A Journal of Solidarity and Sustainability
Vol. 8, No. 12, December 2012|
Luis T. Gutiérrez, Editor
Carrying Capacity and Overshoot: Another Look
This article was originally published in
Approaching the Limits to Growth, 12 May 2011
Reprinted with Permission
In previous articles I've
talked at length about the concepts of "carrying capacity" and
"overshoot". Because these ideas are fundamental to understanding
the predicament in which we humans find ourselves today, I'd like to
present my recent thoughts on them. If you have comments, you may
email me at
email@example.com. Or, if you prefer, you may find me
on Facebook under the name Bodhi Paul Chefurka, where this
article was first published as a pair of notes.
Carrying 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". Unfortunately that
definition becomes more nebulous the closer you look at it – especially
when we start talking about the planetary carrying capacity for
humans. Ecologists claim that our numbers have already surpassed
the carrying capacity of the planet, while others (notably economists
and politicians...) claim we are nowhere near it yet!
I think the confusion arises because we intuitively conflate two very
different understandings of the phrase. I call them the “outside” view
and the “inside” view.
The “outside” view of carrying capacity (I call it CCo) is the view of an observer who
adopts a position outside the species in question. It’s the typical
analytic/synthetic view of an ecologist looking at the reindeer on St.
Matthew’s Island, or at the impact of humanity on other species and its
own resource base. CCo is 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 deductive reasoning. From this point of view
humanity passed CCo a while
ago. It probably happened between 1850 and 1950, depending on what
factors you draw into your assessment, but we certainly passeed it
before 1975. As you will see below, my estimate for when we
passed it is surprisingly early - well before we were aware that it
might be a problem.
The “inside” view of carrying capacity (I call it CCi) is carrying capacity as seen
by a member of the species in question. Rather than arising from an
analytical assessment of the overall situation, it is an instinctual,
experiential judgement limited strictly to the population of one's own
species. All that matters in this view is how many of our own species
will be able to survive to reproduce. If that number is still rising,
we judge that we have not yet passed CCi.
From this point of view humanity has not yet hit CCi, since 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.
In the case of humans it’s possible to shift our perspective from the CCi view to the CCo view through education, but for
most people who are simply living their daily lives, the CCi view dominates their
understanding of the human condition. It’s the instinctual view, after
all, and is therefore a primary driver of our behaviour - and one that
can be only weakly modified by reason.
When a species bumps up against CCi
their population begins to decline. Humanity is now in the
uncomfortable region between CCo and CCi. Outside observers (we
Martian ecologists) have detected the overshoot condition, but the
population as a whole has not identified it yet. As we approach ever
closer to CCi, more and more
ordinary people are recognizing the problem as the symptoms become more
obvious to casual observers. The problem is, of course, the fact that
we've already been in overshoot (above CCo) for quite a while.
When we say that humans have “expanded our carrying capacity” through
technological innovation, we are using our "inside" voice. From the
experiential, subjective, species point of view, we have indeed made it
possible for the environment to support ever more people. This is the
only view that matters at the biological, evolutionary level. In humans
it is this perspective that encourages constant innovation in the face
The combination of our immense intellectual capacity for innovation and
our biological inability to step outside our chauvinistic,
anthropocentric perspective has made it impossible for us to avoid
landing ourselves in our current insoluble global ecological
Note: For the academically
inclined, the "inside/outside" distinction I develop here is also used
by anthropologists who observe other cultures. Anthropologists
call them the "Emic" and "Etic" viewpoints. Emic is the native's
viewpoint (that I call "inside"), while Etic is the observer's
viewpoint (that I call "outside").
When a population surpasses its carrying capacity it enters a condition
known as overshoot.
Because 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 oil,
Population growth in the animal kingdom tends to follow a logistic
curve: an s-shaped curve that starts off low when the species is first
introduced to an ecosystem, then at some later point rises very fast as
the population becomes established, then levels out as the population
saturates its niche.
Humans have been on the front end of our logistic curve for our entire
history. Our population has risen very slowly over the last couple of
hundred thousand years, as we (very) gradually developed the adaptive
skills we needed in order to deal with our varied and changeable
environment (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, we
would probably be following the green curve in the chart below, and
would be well on our way to achieving balance with the energy flows in
the world around us, with our numbers settling down somewhere aroud two
billion. This is the road not taken.
The road we have taken is the one in red, on which our numbers and
consumption have been driven well past the world's long-term carrying
capacity, deep into overshoot territory. As we partied hearty since
1900, we have degraded the flow-based carrying capacity of the
biosphere by messing up the earth, air and water. As a result, when the
party ends, the resulting correction will take us back well below a
carrying capacity of two billion, since that no longer exists - we have
already "eaten" a big chunk of it while we were in overshoot.
The inertia of the correction will carry us even below the remaining
carrying capacity, which I estimate might be enough to support about
1.25 billion people.
I don't think we can do anything to prevent the correction. We may have
been in overshoot since 1920 or so, based on our usage of fossil fuels
Graphic by Gail Tverberg: http://ourfiniteworld.com
Of course we've learned an awful lot since 1920. If we'd been on the
green curve all along we wouldn't have developed wind or solar power,
or large-scale hydro. We need to leverage as much of this knowledge as
we can - not to prevent the Big Red Spike (we can't, it's already been
happening for a hundred years) but to try and slow down the descent a
bit, and maybe prevent the Dead Cat Bounce a century from now.
Oh, where did I pluck the carrying capacity of 2 billion from? It was
the population of the planet in 1925, when we were already using some
coal but had just begun to exploit the other two phases of carbon:
liquid oil and natural gas. It seems as good a place as any to drive in
my picket pin.
The details are all speculative, and open to change based on what
assumptions you choose to make. The overall shape of the curve, though,
will define the human operating environment over the next century or
In conclusion, I would offer the following thoughts to those who are
interested in trying to save civilization.
Politics has nothing to offer this situation. The biophysical driver of
fossil fuel energy transcends all political boundaries and philosophies.
Fossil fuels have supplied 89% of the total primary energy used
throughout the world over the last 55 years, and supply 87% of the
energy used today, according to BP Statistical Review 2011).
Agriculture started the ball rolling about 8,000 years ago, and got our
population to 1.8 billion in 1900 without fossil fuels. But the
logistical curve didn't take off until the serious use of fossil fuels
began in about 1900. Without fossil fuels we'd have seen some
additional growth, but nothing like what we've seen in the last 100
years. That's the message of the green line in the above graph.
To a first approximation, everything we've done since 1900, including
agriculture, has been the result of our use of fossil fuels.
That's the message of the red line.
We are too close to the inflection point (~20 years or less), and have
built too much fossil-dependent infrastructure, for renewables to do
more that act as a buffer in some places against the most egregious
effects of the decline.
"He who knows he has enough is rich."|
Lao Tzu, China, 6th century BCE
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