Ross McCluney*, for the Audubon Society’s Population Internet
List Server 8/30/99 also published as a connection to the AESP web page
at http://www.vicnet.net.au/~aespop/welcome
When environmentalists say that the world is overpopulated, they mean
that the environmental consequences of an excessively high human population
are unacceptable. This leads to the question of what are unacceptable consequences,
and the related question of how many people can the Earth really support.
I've been pondering these questions for a while. Joel Cohen wrote a
book on the subject (How many people can the Earth Support?, W.
W. Norton, 1995). In an astounding Appendix 3, Cohen tracks the estimated
answers to this question from the earliest one listed (1679) to 1994. Estimates
range from the very small (0.5 billion by Ehrlich) to the very large (a
billion billion, obtained by assuming that heat removal is the only limitation,
resulting in a population of around 500 people per square meter and an
outer skin temperature of the structures containing those people of 2000°C).
Most of the variation over the years was due to the limited scientific
knowledge available to earlier prognosticators, and to very widely varying
assumptions regarding what physical or biological factor forces the limitation.
Assumptions are necessary whenever we predict the future. One needs
to make the best educated guess possible concerning what the future consequences
are for projected future changes in policy, behavior, and/or environmental
effects.
Ecologist and Agronomist David Pimentel of Cornell wrote a short article
on the subject for the March/April 1999 (Vol. 5 No. 3) issue of Pop!ulation
Press from the Population Coalition. (Available at www.popco.org.
Click on "Population Press" and "Newsletters & Publications" then look
under the heading for the April 1999 issue.) 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.
To explore this further, we need to consider just what "carrying capacity"
means and we need to look at other issues that may be important in estimating
the maximum human population which not only can be supported by
the Earth but which should be supported.
To begin with, it must be pointed out that humans already pre-empt some
40% of primary photosynthetic production on Earth. This is an astounding
figure, and illustrates how far humans have already gone in taking control
of Spaceship Earth and altering the most fundamental life processes taking
place on it.
A substantial increase in human population would therefore not be possible,
except 1) at the expense of other life-forms with which humans compete
and on which humans also depend, or 2) by finding some means to reduce
this human impact on other life forms while human population continues
to grow. The latter of these alternatives can only be considered wishful
thinking, a radically non-conservative philosophy saying what a few misguided
economists have said, that the human mind can always invent some technological
or other solution to future problems; therefore we should not worry about
them so much. This refrain was stated explicity by Ted Koppel at the beginning
of a recent ABC TV program on genetic engineering.
The first of these alternatives was put in another way by Alan Thornhill,
an ecologist at Rice University, when he said that we are systematically
converting non-human biomass into human biomass. The obvious conclusion
is that at some point there will be insufficient non-human biomass for
humans to eat and sustain life. Of course many other life-support systems
will break down before we reach this point. However, in certain regions
of the Earth, this point has already been reached and people are starving
for lack of food from their own local growing regions.
Of course there are ultimate physical limits to how many people the
Earth can support. Al Bartlett puts it this way: "World population in 1975
was estimated to be four billion and it was growing at the rate of 1.9%
per year.... [If world population growth continues at this rate, it] would
grow to a density of one person per square meter on the dry land surface
of the earth (excluding Antarctica) in 550 years [, a population of 139
thousand billion people.].... Since it is obvious that people could never
live at a density of one person per square meter over the land area of
the Earth, it is obvious that the Earth will experience zero population
growth (ZPG) at some point." Short of this limit, the question is not so
simple to answer. This was clearly illustrated in Joel Cohen's book. Finding
an exact value for the upper limit of human numbers on Earth is a very
slippery slope. One thing is clear from the literature, though. The number
depends very much on two variables, one easy to define, the other more
illusory.
The first variable is the degree of affluence of the people "supported"
by the Earth. We Americans know a lot about affluence, and our environmental
organizations spend a great deal of time trying to reduce the environmental
impacts of the consequences of our per capita affluence, through better,
less polluting technology (and more efficient energy consumption), as well
as through some lifestyle changes, such as recycling and improved business
practices.
The second variable is more difficult to pin down. It has to do with
the future success of technology and new sources of energy in supporting
an expanding population with expanding affluence. Those less concerned
about population limits tend to believe that we can "technologize" our
way out of any problem. So this second variable has to do with how successful
the technologizing process will be over the next several decades, and how
well it can minimize the life-threatening impacts of increasing numbers
of people with increased affluence.
Added to this second variable, and to some extent part of it, is the
question of human adaptability. Can we be relatively happy and content
as a species with fewer other species in the "natural" world around and
with declines in green spaces? Perhaps the vision of those unconcerned
about unlimited growth is that parks can be set aside to insure access
to areas of at least seemingly unspoiled beauty, where at least the few
remaining humans so inclined can get out and have a relatively traditional
outdoors camping experience. Or perhaps it is the vision of some inventive
Japanese who created an indoor ski slope not far from Tokyo, so skiers
don't have to travel great distances to go snow skiing, and so they can
do it year round, indoors.
Setting this sub-issue aside for the moment, assuming that we are so
adaptable that we don't have to worry too much about retaining large scenic
vistas and big sky wilderness areas for human enjoyment, we are still left
with that difficult remaining variable concerning how far we can push the
limits to growth with technological means alone. People apparently can
live at very high population densities. Hong Kong and Tokyo come to mind.
Of course such high densities require fairly large areas in addition to
those cities—for the growing of food, the disposal of wastes, and the acquisition
of other resources. The impacts of high Japanese population densities are
felt far from Japan’s shores.
Here’s what Bruce Sundquist has to say about food-growing limits to
human population numbers:
The lifetime of past civilizations correlates well with their topsoil
resources. Civilizations that had a river system that constantly replenished
topsoil resources always lasted far longer than civilizations that did
not. I argue that topsoil is still the limiting factor for modern-day civilizations
as well. Below I run through a very rough outline of my arguments to give
people an idea of key facts and figures. I could provide more specifics
and references to those who need them. Ignored are such niceties as human
rights, biodiversity, aesthetics etc. Man is considered as purely a mindless
animal consuming food. This is the way to arrive at the most optimistic
conclusions possible.
Irrigated land provides roughly 40% of the world's soil-based food supply.
In the opinion of experts in the field, the ultimate fate of the world's
irrigation systems will be much the same as the fate of irrigation systems
of old--barren salt flats. This is apparently because few systems are underlain
by drainage tiles for draining water away. Irrigation-system growth was
one of the three main reasons why growth of global food supplies kept up
with global population growth over the past 4 decades. Presently however,
creation of new systems is roughly balanced by (and probably less than)
the rate of irrigation-system abandonment due to salination and reallocation
of water supplies to urban uses. The rate of abandonment is sure to increase
dramatically in decades ahead because it takes some time for the effects
of salt buildup to appear, and most irrigation systems are only decades
old.
If one takes the most optimistic data on grassland photosynthesis and
data on how much meat is produced per ton of grass and ton of grain, it
becomes clear that the world's grasslands are overgrazed by a factor of
about 2. This is easy to see from river-sediment data. Rivers draining
the world's arid grasslands are "turbid" and remove several times as much
sediment per acre per year as average sediment loss rates from average
developed land. Most grazing-land sediment is sub-soil from erosion by
gullies and stream banks, so actual topsoil losses may not be much greater
than on croplands, but the basic erosion mechanisms (gullies, stream-bank
erosion) are usually indicative of topsoil erosion on a massive scale--i.e.
over grazing.
If one adds up the rates of topsoil deposition in oceans, river bottoms,
dam backwaters and alluvial plains; then adds topsoil losses due to wind
erosion, salination of irrigation systems, urbanization, and several other
minor effects; then subtracts off topsoil losses from forest lands and
urban lands, one gets a net topsoil loss rate from agricultural land of
roughly 100 billion metric tonnes per year (100 Gt/year)--at least 5 times
the rate of natural topsoil-creation on agricultural lands.
The global inventory of cropland topsoil is about 6500 Gt. Grazing land
topsoil inventories are perhaps three time that (though an acre of grazing
land is only about 1/6 as dollar-productive as an acre of cropland). The
global inventory of top soil on potential (not yet used) croplands is several
times 6500 Gt, but considering only potential croplands that can be cropped
sustainably, the potential inventory is only perhaps 10 percent that of
existing croplands--barely enough to replenish croplands abandoned due
to degradation and urbanization for a few decades. This perhaps explains
why the global cropland inventory has been constant since the early 1980s.
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.
Neglected here is cropland productivity growth due to genetic advances
and increased use of fertilizer--the other two effects that largely supported
population growth during the past 4 decades. However both of these effects
are now close to saturation, so one should not expect really substantive
increases in maximum population values from either of these effects. Increased
use of pesticides to attempt to reduce crop losses from the present 10-20%
of total production has never shown the ability to cut crop losses to pests,
probably because increasing use of monocultures and shrinkage of the global
plant gene-pool have worked to counteract whatever benefits pesticides
might otherwise be expected to offer.
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.
And let us not forget hydroponics that some say permit a global population of 50 billion or so. Hydroponics is useful for producing the more expensive foods (fruits and vegetables) for wealthy First-World people. But the idea of using hydroponics to produce complete diets of average First World people, to say nothing of Third World people, strains credulity. Imagine how many fluorescent light bulbs would be needed to replace the sunlight over all the croplands of the world.
This contribution illustrates how difficult it is to pin down an exact
number for the carrying capacity of the Earth for humans, and illustrates
how complex the human life-support system is and shows the kinds of assumptions
that must be made to produce an estimated maximum population figure. The
energy component of population estimates cannot be ignored. In the United
States, for every calorie of food energy consumed by humans, on the average
it takes 10 calories of fossil fuel energy to grow the food transport it
to the table and prepare and package it. In the United States the ratio
of fossil fuel to solar energy in the food you consume is 10 to 1!
Using his methods Sundquist comes up with sustainable human population
estimates ranging from 0.6 to 1.2 billion, well below the current 6 billion
figure, based on topsoil loss estimates and other factors. The low figure
of 0.5 billion mentioned by Cohen came from this statement by Paul Ehrlich
in 1971: "There are 3.6 billion human beings on the face of the Earth.
According to our best estimates, there are somewhere between three and
seven times more people than this planet can possibly maintain over a long
period of time." [Paul R. Ehrlich, "The Population Crisis—Where We Stand,"
Population, Environment, and People, Noel Hinrichs, ed., McGraw-Hill,
NY, 1971, pp. 8-16.]
"Ecological Footprint" is a term coined by Mathias Wackernagel with
The Task Force on Planning Healthy & Sustainable Communities at The
University of British Columbia. It symbolizes the amount of productive
land required to sustain human life according to different economies. Using
the "Concept of Appropriated Carrying Capacity for Measuring Sustainability",
the method uses a formula for the calculation of land areas required for
human activities, choosing ethanol as the renewable substitute for fossil
fuel and assuming an ethanol productivity per hectare of an optimistic
80 gigajoules per hectare per year [Gj/(ha-yr)] on biologically productive
land. This kind of measurement makes it possible to quantify and compare
energy use across different species and different human societies.
Wakernagel data on Holland gives its population in 1997 as 15,697,000,
with an ecological footprint of 5.3 hectares per capita, in a country with
only 1.7 hectares per capita available. This means that the Dutch population
is using 83,194,100 hectares of bologically productive land altogether
in a country that only has 26,684,900 hectares of such land available.
Obviously Holland is importing a great deal of energy and materials produced
by biologically productive land located elsewhere.
Although these methods seem to be the best around for the time being,
the operational definitions are necessarily still very crude. Of many possible
reasons for this, the main one would be the approximate nature of the information
available for each country. As well as lacking detailed information in
many areas, the world lacks standardized definitions in terms of quantity
and type of oil consumption, variations in quality of biologically productive
land, variations in quality of biologically productive sea, and the amount
of land necessary for preservation of wilderness biodiversity.
Members of the Wackernagel team have estimated that there are approximately 1.7 hectares of biologically productive space (with world average productivity) available per world citizen. On the basis of their formula it has been calculated by the Wackernagel team that if global population continues to grow as expected, in 2030 a predicted world population of 10 billion people will each have an average of only 0.9 hectares of productive land available. (This does not take into account the probability of more soil degradation.) Here is an example of how population size increases demand on "nature's productivity."
--- Sheila Newman, notes from post graduate work in progress, email:
smnaesp@alphalink.com.au
Keeping these issues in mind, let’s look at another one of the assumptions
needed. Sundquist ignored human rights, biodiversity, and aesthetic appreciations
of natural beauty, considering humans only as mindless animals consuming
food. Let’s put these variables back into the equation and ask how many
the Earth can support at various levels of human affluence.
Ecologist and Agronomist David Pimentel of Cornell University has estimated
that the Earth can support from 1 to 2 billion people with an American
standard of living, good health, nutrition, prosperity, personal dignity,
and freedom. Using his 2 billion figure as the benchmark, several additional
estimates of population limits can be offered.
Accurate numbers for each of these possible scenarios, or assumptions
concerning living conditions, are very difficult to come by. I have done
no original research in exploring this topic, and only offer the following
as food for thought. If you are interested in having more detailed knowledge,
you might like to read David Cohen’s very well researched and scholarly
book on the subject.
Few rational people would claim that the Earth could support a population
density of one person per square meter of land area, a population of some
140,000 billion people. Where would you grow the food? Short of that absurd
limit, let us look at somewhat more reasonable estimates. These are my
"rabbit out of the hat" estimates only, intended to provoke thought and
illustrate the difficulties of coming up with good future predictions.
There is at least one conclusion that can be drawn from these guesses,
however. Trying to answer the question of how many people the Earth can
support only brings another question, "What kind of world do you want?"
Maximum Global Population Guesses
Each of these assumes that the current depletion of fossil fuel reserves
has continued to completion. No fossil fuels are left, except possibly
for a small stock, priced high, and used for limited durable uses such
as new plastic production and for some pharmaceuticals.
1. Everyone at the current U.S. standard of living and with all the
health, nutrition, personal dignity and freedom that most Americans currently
enjoy [Pimentel, 1999] 2 billion
2. Everyone at the same affluence level as in 1, but with few restrictions
on commerce, pollution, land use, personal behavior (within current law),
etc. Basically a libertarian, laissez faire economy, with only limited
environmental restrictions. This points out that there is a population
price to pay for the current American way of commerce. 0.5 billion
3. Everyone at the same affluence as indicated in 1, but with many and
onerous restrictions on freedoms relative to behaviors leading to environmental
degradation. In order to accommodate population levels greater than 2 billion,
restrictions such as the following would have to be instituted: Massive
recycling. Driving restrictions (gasolene rationing, fuel rationing even
to mass transit systems). Restrictions on the transport of food (food transported
no more than 100 miles for example to its point of retail sales). Prohibitions
against cutting of trees on one’s property. Limitations on the burning
of fossil fuels in order to save these complex molecules for more valuable
or durable uses, such as in the manufacture of plastics and pharmaceuticals.
Limitations on the areas of open spaces that can be converted to renewable
energy power plants, such as solar thermal, solar photovoltaic, and wind
energy systems. This latter results from the need to preserve natural areas
for atmospheric oxygen generation and food growing. Of course many rooftops
can accept solar energy systems and this scenario basically assumes a nearly
complete saturation of coverage of roof tops and covers over parking lots
for solar energy production. 4 billion
4. Only people in the U.S. and Europe at current level of affluence.
Everyone else at the current prosperity level of Mexico 6 billion
5. Everyone in the world at Mexico’s current prosperity level 20 billion
6. Everyone in the world at the current prosperity level of Northwest
Africa 40 billion
Of course the point of this exercise is to point out that if we wish
to grow the world population to the UN projection of about 12 billion near
the middle of the next century, such growth will have to come at the expense
of many things, not the least of which is compassion for people less fortunate
than we in the U.S.
It also shows somewhat clearly what I have been saying for over 30 years,
that increasing population density is inextricably linked to loss of freedom
and losses of choice. In the worst of the above scenarios, we can forget
the Bill of Rights. This was pointed out recently by M. Boyd Wilcox in
his article, "March 27, 1999: On the anniversary of the Rockefeller Report,
Overpopulation Dilutes Democracy," which appeared in the March/April issue
of Pop!ulation Press. A quote from that article: "One need not pander to
Malthusian or apocalyptic thinking to ask in all seriousness whether the
biosphere can survive another century like this one. Arguably, it is not
biological survival of the human species that is in danger so much as it
is the moral or spiritual survival of what it means to be human and to
be part of a complex living community. We cannot count the ways in which
human identity, imagination, and esthetic appreciation depend on the richly
textured landscape of nonhuman nature. What but unbridled hubris could
let us think that what we consider human nature will survive if we despoil
all of nonhuman nature?"
This exercise also leads to the conclusion of a friend of mine, Paul Jindra, who says that in the end it is not the rule of law that controls human behavior and destiny but the rule of numbers. As the population grows, freedoms of behavior and choice, and niceties such as human rights, of necessity, take a back seat to more fundamental struggles of humans to survive, by whatever means possible.
So we come to this inescapable conclusion when we try to answer the
question of how many people the Earth can support, the answer can only
be a counter question: "What kind of world do you want?" If you want a
world where Americans can continue living pretty much as they are, then
global human population is likely to be substantially less than it is now.
If you wish to keep the current American standard of living where it is,
while allowing the rest of the world to grow substantially in numbers,
the consequence is to doom that "other world" to perpetual misery and lost
expectations, while doggedly holding on to the American way of life as
desperately as possible.
This is not a world I want for my children and grand children, nor is
it one I think anyone else would care to see. The only response, therefore
is to drop all these time and energy-wasting lost-cause efforts to keep
both freedoms of fertility, immigration, and affluence while growing world
human population Instead we should work to reduce the number of
humans living on Earth. This can be done peacefully by conscious action,
or it can be done involuntarily, as is happening currently in Zimbabwe,
with its 30% level of AIDS infection.
To a great extent, the arguments advanced above are like having a multidisciplinary
group of scholars argue for days concerning how many angels can dance on
the head of a pin. We share this world with other creatures, none of which
is capable of talking to us directly. Dennis Philips asks: "What if they
could?"
The choice of which path to take into the Brave New World is ours, as
a species, to make. We can proceed blindly growing, as we now are, and
hope against hope that somehow diseases like AIDS (to other people, in
other parts of the world), wars, and lowered fertility rates due to improved
affluence will reduce the most rapidly growing populations without affecting
us in our own seemingly distant part of the world. We have to acknowledge,
however, that such a philosophy is not a compassionate one, and says something
I’d not like to say about the kind of world we want for our fellow humans.
Due to transportation and communication technologies, however, this isolationist
position probably cannot really insulate us from the terrible consequences
of such a policy. Diseases spreading like wildfire over much of Africa,
or anywhere else, are not likely to remain there.
Alternatively, we can take conscious steps now, and become true to the special "gift" of intelligence that has come to us above all other species, using that intelligence to reduce our numbers providing a better future world for all. The choice is ours to make. It takes only education and persistence.
—
*Dr. McCluney is Principal Research Scientist with the Florida Solar Energy Center in Cocoa, FL, a research institute of the University of Central Florida in Orlando. These are his views and not those of the Florida Solar Energy Center or the University of Central Florida. ©1999 Ross McCluney.