Some of the main pressures on sustainable population levels seem to be
- Depletion of topsoil and salination of topsoil
- Depletion of fisheries.
- Reduction/ loss of fossil fuel resources.
I came to this discussion with the idea that the point at which everyone can still eat wild fish regularly is an important bench mark for sustainable population. That is clearly a lower level of population than we have at present. The extent to which our present population and living standard is dependent on oil for food production, preservation and distribution may also be a critical factor.
Reading Jared Diamond's book and other material quoted below suggests that topsoil depletion is far more important than I would have expected.
David Pimentel made the fundamental assumption that all the people in the world would have the current average standard of living in the United States. His estimate called for a substantial reduction in the current human population, indicating that the Earth’s carrying capacity is inadequate to support the current world population at our relatively high standard of living. His numerical estimate was from 1 to 2 billion people. Since current world population is around 6 billion, the obvious question is how we are currently supporting a larger population than Pimentel’s estimate. The answer is that most of the current six billion people have a much lower standard of living than the one assumed by Pimentel in making his projection. Also, supplemental sources of energy, in addition to our daily budget of energy from the sun, are supporting a population that cannot be supported once these (fossil fuel) energy sources are depleted, as is projected to happen before the end of the next century. Any estimate of long term carrying capacity and human numbers must assume the absence of substantial quantities of fossil fuel resources...
1 have been working on the steady-state problem for over a decade now, but considering mainly topsoil as the limiting resource. I now have annotated reviews of the global literature on topsoil loss, forest land degradation, grazing land degradation, irrigated land degradation, and fishery degradation. It has become clear that there is no need to worry about energy— agricultural top soil and those dependent on it will vanish long before the last barrel of liquefied coal is gone...
I am still working on how to apportion global agricultural topsoil losses between croplands and grazing lands, but I suspect the gross rates are not that much different. This would suggest a cropland topsoil loss rate of 50 Gt per year from a maximum inventory (actual plus potential) of at most 7000 Gt, suggesting a lifetime of human civilization of 7000/50 or 140 years. But now consider that once topsoil depths drop below the depth of the root zone (about 6 inches) cropland erosion becomes nearly irreversible and increases rapid. Current optimistic average depths of cropland topsoil are not over 11 inches, and some data say several inches less. So civilization has only about half of its topsoil to spend down before things get really bad. This gives a lifetime for human civilization of about 70 years--barely one human lifetime, and just an eye-blink in terms of human history.
How is this analysis translated into the number of people that the Earth can support sustainably? Assume that net agricultural topsoil loss rates are directly proportional to human population--an assumption that correlates well with global variations in topsoil loss. In order to reduce gross agricultural topsoil loss to the natural rate of agricultural topsoil creation, the Earth's population would need to fall to about a fifth of its present value--perhaps 1.2 billion. Escalation of irrigated land degradation due to salination could drop this figure to well under one billion...
A far more likely steady-state scenario than human population falling to 1.2 billion is that cropland topsoil is largely destroyed, and the Earth becomes a waste land with populations held constant by war, disease, hunger, suicide and genocide. The productivity of sub-soil is not well known, though it is probably not over 10 percent of the productivity of topsoil. Hence the maximum population under the far more realistic steady-state scenario is probably under 10 percent of the maximum population that a not-erosion-limited, topsoil-based civilization can sustain--possibly 0.6 billion...
Fisheries have been neglected in all this. The problem with fisheries is that Man keeps fishing further and further down the oceanic- and fresh-water food chains, and the lower we go the more dispersed fisheries become. At the dispersion value of the open ocean where about 75 percent of oceanic life-creation occurs, fuel costs for fishing boats per ton of fish harvested increases by about a factor of 100 from present-day values. And present-day fishing-boat fuel costs are already a significant portion of the price people now pay for fish. Aquaculture imposes yet another demand on world grain supplies. So although it may provide a positive contribution to protein sources, its contribution to caloric supply is probably negative. And consider that aquaculture usually entails destruction of coastal wetlands, estuaries and mangrove swamps, all of which provide vital breeding grounds for 80-90 percent of ocean fish, and the frequently-diseased fish in ocean aquaculture pens often escape and devastate populations of their wild cousins. So it is not clear that aquaculture provides a net benefit of any kind.
Bruce Sundquist, Carrying Capacity Committee, Allegheny Group, Sierra Club
Climate change doesn't seem to be factored in.
These limits to population are now being talked of outside the world of population specialists.
Someone dumped some topsoil into a field I can see from my window two years ago. The pile is still sitting there, smooth and rounded by wind and rain, bare and brown, with one or two sad looking weeds growing on it. On only one part, under a tree that some leaves have fallen on, has grass grown.
If the best projections show that our present population won't be sustainable in future, then we need to plan how to manage topsoil and fisheries and our own population levels much better, with the aim of doing it without resource wars and starvation.