[Revised on 8/30/10, 9/9/10, 9/20/13]

Lately it appears more people are beginning to consider the problem of human population size as it relates to carrying capacity.  Nevertheless, misunderstanding abounds.  Since my focus is elsewhere right now, and I have no plan to write on this topic for publication in the near future, I offer a few thoughts here.  This is merely a quick sketch with much detail, some points of logic, and additional convincing evidence omitted.  I may flesh it out sometime in the future. 

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Have we exceeded the earth's carrying capacity for humans?  Many observers conclude there is no way to answer the question with any confidence.  I believe their view stems from an assumption which fails to hold up to analysis.  It is the common notion that, through human ingenuity, we've regularly been able through the course of history to increase carrying capacity. 



This idea stems in part from the correct observation that the advent of agriculture and our later use of fossil fuels were central among those developments which allowed us to grow the human population as enormously as we have.  It does not follow, however, that these developments increased carrying capacity. 



The error here is in failing to account properly for overshoot.  It is well established that animal populations sometimes overshoot or grow beyond carrying capacity.  It is simple enough to demonstrate this has happened with the human population.  Agriculture and fossil fuels have not increased carrying capacity; they have merely led to our overshooting it, our numbers supported entirely by what William Catton calls "phantom carrying capacity."  It is not carrying capacity at all, and is only temporary. [ 1]



This is especially easy to see with regard to fossil fuel depletion.  Fossil fuels are a finite resource.  Relying on them, therefore, to support global food production can only be temporary. 



But there are some less widely recognized observations which also support my point. ( Recall here the definition of overshoot referenced above.)  First, agriculture as we know it has always been unsustainable.  It has brought with it soil erosion and an inevitable depletion of soil nutrients at rates far faster than their natural rates of renewal.  This is comparable to our depletion of finite resources such as oil.  It may have taken ten thousand years for us to see this, but that is barely an eye blink in human history. [ 2] [3]



Second, consider that none of the processes that have allowed our numbers to explode has come without progressive cost to the web of life.  We know well enough about the environmental impacts of extracting and burning fossil fuels.  Less discussed is the cost of agriculture to other species.  Cultivation agriculture means the elimination of all life from a piece of land, turning it then exclusively to human use via one or a few crops. [ 4] [5] Multiply this by over a billion hectares and we see clearly how agriculture has been the primary driver of today's highly accelerated extinction rates.  These extinctions are believed by many experts to signal a Sixth Mass Extinction of species in Earth's history.  This is the direct destruction of Earth's life support systems.  Indeed, it is well accepted that this extinction event began (or entered a new, accelerated phase) upon the human transition to agriculture. 

Third, in all species population follows food supply.  Natural limits on food supply hold population sizes within appropriate limits. ( Under normal circumstances, this works, by the way, with no particular suffering.)  But by adopting agriculture we circumvent this normal process, thereby inevitably growing our numbers far beyond carrying capacity. 

This point is often misunderstood, perhaps because it has seldom been thoroughly explained.  Additionally, many people have trouble accepting (a) that humans are subject to the same natural processes as other species, and (b) that those processes worked perfectly well for us for nearly all of human history prior to civilization's stepping in and interfering. [ 6]

Not only have we not increased carrying capacity, we have decreased it.  It's simple ecology.  We depend on the web of life for our own survival.  When a species consumes resources faster than they are renewed, degrading the habitat on which it depends, it erodes carrying capacity. 

The damage we have done to the biosphere and the web of life has temporarily allowed us to grow our numbers but has reduced carrying capacity.  This may be hard to believe when we consider that for nearly all of human history, prior to exploding into the billions, our numbers never exceeded more than a few million.  It underscores the shocking degree by which we've overshot carrying capacity. 

No, there is no clear evidence we have ever increased carrying capacity. [ 7] Rather, basic principles of ecology reveal we have managed only to overshoot it by an incredible margin. 



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For some underlying fundamentals of carrying capacity please see the article Six Steps to "Getting" the Global Ecological Crisis. 



[1] For much more see Catton's classic text, Overshoot. 



[2] For more detail see Agriculture: Unsustainable Resource Depletion Began 10,000 Years Ago by soil scientist Peter Salonius. 

[3] Note the interacting nature of factors involved in overshoot.  For instance, also putting fossil fuel use in the category of "phantom carrying capacity" is it's direct link to life-threatening climate change. 

[4] For an illuminating description of this process see Lierre Kieth's The Vegetarian Myth. 

[5] On a large enough scale, even a major improvement such as permaculture-based horticulture would seem to suffer from a similar problem of transforming the land excessively for human consumption.  It is intended, however, to be practiced on a scale small enough -- and with ethical constraints against large populations -- that it might prove sustainable under conditions of a much smaller human population.  This is not to suggest it as an "alternative" to agriculture, without which humans did quite well for most of our history.  As an approach to small-scale gardening -- something practiced by many hunter-gatherers -- it makes great sense.  Once we begin to depend, though, on growing food as our primary means of subsistence, upping the scale by altering large tracts of habitat, taking control of our food supply by creating and storing unnecessary surpluses, the problems start. ( I am aware of nothing concrete, by the way, built into the practice of permaculture which would prevent population growth.  I welcome information to the contrary.) 

That said, should today's civilization come to an end, it is plausible that there may be a transitional period when permaculture and related approaches to food production will be essential to many of those shifting toward true sustainability. 

[For more recent thoughts on horticulture, see the updates under this post.] 

[6] References include the journal articles available on Russ Hopfenberg's site where you can also view this slide presentation. 

[7] Among the only arguable exceptions might be a few very limited, very early developments such as our learning to control fire (which may go back a million years or more) and the use of certain tools.  Yet even these technologies may not strictly qualify as exceptions as they likely directly or indirectly did cause some damage to ecosystems. ( Recall the definition of carrying capacity referenced above.)  Of course this was essentially imperceptible compared to what came later.