There’s often a lot of talk and speculation about World Population growth. Just like trying to figure out how much oil there is in the world, the whole exercise is very confusing and difficult.
Here’s what I got from the latest book I am reading… it’s very good and I recommend it to anybody.
Consilience by Edward 0. Wilson pg 306 onwards (forgive my typo’s)
The global population is precariously large, and will become much
more so before peaking some time after 2050. Humanity overall is im-
proving per capita production, health, and longevity. But it is doing so
by eating up the planet’s capital, including natural resources and bio-
logical diversity millions of years old. Homo sapiens is approaching the
limit of its food and water supply. Unlike any species that lived before, it
is also changing the world’s atmosphere and climate, lowering and pol-
luting water tables, shrinking forests, and spreading deserts. Most of the
stress originates directly or indirectly from a handful of industrialized
countries. Their proven fórmulas for prosperity are being eagerly adopted
by the rest of the world. The emulation cannot be sustained, not with the
same levels of consumption and waste. Even if the industrialization of
developing countries is only partly successful, the environmental after-
shock will dwarf the population explosion that preceded
Some will, of course, call this synopsis environmental alarmism.
I earnestly wish that accusation were true. Unfortunately, it is the
reality-grounded opinion of the overwhelming majority of statured
scientists who study the environment. By statured scientists I mean
those who collect and analyze the data, build the theoretical models,
interpret the results, and publish articles vetted for professional jour-
nals by other experts, often including their rivals. I do not mean by
statured scientists the many journalists, talk-show hosts, and think-tank
polemicists who also address the environment, even though their opi-
nions reach a vastly larger audience. This is not to devalue their profes-
sions, which have separate high standards, only to suggest that there
are better-qualified sources to consult for factual information about
the environment. Seen in this light, the environment is much less a
controversial subject than suggested by routine coverage in the media.
Consider, then, the assessment made through the mid-1990s by
the statured scientists. Their quantitative estimates differ according to
the mathematical assumptions and procedures variously used, but
most still fell within limits from which trends can be projected with
By 1997 the global population had reached 5.8 billion, growing at
the rate of 90 million per year.
In 1600 there were only about half a billion people on Earth,
and in 1940, 2 billion.
The amount of increase
during the 1990s alone is expected to exceed the entire population
alive in 1600.
The global growth rate, after reaching a peak during the
1960s, has been dropping ever since.
In 1963, for example, each
woman bore an average of 4.1 children.
In 1996 the number had declined to 2.6.
In order to stabilize the world population, the number
must be 2.1 children per woman (the extra 0.1 allowing for child mor-
Long-term population size is extremely sensitive to this replace-
ment number, as shown by the following projections.
If the number were 2.1, there would be 7.7 billion people on Earth in 2050, leveling
off at 8.5 billion in 2150.
If 2.0, the population would peak at 7.8 billion,
then drop by 2150 to 5.6 billion, the total in the mid-1990s.
If 2.2, it would reach 12.5 billion in 2050, 20.8 billion in 2150;
and if 2.2 could miraculously be maintained thereafter,
the human biomass would eventually equal the weight of the world and then,
after a few millennia, expanding outward at the speed of light, it would exceed the mass
of the visible universe.
Even if the global birth rate were reduced dras-
tically and immediately, say to the Chinese goal of one child per
woman, the population would not peak for one or two generations.
The overshoot is ensured by the disproportionate number of young
people already in existence, who look to long lives ahead.
How many people can the world support for an indefinite period?
Experts do not agree, but a majority put the number variously between
4 and 16 billion. The true number will depend on the quality of life
that future generations are willing to accept. If everyone agreed to be-
come vegetarian, leaving nothing for livestock, the present 14 billion
hectares of arable land (3.5 billion acres) would supply about 10 billion
If humans utilized as food all the energy captured by plant
photosynthesis, some 40 trillion watts. Earth could support about
16 billion people.
From such a frag would have to be excluded.
Even if, by force majeure, the population levels off at well under
10 billion by mid-century, the relatively extravagant lifestyle now en-
joyed by the middle classes of North America, Westem Europe, and
Japan cannot be attained by most of the rest of the world.
The reason is that the impact of each country on the environment is multiplicative.
It is dependent, in a complex manner, on the formula called PAT: pop-
ulation size times per capita affluence (hence consumption) times a
measure of the voracity of the technology used in sustaining consump-
The magnitude of PAT can be usefully visualized by the “ecolog-
ical footprint” of productive land needed to support each member of
the society with existing technology. In Europe the footprint is 3.5
hectares (a hectare is 2.5 acres), in Canada 4.3 hectares, and in the
United States 5 hectares. In most developing countries it is less than
half a hectare. To raise the whole world to the U.S. level with existing
technology would require two more planet Earths.
It matters little that North Dakota and Mongolia are mostly empty.
It makes no difference that the 5.8 billion people in the world today
The PAT formula for estimating impact of population on the environ-
ment was developed originally by Paul R. Ehrlich and John P. Holdren
in “Impact of population growth,” Science, 171; 1212-17 (1997) and has
been discussed in many aspects since. “It is a rough approximation, since
the three multiplicative factors are not independent… It is especially
useful in assessing global impacts, where we normally must fall back on
using per-capita energy use in place of AT’: Paid Ehrlich, “The scale of
the human enterprise,” in Denis A Saunders et al.. Nature Conservation
3: Reconstruction of Fragmented Ecosystems (Chipping Norton, NSW,
Australia: Surrey Beatty & Sons, 1993), pp. 3-8.