By John Feeney:
[Original version published at The Oil Drum; Revised here for clarity - 11/12/07, 11/17/07, 5/31/08, 8/12/08]
Some of us who examine and discuss environmental matters are constantly puzzled and frustrated by the seeming inability of elected officials, environmental organizations, and environmental and political writers to “get” the nature of our ecological plight. Could it be they’re simply unaware of the ecological principles which enable one to understand it?
Since some undoubtedly are, and in light of the warnings in the UN’s latest report on the state of the global environment, here is a brief list of axioms and observations from population ecology with which everyone should be familiar. Most are taught in introductory level ecology and environmental science classes. They appear sequentially, so the reader can step logically through a progression which should make clear some of the fundamental elements of the global ecological challenge before us:
- A finite earth can support only a limited number of humans. There is therefore a global “carrying capacity” for humans. A typical definition of carrying capacity (PDF) is “The maximum number of animals that a specific habitat or area can support without causing deterioration or degradation of that habitat.”
- Axiomatically, a population which has grown larger than the carrying capacity of its environment (e.g., the global ecosystem) degrades its environment. It uses resources faster than they are regenerated by that environment, produces waste faster than the environment can absorb it without being degraded, and otherwise damages the functioning of its environment. Such a population is said to be in “overshoot.”
- Al Bartlett sometimes writes, “A SELF-EVIDENT TRUTH: If any fraction of the observed global warming can be attributed to the activities of humans, then this constitutes positive proof that the human population, living as we do, has exceeded the carrying capacity of the Earth.” The same can be said of much of the rest of the extensive and growing human-caused ecological degradation we see today, including the breakdown of the web of life indicated by the ongoing Sixth Extinction. It is symptomatic of having exceeded the earth’s capacity to sustain our current numbers for the long term. It is, in fact, proof that under current conditions we have done so. Here’s why: Recall the definition of carrying capacity: “The maximum number of animals that a specific habitat or area can support without causing deterioration or degradation of that habitat.” Now consider that most serious, global environmental problems we see today result from our consumption of various resources. The Sixth Extinction, for instance, results in large part from our consumption of habitat on which species depend. Since total consumption = population size x average per capita consumption, then if we agree that the loss of habitat and consequent loss of biodiversity constitute “deterioration or degradation,” we can recognize that, living as we do (per capita consumption), our numbers have passed the point at which the biosphere, our global habitat, can sustain us without suffering degradation — precisely the state of overshoot.  
- It’s axiomatic, as well, that a population can only temporarily overshoot carrying capacity. It will subsequently decline in number, to return to a level at or below carrying capacity. That is, though a population may grow in size until it is too large for existing resources to sustain it, it must subsequently decline.
- Because it degrades it’s environment, a population in overshoot erodes existing carrying capacity so that fewer members of that species will be supported by that habitat in the future.
- Among other factors, our extraction of nonrenewable resources such as oil and coal has allowed us temporarily to exceed the earth’s carrying capacity for our species. As these supplies are drawn down, if alternatives do not keep pace, we will struggle to maintain our present numbers. Likewise, if we remain in overshoot as a result of our numbers and existing or continuing ecological degradation, the number of humans will, of necessity, subsequently come down. Whether we have a hand in voluntarily and humanely bringing them down, or simply let nature manage the whole thing for us, is up to us.
It seems unlikely anyone could fully comprehend the six steps above, and still deny we face a grave, worldwide ecological crisis. But for some, self gain or political ideology tied closely to self-image might be enough to fuel such denial. For others, I hope this little essay is informative.
For an in-depth analysis of the same and related issues try William Catton’s Overshoot.
 Indeed, we can alternatively think of carrying capacity as the sustainable “load” or total consumption level, which is, as mentioned, the product of population size and per person consumption. More sophisticated demonstrations of the impact of population on the environment make use of the related, slightly more complex equation, I = PAT, or its even more sophisticated elaboration, the STIRPAT equation. See this link for details. For our purposes in this essay, the simple multiplication of population size and per capita consumption will suffice.
 Though for humans, carrying capacity may vary somewhat as a function of how we live, no matter how we live we cannot eliminate carrying capacity constraints. It seems unlikely, for instance, that even a hypothetical complete switch to renewable energy, as essential as it ultimately is, would, in itself, drop humanity back to within the limits of carrying capacity. At a time when humans are estimated globally to usurp as much as 40% of all products of terrestrial photosynthesis (link 1 (PDF), link 2, link 3), and when groundwater depletion, habitat destruction and attendant species extinction, extreme overfishing, the global spread of chemical toxins, increased risk of disease due to growing population size, density, and mobility, and the depletion of an array of non-energy resources constitute a large portion of our ecological challenge, we would likely remain in overshoot due to our sheer numbers.
In the end, as we’ve learned from the problem of peak oil, it seems we have never so much extended carrying capacity as learned increasingly and perilously how to overshoot it.