality. In the paper Scenario 9 shows about two thirds of the population dying off in the early 2040s due e.g. to starvation but that still puts us on a course to add ~2 billion extra people before the event. However, one only needs to consider the current death rate from Ebola (50%) and the fact that conventional oil production is already in decline with “all liquids” almost certainly in decline by 2020 (which will place huge pressure on food production) to see that setbacks in population growth may occur earlier (and thus that billion or two extra by 2040 may be a little optimistic). Another picture of a likely reality may be seen in the recent review of the Limits to Growth scenarios by Graham Turner at Uni of Melbourne:

This paper was the basis for an article in The Guardian:

Still, even from this view, demographic momentum shows a powerful effect – they still see some billions of people in existence by 2100 – although it would be a much more brutal existence.


28 10 2014

Thanks Corey, interesting stuff! I have posted your paper on my favourite website:


28 10 2014

I do not call the the I=PAT equation in question but would argue that there is a weakness in the analysis (at least as it is summarised here). Which elements of the equation need attention depends very much on the particular population you are looking at. In much of the developed world population growth is largely fuelled by immigration so the benefits of reduced fertility and negative population growth is lost. In the underdeveloped world population growth tends to continue unchecked leading to political instability (the Arab Spring is one instance).
The weakness in the argument is that it fails to acknowledge that a key part of the problem is that political leaders are still very much locked into an economic growth paradigm – until we get off the growth merry go round no serious efforts will be directed to developing a stable state economy with population reduction as its central organizing feature.


28 10 2014
“Elephant in the room” no more, models show even one-child policy is no quick fix - Environment Institute Blog

[…] “The corollary of these findings is that society’s efforts towards sustainability would be directed more productively towards reducing our impact as much as possible through technological and social innovation,” says Professor Bradshaw. He discusses the publication and the sticky issue of population growth over at his blog, […]


28 10 2014
Barry Brook

Reblogged this on Brave New Climate and commented:
I’m reblogging a post from @conservbytes (Corey Bradshaw) about a new paper we have out today on human population growth and environmental problems. There’s a lot of media coverage about it too!

BNC readers will be amused to know that it was based on a BNC blog I did on population and climate a few years ago.

If you want a PDF copy of the full paper, let me know.


30 10 2014
Chris Stephens

I would appreciate a pdf copy of your recent paper. I’m giving a paper on local area population projections in York(England) next month and would like to refer to this work. I guess you’ve got a lot of requests.Thanks very much for your help.


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