Suppose that on the time scale of 100,000 years or more (about equal to the time since the most recent human genetic bottleneck), the stable carrying capacity of the earth for people is only 0.1-1% or less of today's population, i.e. somewhere between 6,000,000 to 60,000,000 people.   Although at first glance this seems an absurdly small number, there is some basis for it (1).  

The time scale is almost unimaginably long for us, so our naive intuitions are not that much help.  While this is not evidence, it allows us to consider the possibility.  A reader might well consider it as some form of speculative science fiction.   We are asking whether there are non replaceable nonrenewable resources which will be degraded over such a long period.   In terms of losses of biodiversity, deforestation, topsoil, essential minerals, etc., living at a reasonable level of technology and economic development, what population is consistent with a (meta)stable world, on the time scale say of 100,000 years.   

I say metastable rather than stable, because otherwise the question seems ill founded or badly posed.  The ecology of the world, with meteors, supernovae, solar changes, climate changes, vulcanism, etc., actually may not be stable on this time scale, independent of the existence of humans altogether.  In addition, because the level of acceptable environmental alteration is an input variable into any such models, the very notion of carrying capacity here is logically and mathematically suspect, i.e. ill defined(2).    Nonetheless, speaking of the slow, population dependent degradation of the environment, it is pretty clear what I am referring to.   Examples might include the deforestation of ancient Greece and Crete.

Although, at this point the basis is very weak, the very idea allows us to think in a different way, on long time scales, and with an ultimate view that only one descendant might eventually stand for every 100-1000 of us alive today.  Alternatively, in terms of critical natural resources, we might think of ourselves as consuming them at a rate of 100-1000 years future supply for each current year's use.

There is some evidence that some human induced deforestation and changes of patterns of game and other animals were already evident at the dawn of the historical period, about 5,000 years ago, when the earth's human population was probably still less than 60,000,000.   This will be considered in subsequent posts.  

Another approach is to work upwards from what might be considered a lower bound.  Supposing the environmental impact of indigenous new stone age people to be acceptable, guessing about 6,000,000 - 30,000,000 ancestors alive 10,000 years ago and multiplying by a factor of 100-1000 for the increased ecological footprint of each person in one of today's technologically advanced societies, we arrive at 3E7/10E2.5 = 10E5 or about 100,000 as a pretty safe lower bound for the temporally extended stable carrying capacity of the earth, for humans living the way we might hope.

As expected, this puts a radically different spin on current events.  First of all, the world is viewed as not just a little overpopulated, but unstable and wildly so.   The probability of a catastrophic ecological crash in the next century or two increases substantially.  The argument has been made that such an ecological  crash has already begun.  This, of course, is very depressing, and in the short term seems to leads to all sorts of psychological ill effects and unbalanced statements.  This is at least partially due to the magnitude of the expected change and its apparent inevitability.  Basically the various stages of reaction to grief are experienced, i.e. denial, anger, bargaining, depression, etc. before some sort of mental balance is restored, both personally and publicly.

Possible solutions begin with recalling the relatively immense amounts of time we are dealing with.  Even to do something substantial in this direction in 1000 years would be very worthwhile, although the sooner the better, obviously.  Furthermore, there are very substantial non obvious technological changes which can be made.   However, these ideas look like science fiction today.

For example Marvin Minsky has informally estimated that the smallest living form with a level of mental complexity similar or greater than our own (representing say 10^15 bits of information changing at 10^12 bits/second would be about (10^4 nm)^3 or (10 micron)^3, i.e. about the size of a single human cell (3).   So we could probably engineer entities as smart and even as emotionally and linguistically aware as Shakespeare in about the volume of  a paramecium (about 100 micron)^3 in a few thousand years, with complete backup of all mental contents, and a very long lifespan, say about 500 years.  Furthermore such an organism's brain might work thousands of times faster than our own.  

One could even imagine a a civilization based on a graded series of people of different sizes, starting with a few of  ourselves (large size, used for physical labor), proceeding downward to pygmy sized, cat sized, mouse sized, insect sized, etc., down to the minimal size referred to above.  Maybe beings of different sizes would aggregate as "tribes", with geometrically more of the smaller ones.   In this way we could remain within the ecological  limits mentioned above, while still not becoming too lonely.  In a sense, the rate limiting obstacle to the development of this type of society is not technology but security.  Who would be the first to allow their children to be "downsized" in a hostile world such as we have today?

Returning from such dizzying speculation, we see our current problems in a new light.  Our task is to develop the social structures to allow for non catastrophic reduction of the population size of today's humans to eventually take place.  Beginning in the very difficult present, we would like to establish Utopian  structures based on respect for others, mutual security and development, as we are largely already trying to do.  In the meantime we would like to reduce the suffering which we see so much of, in the standard way we have developed via our best current institutions.  We also want to conserve as much as possible, to leave as much as we can for the more stable and balanced civilization(s) to come.   

Unfortunately, like all Utopian schemes, this appears impossible at present.  Nonetheless, it appears necessary.

One of the big differences to arise from this longer view is how we view the undeniable present day competition for food and resources amongst ourselves, namely as a temporary phenomenon.  Presumably, land hunger will diminish, (although this is not certain) as we anticipate  that our current territory will eventually be inhabited at a (mass normalized) population density of say 0.3% of what we see today.     If there are so many fewer people, one might expect that they would appreciate their neighbors more, although again, this is not certain.  

Admittedly, this is not the only possible  outcome of an ecologically stable society.   After all ego and insecurity could still lead to willful negligence and worse.  Sparse populations of stone age peoples killed each other all too often, and our egotism cannot simply be overcome by these sorts of technological fixes.    But it is a possible longer term physical subgoal, when other goals seem either nonexistent or ecologically non sustainable.

Notes
1.  This work all derives from a speculative estimate published in Nature about 15 years ago of the expected lifespan of the human species, and some subsequent correspondence.  I seem to recall that the number of 600,000 as the optimal number of humans in a technologically advanced stable earth was advanced there, but I am still searching for the author(s) and the references.  
2.  See  Joel Cohen  at www.environmentalreview.org/vol03/cohen.html for an opposing view as to whether or not the idea of carrying capacity makes any sense at all.  Summary at   www.overpopulation.com/faq/Natural_Resources/carrying_capacity.html
3.  Such an organism would not resemble a computer in architecture.   Rather, it might be built along the lines of neural architectures offered by Walter J. Freeman and colleagues.   Minsky has also made the point that any autonomous engineered organisms such as this would require emotions.   I expect that we could design organisms like this within a few centuries to a few millenia.

 Last updated 05/09/2005