since the writing of Thomas Malthus in the early 1800s, and especially
since Paul Ehrlich’s publication of “The
Population Bomb” in 1968, there has been a lot of learned
skull-scratching over what the sustainable human population of Planet
Earth might “really” be over the long haul.
question is intrinsically tied to the issue of ecological overshoot so
ably described by William R. Catton Jr. in his 1980 book “Overshoot:The
Ecological Basis of Revolutionary Change”. How much have we
already pushed our population and consumption levels above the
long-term carrying capacity of the planet?
this article I outline my current thoughts on carrying capacity and
overshoot, and present five estimates for the size of a sustainable
capacity” is a well-known ecological term that has an obvious and
fairly intuitive meaning: “The maximum population size of a
species that the environment can sustain indefinitely, given the food,
habitat, water and other necessities available in the environment."
that definition becomes more nebulous and controversial the closer you
look at it, especially when we are talking about the planetary carrying
capacity for human beings. Ecologists will claim that our numbers
have already well surpassed the planet’s carrying capacity, while
others (notably economists and politicians...) claim we are nowhere
near it yet!
This confusion may arise because we tend to confuse two very different
understandings of the phrase “carrying capacity”. For this
discussion I will call these the “subjective” view and the “objective”
views of carrying capacity.
The subjective view is carrying capacity as seen by a member of the
species in question. Rather than coming from a rational, analytical
assessment of the overall situation, it is an experiential
judgement. As such it tends to be limited to the population of
one's own species, as well as having a short time horizon – the current
situation counts a lot more than some future possibility. The
main thing that matters in this view is how many of one’s own species
will be able to survive to reproduce. As long as that number continues
to rise, we assume all is well – that we have not yet reached the
carrying capacity of our environment.
this subjective point of view humanity has not even reached, let alone
surpassed the Earth’s overall carrying capacity – after all, our
population is still growing. It's tempting to ascribe this view
mainly to neoclassical economists and politicians, but truthfully most
of us tend to see things this way. In fact, all species,
including humans, have this orientation, whether it is conscious or not.
tend to keep growing until outside factors such as disease, predators,
food or other resource scarcity – or climate change – intervene.
These factors define the “objective” carrying capacity of the
environment. This objective view of carrying capacity is the view
of an observer who adopts a position outside the species in
question.It’s the typical viewpoint of an ecologist looking at the
reindeer on St. Matthew Island, or at the impact of humanity on other
species and its own resource base.
the view that is usually assumed by ecologists when they use the naked
phrase “carrying capacity”, and it is an assessment that can only be
arrived at through analysis and deductive reasoning. It’s the
view I hold, and its implications for our future are anything but
a species bumps up against the limits posed by the environment’s
objective carrying capacity,its population begins to decline. Humanity
is now at the uncomfortable point when objective observers have
detected our overshoot condition, but the population as a whole has not
recognized it yet. As we push harder against the limits of the planet’s
objective carrying capacity, things are beginning to go wrong.
More and more ordinary people are recognizing the problem as its
symptoms become more obvious to casual onlookers.The problem is, of
course, that we've already been above the planet’s carrying capacity
for quite a while.
One typical rejoinder to this line of argument is that humans have
“expanded our carrying capacity” through technological
innovation. “Look at the Green Revolution! Malthus was just
plain wrong. There are no limits to human ingenuity!” When
we say things like this, we are of course speaking from a subjective
viewpoint. From this experiential, human-centric point of view, we
have indeed made it possible for our environment to support ever more
of us. This is the only view that matters at the biological,
evolutionary level, so it is hardly surprising that most of our fellow
species-members are content with it.
The problem with that view is that every objective indicator of
overshoot is flashing red. From the climate change and ocean
acidification that flows from our smokestacks and tailpipes, through
the deforestation and desertification that accompany our expansion of
human agriculture and living space, to the extinctions of non-human
species happening in the natural world, the planet is urgently
signalling an overload condition.
have an underlying urge towards growth, an immense intellectual
capacity for innovation, and a biological inability to step outside our
chauvinistic, anthropocentric perspective. This combination has
made it inevitable that we would land ourselves and the rest of the
biosphere in the current insoluble global ecological predicament.
a population surpasses its carrying capacity it enters a condition
known as overshoot. Because the carrying capacity is
defined as the maximum population that an environment can maintain indefinitely,
overshoot must by definition be temporary. Populations always decline
to (or below) the carrying capacity. How long they stay in
overshoot depends on how many stored resources there are to support
their inflated numbers. Resources may be food, but they may also
be any resource that helps maintain their numbers. For humans one
of the primary resources is energy, whether it is tapped as flows (sunlight,
wind, biomass) or stocks (coal, oil, gas, uranium
etc.). A species usually enters overshoot when it taps a
particularly rich but exhaustible stock of a resource. Like
fossil fuels, for instance...
Population growth in the animal kingdom tends to follow a logistic curve. This is an
S-shaped curve that starts off low when the species is first introduced
to an ecosystem, at some later point rises very fast as the population
becomes established, and then finally levels off as the population
saturates its niche.
Humans have been pushing the envelope of our logistic curve for much of
our history. Our population rose very slowly over the last couple of
hundred thousand years, as we gradually developed the skills we needed
in order to deal with our varied and changeable
environment,particularly language, writing and arithmetic. As we
developed and disseminated those skills our ability to modify our
environment grew, and so did our growth rate.
If we had not discovered the stored energy resource of fossil fuels,
our logistic growth curve would probably have flattend out some time
ago, and we would be well on our way to achieving a balance with the
energy flows in the world around us, much like all other species
do. Our numbers would have settled down to oscillate around a
much lower level than today, similar to what they probably did with
hunter-gatherer populations tens of thousands of years ago.
our discovery of the energy potential of coal created what
mathematicians and systems theorists call a “bifurcation point” or what
is better known in some cases as a tipping point. This is a point at
which a system diverges from one path onto another because of some
influence on events. The unfortunate fact of the matter is that
bifurcation points are generally irreversible. Once past such a
point, the system can’t go back to a point before it.
the impact that fossil fuels had on the development of world
civilization, their discovery was clearly such a fork in the road.
Rather than flattening out politely as other species' growth
curves tend to do, ours kept on rising. And rising, and
is a sustainable population level?
we come to the heart of the matter. Okay, we all accept that the
human race is in overshoot. But how deep into overshoot are
we? What is the carrying capacity of
our planet? The answers to these questions,after all, define a sustainable population.
surprisingly, the answers are quite hard to tease out. Various
numbers have been put forward, each with its set of stated and unstated
assumptions –not the least of which is the assumed standard of living
(or consumption profile) of the average person. For those
familiar with Ehrlich and Holdren’s I=PAT equation,
if “I” represents the environmental impact of a sustainable
population, then for any population value “P” there is a
corresponding value for “AT”, the level of Activity and
Technology that can be sustained for that population level. In
other words, the higher our standard of living climbs, the lower our
population level must fall in order to be sustainable. This is
discussed further in an earlier article on Thermodynamic
get some feel for the enormous range of uncertainty in sustainability
estimates we’ll look at five assessments, each of which leads to a very
different outcome. We’ll start with the most optimistic one, and
work our way down the scale.
Ecological Footprint Assessment
concept of the Ecological
developed in 1992 by William Rees and Mathis Wackernagel at the
University of British Columbia in Canada.
ecological footprint is a measure of human demand on the Earth's
ecosystems. It is a standardized measure of demand for natural capital
that may be contrasted with the planet's ecological capacity to
regenerate. It represents the amount of biologically productive land
and sea area necessary to supply the resources a human population
consumes, and to assimilate associated waste. As it is usually
published, the value is an estimate of how many planet Earths it would
take to support humanity with everyone following their current
has a number of fairly glaring flaws that cause it to be
hyper-optimistic. The "ecological footprint" is basically for
renewable resources only. It includes a theoretical but underestimated
factor for non-renewable resources. It does not take into account
the unfolding effects of climate change, ocean acidification or
biodiversity loss (i.e. species extinctions). It is intuitively
clear that no number of “extra planets” would compensate for such
the estimate as of the end of 2012 is that our overall ecological
footprint is about “1.7 planets”. In other words, there is at
least 1.7 times too much human activity for the long-term health of
this single, lonely planet. To put it yet another way, we are 70%
would probably be fair to say that by this accounting method the
sustainable population would be (7 / 1.7) or about four billion people at our current
average level of affluence. As you will see, other assessments
make this estimate seem like a happy fantasy.
Fossil Fuel Assessment
main accelerant of human activity over the last 150 to 200 years has
been fossil fuel. Before 1800 there was very little fossil fuel
in general use, with most energy being derived from wood, wind, water,
animal and human power. The following graph demonstrates the
precipitous rise in fossil fuel use since then, and especially since
by Gail Tverberg
information was the basis for my earlier Thermodynamic
That article investigated the influence of technological energy
(87% of which comes from fossil fuels) on human planetary
impact, in terms of how much it multiplies the effect of each “naked
ape”. The following graph illustrates the multiplier at different
points in history:
fuels have powered the increase in all aspects of civilization,
including population growth. The “Green
Revolution” in agriculture that was kicked off by Nobel laureate
Norman Borlaug in the late 1940s was largely a fossil fuel phenomenon,
relying on mechanization,powered irrigation and synthetic fertilizers
derived from fossil fuels. This enormous increase in food production
supported a swift rise in population numbers, in a classic ecological
feedback loop: more food (supply) => more people (demand) => more
food => more people etc…
the core decades of the Green Revolution from 1950 to 1980 the world
population almost doubled, from fewe rthan 2.5 billion to over 4.5
billion. The average population growth over those three decades
was 2% per year. Compare that to 0.5% from 1800 to 1900; 1.00%
from 1900 to 1950; and 1.5% from 1980 until now:
analysis makes it tempting to conclude that a sustainable population
might look similar to the situation in 1800, before the Green
Revolution, and before the global adoption of fossil fuels: about 1 billion peopleliving
on about 5% of today’s global average energy consumption.
tempting (largely because it seems vaguely achievable), but
unfortunately that number may still be too high. Even in 1800 the
signs of human overshoot were clear, if not well recognized:
there was already widespread deforestation through Europe and the
Middle East; and desertification had set into the previously lush
agricultural zones of North Africa and the Middle East.
to mention that if we did start over with “just” one billion people, an
annual growth rate of a mere 0.5% would put the population back over
seven billion in just 400 years. Unless the growth rate can be
kept down very close to zero, such a situation is decidedly
Population Density Assessment
is another way to approach the question. If we assume that the
human species was sustainable at some point
in the past, what point might we choose and what conditions contributed
to our apparent sustainability at that time?
use a very strict definition of sustainability. It reads
something like this: "Sustainability is the ability of a
species to survive in
damaging the planetary ecosystem in the process." This
principle applies only to a species' own actions, rather than
uncontrollable external forces like Milankovitch
cycles, asteroid impacts, plate tectonics, etc.
order to find a population that I was fairly confident met my
definition of sustainability, I had to look well back in history - in
fact back into Paleolithic times. The sustainability conditions I
chose were: a very low population density and very low energy use, with
both maintained over multiple thousands of years. I also assumed the
populace would each use about as much energy as a typical
hunter-gatherer: about twice the daily amount of energy a person
obtains from the food they eat.
are about 150
million square kilometers, or 60 million square miles of land on
Planet Earth. However, two thirds of that area is covered by
snow, mountains or deserts, or has little or no topsoil. This
leaves about 50
million square kilometers (20 million square miles) that
is habitable by humans without high levels of technology.
A typical population density for a non-energy-assisted society of
hunter-forager-gardeners is between 1
person per square mile and 1 person per square kilometer.
Because humans living this way had settled the entire planet by the
time agriculture was invented 10,000 years ago, this number pegs a
reasonable upper boundary for a sustainable world
population in the range of 20
to 50 million people.
settled on the average of these two numbers, 35 million people.
That was because it matches known hunter-forager population
densities, and because those densities were maintained with virtually
zero population growth (less than 0.01% per year)during the 67,000
years from the time of the Toba super-volcano eruption in 75,000 BC
until 8,000 BC (Agriculture Day on Planet Earth).
we were to spread our current population of 7 billion evenly over 50
million square kilometers, we would have an average density of 150 per
square kilometer. Based just on that number, and without even
considering our modern energy-driven activities, our current population
is at least 250 times too big to be sustainable. To put it another way,
we are now 25,000%into overshoot based on our raw
population numbers alone.
As I said above, we also need to take the population’s standard of
living into account. Our use of technological energy gives each of
us the average planetary impact of about 20 hunter-foragers. What
would the sustainable population be if each person kept their current
lifestyle, which is given as an average current Thermodynamic Footprint
(TF) of 20?
We can find the sustainable world population number for any level
of human activity by using the I
= PAT equation mentioned above.
- We decided above that the maximum hunter-forager
population we could accept as sustainable would be 35 million people,
each with a Thermodynamic Footprint of 1.
- First, we set I (the allowable
total impact for our sustainable population) to 35, representing those
35 million hunter-foragers.
- Next, we set AT to be the TF
representing the desired average lifestyle for our population. In
this case that number is 20.
- We can now solve the equation for P.
Using simple algebra, we know that I = P x AT is
equivalent to P = I / AT. Using that form of the
equation we substitute in our values, and we find that P = 35 /
20. In this case P = 1.75.
number tells us that if we want to keep the average level of per-capita
consumption we enjoy in in today’s world, we would enter an overshoot
situation above a global population of about 1.75 million people. By
this measure our current population of 7 billion is about 4,000 times
too big and active for long-term sustainability. In other words, by
this measure we are we are now 400,000% into overshoot.
Using the same technique we can calculate that achieving a sustainable
population with an American lifestyle (TF = 78) would permit a world
population of only 650,000 people – clearly not enough
to sustain a modern global civilization.
For the sake of comparison, it is estimated that the historical
world population just
after the dawn of agriculture in 8,000 BC was about five million, and
in Year 1 was about 200 million. We crossed the upper threshold
of planetary sustainability in about 2000 BC, and have been in
deepening overshoot for the last 4,000 years.
a species, human beings share much in common with other large
mammals. We breathe, eat, move around to find food and mates,
socialize, reproduce and die like all other mammalian species.
Our intellec tand culture, those qualities that make us uniquely human,
are recent additions to our essential primate nature, at least in
it makes sense to compare our species’ performance to that of other,
similar species – species that we know for sure are sustainable.
I was fortunate to find the work of American marine biologist Dr.
Charles W. Fowler, who has a deep interest in sustainability and the
ecological conundrum posed by human beings. The following two
assessments are drawn from Dr. Fowler’s work.
2003, Dr. Fowler and Larry Hobbs co-wrote a paper titled, “Is
humanity sustainable?” that was published by the
Royal Society. In it, they compared a variety of ecological
measures across 31 species including humans. The measures included
biomass consumption, energy consumption, CO2 production, geographical
range size, and population size.
should come as no great surprise that in most ofthe comparisons humans
had far greater impact than other species, even to a 99%confidence
level. The only measure inwhich we matched other species was in
the consumption of biomass (i.e. food).
it came to population size, Fowler and Hobbs foundthat there are over
two orders of magnitude more humans than one would expectbased on a
comparison to other species – 190 times more, in fact. Similarly,
our CO2 emissions outdid otherspecies by a factor of 215.
on this research, Dr. Fowler concluded that there are about 200 times
too many humans on the planet. This brings up an estimate for a
sustainable population of 35
is the same as the upper bound established above by examining
hunter-gatherer population densities. The similarity of the
results is not too surprising, since the hunter-gatherers of 50,000
years ago were about as close to “naked apes” as humans have been in
2008, five years after the publication cited above, Dr. Fowler wrote
another paper entitled “Maximizing
biodiversity, information and sustainability." In
this paper he examined the sustainability question from the point of
view of maximizing biodiversity. In other words, what is the
largest human population that would not reduce planetary biodiversity?
is, of course, a very stringent test, and one that we probably failed
early in our history by extirpating mega-fauna in the wake of our
migrations across a number of continents.
this paper, Dr. Fowler compared 96 different species, and
again analyzed them in terms of population, CO2 emissions and
time, when the strict test of biodiversity retention was applied, the
results were truly shocking, even to me. According to this
measure, humans have overpopulated the Earth by almost 700 times. In
order to preserve maximum biodiversity on Earth, the human population
may be no more than 10
million people –
each with the consumption of a Paleolithic hunter-forager.
you can see, the estimates for a sustainable human population vary
widely – by a factor of 400 from the highest to the lowest.
The Ecological Footprint doesn't really seem
intended as a measure of sustainability. Its main value is to
give people with no exposure to ecology some sense that we are indeed
over-exploiting our planet. (It also has the psychological
advantage of feeling achievable with just a little work.) As a
measure of sustainability,it is not helpful.
I said above, the number suggested by the Thermodynamic Footprint or Fossil Fuel analysis
isn't very helpful either – even a population of one billion people
without fossil fuels had already gone into overshoot.
leaves us with three estimates: two at 35 million, and one of 10
think the lowest estimate (Fowler 2008, maximizing biodiversity),
though interesting, is out of the running in this case, because human
intelligence and problem-solving ability makes our destructive impact
on biodiversity a foregone conclusion. We drove other species to
extinction 40,000 years ago, when our total population was estimated to
be under 1 million.
leaves the central number of 35
million people, confirmed by two analyses using different data and
assumptions. My conclusion is that this is probably the largest
human population that could realistically be considered sustainable.
what can we do with this information? It’s obvious that we will
not (and probably cannot) voluntarily reduce our population by
99.5%. Even an involuntary reduction of this magnitude would
involve enormous suffering and a very uncertain outcome. In fact,
it’s close enough to zero that if Mother Nature blinked, we’d be gone.
fact, the analysis suggests that Homo sapiens is an inherently
unsustainable species. This outcome seems virtually guaranteed by
our neocortex, by the very intelligence that has enabled our rise to
unprecedented dominance over our planet’s biosphere. Is
intelligence an evolutionary blind alley? From the singular
perspective of our own species, it quite probably is. If we are to find
some greater meaning or deeper future for intelligence in the universe,
we may be forced to look beyond ourselves and adopt a cosmic, rather
than a human, perspective.
do we get out of this jam?
How might we get from where we are today to a sustainable world
population of 35 million or so? We should probably discard the
notion of "managing" such a population decline. If we can’t get
our population to simply stop growing, an outright reduction of over
99% is simply not in the cards. People seem virtually incapable
of taking these kinds of decisions in large social groups. We can
decide to stop reproducing, but only as individuals or (perhaps) small
groups. Without the essential broad social support, such personal
choices will make precious little difference to the final
outcome. Politicians will by and large not even propose an idea
like "managed population decline" - not if they want to gain or
remain in power, at any rate. China's brave experiment with
one-child families notwithstanding, any global population decline will
be purely involuntary.
A world population decline would (will) be triggered and fed by our
civilization's encounter with limits. These limits may show up in
any area: accelerating climate change, weather extremes,shrinking food
supplies, fresh water depletion, shrinking energy supplies,pandemic
diseases, breakdowns in the social fabric due to excessive
complexity,supply chain breakdowns, electrical grid failures, a
breakdown of the international financial system, international
hostilities - the list of candidates is endless, and their interactions
are far too complex to predict.
In 2007, shortly after I grasped the concept and implications of Peak
Oil, I wrote my first web article on population decline: Population:
The Elephant in the Room. In it I sketched out the picture of
a monolithic population collapse: a straight-line decline from today's
seven billion people to just one billion by the end of this century.
As time has passed I've become less confident in this particular
dystopian vision. It now seems to me that human beings may be
just a bit tougher than that. We would fight like demons to stop
the slide, though we would potentially do a lot more damage to the
environment in the process. We would try with all our might to
cling to civilization and rebuild our former glory. Different
physical, environmental and social situations around the world would
result in a great diversity in regional outcomes. To put it
plainly, a simple "slide to oblivion" is not in the cards for any
species that could recover from the giant Toba volcanic eruption in just
there are those physical limits I mentioned
above. They are looming ever closer, and it seems a foregone
conclusion that we will begin to encounter them for real within the
next decade or two. In order to draw a slightly more realistic picture
of what might happen at that point, I created the following thought
experiment on involuntary population decline. It's based on the idea
that our population will not simply crash, but will oscillate (tumble)
down a series of stair-steps: first dropping as we puncture the limits
to growth; then falling below them; then partially recovering; only to
fall again; partially recover; fall; recover...
I started the scenario with a world population of 8 billion people in
2030. I assumed each full cycle of decline and partial recovery would
take six generations, or 200 years. It would take three
generations (100 years) to complete each decline and then three more in
recovery, for a total cycle time of 200 years. I assumed each decline
would take out 60% of the existing population over its hundred years,
while each subsequent rise would add back only half of the lost
In ten full cycles - 2,000 years - we would be back to a sustainable
population of about 40-50 million. The biggest drop would be in the
first 100 years, from 2030 to 2130 when we would lose a net 53 million
people per year. Even that is only a loss of 0.9% pa, compared to our
net growth today of 1.1%, that's easily within the realm of the
conceivable,and not necessarily catastrophic - at least to begin
As a scenario it seems a lot more likely than a single monolithic crash
from here to under a billion people. Here's what it looks like:
It's important to remember that this scenario is not a
prediction. It's an attempt to portray a potential path down
the population hill that seems a bit more probable than a simple, "Crash! Everybody dies."
also important to remember that the decline will probably not happen
anything like this, either. With climate change getting ready to
push humanity down the stairs, and the strong possibility that the
overall global temperature will rise by 5 or 6 degrees Celsius even
before the end of that first decline cycle, our prospects do not look
even this "good" from where I stand.
assured, I'm not trying to present 35 million people as some kind of
"population target". It's just part of my attempt to frame what
we're doing to the planet, in terms of what some of us see as the
planetary ecosphere’s level of tolerance for our abuse.
The other potential implicit in this analysis is that if we did drop
from 8 to under 1 billion, we could then enter a population free-fall.
As a result, we might keep falling until we hit the bottom of Olduvai
Gorge again. My numbers are an attempt to define how many people might
stagger away from such a crash landing. Some people seem to
believe that such an event could be manageable. I don't share
that belief for a moment. These calculations are my way of getting
that message out.
I figure if I'm going to draw a line in the sand, I'm going
to do it on behalf of all life,
not just our way of life.
can we do?
To be absolutely clear, after ten years of investigating what I
affectionately call "The Global Clusterfuck", I do not think it can be
prevented, mitigated or managed in
any way. If and when it happens, it will follow its own
dynamic, and the force of events could easily make the Japanese and
Andaman tsunamis seem like pleasant days at the beach.
most effective preparations that we can make will all be done by
individuals and small groups. It will be up to each of us to
decide what our skills, resources and motivations call us to do.
It will be different for each of us - even for people in the same
neighborhood, let alone people on opposite sides of the world.
I've been saying for a couple of years that each of us will each do
whatever we think is appropriate to the circumstances, in whatever part
of the world we can influence. The outcome of our actions is ultimately
unforeseeable, because it depends on how the efforts of all 7 billion
of us converge, co-operate and compete. The end result will be
quite different from place to place - climate change impacts will vary,
resources vary, social structures vary, values and belief systems are
different all over the world.The best we can do is to do our best.
Here is my advice:
- Stay awake to what's happening around us.
- Don't get hung up by other people’s "shoulds and
- Occasionally re-examine our personal values. If
they aren't in alignment with what we think the world needs, change
- Stop blaming people. Others are as much victims of the
times as we are - even the CEOs and politicians.
- Blame, anger and outrage is pointless. It wastes
precious energy that we will need for more useful work.
- Laugh a lot, at everything - including ourselves.
- Hold all the world's various beliefs and "isms"
lightly, including our own.
- Forgive others. Forgive ourselves. For everything.
- Love everything just as deeply as you can.
what I think might be helpful. If we get all that personal stuff right,
then doing the physical stuff about food, water,
housing,transportation, energy, politics and the rest of it will come
easy – or at least a bit easier. And we will have a lot more fun doing
I wish you all the best of luck!
May 16, 2013