Footprints, Limits, and Human Carrying Capacity

Lately, I’ve been thinking about footprints.  I live in the Rocky Mountains near the Pike National Forest and often hike or ride my horse in the forest.  Counting my wife and neighbors about a dozen of us tramp through the woods on a regular basis.  We mostly followed faint game trails that over the years have become miles of well established two foot wide tracks through the Ponderosa, Lodgepole and Aspen environments of the adjacent public lands.

In the grand scheme of things, it’s a relatively small footprint.  A slight loss of carbon sink and still too faint to cause any erosion damage.  Judging by the animal prints and scat, it’s safe to say that that the local fauna also make frequent use of our primitive highway system.  Still, however faint, our impact is still visible, clear evidence of our small tribes foot and hoof fall on the local flora.

Out of the forest, back in the neighborhood, our footprint grows ever larger.  Our homes cast their own footprint on the land.  The roads that serve those homes cast an exponentially larger footprint.  In Colorado, the electricity that serves those homes casts a carbon footprint of 1.8 lbs per kwhr and most of us contribute addition carbon with propane furnaces and supplemental wood heating.  We do a bit better with water.  We draw water from shallow wells that tap into the first water flowing out of the eastern slope of the continental divide and we return 85% of the that water to the local aquifer through our septic systems.   When you factor in some of our 100 mile round trip commutes to Denver and the vast global supply chain that delivers pineapples to our households in mid winter, you begin to just get a glimmer of the immense footprint that our small mountain tribe casts upon the world.

The concept of “footprint” used as a way to measure humanity’s impact on the earth first gained traction in the 1990’s when Rees and Wackernagel introduced the idea of “ecological footprint”.  The ecological footprint was and is an attempt to quantify the amount of land required to supply the world’s population with what they consume.  Recently we have added “carbon footprint” and “water footprint” to our lexicon ecological metrics.

Footprints are a very helpful way to visualize environmental impacts. In essence they are direct or indirect references to limits or to the concept of limits.  They help me to frame my mountain tribe and it’s forest tracks within the context of the 6.7 billion human inhabitants of earth hell-bent for growth and the pursuit of happiness in a closed and limited eco-system.

One way to think about limits is through the concept of carrying capacity.  For example, the carrying capacity of a biological species in a closed environment like an island is the population size of the species the environment can be sustained indefinitely, given the food, water and other natural resources available.  The concept of human carrying capacity is a bit more complex since one has to factor in the possibility of leveraging technology to increase the earth’s carrying capacity.  You also have to consider the equal possibility of unintentionally leveraging technology to decrease the earth’s carrying capacity.  Since the free market does not see any “carrying capacity price signals”, technological impacts on organic or natural carrying capacity tend to be skewed toward the negative.  William Catton, author of “Overshoot: The Ecological Basis of Revolutionary Change”, defines human carrying capacity not just in terms of population but also in terms of humanity’s “load” on the environment.

[Human] Carrying capacity needs to be understood as the maximum load an environment can permanently support (i.e., without reduction of its ability to support future generations), with load referring not just to the number of users of an environment but to the total demands they make upon it. For human societies, as for populations of other species, the relation of load to carrying capacity is crucial in shaping our future. Public comprehension of the concepts of carrying capacity and load is both vague and inadequate, and the need to correct these deficiencies is urgent.

When load comes to exceed carrying capacity, the overload inexorably causes environmental damage; then the reduced carrying capacity leads to load reduction (i.e., a crash). – William Catton

In the world of flora and fauna, a species will sometimes stumble upon an environment rich in nutrients creating a large and temporary surplus in carrying capacity.  The usual result is a sharp increase in population leading to an overshoot and a deficit in carrying capacity.  Tragically, this causes both a sharp degradation of carrying capacity and a total population collapse.

Whether humanity will suffer the same fate is subject to debate.  Some predict extinction while others believe that technology will continue to keep us safely in a state of carrying capacity surplus.  While either extreme is possible, I think the truth will end up somewhere north of extinction.

Our greatest danger today is that we rely too little on natural or organic carrying capacity and too much on borrowed or specious carrying capacity.  To use a quasi-mathematical formula:

Human Carrying Capacity = Organic Carrying Capacity + Specious Carrying Capacity

where (in simplest of terms),

Organic Carrying Capacity is a function of:

  • the biosphere
  • surface water and hydrologic cycle
  • the atmosphere
  • solar irradiation
  • top soil

and, the Specious or Borrowed (from the past) Carrying Capacity is a function of non-renewable resources such as:

  • fossil fuels
  • minerals
  • groundwater from slow recharge aquifers

In the case of human carrying capacity, both categories are acted on by technology and our economic systems of finance and trade.  The population explosion starting with the industrial revolution was made possible by the massive technological leveraging of non-renewable resources to create a temporary and specious carrying capacity surplus. The same technological advances caused and continue to cause a concurrent and accelerating degradation of our organic carrying capacity.  As a result, human carrying capacity now rests on an unsustainable house of cards.

As our total population and standard of consumption(load) have taken us back into a condition of carrying capacity deficit and we are now on the brink of collapse.  The tipping point will depend on Liebig’s Law which states that “whatever necessity is least abundantly available in an environment sets the environment’s carrying capacity”.


In our case, the Liebig trigger could be the peaking of oil production, food or water limitations, or a myriad of environmental ills that continue to further degrade organic carrying capacity.  It may even be systemic breakdown in our economy – call it “peak debt”.

Whatever the initial trigger, the question is how we will react.  Will we recognize it for what it is and navigate our way to a new state of equilibrium and balance, or frantically cling to “growth” as we compete for resources in a race to extinction?

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One response to “Footprints, Limits, and Human Carrying Capacity

  1. Pingback: Topics about Energycrisis » Archive » Footprints, Limits, and Human Carrying Capacity

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