Bill Rees comment

How it all began

Ecologists might suggest using ‘carrying capacity’ as a starting point; the question then becomes: “how many people could the subject city/region/country support indefinitely at a defined standard of living if isolated from the rest of the world?”

As an operational concept ‘carrying capacity’ seems innocuous enough and the implicit logic isn’t all that hard to grasp—but don’t be seduced by musical alliteration or the simple meaning of words. In some quarters, talking ‘carrying capacity’ has the effect of blasphemous cursing in church.

I learned this the hard way. Early in my academic career I was assigned to a motley task force of economists, sociologists, engineers, hydrologists and geographers who were to set out the terms for new interdisciplinary research centre on regional water resource management. Since we hardly knew each other, the Chair cleverly decided that task force members would deliver short seminars to the group on any of their research interests that might be relevant to the proposed centre. I was a wet-behind-the-ears ecologist and had yet to establish a research program, but thought I might be able to make the case that human carrying capacity should be a core concept for any research institute focused on sound resource management.

The hazards of siloed learning

Turns out this was a bad idea. My audience of economists, and assorted other techno-optimists received my arguments politely enough but one task-force member, who just happened to be one of Canada’s most prominent resource economists, took me aside and asked to meet privately later. Pleased with the apparent recognition I agreed enthusiastically. And naively—at our subsequent get-together, my colleague, grave of face, told me point blank (albeit with the greatest of professional respect and courtesy) that, should I pursue my interest in human carrying capacity, my academic career would likely be a Hobbesian “nasty, brutish, and short.”

Whoa!

To say I was taken aback is the grand epitome of understatement. I was new to the academic game but I was also a highly-educated life-scientist, an ecologist who thought he knew a thing or two about ecosystems dynamics including the potential relevance of carrying capacity to human well-being. It seemed obvious from an ecosystems perspective that there were potential constraints on the number of people a region or nation could support!

Seems I was dead wrong and my eco-paradigm irrelevant. I hadn’t learned from the confines of my own formative academic silo that economists had long abolished carrying capacity from consideration in contemplating the human future. In fact, at the time, they were vigorously engaged in burying the ground-breaking 1972 Club of Rome/MIT report on Limits to Growth in the graveyard of failed analyses.

My colleague elaborated in exquisite detail on his dire warning. Basically, he was incredulous that I, as a scientist, could hold such backward views on human potential. Surely I didn’t imagine, as had the notorious Paul Ehrlich,[1] that humanity might be looking at future food shortages? Wasn’t I aware that farmers would produce more of ‘whatever’ if scarcity-induced market price increases stimulated them to do so? And what about the inexorable march of technology? After all, the green revolution was already surging ahead producing crop yields beyond anyone’s previous imagining.

As for my argument that our regional population or economy might someday be constrained by local shortages of water, energy or some other crucial non-food resource, he advised me to bone up on the basic theory and historical role of trade. It seems any region/country can exchange its surpluses of resource ‘a’ (or perhaps some more abstract service) for supplies of scarce resource ‘b’, thus freeing itself—and presumably its trading partner—from their respective regional limits. Moreover, trade theory shows that if each country specializes in making those few products it produces most efficiently, and trades for everything else, manufacturing costs and consumer prices for everything will be at their lowest and global production maximized—i.e., there’ll be more stuff for everyone at affordable prices!

In short, my colleague argued, greater global economic integration and freer trade can relieve any local supply bottlenecks, benefit all trading partners and elevate global living standards. And, by the way, he added, if market push really comes to scarcity shove, rising prices will stimulate some genius to invent a substitute for any depleting good or service provided by nature. It seemed that if environmentalists and other nay-sayers would just get out of the way, the economic efficiency gains of globalization and trade, abetted as needed by human ingenuity, could extend rising material standards to growing numbers of eager ‘consumers’ indefinitely.[2] What’s not to like? Carrying capacity be damned; population/economic growth can continue unabated.

My economist friend kindly offered me a list of core readings reinforcing this perspective.

Defeat and resurrection

Put yourself in my position. I was a relatively shy untenured and untested newbie; I had never studied economics. In fact, my cloistered scientific education had sheltered me completely from the social sciences and humanities with their boldly breathtaking visions of human potential. Moreover, I was being confronted by an established—famous even—senior economist who spoke with great conviction and authority in advancing arguments that, frankly, seemed fatal to my naïve notions of human carrying capacity. (It didn’t help that he was also in a position to see his Hobbesian threat to my future come to fruition.) I left that private consultation deflated, disorientated, tail lodged firmly between my legs.

It took a few days for my ego to heal and a few days more before I became aware that something was gnawing away like a hungry maggot in the apple of my unconscious.[3] Perhaps my education had not been totally wasted. I had been steeped in the tradition of scientific skepticism, the idea that all beliefs, assumptions and even complete theories should be subject to critical examination, tested on their merits. Mere authority, however aggressively expressed, was no bulwark to critical thinking, reasoned analysis or contrary evidence. Something born of these logical chisels must have been chipping away at my colleague’s thesis even while I slept.

In any case, I began to feel that something was deeply biophysically awry with his prescriptions. Gaining confidence, I began consciously to revisit that debilitating economics lesson and discovered bits of logical wiggle room. Then in a flash, that subliminal thought-worm broke through. I had a classic, wake-up-in-the-middle-of-the-night, an ‘eureka’ experience, a coming to consciousness that fully resolved my dilemma.

Which brings us back to consideration of carrying capacity so maybe it’s time to elaborate a bit more on what we are talking about.

It seems that in my seminar presentation I had run afoul of the standard definition of carrying capacity, to wit: “the average maximum population[4] of a given species that can persist in a particular habitat without permanently impairing the productive capacity of that habitat.” Game managers know that too many deer or elk may deplete their forage. Any old-timey dairy farmer knows you can’t put too many cows on the back forty or they will wreck the pasture.[5] Do we really think humans are any different? Well, actually, my colleague was correct on one level; unlike grass-fed dairy cows or elk, people needn’t be dependent on local productivity. Since imported goods can eliminate local resource constraints and technology can invent resources, economists might well indeed argue that ‘carrying capacity’, as usually defined, does not apply to humans.

But wait just a minute…

What if we invert the traditional carrying capacity ratio? Instead of asking how large a human population a particular region or country might support, let’s ask instead how much productive area is needed to support the subject population, regardless of the location of the relevant ecosystems or whatever the technological sophistication of the population.

EUREKA!

Every living person and population is drawing on ecosystems somewhere. Simply inverting the carrying capacity definition acknowledges humans’ direct dependence on productive land and water ecosystems even if these systems are scattered all over the planet. Flipping the ratio accounts for trade, and the material content of traded commodities reflects whatever technological wizardry is in place at the time of the analysis. If we can quantify the relevant material flows and convert them to a corresponding land/water area, we have a way to put carrying capacity firmly back onto the sustainability agenda. After all, there is only a finite—and diminishing—area of productive land- and waterscape in any region or nation (and even on our whole planet of ever more people each with increasing energy and material demands). We can use this approach to dissolve any objections to human carrying capacity based on trade and technological fixes; carrying capacity again becomes a vital sustainability consideration.

Conscious of it or not, you have an ecological footprint

The first question of human ecology should be: “Just how much of Earth’s productive land and water surface
area is necessary to support just me in the style to which I am accustomed?”

To bring this back down to Earth, let’s try a simple mental exercise. Imagine what would happen to the folks in your hometown or favourite city if it were enclosed in a giant, impermeable glass capsule or bell-jar. You don’t have to be a genius to realize that the isolated population would simultaneously suffocate and starve to death within a few days. The city has grossly insufficient ‘carrying capacity.’

Now suppose you had data on the average productivity of the world’s ecosystems and a full accounting of your urban population’s material consumption and carbon emissions. This would enable an estimate of how large that bell-jar would have to become to contain, not only the city’s built-up area, but also the area of average land and water ecosystems sufficient to satisfy the human population’s demand for food and fibre and to assimilate its carbon waste. You would find that, to maintain current levels of consumption and carbon emissions alone, modern high-income cities effectively appropriate an external ecosystem area several hundreds or even thousands of times larger than their political or built-up areas. The geographic area of a city—that dot on the map—is only a tiny fraction of the complete or functional urban human ecosystem. Urban populations vastly overshoot their cities’ internal or domestic carrying capacities; in effect, cities run massive ‘ecological deficits’; their populations are utterly dependent for survival on ecological surpluses and waste assimilation capacities imported from distant ‘elsewheres’.

In 1990, while writing my first formal paper on the implications for urban sustainability of what I was calling ‘regional capsule’ analysis, my old computer crashed. Its replacement came in a slim mini-tower and I quickly came to appreciate the much smaller desk-top ‘footprint’. Pure serendipity! That’s what I was writing about—the ecological footprints of human populations on Earth. A population’s eco-footprint is the total area of land and water ecosystems that the population requires, on a continuous basis, to supply its bioresources (food and fibre) and to assimilate its (mostly carbon) wastes, wherever on Earth those supportive ecosystems are located. It took just a few seconds to replace every reference to ‘regional capsule’ in the paper with ‘ecological footprint.’ That article became the most cited in the journal’s history no doubt due, in part, to the communicative power of the footprint metaphor.

The human ‘ecological footprint’ (EF) has since become one of the world’s best-known (un)sustainability indicators (a Google’s primary search for ‘ecological footprint returns 200,000,000 to 300,000,000 hits).[6] Three and a half decades since publication of that first article, the method is still going strong and the national footprint accounts[7] still improving.

Of course, ecological footprinting still has weaknesses which mostly result in under-estimates of total human impact. The method was designed to estimate the human appropriation of biocapacity (productive ecosystem area), so EF estimates do not incorporate such effects as biodiversity loss, micro-plastic contamination or other forms of pollution—no single index can capture everything.

But does this really matter? What the method does show is that no city could survive more than a few days in isolation; that many whole countries are running significant eco-deficits; and that the aggregate, mostly urban, human EF exceeds available global biocapacity by at least 78%. This means that humanity has vastly overshot the long-term carrying capacity of Earth; the growth and maintenance of the human enterprise today is being ‘financed’ by depleting stocks of essential bioresources (not to mention non-renewables) needed tomorrow. We are rapidly draining Earth’s battery and there are no nearby magical charging stations.

It should be obvious that overshoot is ultimately a terminal condition. Figure 1 shows that humanity is nearing the peak of a one-off population boom-bust cycle and will contract. We have even blown the chance to halt the descent near historic carrying capacity via ‘one-planet living’ (overshoot can permanently degrades carrying capacity). Any future ‘civilization’ will occupy a much-diminished planet.

 

Figure 1: Boom-bust and the loss of future carrying capacity

This assessment may seem over the top, particularly to those who haven’t noticed the problem yet (which probably means they live in a rich country). Others elsewhere are already suffering the consequences of overshoot and the rest of us will eventually too. The Gaian ecosphere is a complex thermodynamic system characterized by threshold effects (tipping points) and systemic lags between cause and effect. (For example, the global heating we are experiencing today is the result of carbon dioxide emissions released decades ago.)[8] Humans managed temporarily to neutralize the negative feedbacks—food scarcity/starvation, overcrowding, other resource shortages, disease, war—that historically held our populations in check but Gaia will have her revenge as the systems evolves. Wait for the resurgence of natural negative feedbacks and unseen thresholds—increasing climate chaos, crop failures, energy and water shortages, new diseases, another pandemic(s)—and the subsequent geopolitical turmoil. (Have you been reading the headlines lately?)

And remember it’s ultimately all about carrying capacity.

Surely that should be enough to pour vinegar into economists’ wine.

[1] The late Paul Ehrlich was famous as an entomologist and infamous as author of The Population Bomb. Initially ignored, Ehrlich’s tract would eventually “…become one of the most influential books of the 20th century—and one of the most heatedly attacked.”

[2] At that time I had never heard of ‘human exceptionalism’ and so was unaware that neoliberal economics considered the economy and the ‘environment’ to be separate, essentially independent systems. In this fantastical framing, latter cannot constrain the former.

[3] It turns out that 95%+ of human thought processes occur beneath consciousness.

[4] ‘Average maximum’ because the instantaneous carrying capacity is constantly fluctuating with environmental conditions.

[5] One reason beef prices have risen so much in North America is that extended drought has reduced the carrying capacity of range lands and hence cattle ranchers’ herd sizes.

[6] Much of the credit is due to continuous methodological and practical improvements by my graduate students, particularly Dr Mathis Wackernagel. Mathis co-founded and became the first President of the Global Footprint Network (GFN) in 2003. GFN’s website (

Global Footprint Network

) has become an invaluable data-rich resource for students, teachers and EFA practitioners everywhere.

[7] Now maintained at York University in Toronto.

[8] More immediately, the Israel/US vs Iran war choked off the Strait of Hormuz nearly two months ago on 28 February 2026. The inflationary effects of higher fuel and some commodity prices (set in the global marketplace) were felt almost immediately everywhere. Within weeks many poorer countries were also severely hampered by supply problems but wealthy countries are just beginning to suffer physical shortages. These countries were initially buffered by such things as sheer wealth, national petroleum reserves and loaded ships in transit at sea—all complex systems lag effects— but the last cargoes of oil, gas, and other supplies have now been delivered and there are no more en route. It will take months for the full impacts of broken supply chains and crucial shortages to be felt and many more months to recover, assuming recovery is possible.