or many people the word "overpopulation" conjures
up an image of Bangladesh, but a truer definition of overpopulation might
lead us to think of Britain instead. While Bangladesh's population is growing
12 times faster than Britain's, Britain's population growth, in conjunction
with its profligate lifestyle, contributes 3.5 times as much carbon dioxide
to the global atmosphere and hence to global warming. People in Britain
do not live on an island nearly as much as they like to suppose. They depend
on an overseas "shadow ecology" several times their country's
size for one-third of their food. Each time a British citizen lifts a forkful
of food made with ground nuts, he or she lends a hand to hoes spreading
desertification thousands of kilometers away. How many people are good for
Britain, one might well ask, and how many Britishers are good for the world?
As the world struggles to support 5.5 billion people
- a number that is expected to double by the year 2050 - certain observers
are asking some supersize questions. How many people can our Earth support,
both now and into the long-run? In other words, what is the Earth's carrying
capacity?
The basic idea of carrying capacity can be represented
by the equation, I = PAT, propounded by Professor Paul Ehrlich and Dr. Anne
Ehrlich of Stanford University. I stands for environmental impact, P for
population, A for affluence or per-capita consumption, and T for the environmentally
harmful technology that helps create affluence. These three factors compound
each other's impact. No matter what kind of technology a society has, no
matter how many goods it consumes or how much waste it produces, no matter
how much poverty or inequality it allows, its impact on the environment
will be proportional to the size of its population.The Ehrlichs' equation demonstrates why developing nations,
with large populations but limited economic advancement, can generate a
vast impact on the environment. Likewise, it shows why developed countries
with relatively small but technologically sophisticated and highly affluent
populations can also dramatically affect the environment.To understand how the equation works, suppose that,
by dint of exceptional effort, humankind managed to reduce its per-capita
consumption of environmental resources (A in the equation) by 5 percent;
and to improve its technologies (T) so that they caused 5 percent less environmental
injury on average. This would reduce humanity's total impact (I) by roughly
10 percent. But as the Ehrlichs point out, unless global population growth
(P) were restrained at the same time, it would bring the total impact back
to the previous level within less than six years.
According to the Ehrlichs' definition, carrying capacity
amounts to "the number of people that the planet can support without
irreversibly reducing its capacity to support people in the future."
While this global-level definition can be applied to individual nations
as well, carrying capacity is a far more complex affair than the Ehrlichs'
equation and definition might imply. It's a function of factors that reflect
food and energy supplies, ecosystem services (such as providing freshwater
and recycling nutrients), human capital, people's lifestyles, social institutions,
political structures, and cultural constraints, among many other factors,
all of which interact with each other.Some observers (often ecologists) say that carrying
capacity not only can constrain a country's population growth, but also
can limit it absolutely. Others (often economists) assert that carrying
capacity is a flexible affair, subject to endless expansion through technology
and policy changes. In either case, two things are clear: carrying capacity
is ultimately determined by its most restrictive component, and human communities
must learn to live off the "interest" of environmental resources
rather than off the "principal."
Given the amount that humans consume and the environmentally
harmful technology they use, many areas already operate beyond their capacity.
Consider humanity's capacity to feed itself. According to the World Hunger
Project at Brown University, current agro-technologies could sustainably
support 5.5 billion people from the planetary ecosystem, provided food were
distributed equally worldwide and everyone lived off a vegetarian diet (the
1993 global population is already 5.5 billion). If populations derived 15
percent of their calories from animal products, as do many people in South
America, the total would decline to 3.7 billion. If they gained 25 percent
of their calories from animal protein, as do most people in North America,
the Earth could indefinitely support only 2.8 billion people.True, we must hope that many advances in agro technologies
are still to come, but consider the population/ food record over the past
four decades. From 1950 to 1984, thanks largely to remarkable advances in
Green Revolution agriculture, world grain output increased 260 percent,
raising per-capita production by more than one-third. From 1985 to 1992,
however, crop yields hardly increased even though the world's farmers invested
billions of dollars.Plant breeders and agronomists, it appears, have run
out of technological innovations. Meantime, the world population has grown
by almost 13 percent, adding 625 million people to be fed. As a result,
grain output per person has declined by nearly 9 percent. To put it another
way, every 15 seconds another 45 people arrive on the planet; during the
same 15 seconds, the planet's stock of arable land declines by one hectare.
We have not fed hungry people more, we have fed more hungry people.If and when the world population swells to 10 billion
(the United Nations predicts it could do so, given moderate growth, by the
year 2050), we shall have to produce nearly three times more calories per
hectare than we do today. To supply that much food, all the world's current
croplands will have to yield as much as lowa's best cornfields, or three
times the present world average. In developing countries, cropland will
have to more than double by 2050 in order to feed growing populations, yet
all too few new areas can be farmed. From 1950 through 1980, cropland annually
expanded by 0.5 percent per capita despite a growing population. Since then,
however, population growth has outpaced the expansion of farms: arable land
per capita has declined by 1.9 percent per year. Similarly, irrigated lands,
which supply one-third of our food from one-sixth of our croplands, grew
by 2 to 4 percent per year from 1950 through 1980, but at only 1 percent
per year after 1980. Because carrying capacity is tied to the environment,
it can apply to local economies and societies as well. Consider employment.
Today the developing world's work force numbers around two billion. Of these,
more than one quarter are unemployed or grossly underemployed - more than
the developed world's entire work force. By the year 2025, the developing
world's labor force is projected to surge to well over 3 billion. To employ
the new worker multitudes, let alone those without work, the developing
world will have to annually create nearly 40 million new jobs during the
1990s. The United States, with an economy half as large again as the entire
developing world's, often has trouble generating another two million jobs
each year.
For a specific example of carrying capacity, let's take
a quick look at Kenya, where I lived for 24 years and where I conducted
a good deal of research on carrying capacity. Kenya's 1993 population is
projected to expand from 28 million to 125 million by the time zero growth
is attained some time in the 22nd century. Yet the Food and Agriculture
Organization calculates that even if the nation employed Western Europe's
high-technology agriculture, it could support no more than 52 million people
off its own lands. Worse, half of all Kenyans are 15 years old or younger
and soon will be of childbearing age. Even if Kenya were to achieve the
two-child family forthwith, the population would still double, relying on
steadily increasing amounts of outside food to support itself.Unfortunately, because Kenya's population is growing
so quickly - by 3.7 percent per year - its per-capita economy has grown
less than 2 percent in recent years. Worse yet, Kenya's terms of trade have
declined throughout the past ten years until they are barely positive, leaving
the country diminishing means to purchase food abroad. To buy enough food
to meet its fast-growing needs, the country's export economy will have to
flourish as never before. Worst of all, Kenya will have to do so despite
the fact that its forests have almost disappeared, water for irrigation
is badly depleted, and much topsoil is gone with the wind. Other nations currently able to ensure their food supplies
would do well to take notice, for the roots of Kenya's dilemma stretch back
to when it was still capable of feeding itself and populations were just
starting to grow rapidly. To improve its prospects at this late date, Kenya
will have to immediately and vigorously slow down its population growth.
By achieving the two-child family in 2010 instead of the projected 2035,
it could hold its population to 72 million, 53 million fewer than expected.
Mexico is an equally telling example. As the first developing
country to take up Green Revolution agriculture, Mexico quadrupled its grain
production between the mid-1950s and the mid-1980s. But its population also
has grown exceptionally, from 27 million in 1950 to 90 million today. This
population boom, together with increasing environmental degradation, has
all but offset the benefits of the Green Revolution. Mexico once again imports
more food than it exports.In short, Mexico cannot support - feed, employ, house,
generally care for - its present populace, let alone ever greater numbers.
Because Mexico's population is relatively young, like Kenya's, it should
grow to a total of 138 million people by the year 2025, despite some recent
striking success in the family-planning field. The total number of unemployed
or underemployed people in the workforce should grow from 15 million today
to 20 million as early as the year 2000. Though Mexico's economy performed
remarkably for much of the period 1960 -1980, the average Mexican earned
less in 1990 in real wages than he or she did in 1970. To make matters worse,
the economy has generally stagnated or even contracted since 1980 while
the population has grown by well over one quarter. Many Mexicans in the year 2000 could be poorer than
they are today. To keep pace with the growing work force, Mexico will have
to create half as many jobs as the United States off an economy only one-thirtieth
the size, and by investing as much as $500 billion by the year 2000. It
would be optimistic to foresee that "only" 20 million Mexicans
will lack proper employment in the year 2000. According to Mexican political
leader Dr. Jorge G. Castenada, "The consequences of not creating nearly
15 million jobs in the next 15 years are unthinkable. The youths who do
not find them will have only three options: the United States, the streets,
or revolution.'' If they head for the United States, they will impose significant
costs on their hosts. Americans now pay more than $3 billion to supply social
services to more than 1 million immigrants per year, most of them Hispanics.
Finally, let us consider the United States. Each American
consumes 47 times more environmentally based goods and services than does
a Chinese. According to Professor David Pimentel of Cornell University,
feeding each American requires at least 1,500 kilograms of agricultural
products; a Chinese requires less than 600 kilograms An American consumes
8,000 liters of oil-equivalent fossil fuel per year; a Chinese consumes
one-twentieth as much. The average American family is comprised of two children,
but when we factor in how many natural resources these children consume
and compare the American lifestyle with the global average, then the average
American family, in "real world" terms, contains something like
30 to 40 children. Like all other developed countries, however, the United
States does not have even the basic makings of a population policy. Though
it often criticizes developing countries that fail to implement their population
policies with sufficient vigor, the United States shows no signs of asking
itself what its carrying capacity might be. Lindsey Grant, a noted population
expert, estimates that in order to sustainably support the economy at today's
levels without depleting natural resources, the U.S. population should number
between 125 and 150 million, or about the size it was in the 1940s. A leading
ecologist, Robert Constanza, puts the figure at 85 - 170 million depending
on per-capita consumption. Pimentel calculates 40 - 100 million for a self-sustaining
society with a quality environment. The Ehrlichs estimate around 75 million,
about the size of 1900.Those totals derive from environmental considerations.
If Americans were prepared to reduce their standard of living in material
terms, then the country could support more people - plain enough. What is
not so plain is how to calculate an acceptable lifestyle. Do Americans want
to increase their consumption in perpetuity? Should the United States maintain
its economic pre-eminence for all time? The answers reflect personal tastes
and aspirations, so one person's crystal ball is no less clouded than another's.
Many Americans are recognizing that the good life does not depend entirely
on material possessions. But of what does it consist? Alas, there are no
worthwhile criteria, no "ideal standards of living," by which
we can currently calculate - and strive for - an optimum population.Finally, let us recognize that even if a country takes
the pioneer step of estimating its optimum population, that will be no more
than a start. Suppose the United States decided that its optimum population
would amount to, say, 100 million. (Of course, the total could eventually
be increased if the country found ways of sustainably supporting more people
through innovative technology, for instance; or it could be decreased if
the country recognized that 100 million would still deplete resources.)
A far greater challenge would be for Americans to get from here to there.
The obstacles to achieving zero growth, let alone to slowly reducing the
population, would be immense. Think of what minorities, chambers of commerce,
political leaders, economists, and certain religious bodies would say, as
well as others with vested interests that proclaim the infinite virtues
of infinite growth (albeit in a finite world)?Nevertheless, in certain key respects, the prospect
need not be so daunting. To get down to 150 million would require no more
than a century-long birth rate of 1.5 children per woman (today it's 2.0),
a rate that has already been adopted by Germany, Italy, Austria, Greece,
Spain, Portugal and Japan. A solid start could be achieved by eliminating
teenage pregnancies, a generally unwanted phenomenon that costs the United
States $25 billion a year in support services.Reducing populations would surely be the most complex
and difficult undertaking in the history of human society - though it would
demonstrate this generation's commitment to the next and help assure humanity's
well-being into the indefinite future. Who knows? Americans might soon find
they're turning a profound problem into a glorious opportunity. The first
step would involve the most adventurous, the most creative, and the most
incisive environmental measures that humans have ever taken. Let us get
on with thinking the unthinkable, rather than letting the force of environmental
circumstances do our thinking for us. 
Norman Myers is a Visiting Fellow at Green College, Oxford University.
He has field experience in over 80 countries and has published more than
250 professional papers in scientific journals and several hundred popular
articles. For a first-rate review of carrying capacity, see The Population
Explosion by Ann and Paul Ehrlich (Simon and Schuster, 1990).
