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Will Limited Land, Water, and Energy Control Human
Population Numbers in the Future?
David Pimentel &Michele Whitecraft &Zachary R. Scott &Leixin Zhao &
Patricia Satkiewicz &Timothy J. Scott &Jennifer Phillips &Daniel Szimak &
Gurpreet Singh &Daniela O. Gonzalez &Tun Lin Moe
#Springer Science+Business Media, LLC 2010
Abstract Nearly 60% of the world’s human population is
malnourished and the numbers are growing. Shortages of
basic foods related to decreases in per capita cropland,
water, and fossil energy resources contribute to spreading
malnutrition and other diseases. The suggestion is that in
the future only a smaller number of people will have access
to adequate nourishment. In about 100 years, when it is
reported that the planet will run out of fossil energy, we
suggest that a world population of about two billion might
be sustainable if it relies on renewable energy technologies
and also reduces per capita use of the earth’s natural
resources.
Keywords Sustainable world population .Fossil fuels .
Population growth .Agricultural land degradation
Introduction
Developed and developing nations need to provide a good
quality life for their people while coping with rapid
population growth, but “Population is the issue no one
wants to touch”(Meadows 2000). The current world
population is about 6.8 billion. Based on the present growth
rate of 1.2% per year, the population is projected to double
in approximately 58 years (Chiras 2006;PRB2008).
Because population growth cannot continue indefinitely,
society can either voluntarily control its numbers or let
natural forces such as disease, malnutrition, and other
disasters limit human numbers (Bartlett 1997–98; Pimentel
et al.1999). Increasing human numbers especially in urban
areas, and increasing pollution of food, water, air, and soil
by pathogenic disease organisms and chemicals, are
causing a rapid increase in the prevalence of disease and
human mortality (Murray and Lopez 1996; Pimentel et al.
2007). Currently, more than 3.7 billion humans are
malnourished worldwide—the largest number ever (WHO
2005a,b).
The planet’s numerous environmental problems highlight
the urgent need to evaluate available land, water, and energy
resources and how they relate to the requirements of a rapidly
growing human population (Pimentel and Pimentel 2008). In
this article we assess the carrying capacity of the Earth’s
natural resources, and suggest that humans should voluntar-
ily limit their population growth, rather than letting natural
forces control their numbers (Bartlett 1997–98; Ferguson
1998; Pimentel et al.1999). In addition, we suggest
appropriate policies and technologies that would improve
standards of living and quality of life worldwide.
Population Growth and Consumption of Resources
All of our basic resources, such as land, water, energy, and
biota, are inherently limited because of human abundance.
At the current growth rate of 1.2% the world’s population
will double to 13 billion in 58 years (PRB 2008).
The U.S. population doubled during the past 70 years
from 151 million to more than 305 million, and based on
current growth of approximately 1.1% per year (USCB
2002,2009) is projected to double again to 600 million in
the next 64 years. China’s population is 1.3 billion, and
despite government policy permitting only one child per
D. Pimentel (*):M. Whitecraft :Z. R. Scott :L. Zhao :
P. Satkiewicz :T. J. Scott :J. Phillips :D. Szimak :G. Singh :
D. O. Gonzalez :T. L. Moe
College of Agriculture and Life Sciences, Comstock Hall,
Cornell University,
Ithaca, NY 14853, USA
e-mail: dp18@cornell.edu
Hum Ecol
DOI 10.1007/s10745-010-9346-y
couple, is still growing at an annual rate of 0.6% (PRB
2008). Note that the rate of population growth in the U.S. is
nearly double that of the Chinese population.
In addition to limitations due to population size, high
per capita consumption levels in the United States and
other developed nations put further pressure on natural
resources. Americans consume several times more goods
and services because of relatively abundant per capita land,
water, energy, and biological resources, as compared to the
Chinese (PRB 2008). However, industrialized China emits
more carbon dioxide than the U.S., largely due to its heavy
use of coal (NEAA 2008). Achieving an average European
standard of living ($35,000 per capita/yr.) or an average U.S.
standard of living ($45,000 per capita/yr.) appears unrealistic
for most countries because of serious shortages of basic
natural resources (PRB 2008). This does not imply that both
developed and developing countries cannot use their
resources more efficiently than they are at present through
the implementation of appropriate policies and technologies.
Thus far, Americans enjoy relative affluence because of
fertile cropland, abundant water, and cheap fossil energy. If
the U.S. population continues to expand as projected,
however, resource shortages similar to those now being
experienced by China and other developing nations will
become more common (Table 1), and accelerated declines
in living standards are likely.
Status of World Environmental Resources
The quantity and quality of cropland, water, energy, and
biological resources determine the current and future status
of the support services for human life. There are measurable
shortages of fertile land, water, and fossil energy in many
regions of the world, making it appropriate to ask, “Are we
consuming too much?”(Arrow et al.2004).
Land Resources
More than 99% of human food (calories) in the world is
derived from the terrestrial environment. Although only
0.3% comes from the oceans and other aquatic ecosystems
(FAO 2003), even these resources are being stressed near to
breaking point.
As Botsford et al.(1997) note “the global marine catch
is approaching its upper limit”and “management has failed
to achieve a principal goal, sustainability.”Worldwide, food
and fiber crops are grown on 11% of the Earth’s total land
area of 13 billion hectares. Globally, the annual loss of land
to urbanization and highways ranges from 10 to 35 million
hectares (approximately 0.5%) (Döös 1994). Much of this
land is prime cropland, including prime coastal and river
valley land (Döös 2002; Ho and Lin 2004).
In 1960, when the world population numbered about 3
billion, approximately 0.5 ha of cropland was available
per capita worldwide. This half a hectare is needed to
provide a diverse, healthy, nutritious diet of plant and
animal products—similar to the typical diet in the United
States and Europe (Giampietro and Pimentel 1994). The
average per capita world cropland is now only 0.22 ha, or
about 40% the amount needed according to industrial
nation standards (Table 1).
Grain production, the most efficient use of cropland
because it provides more than 80% of world food, has not
kept pace with population growth (Cassman et al.2003).
Grain consumption per capita has declined 12% from its
peak in 1984 to 2006 (Kondratyev et al.2003; Earth Policy
Organization 2008). If the amount of grain land remains the
same in 2050 as it was in 2000, grain land per capita will
shrink from 0.10 to only 0.07 ha due to population growth
(Larsen 2003). Already, in China the amount of available
cropland is only 0.10 ha per capita, and rapidly declining
due to continued population growth and extreme land
degradation (Pimentel and Wen 2004). This shortage of
productive cropland is one underlying cause of current
worldwide food shortages and poverty (Leach 1995;
Pimentel and Pimentel 2008).
Currently, Americans consume a total of nearly
916 kg/yr per capita of food products, (USDA 2007).
While the Chinese consume less per capita, by all
measurements, they have reached or exceeded the limits
of their agricultural system (Pimentel and Wen 2004). Their
reliance on large inputs of fossil-fuel based fertilizers—as
well as other limited inputs—to compensate for shortages of
arable land and severely eroded soils will present severe
problems in the future (Wen and Pimentel 1992).
Since cropland has become relatively scarce worldwide,
farmers will need to produce increasing amounts from what
is currently available. This intensification exacerbates land
degradation, including water and wind erosion, and the
Table 1 Resources used and/or available per capita per year in the
United States, China, and the world to supply the basic needs of
humans (FAO 1998,2006; Goklany 2001)
Resources U.S. China World
Land
Cropland (ha) 0.59 0.10 0.22
Pasture (ha) 0.79 0.30 0.52
Forest (ha) 1.01 0.15 0.61
Total 3.06 0.71 2.00
Water (liters×10
6
) 1.7 0.45 0.60
Fossil fuel
Oil equivalents (liters) 9,500 700 2,100
FAO ( 1998,2006); Goklany (2001)
Hum Ecol
salinization and water-logging of irrigated soils (Kendall
and Pimentel 1994; Crosson 1997), threatening most crop
and pasture land worldwide (Fischer et al.2005). World-
wide, more than 10 million hectares of productive arable
land are heavily degraded and abandoned each year
(Pimentel 2006).
The urgent need for more agricultural land accounts for
60% to 70% of deforestation now occurring worldwide
(Myers 1990; Butler 2009), which, in turn, is the prime
cause of soil degradation and loss of freshwater in South
America and Asia and a major contributor to soil
degradation on other continents (Oldeman et al.1990).
The vast majority of any future cropland expansion is
expected to occur in Latin America and sub-Saharan Africa,
leading to massive losses of the world’s remaining
tropical and temperate forests, savannahs, and grasslands
(Kloverpris et al.2008)
Current erosion rates of agricultural land by wind and
water, the most serious causes of soil loss and degradation
(Oldeman et al.1990), are greater than ever (Pimentel
2006). On average, humans have increased soil erosion at
least tenfold from what is geologically normal, with some
areas eroding at a thousand times the normal rate
(Montgomery 2007). Soil erosion on cropland ranges from
about 13 tons per hectare per year (t/ha/yr) in the United
States to 40 t/ha/yr in China (Pimentel and Wen 2004).
Worldwide, soil erosion averages approximately 30 to 40 t/
ha/yr, or about 30- to 40-times faster than the replacement
rate (Pimentel 2006). Soil eroded by wind in Africa is
detected in Florida and Brazil each year (Pimentel 2000).
Erosion adversely affects crop productivity by reducing
the water-holding capacity of the soil, water availability,
nutrient levels and organic matter in the soil, and soil depth
(Sanchez 2002). Croplands on steeper slopes are especially
at risk for accelerated erosion. Over the next century, 33%
of the steepest of U.S. cropland is projected to fall out of
production due to erosion (Montgomery 2007). Estimates
project that agricultural land degradation alone can be
expected to depress world food production between 15%
and 30% by the year 2020 (Crosson 1997; Pimentel 2000).
The global economic cost of soil erosion is estimated to be
about $400 billion per year (Lal 1997), emphasizing the
need to implement known soil conservation techniques,
including biomass mulches, no-till, ridge-till, terracing,
cover crops, grass strips, crop rotations, or combinations
of all of these. All these techniques essentially require
keeping the land protected from wind and rainfall energy
with some form of vegetative cover.
The current high erosion rates throughout the world are
of great concern because of the slow rate of topsoil renewal;
it takes approximately 500 years for 2.5 cm (1 in.) of
topsoil to form under agricultural conditions (Troeh et al.
2004a). The U.S. is losing soil at ten times the rate of
sustainable replacement, and the rate is higher in the rest of
the world (NAS 2003).The effects of climate change are
expected to lead to increased intensity of storm events
worldwide (SWCS 2003), with predictions of 20% to
almost 300% increases in erosion rates in some areas as a
result of high-intensity rainfall (Nearing et al. 2004;
Montgomery 2007).
The fertility of nutrient-poor soil can be improved by
large inputs of fossil-based fertilizers. The global doubling
of grain yields from 1961 to 2000 can be partially attributed
to the 700% increase in fertilizer use during the same period
(Matson et al. 1997; Tilman et al.2001). This practice,
however, increases dependency on limited fossil fuels
necessary to produce these fertilizers.
If the U.S. population were reduced from the current 305
million to 200 million, per capita cropland would increase
to about 0.7 ha (USDA 2007). Using more crop rotations,
cover crops, grass strips, mulches and other types of soil
conservation technologies will require additional cropland.
Still the U.S. should have ample cropland available for
domestic food production, plus some for export.
Water Resources
The present and future availability of adequate supplies of
freshwater for human and agricultural needs is already
critical in many regions, like the Middle East (Postel 1997).
Rapid population growth and increased total water con-
sumption are rapidly depleting available water. Between
1950 and 1995, per capita availability of freshwater
worldwide declined by about 70% (Gleick 2008–2009).
All vegetation requires and transpires massive amounts
of water during the growing season. Agriculture uses more
water than any other activity on the planet. Currently, 70%
of water removed from all sources worldwide is used solely
for irrigation (Pimentel and Wilson 2004). Of this, about
two-thirds are consumed by plant life (non-recoverable)
(Postel 1997). For example, a corn crop that produces about
9,000 kg/ha of grain uses about 7 million liters/ha of water
during the growing season (Pimentel and Pimentel 2008).
To supply this much water, approximately 1,000 mm of
rainfall per hectare—or 10 million liters of water—is
required. The estimated “minimum amount of water
required per capita for food is about 400,000 l per year
worldwide and in the United States, the average amount of
water consumed annually in food production is 1.7 million
liters per capita per year”(Sustainable World Water 2002).
Most of the 1.7 million liters is for irrigated food
production.
Water resources and population densities are unevenly
distributed worldwide. Even though the total amount of
water made available by the hydrologic cycle is enough to
provide the world’s current population with adequate fresh
Hum Ecol
water—according to the minimum requirements cited
above—most of this total water is concentrated in specific
regions, leaving other areas water-deficient. Water demands
already far exceed supplies in nearly 80 nations of the world
(Gleick 1993). In China, more than 300 cities suffer from
inadequate water supplies, and the problem is intensifying as
the population increases (Berk and Rothenberg 2003). In arid
regions, such as the Middle East and parts of North Africa,
where yearly rainfall is low and irrigation is expensive, the
future of agricultural production is grim and becoming more
so as populations continue to grow. Political conflicts over
water in some areas have even strained international relations
between critically water-starved nations (Gleick 1993).
Because of their slow recharge rates, usually between 0.1%
and 0.3% per year (Wheal 1991;Covich1993), groundwater
resources must be carefully managed to prevent depletion.
Yet, groundwater resources are also mismanaged and over-
tapped. In the state of Tamil Nadu, India, groundwater levels
declined 25–30 m during the 1970s as a result of excessive
pumping for irrigation (UNFPA 1991;Pimentel2002). In
Beijing, the groundwater level is falling at a rate of about
1 m/yr; while in Tianjin, China, it drops 4.4 m/yr (Postel
1997). In the United States, aquifer overdraft averages 25%
higher than replacement rates. In an extreme case such as the
Ogallala aquifer under Kansas, Nebraska, and Texas, the
annual depletion rate is 100% to 140% above replacement
(Ehrlich and Ehrlich 1997). In parts of Arizona, water in
some aquifers is being withdrawn 10-times faster than the re-
charge rate (Gleick et al.2002).
Desalinization of ocean water is not a viable source for
freshwater needed by agriculture, because the process is
energy intensive and, hence, economically impractical. A
desalinization system in East Africa, for example, reports
70% less energy than other systems, yet still requires
2.3 kwH (nearly 2,000 kcal) per cubic meter (1,000 l) of
water (Gilau et al. 2007). The amount of desalinized water
required by 1 ha of corn would cost $14,000, while all other
inputs, like fertilizers, cost only $500 (Pimentel et al.
2004). This figure does not even include the additional cost
of moving large amounts of water from the ocean to inland
agricultural fields.
Another major threat to maintaining ample fresh water
resources is pollution. Considerable water pollution has
been documented in the United States (USCB 2008), but
this problem is of greatest concern in countries where water
regulations are less rigorously enforced or do not exist.
Developing countries discharge approximately 90% to 95%
of their untreated urban sewage directly into surface waters
(WHO 1993; Pollution Problem 2009). Downstream, the
polluted water is used for drinking, bathing, cooking, and
washing (WHO 1992).
Overall, approximately 95% of the water in developing
countries is polluted (WHO 1992). There are, however, also
serious problems in the United States. The Environmental
Protection Agency (EPA 1994) reports that 40% of U.S.
lakes are unfit for swimming due to runoff pollutants and
septic discharge.
Pesticides, fertilizers, and soil sediments as well as some
100,000 different chemicals applied to crops (Nash 1993)
pollute water resources when they accompany eroded soil
into a body of water. In addition, industries all over the
world often dump untreated toxic chemicals into rivers and
lakes (WRI 1993;WHO1993). Although some new
technologies and environmental management practices are
improving pollution control and the use of resources, there
are economic and biophysical limits to their use and
implementation (Gleick 1993).
Food Production
Reducing the calorie intake from about 3,747 kcal per
day to about 2,300 kcal would improve the health of the
U.S. population. Moreover, a primarily plant-based
Mediterranean diet of minimally processed foods and
seasonal and locally produced foods is highly recom-
mended (Willett et al.1995).
What has been thought of as waste (manure) from
livestock will be a valuable source of nutrients for crop
production and an energy source in the form of biogas (Ro
et al. 2007; Cantrell et al.2008). All livestock should be
moved back on farms to make use of the manure and
produce biogas.
The primary plant foods in the future will probably be
rice, wheat, corn, potatoes, soybeans, cabbage, and a few
other plant foods. All of these crops can be produced
without any liquid fuel except for the 60 l of ethanol per
hectare to run small tractors (Pimentel et al.2008a). For a
population of 200 million, estimated as sustainable for the
U.S. in the absence of fossil fuels, the area of cropland
needed would be 100 million ha, based on 0.5 ha/capita.
This reflects the present use of cropland in the U.S., and is
estimated to be applicable to a fossil fuel-free future
because lower crop yields could be offset by a decrease in
the consumption of meat.
The ethanol required to operate the tractors would therefore
amount to 60 billion liters. Although with current conven-
tional corn technology the energy balance in producing
ethanol is negative, if an organic corn production technology
were employed similar to that in Table 2, then the energy
balance would become positive (output exceeds input).
Growing corn this way is likely to yield a useful 2,000 l of
ethanol per ha, so the area of corn needed to produce the
ethanol would be 30 million ha. The total of 130 million ha
is less than the total area of cropland in the USA, but only
ethanol needed for producing crops has been accounted for,
and liquid fuels will be needed for other reasons.
Hum Ecol
It may be noted that although processing the 8,000 kg/ha
of corn shown in Table 3into 2,000 l of ethanol would
require an estimated 9 million kcal of non-liquid energy,
this is mostly heat energy that could come from wood or an
electrical energy source.
The 30 million ha needed for ethanol production adds
30% to the 100 million ha of cropland needed to grow
crops for consumption. Using draft animals would require
much the same actual area. Two teams (four horses) would
be needed to manage 20 ha in a practical manner, and this
would require 7 ha of land to provide the corn, hay and
pasture needed for the horses (Morrison 1946; Ferguson
2008). Thus 100 million hectares would require 35 million
hectares of land for horses. This is a larger area but only 3%
would be cropland (Morrison 1946), i.e., about 1 million ha
of cropland; so horse power could be a better choice.
Of the 400 million draft animals used in world
agriculture, 19 million are located in sub-Saharan Africa,
primarily oxen (Muvirimi and Ellis-Jones 1999). Although
there is a tradition of using oxen and horses, donkeys and
cows will increasingly be used for draft power (Muvirimi
and Ellis-Jones 1999).
Nitrogen nutrients should be produced using nitrogen-
fixing legume crops, such as vetch. The U.S. uses 13 million
tons of commercial nitrogen per year (USDA 2007). Currently
biological nitrogen fixation in the U.S. yields approximately
14 million tons of nitrogen per year (Pimentel 1998). Even
with a major reduction in livestock population, possibly 300
million tons of nitrogen could be produced on farms. Note that
since 1950 most livestock manure is not produced on farms
(NAS 1989). One ton of cow manure has 6 kg of nitrogen, and
can only be transported slightly more than 8 km before the
nitrogen energy benefits in the manure equal those of
inorganic fertilizer (Wiens et al.2008), which is why
livestock that are grass-fed must be managed on the farm.
Nitrogen fertilizer can be produced employing electrolysis
but requires 8,800 kcal/kg of electrical energy (IFFCO 2008).
This electrical energy translates into 26,400 kcal. The energy
required for nitrogen production using electrical energy is
reportedly much higher than producing nitrogen fertilizer
using natural gas, which requires 16,000 kcal per kilogram
of nitrogen (Patzek, T., Personal Communication, 2009).
Energy Resources
Over time, people have relied on various sources of power
ranging from human, animal, wind, tidal, and water energy,
to wood, coal, gas, oil, and nuclear sources for fuel and
power. Fossil fuel energy permits a nation’s economy to
feed an increasing number of humans, as well as to improve
the general quality of life in many ways, including the
protection from numerous diseases.
About 473 quads (1 quad= 10
15
BTU or 1,055×10
18
Joules) from all energy sources are used worldwide per year
(International Energy Annual 2007). Increasing energy
expenditure is caused by rapid population growth, urbaniza-
tion, and high resource consumption rates (Table 1). Increased
energy use also contributes to environmental degradation
(Pimentel and Pimentel 2008). Energy use has been growing
even faster than world population. From 1970 to 1995, energy
use was increasing at a rate of 2.5% (doubling every 28 years)
whereas worldwide population grew at only 1.7% (doubling
every 42 years) (PRB 1996; International Energy Annual
1995–2007). Current energy use is projected to increase at a
rate of 2.2% (doubling every 32 years) compared with a
population growth rate of 1.2% (doubling every 58 years)
(PRB 2008; International Energy Annual 2007).
Table 2 Energy inputs and costs of corn production per hectare
(8,000 kg corn) in the United States and potential for reduced energy
inputs compared with 8,228 kcal X 1000 total energy inputs for
conventional United States corn tillage. Pimentel et al.(2008a,b);
Pimentel and Patzek (2008)
Inputs Quantity kcal×1000
Labor 15 h 608
Machinery 10 kg 185
Ethanol 60 L 684
Nitrogen Legumes 1,000
Phosphorus 45 kg 187
Potassium 40 kg 130
Lime 600 kg 169
Seeds 21 kg 520
Irrigation 0 0
Herbicides 0 0
Insecticides 0 0
Electricity 13.2 kWh 34
Transport 75 kg 25
Total 3,542
Table 3 Potential renewable energy for the United States
Energy technology Current quads Projected (2100) quads
c
Biomass 3.3
a
7
Hydroelectric 2.9
a
5
Geothermal 0.3
a
3
Solar thermal 0.06
b
10
Photovoltaics 0.06
b
10
Wind power 0.3
a
8
Biogas 0.001
b
0.5
TOTAL 6.8 43.5
a
EIA (2008)
b
USCB (2008)
c
Calculated from Pimentel (2008)
Hum Ecol
Although about 50% of all the solar energy captured by
photosynthesis worldwide is used by humans, it is still
inadequate to meet all of humanity’s need for food
(Pimentel and Pimentel 2008). To make up for this
shortfall, about 473 quads of fossil energy (oil, gas, and
coal) are utilized worldwide each year (International Energy
Annual 2007). Of this, 109 quads are utilized in the United
States (USCB 2008). The U.S. population consumes 70%
more fossil energy than all the solar energy captured by
harvested U.S. crops, forest products, and other vegetation
each year (Pimentel et al.2008b). Industry, transportation,
home heating, and food production account for most of the
fossil energy consumed in the United States (USCB 2008).
Per capita use of fossil energy in the United States is 9,500 l
of oil equivalents per year, more than 13 times the per
capita use in China (Table 1). In China, most fossil energy
is used by industry, but approximately 25% is used for
agriculture and the food system (Pimentel and Wen 2004).
Developed nations annually consume about 70% of the
fossil energy worldwide, while the developing nations,
which have about 75% of the world population, use only
30% (International Energy Annual 2007). The United
States, with only 4.5% of the world’s population, accounts
for almost 25% of the world’s carbon emissions from fossil
fuels (West 2008; PRB 2009).
Several developing nations that have high rates of
population growth are increasing fossil fuel use to augment
their agricultural production. In China, there has been a
100-fold increase in fossil energy use in agriculture for
fertilizers, pesticides, and irrigation since 1955 (Pimentel
and Wen 2004).
Fertilizer production on the whole, though, has declined
per capita by more than 22% since 1991, especially in the
developing countries, due to fossil fuel shortages and high
prices (IFIA 2008).
World oil production has peaked and the world’s supply
of oil is projected to last approximately 40 years, if use
continues at current rates (Energy Information Agency
2008). The earth’s natural gas supply is projected to peak at
2020 and coal is projected to peak at 2025 (Energy
Information Agency 2008). In the U.S., natural gas supplies
are already in short supply and it is projected that the U.S.
will deplete its natural gas resources in about 40 years (W.
Youngquist, Personal Communication, petroleum geologist,
Eugene, Oregon, 2008).
Both the production rate and proven reserves of oil and
natural gas have continued to decline. In the United States,
oil and natural gas production will be substantially less in
20 years than it is today. Neither is now sufficient for
domestic needs, and supplies are imported in increasing
amounts yearly (USCB 2008). Analyses suggest that at
present (2008) the United States has consumed about 90%
of its recoverable oil so that we are currently consuming the
last 10% of domestic reserves. The United States is now
importing about 60% of its oil (USCB 2008).
At present, electricity represents about 34% of total U.S.
energy consumption (USCB 2008). Nuclear power produc-
tion of electricity contributes about 20% and has some
advantages over fossil fuels because it requires less land
than coal-fired plants and does not contribute to acid rain
and global warming.
All chemical and nuclear energy consumed ultimately
winds up as heat in the environment. The Second Law of
Thermodynamics limits the efficiency of heat engines to
about 35%. This means that approximately two-thirds of
the potential energy in fuel, whether chemical or nuclear, is
converted into heat, while the remaining one-third is
delivered as useful work (and, eventually, also converted
into heat).
More efficient end-use of electricity can reduce its costs
while at the same time reducing environmental impacts.
Commercial, residential, industrial, and transportation sec-
tors all have the potential to reduce energy consumption by
approximately 33% while saving money (American Phys-
ical Society 2008;NASA/C3P2008). Some of the
necessary changes to reduce consumption would entail
more efficiently designed buildings, appliances, and indus-
trial systems (American Physical Society 2008; NASA/C3P
2008).
Using available renewable energy technologies, such as
biomass and wind power, an estimated 29 quads of energy
can be supplied in the U.S. with the full implementation of
eight different renewable energy technologies (Table 3)
(Pimentel et al.2002). Worldwide we estimate that the need
for 200 quads of renewable energy could be produced from
20% to 26% of the land area (Yao Xlang-Jun, personal
communication, Cornell University, 1998; Exploring the
Future 2001; Zweibel et al.2007). Daily et al. (1994),
Desvaux (2009), and Mann (2009) all suggest an optimum
population for the earth of about 2 billion people. We
suggest that a self-sustaining renewable energy system
producing 200 quads of energy per year for about this
number would provide each person with 5,000 l of oil
equivalents per year (half of America’s current consump-
tion per year but an increase for most people of the world).
However, the appropriation of over 20% of the land area for
renewable energy production will further limit the resilience
of the vital ecosystem that humanity depends upon for its
life support system.
U.S. house size could be reduced from the current
average of 2,500 sq.ft. to about 1,000 sq. ft., as it was about
60 years ago and is currently in the British Isles (USCB
2008). Heat would come from wood fuel in the northeast
and north-central states. About 2 ha of forest would be
needed per home, which would provide about 6 tons of
wood fuel per year, adequate for a 1,000 sq. ft. well
Hum Ecol
insulated home. In low rainfall regions where there is little
wood fuel available, wind power or photo-voltaics will be
used for heat. Here, the problem of intermediacy of energy
supply can be offset by storing the heat in large hot-water
or sand tanks.
Biological Resources
In addition to land, water resources, crops and livestock
species, humans depend on the presence and functioning of
approximately 15 million other species existing in agro-
ecosystems and nature (McNeely 1999). More than 60% of
the world's food supply comes from rice, wheat, and corn
species (Brown 2008) and as many as 20,000 other plant
species are used by humans for food (Vietmeyer 1995).
Humans have no technologies that can substitute for the
food—and some medicines—that some plant species in
wild biota provide. Plants, animals, and microbes also carry
out many essential activities for humans, including polli-
nation of crops and wild plants, recycling manure and other
organic wastes, degrading chemical pollutants, and purify-
ing water and soil (Pimentel et al.1997). Humans, again,
have no synthetic substitutes for such ecosystem services
(Daily and Ehrlich 1996).
Pest insects, pathogens, and weeds destroy crops and
thereby reduce food and fiber supply. Despite the yearly use
of 3.0 million tons of pesticides and other controls
worldwide, about 40% of all potential crop production is
lost to pests (Oreke and Dehne 2004). Specifically, in the
United States, about 0.5 million tons of pesticides are
applied each year, yet pests still destroy about 37% of all
potential crop production. Estimates suggest that pesticide
use could be reduced by 50% or more, without any
reduction in pest control and/or any change in cosmetic
standards of crops, through the implementation of sound
ecological pest controls, such as crop rotations and
biocontrols (Pimentel 1997).
Approximately one third of the United States’and
world’s food supply relies either directly or indirectly on
effective insect pollination (Science Daily 2008). Honey-
bees and other wild bees play an essential role in
pollinating U.S. crops. They also are vital for pollinating
natural plants.
Worldwide, environmental pressure from the human
population is the prime destructive force and the primary
cause of reduced biodiversity (Pimentel et al.2006).
Humans currently occupy 95% of the terrestrial environ-
ment with either managed agricultural and forest ecosys-
tems or human settlements (Western 1989). The major
focus of world biological conservation has been on
protecting national parks that cover only 3.2% of the
world’s terrestrial area (Reid and Miller 1989). However,
most species diversity occurs in managed terrestrial
environments, so increased efforts should be devoted to
improving the sustainability of agricultural and forest
ecosystems (Pimentel et al. 1992).
Resources and Human Diseases
As world population increases and resources are limited,
human health suffers. Populations living in polluted regions
are prone to infectious diseases, including tuberculosis,
diarrhea, and parasitic worms. WHO (2008) reports that the
world’s annual death rate is 58 million. Diseases, including
malnutrition, cause 18 million deaths per year. Poverty in
developing countries reduces the availability of foods and
increases malnutrition. WHO (2005a,b) reports nearly 60%
of the world population is malnourished.
About 90% of the diseases occurring in developing
countries result from a lack of clean water (WHO 1992).
Worldwide, about 4 billion cases of disease are contracted
from water and approximately 50 million deaths result from
all diseases from water, food, air, and soil each year (WHO
2004). About 6,000 people die each day from a lack of
access to clean water (Rijsberman 2004). Shistosomiasis
and malaria, common diseases throughout the tropics, are
examples of parasitic diseases associated with aquatic
systems (Hotez and Pritchard 1995).
Intestinal parasites introduced through contaminated food,
water, and soil, impact health by reducing intake of nutrients
in various ways (Shetty and Shetty 1993). Hookworms, for
instance, which thrive in contaminated moist soils in the
tropics, can remove up to 30 ml of blood from a person in a
single day, leaving them weak and susceptible to other
diseases, including HIV/AIDS, which affects up to 37% of
the population of some countries (Hotez and Pritchard 1995;
Stillwaggon 2006). From 5% to 20% of an infected person’s
daily food intake is used to offset other illnesses and physical
stress caused by disease, thereby diminishing his/her
nutritional status (Pimentel and Pimentel 2008).
Transition to an Optimum Population with Appropriate
Technologies
The human population has enormous potential for rapid
growth because of the young age distribution both in the
U.S. and throughout the world (PRB 2008). Future
population growth is highly dependent on the path that
future fertility takes (WPP 2006). If the whole world agrees
on and adopts a policy of only two children per couple, it
would be more than 100 years before the world population
finally stabilizes at approximately 13 billion (Weeks 1986).
A more daunting projection is that if fertility remains at
present levels, the population will reach nearly 13 billion by
2050 (Cohen 2003).
Hum Ecol
However, a population policy ensuring that each couple
produces an average of only one child would be necessary to
achieve the goal of reducing world population from the
current 6.8 billion to an optimal population of approximately 2
billion in slightly more than 100 years. Even with the United
Nation’s projection of declining fertility rates, population will
reach 9.1 million by mid-century (UN 2005).
Our suggested 2 billion population carrying capacity for
the Earth is based on a European standard of living for
everyone and sustainable use of natural resources. For land
resources, we suggest 0.5 ha of cropland per capita (the
level that existed in 1960) with an intense agricultural
production system (~8 million kcal/ha) and a diverse plant
and animal diet for the people. In addition, approximately
1.5 ha of land would be required per capita for a renewable
energy system. At the same time, the goal would be to have
approximately 1 ha each for forest and pasture production
per capita. Of course, all current land degradation associ-
ated with soil erosion would have to stop (Pimentel et al.
1995), but technologies are currently available for soil
conservation in agricultural and forest production which
only need to be implemented (Troeh et al. 2004b). In recent
years there has been a growth in the adoption of
“conservation tillage”and “zero-tillage”systems. These
systems maintain a crop residue cover of the soil, rather
than leaving the soil unprotected, as in conventional
production, which increases organic matter decomposition
that contributes to global warming.
Balancing the population-resource equation will be
difficult because current overpopulation, poor distribution
of resources, and environmental degradation are already
causing serious malnourishment and poverty throughout the
world, especially in developing countries (Gleick 1993;
WHO 2005a,b). Wheat demand is projected to increase by
40% by 2020 and most demand will be from developing
countries (Pingali and Rajaram 1999). The current shift to
wheat and rice based diets is most apparent in developing
countries, which are projected to have a per capita increase
of 6%, from 62 kg per capita in 1993 to 66 kg per capita in
2020 (Rosegrant et al.1999). In the future, more food will
have to be produced on less land and with fewer water
resources. Based on the estimate that 0.5 ha per capita is
necessary for an adequate and diverse food supply, it would
be possible to sustain a global population of approximately
2 billion humans. Cropland land is being degraded and lost at
a rate of more than 20 million ha per year. At this rate, in just
42 years there will be sufficient arable land for a population of
only 2 billion. It is criticalto adopt soil and water conservation
techniques to protect the soil resources that currently produce
more than 99.7% of the world’sfood.
A reduction in the world population to approximately 2
billion, in addition to a reduced per capita consumption
rate, would help reduce the current severe pressure on
surface and groundwater resources and decrease water
pollution. If water shortage and pollution problems were
reduced, agricultural production would improve and degra-
dation of aquatic ecosystems would decline. Appropriate
technologies that conserve soil and water resources, and
reduce pollution in soil, water, and atmospheric resources
would help avert the alarming extinction rates of almost all
species (Kellert and Wilson 1993), which in turn would
protect and preserve most of the essential functions
provided by natural biodiversity (Pimentel et al.1997).
With the exhaustion of fossil fuels and associated
increases in costs and pressure from global climate change,
significant changes will also have to take place in energy
use and practices. Fossil fuel shortages and global warming
problems will force a transition to renewable energy
sources within the next 100 years. Research on ways to
convert solar energy into usable energy, for example, and
research to develop other new power sources will have to
be given a much higher priority. Although many solar
technologies have been investigated, most are only in
limited use. The most promising of renewable sources of
energy include: solar thermal receivers, photo-voltaics,
solar ponds, wind-power, hydropower, and biomass
(Pimentel 2008).
Global warming caused by CO
2
and other greenhouse
gases is a major challenge facing humans (Vorosmarty et al.
2000). The IPCC (2007) states that “warming of the climate
system is unequivocal, as is now evident from observation
of increase in global temperature, widespread melting of
snow and ice and rising global average sea level.”
Greenhouse gases are essential to maintaining a reasonable
temperature on earth; without the gases, the planet would
be so cold as to be uninhabitable. However, an excess of
greenhouse gases can raise the temperature of the planet to
high levels. Efforts underway to reduce the CO
2
emissions
have been to date unsuccessful. A 1% increase in the
population results in a 1% increase in carbon dioxide
emissions. Consumption of energy and other human
activities contribute to the greenhouse gas problems (Shi
2003).
The adjustment of the world population from 6.8
billion to 2 billion could be achieved over approximately
a century, but only if the majority of people agree that
protecting human health and welfare is vital, and are
willing to work towards a stable quality of life for
themselves and their children. Although a rapid reduction
in population numbers to 2 billion humans could cause
social, economic, and political problems, continued rapid
growth to 13 billion people will result in a dire situation
with major starvation and disease outbreaks. Worldwide
catastrophic health and environmental problems will
reduce human numbers but with major disturbances in
human lives and welfare.
Hum Ecol
Conclusion
Clearly, the world’s human population cannot continue to
increase indefinitely. Natural resources are critically limited,
and there is emerging evidence that natural forces are already
starting to control human population numbers through
malnutrition and other severe diseases. More than 3.7 billion
people worldwide are malnourished, and 3 billion are living in
poverty; grain production per capita has been declining since
1984; irrigation per capita has been declining since 1978;
arable land per capita has been declining since 1948; fish
production per capita has been declining since 1980; fertilizer
supplies essential for food production have been declining
since 1989; loss of food to pests has not decreased below 52%
since 1990; and pollution of water, air, and land has increased,
resulting in a rapid increase in the number of humans suffering
from serious, pollution-related diseases.
Fifty-eight academies of science, including the U.S.
National Academy of Sciences, recognize that “Humanity
is approaching a crisis point with respect to the interlocking
issues”of population, natural resources, and sustainability
(NAS 1994). The report points out that science and
technology have a limited ability to meet the basic needs
of a rapidly growing human population with rapidly
increasing per capita demands. Unfortunately, most indi-
viduals and government leaders appear unaware, unwilling,
or unable to deal with the growing imbalances between
human population numbers and the energy and environ-
mental resources that support all life. The interdependence
among the availability of life-supporting resources, indi-
vidual standard of living, quality of the environment,
environmental resource management, and population den-
sity are neither acknowledged nor easily understood.
Although we humans have demonstrated effective environ-
mental conservation in certain cases (e.g., water), overall
we have a disappointing record in protecting essential
resources from over-exploitation in the face of rapidly
growing populations (Pimentel and Pimentel 2008).
Historically, decisions to protect the environment have
been based on isolated crises and catastrophes. Instead of
examining the problem in a holistic, proactive manner,
these ad hoc decisions have been designed to protect and/or
promote a particular resource or aspect of human well-
being in the short-term. Our concern, based on past
experience, is that these urgent issues relating to human
carrying capacity of the world may not be addressed
holistically until the situation becomes intolerable or,
possibly, irreversible.
Through the use of a population policy that respects
individual rights, and effective resource use policies, as
well as science and technology to enhance energy
supplies and protect the integrity of the environment,
an optimum population of 2 billion people can be
achieved. With a concerted effort, fundamental obliga-
tions to ensure the well-being of future generations can
be attained within the twenty-first century. Individuals
will then be able to live free from poverty and starvation,
in an environment that is capable of sustaining human
life with dignity. We must avoid allowing the human
population to continue to increase beyond the limit of the
Earth’s natural resources, which will inevitably lead to
increased disease, malnutrition, and violent conflicts over
limited resources.
Acknowledgements The authors would like to acknowledge the
following people for reviewing and offering useful comment on an
early draft of this paper:
Jack Alpert
Stanford University
Stanford, California
Joachim Braun
International Food Policy Research Institute
Washington, DC
Jason Brent
jbrent6179@aol.com
Las Vegas, Nevada
Joel E. Cohen
Laboratory of Populations
Rockefeller University & Columbia University
John Coulter
Sustainable Population Australia
ACT, Australia
Andrew R.B. Ferguson
Optimum Population Trust
Oxfordshire, UK
Gary Fick
Crop and Soil Science
Cornell University
Bernard Gilland
Espergaerde, Denmark
Tiziano Gomiero
Dept. of Biology
Padua University
Robert Goodland
RbtGoodland@aol.com
Arlington, VA 22207, USA
Stefan Hellstrand
Department of Urban and Rural Development
Swedish University of Agricultural Sciences
Ben Ho
Johnson Graduate School of Management
Cornell University
Dr. Ray G. Huffaker
Washington State University
huffaker@wsu.edu
Timothy LaSalle
The Rodale Institute
Kutztown, PA 19530
Ron Leng
Nutritional Biochemistry
University of New England
Armidale, NSW, Australia
Philip McMichael
Development Sociology
Cornell University.
Hum Ecol
Mario Molina
Prolongación Paseo de los Laureles #458
Despacho 406
Colonia Bosques de las Lomas
C.P. 05120 México, D.F.
Maurizio G. Paoletti
Dipartimento di Biologia, Università di Padova
Padova, Italy
John F. Rohe
Colcom Foundation
Pittsburgh PA 15222
Norman Uphoff
Professor of Government
Cornell University
Mathis Wackernagel
Global Footprint Network HQ
Oakland, CA 94607-3510 USA
Walter Youngquist
Petroleum Consultant
Eugene, OR
We wish to express our sincere gratitude to the Cornell Association
of Professors Emeriti for the partial support of our research through
the Albert Podell Grant Program.
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