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3
GLOBAL CHANGE AND CARRYING CAPACITY:
IMPLICATIONS FOR LIFE ON EARTH
Paul R. Ehrlich, Gretchen C. Daily, Anne H. Ehrlich,
Pamela Matson, and Peter Vitousek
Determining the long-term carrying capacity of Earth--that is, the
number of people that the planet can support without irreversibly re-
ducing its ability to support people in the future--is an exceedingly
complex problem. About all we can be sure of now is that, with present
and foreseeable technologies, the human population has already exceeded
that capacity. Even today's 5.2 billion people can only be supported by
a continuing depletion of humanity's one-time inheritance from the
planet: nonrenewable resources including deep, rich agricultural soils,
Fossil" groundwater, and the diversity of nonhuman species.
Carrying capacity is a function of characteristics of both the human
species and the planet. Through cultural evolution, human beings may
quickly shift their demand for and ability to extract different resour-
ces. At the same time, natural and anthropogenic processes change the
distribution and abundance of resources in the short and medium term.
This paper addresses the latter aspect of carrying capacity: the influ-
ence of global change on the planet's capability to support people over
the next 20 to 100 years.
Carrying capacity can be broken down into a number of interacting
elements, including food, energy, ecosystem services (such as provision
of fresh water, flood control, and recycling of nutrients; Ehrlich and
Ehrlich, 1981), the epidemiological environment, social structure,
politics, and culture. Applying the reasoning of Liebig's ''law of the
minimum," overall carrying capacity is determined by whichever component
yields the lowest carrying capacity. Much of our discussion here focuses
on food because, although it may not ultimately be the limiting resource
of human population size, food production is a crucial factor that is
very sensitive to global change. In addition, basic human nutritional
requirements are relatively inflexible and easy to quantify in contrast
to other elements: there is no substitute for food.
It is especially critical to evaluate carrying capacity now because
the human population has clearly exceeded local and regional carrying
capacities in many parts of the world (FAO, UNFPA, and IIASA, 1982), as
shown by an increasing failure of food production to keep pace with
population growth. For the first time ever, moreover, carrying capacity
has been exceeded globally. Furthermore, human population pressure is
reducing carrying capacity directly through the unsustainable use and
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consequent destruction of natural habitat and agricultural land (Brown,
1988; Ehrlich and Ehrlich, 19889.
The human population has indirect impacts on carrying capacity as
well. The magnitude of these impacts can be evaluated as the product of
three interacting, multiplicative factors, of which population size is
one. The other two factors are per capita consumption of resources (a
measure of affluence) and some measure of the environmental damage
generated by technologies used to provide each unit of consumption
(Holdren and Ehrlich, 1974~. Indirect impacts are causing global
environmental changes that themselves influence the number of people the
Earth can support. Of these changes, the greatest potential consequences
for carrying capacity appear to reside in anthropogenic changes in the
global climatic system.
Population growth thus contributes to a widening gap between the
quantity of resources, especially food, needed by the human population
and the amount that can be extracted from the planet. In the following,
we discuss the reduction in carrying capacity that can be expected to
result from direct human impacts on resources and the environment and
from our indirect impacts on the climatic system.
DIRECT HUMAN IMPACTS
The Stanford Carrying Capacity Project has estimated that the human
population now uses directly, coopts, or has destroyed approximately 40
percent of global net primary productivity on land, the basic food supply
of all terrestrial animals (Vitousek et al., 1986~. Humanity is not only
exercising increasing control over this global food supply but is also
undermining the capacity of photosynthesizing organisms to produce it.
The direct human impact on carrying capacity is especially evident
on marginal land at both extremes of the moisture gradient. Arid and
semiarid regions, particularly in Africa, are suffering severe degrada-
tion through desertification. A total of 27 million hectares of land--
an area the size of the state of Colorado--completely lose economic
utility each year because of excessive human impact (UNEP, 1987~.
Waterlogging and salination lead to 200,000 to 500,000 hectares of
irrigated land coming out of production annually (Goldsmith and Hildyard,
1984~. The carrying capacity for human beings of all this land is
essentially reduced to zero. When land deteriorates to this extent in
poor countries, its inhabitants are forced either to join masses of
displaced peasants in swelling urban slums or to migrate onto other
marginal land where the cycle repeats itself.
Similarly, partly in response to population pressures, human beings
are moving in ever-greater numbers into tropical moist forests (TMFs),
where rapid deforestation and unsustainable agricultural practices render
this land economically useless as well (Raven, 1988~. Population growth
among traditional shifting cultivators also threatens TMF by accelerating
the slash-and-burn cycle to the point that the forest lacks time to
recover between cuttings (Ehrlich et al., 1977~.
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Currently, only about half of Earth's original 16 million km2 Of TMF
remains, and this is being severely disturbed (through intensive logging
and slash-and-burn agriculture) or completely cleared at an annual rate
of roughly 200,000 km2 (Myers, 1988~. Unless these patterns change, in
40 years relatively undisturbed TMF will be restricted to scattered
fragments on steep hillsides and a few ''islands"' in Amazonia, the Congo
basin, and Southeast Asia.
Such wholesale destruction of ecosystems reduces or eliminates
services they once provided to people living both within and far from
them. In addition to undergoing severe soil erosion; some badly de-
forested regions (where evapotranspiration is greatly reduced) suffer
locally drier climates. In the Panama Canal area, as in some other
tropical regions, there has been a steady decline in rainfall associated
with the removal of most of the forest cover (Myers, 1988~. Reduction
in the recycling of water within the ecosystem may thus set up a posi-
tive feedback system that accelerates the loss of TMF.
The recent catastrophic flooding in Bangladesh can be attributed in
part to massive deforestation in the Himalayas (Swaminathan, 1988), a
phenomenon closely tied to population growth. The consequences of the
loss of tropical biodiversity on carrying capacity are even more wide-
spread. Industrialized countries rely heavily on tropical species for
genetic material needed for the maintenance and improvement of strains
of crops now in production (Myers, 1983) and for the development of new
crops that could improve diets of human populations in the tropics
(Ehrlich and Ehrlich, 1981~.
Even more threatening than these direct effects of the human popu-
lation on local and regional carrying capacities are human impacts that
operate by changing global systems indirectly. The most important of
these (but far from the only one) involves exacerbation of the natural
long-term trend of interglacial warming.
GLOBAL WARMING
Anthropogenic climate change has been a matter of deep concern among
environmental scientists for more than 2 decades (Bryson and Wendland,
1968 ; Ehrlich, 1968 ; Ehrlich and Ehrlich, 1970; SCEP, 1970 ; Ehrlich et
al., 1977~. The consensus among atmospheric scientists now is that the
increased injection of greenhouse gases into the atmosphere due to human
activities has already committed the planet to a warming of at least 1 or
2°C (Abrahamson, 1989~. Furthermore, there seems to be little prospect
of curbing future emissions sufficiently to prevent an average tempera-
ture rise of 3 to 4OC? or even more. To put this into perspective, con-
sider that the average surface temperature of Earth during the last ice
age was only 5°C cooler than it is today (Schneider, 1988~!
While the climatic effects of such a warming cannot be predicted with
accuracy, computer models indicate that among the more likely results
will be a decrease in water availability in the world's major grain
belts. In addition, it is agreed that climate change will occur at a
rate unprecedented in recorded history--possibly 10 to 50 times faster
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than the average natural rates of change following the last ice age
(Schneider, 1988~.
This degree and pace of change will inevitably cause major disrup-
tions in world agriculture. Shifting climate belts will require major
adjustments in irrigation and drainage systems at a cost of as much as
$200 billion worldwide (Poster, 1987~. Farmers will have to switch to
drought-resistant crops where possible, thereby incurring reduced yields
(drought-tolerant grains have an average yield less than half that of
corn) (Brown, 1988~. Drought-reduced harvests, like those of the late
1980s, can be expected to occur with greater frequency and severity.
Northward migration of temperature/rainfall belts that are favorable
for grain production may at first glance appear beneficial to agriculture
in regions like Canada and the northern part of the Soviet Union, where,
low temperatures and growing season frosts are limiting factors. But in
many of those areas, thin, infertile soils will severely constrain pro-
ductivity (Jenny, 1980~.
Similarly, an increase in carbon dioxide concentration may enhance
potential productivity, but it is doubtful that this will yield a net
benefit in the face of so many other limitations. Higher temperatures
and more carbon dioxide may unfavorably change relationships between
crops and their pollinators, competitors, or pests. Finally, the un-
willingness of governments to take many of the steps necessary to deal
with nearly certain and unprecedented change will result in considerable
delay and will exacerbate the socioeconomic problems involved in making
adjustments.
Humanity has few options in making such adjustments to the projected
greenhouse warming. The negative impact of climate change on global
carrying capacity is not likely to be offset by increased agricultural
yields through bringing more land into production or through increased
fertilizer use. The potential for increasing the world's cultivated area
is slim--the land area planted in grain worldwide has actually declined
by about 7 percent since 1981 (Brown, 1988), due mainly to three changes:
abandonment of deteriorated land; conversion of cropland to nonfarm uses,
especially in densely populated regions; and set-asides in the United
States.
The primary prospect for expanding food production thus rests with
the potential for increasing yields through more intensive cropping,
increased fertilizer use, or development of more productive strains.
While it is still unclear how much higher yields can be raised (within
economic constraints--inputs are limited by costs), no marked increases
are foreseen in the near future as each of these avenues is approaching
saturation under current economic conditions (Brown, 1988~.
Global warming will also cause a rise in sea level due first to
thermal expansion of the oceans and later to the melting of ice caps in
polar regions, where the projected temperature rise is expected to be
most dramatic. The predicted sea-level rise of as much as 1.4 to 2.2
meters by the end of the next century (Jacobson, 1988) will not only
decrease food production through flooding of agricultural land, but will
also displace millions of people from their homes and livelihoods.
Damage to fisheries from inundation of wetlands that support them will
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adversely affect the nutrition of people who are heavily dependent on
that food resource.
Coupled with land subsidence due to natural processes and the
extraction of oil and groundwater, sea-level rises in some localities
will be much higher than the average. Low-lying, fertile, and sometimes
heavily populated deltas (e.g., the Brahmaputra/Ganges and Nile deltas)
are likely to be submerged first. In Bangladesh and Egypt alone, an
estimated 46 million people may be threatened by flooding (Jacobson,
1988).
Much larger areas of coastal land will become unsafe for human
habitation because of the threat of storm surges carrying far inland.
Developed countries, although more capable of resisting the rising seas,
will not be immune. Holland may have to flood some of its reclaimed
agricultural land with Rhine River water to prevent saltwater intrusion
into groundwater supplies (Schneider, 1988~. In Florida, much of the
Everglades will be lost (with deleterious effects on fisheries), aquifers
will be salinized, and large areas will be made much more vulnerable to
storm damage. The increased frequency and severity of natural disasters
(e.g., drought, storms, and flooding) associated with global warming, at
a time when ecosystems are already stressed, will further reduce Earth's
carrying capacity by decreasing the land area suitable for agriculture
and human habitation.
An increased frequency of drought would also render food production
less predictable, thereby reducing carrying capacity. Such reductions
would be very serious, inasmuch as humanity is unable to feed itself
adequately under current production and distribution systems.
A study by the Alan Shawn Feinstein World Hunger Program at Brown
University (Kates et al., 1988) estimated that, even if food were
equitably distributed (and nothing diverted to livestock), the all-time
record food production of 1985 could have provided a minimal vegetarian
diet to about 6 billion people, a number projected to be exceeded within
the next decade. The same global harvest, allowing a diet with about 15
percent animal products, could feed some 4 billion people. A diet
consisting of 35 percent animal products, similar to that consumed by
most North Americans and West Europeans today, could be provided to only
about 2.5 billion people--less than half of today's population. These
estimates assume a 40 percent loss of the food harvested to pests and
wastage before consumption, a Food and Agriculture Organization estimate
that may be somewhat high. But even if that figure were 20 percent, it
would not permit anything but an adequate vegetarian diet for today's
population.
MODELING GLOBAL CHANGE AND FOOD SECURITY
To examine the possible effect of climate change on food production,
we constructed a simple global model (for details, see G. C. Daily and
P. R. Ehrlich, "An exploratory model of the impact of rapid climate
change on the world food situation," in preparation) that simulates
population growth, annual agricultural output, annual consumption, and
the frequency and severity of unfavorable weather patterns such as
r
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occurred in 1988. The model determines the amount of food available for
consumption (production plus carry-over stocks) in each year over a 20-
year period. For all runs of the model, we assumed that average
increases in grain production would keep up with population growth (1.7
percent annually). In years with favorable weather, we assumed that a
surplus of 50 million metric tons of grain was produced. We then varied
the frequency and severity of unfavorable weather patterns.
Under our most "optimistic" scenario, unfavorable climatic events
occurred on average once every 5 years and caused a 5 percent reduction
in grain harvest, roughly the magnitude of the climate-caused drop in
1988. Under our most "pessimistic" scenario, the mean time between
unfavorable climatic events was 3.3 years, and each event caused a 10
percent drop in grain production below the trend.
In order to simulate the feedback between availability of food and
population size, it was assumed that a food deficit of 1 metric ton of
grain resulted in two incremental deaths. Roughly three people are
supported by each ton of production now, but about one-third of all grain
is fed to animals, so compensation is theoretically available by
consuming more grain directly.
Actual death rates might, of course, be raised further than this
indicates. In the real world, undernutrition occurs mainly among the
poorest people, perhaps the bottom one-quarter or one-fifth of the
population. This group bears the brunt of any deficits, while the rest
usually can maintain adequate diets (although probably at higher prices).
Because of the disproportionate burden on the poor, disease and hunger
may take a heavier toll on them than our all-or-nothing simplification
suggests.
Results of the model suggest that the optimistic scenario (a 5 per-
cent reduction in grain harvest on average twice per decade) would not
lead to complete depletion of world grain stocks, although world food
security would be threatened. These reductions would have little effect
on overall population growth. Under the pessimistic scenario (10 percent
reductions on average three times per decade), however, severe deficits
in grain stocks occur about twice per decade, each causing the deaths of
between 50 and 400 million people.
Weather patterns that might cause such drops include, for instance,
repeats of the 1988 North America/China drought event, with the same or
greater severity, or totally different patterns involving other areas.
In short, we have not incorporated the question of the pattern of crop
failures that would lead to declines in grain production. We also have
not considered compensatory actions such as bringing set-aside land in
the United States back into production, conversion from feed to food
crops, or the general intensification of agricultural activity that would
result from increased demand for food, except to the degree they are
subsumed in our "constant average increase" assumption.
We have also perhaps been pessimistic in not incorporating the
possibility of increases in production because of technical innovations
stimulated by famines. On the other hand, some of our implicit assump-
tions about carrying capacity are optimistic. We have not, for example,
incorporated additional drops in harvest due to social breakdown related
to famines, the spread of disease through malnourished (and thus
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immune-compromised) populations, or inappropriate aid programs that
damage the agricultural sectors of recipient nations.
Indeed, most of our basic assumptions could be considered very
optimistic. For instance, agricultural production is no longer keeping
pace with population growth in Africa or Latin America. Furthermore, we
have assumed that (climatic change aside) production can be kept growing
for 2 decades more in spite of massive erosion of topsoil, increased
waterlogging and salinization in irrigated areas, dropping water tables,
deforestation leading to regional drought and flooding, desertification,
accelerating conversion of land to nonagricultural uses, and so forth.
The model is, of course, simply an aid to thinking about the possible
consequences if short-term climatic change were to cause drops in grain
production of a magnitude roughly comparable to those known to have been
caused before, and considering the rest of the system to be essentially
Surprise free." Our results are not predictions; they are simply
indications of the nature of problems that may occur if the global
warming leads to an increased frequency and severity of climatic events
deleterious to agriculture.
CONCLUSIONS
The population-food system has no '"fail-safe" backup mechanisms
designed into it, even if climates should remain very favorable to food
production. We depend on the statistical ''cushion'' that adverse weather
and unusual pest outbreaks do not occur everywhere at once. To the
degree that global food production becomes more concentrated (as in North
America), humanity becomes more vulnerable. There is no time to be lost
in moving toward population shrinkage as rapidly as is humanly possible;
the momentum of population growth ensures that human numbers cannot start
to decline as a result of reduced fertility in less than half a century
under any realistic assumptions.
Not only is population control required, but governments and
societies must bend their efforts to reduce the rate of global climatic
change (Ehrlich, 1988~. The 1988 drought spurred the U.S. Congress into
action, but bills first introduced in 1988 were still under debate in the
spring of 1989 when the prospect of another drought-reduced harvest in
North America seemed very real. Concerted action to start reducing the
emission of greenhouse gases is needed now because (1) the resistance to
implementing changes is so great and (2) the lead time on many effective
actions will be a decade or more. The problem is especially acute, since
leaders in the rich nations have largely failed to realize the magnitude
of the changes necessary if the warming is to be significantly slowed.
Action in rich nations is needed also to ensure that poor nations will
have some chance to develop through use of their indigenous energy
resources (Ehrlich and Ehrlich, 1989~.
For the indefinite future, Homo sapiens will face major challenges in
supplying everyone with adequate diets. Production must be increased
while at the same time curbing the destruction of irreplaceable soils,
overdrafts of "fossil" groundwater, and the destruction of biodiversity.
Much more effort should go into reducing wastage of food between field
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and stomach, strengthening the agricultural sectors of poor nations in
ways that promote their food security, and improving the equity of food
distribution. Even if all of these daunting ecological, economic,
social, and political tasks can be tackled simultaneously, there is no
guarantee of success.
Only one element of carrying capacity--food--has been examined in
this paper, and many of the complex interactions in the population-food-
climate complex have not even been explored. Nonetheless, our
preliminary analysis suggests that there is no room for complacency
whatsoever.
ACKNOWLEDGMENTS
This work is cosponsored by The Center for Conservation Biology of
the Department of Biological Sciences and the Morrison Institute for
Population and Resource Studies, Stanford University. We thank Susan
Harrison for helpful comments on the manuscript.
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Representative terms from entire chapter:
food production
