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Population vs. consumption: it’s not either/or, and for both, less is more

Climate change is often thought of as a technological problem: humans are using a means of energy production that pollutes.  A parallel school of thought emphasizes the lifestyle-cum-ethical issue of high consumption (with its global inequalities).  What if it is actually a demographic problem, one of overpopulation?

Ever since the early 1970s – the era of “The Limits to Growth” (Club of Rome, 1972) and “The Population Bomb” (Paul R. Ehrlich and Anne Howland Ehrlich, 1968) – it has been clear to those who grasp the concept of exponential growth that human population would eventually, indeed soon, overwhelm Earth’s resources, carrying capacity and balances.  The MIT modellers of “The Limits to Growth” even put a timeframe on it ­– the late 2020s.  It was not clear which key resource would run short first, be it land, water, minerals, fossil energy, forest resources, or fertilizer.  (In fact food production has proven much more elastic than feared; but the Green Revolution bought time only at the cost of greater use of fossil fuels and chemicals in agriculture, which in turn killed a lot of the carbon-retaining microbiome in agricultural soils, thus worsening agriculture’s carbon footprint.)  What was clear was that at least one of these would be outstripped by population growth; and perhaps it was clear to some that the mounting by-products of ever-increasing use of them, even apart from shortages, would somehow poison the biosphere and people.

That is where we are now.  Continuous and accelerating burning of carbon fuels, combined with attenuation of some natural buffers against atmospheric carbon and heating – forests and fallow land, cool and carbon-absorbing ocean water – means we are already in a slow bake that is only going to get worse.

Conversion to non-polluting energy is the theoretical salvation.  Yet with each passing day of inaction on the scale needed, this solution slips away.  (It may have never been an option: according to some like Peter Dynes for example, the Earth’s resources don’t suffice to build all the non-polluting energy systems needed for even a fraction of humanity’s current energy use.  In this plausible view, a profound economic and cultural shift to lower-consumption lifestyles with less energy use is the only way out.)  Also, since it is clear that current atmospheric carbon levels are already enough to eventually kill much of humanity and immiserate the rest – a huge climatic momentum is underway that we are only beginning to grasp – simply stopping emissions is not enough: carbon must be gotten back out of the atmosphere, as a matter of life or death.

What happens if we look at this as an over-population problem rather than an energy-technology or excessive-consumption one?  The increase in carbon burning over the past decades is usually thought to have been driven by so-called economic development in low- and middle-income countries, plus output growth in developed economies.  Some part of it, however, is simply population growth.  Carbon emissions rise with population even where economic development is stalled: even poor people in South Asia or Africa can still buy a few liters of gasoline for their scooters, or kerosene for their lamps; they cut trees for subsistence farming – not on the scale of large cattle ranchers, loggers or miners, but to a certain scale.  Lest this seem to put the onus on poor countries, population swelling in high-consumption (so-called developed) economies and societies causes a much greater emissions rise than fertility among the world’s poor.

The metric that can shed light on this issue ­– how much rise in emissions owes to development vs. to population swelling – is carbon emissions per capita.  A rise in this indicates what you could call intensification of carbon emissions; a steady per-capita rate, in which the absolute increase in emissions tracks population growth, would indicate that emissions growth is population-driven.  (One could refine the analysis by differentiating among the sources of carbon emission, i.e. fossil-fuel use vs. deforestation or wood-burning, for example.)  The probable reality is that there is some of both – some intensification or per-capita increase, and some population-driven.  In at least one estimate, however, the latter greatly outweighs: “The historical increase in emissions is one-fourth attributable to the growth of emissions per person, whereas three-fourths are due to population growth. This striking evidence is not represented in the majority of climate-economic studies, which mostly neglect the environmental consequences of individuals’ reproductive decisions.”[1]

Let’s jump to possible solutions.  Speed is of the essence, because deadly climate breakdown is happening already.  How rapidly could we pivot to reducing carbon emissions, and start to re-absorb atmospheric carbon, with simple population easing as opposed to a switch to non-polluting energy or mass conversion to leaner lifestyles?  Where would a steep level of population easing – an immediate decline in fertility to below replacement level – get us in a few years?

Population forecasts use ‘fertility rate,’ the average number of children the average woman in a given population bears over her lifetime.  The ‘replacement rate’ that maintains the population unchanged would be 2.1 to 2.3 children per woman (the margin over 2 being those who die before child-bearing or otherwise do not reproduce).

I was recently at a conference of population experts, and asked one expert what is the formula for converting fertility rate to population growth (or decline, so let’s say change) rate.  He answered, it’s complicated.  And it is: fertility rate is a snapshot that averages women’s lifetime fertility, whereas population change rate is per unit time.  The latter also contains the variable of death rate (plus immigration and emigration, if the unit of analysis is a country).  It takes more than a back-of-the-envelope calculation to work out how a given change in fertility rate will affect population change rate.  And, to our main question, even more calculation to figure out if a sharp fall in global human fertility and consequent population easing would help get us out of our climate predicament.

Fortunately for our purposes, there is a shortcut.  The Global Footprint Network has an on-line tool for translating fertility rate into ecological footprint.  The intermediate variable of population change rate is in the equation somewhere, though not visible in the tool’s output (the graphs below for example).  The three variables with which users can play are fertility rate, average age of motherhood, and ecological footprint per person (i.e. consumption and lifestyle, averaged in this case over the Earth’s population).  Each of these has a sub-variable of number of “years until change is fully implemented,” i.e. how soon the variable change you program would take effect.

I can’t vouch for the accuracy of the assumptions and algorithms that commute these explanatory variables into “Humanity’s Ecological Footprint,” the outcome variable.  (In the figures below it’s expressed as biocapacity, in which 1 = one Earth’s worth.)  But let’s look at an isolated comparison between consumption decline and fertility decline.  First, the recent past and projection based on current values of the explanatory variables:

Figure 1: past and projection with current values of explanatory variables

Now let’s reduce the ecological footprint per person – lifestyle and technological changes – by about 25%, from 2.77 to 2.1:

Figure 2: Projected overall footprint with per-capita footprint reduced by 25%

Reducing per-capita footprint brings a steep and almost immediate fall, but you would have to reduce it by much more than 25% to bring the total footprint in line with the Earth’s capacity.  And it soon resumes rising, with inexorable population growth.

Next, with footprint per person restored to current values and instead fertility rate reduced by about 25%, to 1.75 children born per woman:

Figure 3:  Projection with fertility rate reduced by 25%

Here the drop is much slower, and does not converge with the Earth’s capacity until well after 2100 – too late.

An immediate takeaway is that reduction of neither per-capita consumption nor fertility suffices on its own, unless reduced at implausibly steep rates. 

(This tool offers a third explanatory variable: increase in average age of mother at birth of first child.  The default, reflected in the above charts, is zero increase.  But even increasing it by an implausible ten years does not reduce the footprint outcome to the Earth’s capacity.)

Finally, let’s combine moderate 25% reductions in the two explanatory variables (fertility and per-capita consumption):

Figure 4: Projection with both consumption and fertility reduced by 25%

This is a bit better, but still forecasts exceeding the Earth’s capacity for many decades.

For the record, to get the aggregate footprint down to the level of Earth’s capacity in short order, hopefully before irreversible ecological harm, and to stay there, according to this tool you need a reduction of per capita footprint by almost 50% – from 2.77 to 1.50 – combined with a fertility reduction of about 30% (from 2.33 children born per woman on average to 1.80.)

We can’t hang too much on this tool’s prognostications without having peered under the hood and closely examined the data and specification of variables.  But, assuming it’s not totally off base, it does illustrate our predicament – and show that both population and consumption are problems to be solved in parallel.

It’s clear that unless and until there is complete conversion to non-polluting energy, population will be a strong determinant of emissions, and consequently population easing would reduce emissions almost in direct proportion.  The question now is, can it do so fast enough?  Global mortality rate in 2022 is about 7.7 per thousand people, or 0.77%, per year.[2]  Over ten years, that would produce about a 7% population decline, if there were no new births.  (In reality, even with a below-replacement fertility rate of only 2.0, population would continue to climb for some years because of demographic momentum – the large cohort just entering reproductive age.)  If emissions accordingly decrease by the same proportion, that does not solve our predicament: we need not a 7% reduction in annual emissions but instead to get to zero emissions and major carbon re-absorption without delay.  Even if such a population easing disproportionately reduced atmospheric carbon – not just cutting emissions but also leaving land fallow or forested, reducing livestock, and maybe some other factors – it would not suffice to reverse our course towards immolation.  Any scheme that trumpets 7% or even 20% emission reductions is hardly a solution: it would have done little good to plug 20% of the gash in the Titanic’s hull – at most buy time and save a few lives.

So emissions reduction through population easing is admittedly too slow for our predicament.  What’s its role then?

Population easing would solve more problems than just carbon build-up: it addresses all the “Population Bomb” and “Limits to Growth” problems of resource shortages and consequent problems of competition and stress.  It also would tackle carbon emissions and build-up in multiple ways: not just directly by easing the number of fossil-fuel customers, but also for example by allowing reduction of fossil-fuel-intensive agricultural methods (necessary to carry the world’s overpopulation, but destructive to the carbon-retaining soil microbiome), and reforestation of agricultural land, including importantly pasturage for livestock.  It would free up resources for conversion to cleaner energy (and reduce its scale and costs), and for adaptation to the climate assaults that are already inevitable.[3]  Less tangibly, it may also ease political stresses, which in turn would enable more constructive climate action.

Lower per-capita emissions need a lead time: personal choices about reducing consumption are worthy (and will prepare those who adopt them for the hard times to come), but our economies and polities limit those choices and have great inertia that even strong political will would need time to pivot.  By contrast, the great worldwide unmet demand for contraception[4] (on the order of 100 million unintended pregnancies per year[5]) could be remedied very quickly, and thus fertility easing could start almost immediately.

And of course, as the early-1970s researchers foresaw, overpopulation stresses all sorts of systems and resources critical for human survival and for the well-being of nature, of which we are a part.  Switching to carbon-free energy, and reduction of current harmful atmospheric carbon, will not palliate that.

Lastly, it seems an interesting fact that uncontrolled population growth is exponential, whereas population easing would tend to be linear: multiple offspring themselves multiply exponentially, whereas a non-existent offspring does not spread non-existence (unless by example).  In other words, fertility decline below replacement level can only avoid exponential growth; it does not in itself bring exponential decline.  Maybe this asymmetry’s implications need to be explored in analysis and policy.  It seems related to the fact that reducing fertility saves a lot of future emissions, cascading over the lifetime of offspring and descendants, though the immediate effect is small: “In 2009 it was calculated that by adopting available ‘eco-friendly’ actions, including meticulous recycling, an American couple could curb their lifetime carbon footprint by 486 tonnes. Simply by having one less child, an American woman would reduce her ‘carbon legacy’ (the summed emissions of herself and her descendants weighted by relatedness) by 9441 tonnes.”[6] 

I suppose I’m writing this article now because the population vs. consumption debate risks settling into opposing camps with ill will and negative stereotypes – self-flagellating ascetics vs. Malthusian misanthropes.  But it’s a false dichotomy: both are genuine problems, action on both is needed, and we should not waste intellectual energy and tolerance debating which is more important and which side is misguided or revealing character flaws.  The good news is, both are good news.  Population easing has manifold benefits even apart from climate, biodiversity and resources; women’s rights and empowerment, for starters.  And unwinding the mindset that consumption equals quality of life speaks for itself – less work and running around, more time for relationships and cultural richness.  Both come down to quality of life; and in both cases, less is more.  This is good news, to be spread.

In a coming article, we’ll look at why population easing is not just dismissed by many, but positively lambasted as somehow anti-humanist; and what advocacy and actions can do about that.  Preview: a simple response to the idea that advocating population easing is tantamount to blaming poor countries for the behavior of rich countries – or even worse, amounts to outright tribalism or racism – is to argue that rich countries should engineer particularly steep population easing, because of their weighty per-capita carbon footprints.  Also to note that less-developed countries’ ‘right to develop’ out of poverty would be best aided by population easing, even in the absence of climate breakdown or other resource stresses.

(This essay is a synthesis of others’ ideas, mostly recent and a few older, plus a very few of my own – which are likely to have occurred to others as well. I particularly acknowledge Kirsten Stade of Population Balance, whose blog advances many of the same arguments and beat me to publication by a few days.  Commendation also to Julian Cribb for aptly stating “The population bomb has exploded.”)

© Blackthorn 2022

[1] Gerlagh, Reyer and Lupi, Veronica and Galeotti, Marzio, Family Planning and Climate Change (2018). CESifo Working Paper No. 7421, Available at SSRN: or


[3] Bongaarts J, O’Neill BC. Global warming policy: Is population left out in the cold? Science. 2018 Aug 17;361(6403):650-652. doi: 10.1126/science.aat8680. PMID: 30115798.

[4] Casterline J and Sinding SW, Unmet need for family planning in developing countries and implications for population policy, Population and Development Review, 2000, 26(4):691-723.  Cited in Bongaarts, J., & Sinding, S. W. (2009). A Response to Critics of Family Planning Programs. International Perspectives on Sexual and Reproductive Health, 35(1), 39–44.

[5] “Worldwide about 99 million unintended pregnancies occur each year—44% of all pregnancies. More than half of these unintended pregnancies end in an induced abortion (56 million per year) and the remainder in unintended births or miscarriages…”  Bongaarts J, Sitruk-Ware R. Climate change and contraception. BMJ Sex Reprod Health. 2019 Oct;45(4):233-235. doi: 10.1136/bmjsrh-2019-200399. PMID: 31615904, p. 233, citing in turn Bearak J. Popinchalk J, Alkema L, et al. Global, regional, and sub-regional trends in unintended pregnancy and its outcomes from 1990 to 2014: estimates from a Bayesian hierarchical model. Lancet Global Health 2018;64:e380–9.

[6] Guillebaud J. Voluntary family planning to minimise and mitigate climate change. BMJ 2016; 353:i2102 doi:10.1136/bmj.i2102, p. 2, citing Murtaugh P, Schlax M. Reproduction and the carbon legacies of individuals. Glob Environ Change 2009;19:14-20.


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