limits to growth lecture

The Limits to Growth
Is there a limit to that size?
How can we predict the size of the human
population in the future?
What are the consequences if the population
grows to exceed that limit?
Finally, can we really answer all these questions?
Is there a limit to the size of the human population?
To answer that, you need some background information.
There are two basic growth models that have been applied to
population growth. The first, called an exponential growth
model, describes growth with no limit. Resources needed to
support growth are considered to be infinitely available.
The second model is growth towards some limiting value,
called the carrying capacity (or K). The model is called
logistic or ‘sigmoid’. Here there is a limit to resource
What does human population growth look like?
Pretty much exponential – but is it really exponential?
If it is, then the intrinsic (or exponential) rate of increase
should be constant. I can’t show you world data, but can
show you what has happened in the U.S.
Human population growth in the United States…
as population size has increased, the intrinsic rate of
increase has slowed...
Extrapolated using censuses from 1790 to 1910, the U.S.
population should have come to equilibrium at ~200 million.
That didn’t happen. Better medicine, immigration, and many
other factors kept the population growing.
Globally, something similar has happened: Growth has
slowed a bit, but the human population continues to grow.
Here’s a table extracted from Limits to Growth – the 30 year
update showing world population, growth rate at each time,
and the resultant number added to the population…
Is there a limit to that growth…
There’s a history associated with the answer.
William Petty, in "Another Essay in Political Arithmetic"
(1683) estimated the K as a density of 2 people for every 5
acres of habitable land. That would yield an estimate of K
(based on current habitable area) of 10 billion.
In 1960, the Club of Rome, a gathering of experts in various
fields related to predicting the future of the human
population, developed a modern estimate of K, and came up
with the same number – 10 billion.
Many other estimates have been made, but there is absolute
agreement that there is a limit – the earth can sustain a
population somewhat larger than the ~7 billion on earth
today, but not too many more.
How do we determine the future size of the human
The tools used can be collected under the umbrella called
demography. [Definition: the study of the age structure and
growth of populations]
There are neat graphical representations that can tell us in
general terms whether a population is growing, holding
steady, or declining, and even give us a general idea about
how fast it’s growing. They’re called demographers’ curves.
On the x-axis of each ‘curve’ is the proportion of the
population in each group. On the y-axis are the age classes
into which the population is grouped
Consider the third question: What are the consequences if the
population grows to exceed that limit (the carrying capacity)?
There’s a history to this question, too.
Thomas Malthus (1766-1834), in his An Essay on the
Principle of Population (1798), used the growth of the English
immigrant population of North America to propose that the
growth of the human population was (at that time)
exponential. However, he noted that the growth of food
production was only linear. Eventually, the population would
exceed the support capacity of its resources. He said that the
result would be “misery and vice”.
Once more there is a 20th century take on what may happen…
We hope that improved technology (biotechnology, genetic
engineering, new pollution control methods,… will solve the
problems. Limits to Growth used a complex model to project
what may happen based on differing scenarios.
What follows are a series of graphs that show you model
output under those differing ‘inputs’.
Scenario 1: ample supplies of non-renewable resources and
improved pollution control technology
Scenario 2: Larger supplies of non-renewable resources,
improved pollution control technology (as before), and
enhancement of agricultural yields/unit area
There’s not much difference in either of those scenarios –
the population ‘crashes’, resources decrease dramatically in
availability, pollution increases until the lack of resources
drives down industrial production. In terms of the human
condition, life expectancy, as well as availability of food and
consumer goods, declines.
The only scenario in which the human population and its
condition don’t markedly decline during the 21st century is
one in which all of the previous steps are taken, and 1)
protection against soil erosion (particularly on agricultural
lands) is enforced, and 2) new technologies to improve
resource use efficiency are developed. Here’s what the
model then looks like:
Finally, are there answers to all those questions?
No! The models are predictions that make enormous
assumptions about our will to limit population growth, our
ability to develop and demand greatly improved pollution
control technology, our ability to use tools like genetic
engineering to increase crop yields (and to accept the
products of that genetic engineering), and our willingness to
subjugate individual ‘rights’ to the common good in areas
like prevention of soil erosion and the adaptation of
resource-efficient technology. Today the hybrid car,
tomorrow ??