The Population Debate Revisited

William E. Rees, June 2022

As climate change accelerates, and other symptoms of overshoot compete for the headlines, the world community finally seems willing to re-visit “the population problem.”

One thing is curiously missing from virtually every article on the topic—an exploration of the bio-evolutionary dynamics of the population conundrum. This conceptual vacuum is characteristic of modern techno-industrial (MTI) culture’s allegiance to human exceptionalism. We do not consider ourselves to be mere animals beholden to the laws of nature.

Such reticence is pure social construct and wholly unjustifiable. Ukrainian-American geneticist Theodosius Dobzhansky once famously argued that “nothing in biology makes sense except in the light of evolution.”  Since H. sapiens is an evolved species, and individual and group behaviours are, in part, heritable qualities shaped by natural selection, it is fair to assert that nothing in human affairs makes sense except in the light of evolution.

Consider just two adaptive behaviors that H. sapiens shares with all other species. Humans have an innate propensity to consume available resources—often to depletion—and a parallel drive to invade and colonize all accessible habitats. Earth is to H. sapiens as a nutrient-rich Petri dish is to colonizing bacteria. The difference is that humans are better than bacteria or any other species at expressing both survival strategies.

Indeed, our competitive superiority is continuously being upgraded by advancing technology—modern factory-freezer trawlers are more effective at depleting available resources than are indigenous river-mouth fish weirs; engineers are continually redefining the meaning of “available” in the petroleum and mineral-mining industries. Consider, too, how the financial sector exploits consumers’ yearning for material gratification. Depleted your paycheck? Not to worry!  Human ingenuity has devised the humble credit card, a pseudo-resource that enables its possessors to consume well beyond their incomes.  

H. sapiens’s spatial conquest of Earth has also been finely tuned for at least 60 millennia. Humans have not only occupied all suitable habitats, but have altered many hostile environments so that we can colonize these as well.  It is a mark of evolutionary success that we have the most extensive geographic range of any vertebrate species (except perhaps for the rats and mice that follow us around).  Does anyone think that if a new verdant continent were discovered we would ignore it on grounds that we’ve screwed up everywhere else?  Elon Musk and others of his “we’ve-gotta-colonize-Mars” ilk are simply playing out humanity’s expansionist bravado.

There is, of course, something deeper at play here. First, like all other species, human populations are capable of exponential growth under ecologically favorable conditions. Second, H. sapiens has a highly successful reproductive strategy: we exhibit all the qualities of typical ‘K’-strategic species (‘K’ = carrying capacity). We are long-lived and highly competitive, and we have a relatively low reproductive rate, exhibit extreme parental care, and enjoy relatively high off-spring survival rates. Thus, at every level of technological achievement, populations of H. sapiens eventually press, often painfully, against local carrying capacities. Bluntly put, humans are potentially unsustainable by nature. This was Malthus’s great insight (now denied or forgotten).

“But wait,” some will protest. “Surely MTI society has ‘evolved’ beyond such primitive considerations!” Not at all—they perfectly describe the trap we have set for ourselves. Anatomically modern humans have been around for at least 200,000 years but for 99.9% of this period our numbers were held in check by natural negative feedback. Our population didn’t reach its first billion until the early 1800s; then, in just two centuries—1/1000th as much time—we ballooned to seven billion (and will likely hit eight billion in 2022). The scientific and industrial revolutions had finally freed the genie of exponential growth from its bottle. Improving population health contributed, but the human explosion was mostly made possible by fossil fuel (FF). Coal, oil, and natural gas provided the energetic means by which MTI society could produce/acquire all the food, fiber, and mineral resources needed to grow the human enterprise.

But now we have a problem. It took massive quantities of extra-somatic energy to make modern society possible, and additional huge quantities are necessary even to maintain it in place.  MTI society is suspended at heady height on a gusher of depletable petroleum and natural gas, and, as the pressure drops, society will likely descend in proportion. Every significant facet of modern society is deeply energy-dependent, and, despite the promotional hype, there are no viable substitutes for most uses of fossil fuels. (This is no doubt a major reason why the world community has taken so little action to curb fossil fuel use with its attendant carbon emission.)

There’s more. Abundant cheap energy did not inflate our numbers by raising long-term carrying capacity, but rather by increasing the rate at which the human enterprise could pillage our finite planet. We are exploiting nature well beyond the ecosphere’s regenerative capacity, depleting cumulated stocks of “natural capital”—fish-stocks, tropical forests, arable soils, groundwater, biodiversity, fossil fuels, etc., etc.—and polluting just about everything. (Even climate change is an excessive waste problem.) This is the very definition of ecological overshoot.

Not everyone is equally culpable. The wealthiest quarter of humanity alone has effectively appropriated most of the planet’s biocapacity. Egregious inequality is, however, a separate socio-political issue—population growth is currently the major contributor to overshoot in all income quartiles. That said, there is a connection: the material footprints of upper-income consumers average ten to 15 times larger than those of the denizens of low-income countries. It follows that the most ecologically significant per capita gain from population reduction would come from policies that accelerate the decline in the numbers of wealthy consumers.   

As matters stand, overshoot means that Earth cannot long sustain even the present population at average material standards.  Even so, the MTI mindset ensures we will continue scouring the planet—our biological imperative is currently being reinforced by a similarly expansionist cultural narrative. Humans will scrape the bottom of the oil barrel and every other resource trove to which we can gain access in a desperate attempt to maintain the growth-based status quo.

Such intransigence risks disaster. When other species experience population booms during ecologically benign periods, the booms are invariably followed by busts—resources run out, and negative feedback kicks in to restore balance. There is no reason to think human population dynamics are any different. For far too long, the world community has avoided confronting both excess consumption and population growth. Climate change inaction and blindness to over-population are two heads of the same hydra of deep cultural denial. All signs are that H. sapiens is nearing the peak of the boom phase in a one-off global population cycle and that bust will follow. While rapid, determined collective action may yet limit the pain (especially to the poor and powerless), the world community has little chance of avoiding a major population correction in this century. A glance at the daily headlines suggests that the Four Horsemen are already saddling up.

Bill Rees



  1. Hi Jon,

    Bill Rees is among the best communicators of the Problematique I know. We met around two decades ago. Please email me, as I have a private question for you. I’ve been an OP activist for over 30 years. kurtzs at ncf Dot ca


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