POPULATION GROWTH AND THE ENVIRONMENT

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POPULATION GROWTH AND THE ENVIRONMENT

• ARE WE OVERWHELMING MOTHER EARTH?

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QUOTE FOR THE WEEK

• Life is what happens when you are making other
  plans John Lennon.

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To a certain extent, awareness of the fragile
nature of our surroundings was triggered by talk
of the population bomb.

• These concerns arose in the late 1960s as
  demographers began to notice quite large annual
  rates of population growth following the end of
  World War II.
• In the U.S. the baby boom that began in 1945
  and continued until around 1960 was a major
  source of concern to those who worried about the
  planet being overrun with people.
• Population growth was, at that time, a reflection
  of new-found prosperity and of confidence in the
  future.
• Fertility (births) is quite sensitive to economic
  outlook and following World War II optimism was
  quite high.

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With the exception of a few years between
1955-1962, the annual addition to the worlds
population was increasing and this trend
continued until the 1990s. At this time, the
annual increments to population began to shrink.
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Wars, Massacres and Atrocities of the Twentieth
Century Year-by-Year Death Toll
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Notice that even though the annual percent growth
(the growth rate) began to fall around 1965, the
actual growth in population continued until
around 1990.This result follows from the fact
that a slightly smaller percentage increase on a
larger population base will continue to imply
larger populations each year until the growth
rate falls enough to overcome the larger number
of people. The period between 1962 and 1990 was
just such a period of transition. Since then we
have been on a trend of reduced annual growth,
but continued growth in total population.
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A focus on total population growth obscures
important differences among regions of the
world.As a general rule, the industrialized
countries of the world are experiencing very low
rates of population growth, while the developing
countries are where we find high rates of
population growth.Consider the following graphs.
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There is an interesting and profound relationship
between income levels in a country and birth
rates.Consider the following graph of per
capita GDP (an indicator of national income) for
a large number of nations.
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Now let us see how national income levels
correlate with annual rates of population growth

• The following graph shows a regression
  relationship between rates of population growth
  and per capita GDP.
• The R2 of 0.4133 tells us that per capita GDP
  accounts for 41 percent of the variation across
  countries in the rate of population growth.

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Now for a few more graphs of population-related
data

• World population by regions of the world.
• Life expectancy in different countries.
• Infant mortality across countries.

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WORLD POPULATION BY REGION
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We see that most of the worlds population
increase is coming from the poorest nations.And
we see that some of the richer nations have a
negative rate of population growth, while many
others are close to the replacement rate (zero
population growth).
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We now turn our attention to the theory of
population dynamics.That is, how might we
model population changes?
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We start by positing a simple model of births.
This model is informed by some general patterns
we know to be pertinent

• Birth rates f(Ef, Y, U, SS)
• Ef years of female education
• Y per capita income
• U degree of urbanization
• SS government social security programs

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We then hypothesize that births vary with respect
to these four factors as below

• Years of female education ?B/?Ef lt 0read this
  to say that as years of female education increase
  we expect birth rates to fall.
• Per capita income ?B/?Y lt 0read this to say
  that as per capita incomes rise we expect birth
  rates to fall.
• Degree of urbanization ?B/?U lt 0 read this to
  say that as a country becomes more urbanized we
  expect birth rates to fall.
• Government Social Security ?B/?SS lt 0 read this
  to say that as the government plays a more
  important role in the economic security of the
  elderly we expect birth rates to fall.

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Now consider a simple model of deaths.

• Death rates f(Y, M)
• Y per capita income
• M access to good medical care

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Deaths as a function of

• Per capita income ?D/?Y lt 0 read this to say
  that as per capita income increases we expect
  death rates to fall.
• Access to medical care ?D/?M lt 0read this to
  say that as better access to medical care
  increases we expect death rates (especially of
  infants) to fall.

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These two general relationships combine to form
a model of the demographic transition.This
model is historical in the sense that it captures
the empirical experience of the industrialized
countries as they moved through three distinct
periods.Phase I high birth and death
ratesPhase II high birth rates but falling
death ratesPhase III low birth and death rates
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Consider (in the following graph) the changing
age structure in the U.S. as we went through our
own demographic transition since 1900.

• At the turn of the 20th century we were an
  agrarian nation with a very large share of the
  work force engaged in what is called primary
  activitiesagriculture, mining, forestry.
• Now we are a post-industrial economy with less
  than 3-4 percent of the labor force engaged in
  primary activities, a somewhat larger share
  engaged in secondary activities
  (manufacturing), and a growing share of the labor
  force engaged in tertiary activities such as
  real estate, insurance, legal affairs,
  transportation, government jobs, communications,
  teachers, etc.
• All of the concern for the loss of manufacturing
  jobs reflects the fact that the U.S. is now post
  industrial in nature and we will never again
  have the level of manufacturing employment that
  we had at the beginning of the 20th century.

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CHANGES IN DEMOGRAPHIC COMPOSITION OVER TIME
UNITED STATES
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In the next slide I show the situation for the 84
poorest countries in the world.

• Notice the strong relation between income rank of
  birth rates
• And the somewhat weaker relation between income
  rank and death rates.

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Next we see the actual data from one developing
countrySri Lanka.
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Let us now consider population dynamics and the
danger of exceeding the natural carrying capacity
of the planetor perhaps our ability to produce
food.This brings us to the work of Parson
Thomas Robert Malthus, a mathematician of the
first rank in his class at Cambridge (1784), but
alas, not a very good empirical researcher. He
was good with mathematical models, but not very
good with data.
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Consider a simple model of population growth

• Assume two periods t and t1. Let t be the end
  of this year, and t1 be the end of next year.
• Population (N) at the end of next year (t1) is a
  function of population at the end of this year
  (t) times some growth factor R.

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That is Nt1 RNt Or we may write it
as R Nt1 / Nt If R gt 1 the population
will increase If R lt 1 the population will
decline If R 1 the population is in a steady
state between these two periods.
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We see here the issues that concerned Malthus.
Specifically, he worried that since food supply
was able to grow at a linear rate (an assumption
based on historical conditions), and population
would grow at an exponential rate (something
derived from mathematics with particular
assumptionsas above), he foresaw immiserization
setting in as hunger and squalor reduced both
fertility rates (Ft) and the survival rates (St)
of the youngand even of adults.Let us now
consider the Malthusian model.
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A linear function for food production might
be Ft1 ?Ft where ? some
constant.Malthus thought population growth
would be exponential until population outstripped
food supply and then starvation would reduce
population. So population would follow a path
such as Pt1 Ptedt where e 2.718
(the base of the natural logarithm)d rate of
growth of population (in percentage) t time
period
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The nice aspect of this formulation is that it
gives us the percentage population change
directly in the form of the parameter d

• Pt1 Ptedt
• So the parameter d can be read directly as the
  percentage change in population between period
  t1 and t.

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Recall our earlier formulation had R Nt1 Nt
so that R is the ratio of next years
population to this years population. R depicts
the actual change in population.

• In contrast, the parameter d is the percentage
  change in population between two periods.

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The parameter d captures all of the factors that
will influence population change. These are (1)
fertility and (2) survival.
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Fertility is largely endogenous to the family,
while survival is largely exogenous.

• The family is in control of fertility decisions,
  while social conditions national economic
  circumstances, access to (and quality of) health
  care, and old-age insurance tend to be external
  to the family.

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Earlier we saw that Malthus considered food
supplies to grow in a linear fashion. Indeed
during the 18th century when he wrote, total food
production had been increasing at about 0.1 -
0.25 annually. And it had been doing this since
at least the 12th century in England. As above,
a linear function for food production might
be Ft1 ?Ft where ? some constant.
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But we could just as easily express food
production (supply) as we have expressed
population. That is, we could say that food
supply (here called G for grain) can be depicted
as Gt1 Gtedt And, as we saw with
population growth, a 0.1 annual increase in
grain production would show up as Gt1
Gte0.001(t)
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We can think of this relationship between food
grain production and population growth in a
broader sense

• Specifically, think of food production as the
  ability of the earth to support people and our
  lifestyle, including our consumption and disposal
  of stuff. To the extent that natural resources
  are degraded or depleted through human activity
  then the ability of nature to keep providing for
  us is diminished. We can degrade the earth just
  as food production can reach its limits.

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In the following graph I depict a situation of a
gradual increase in annual food supplies, a
gradual decline in the rate of population growth
leading to a general increase in grain supplies
per person, then a serious crop failure in year
12, followed by a predictable impact on
population growth rates, and then followed by a
slow recovery in the rate of population growth as
food production recovers.
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In the following graph we see a gradual increase
in food supplies per person, then the crop
failure and accompanying famine, then followed by
a gradual increase in the standard of living as
measured by food availability.But before long
per capita grain consumption (availability)
begins to drop off (in year 18) as population
growth picks up after the famine years.
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The point here is that when the ability of
naturebroadly defined to include food
productionto provide materials and services can
no longer keep up with our demands for those
materials and services then something has to
give.This is what people
worry about under the general heading of
sustainability, or sustainable development.
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Let us now add one more refinement to the simple
Malthusian story

• That is, I will show you how to model fertility
  as endogenous.

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Malthus started with the exponential growth
equation for population, but he
dideventuallycome to recognize positive and
negative checks on population growth. This
is captured by the logistical growth equation
from Verlhurst (and elaborated in biology by
Lotka and Volterra). This is also called a
Sigmoid curve.
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This relation demonstrates that population growth
is itself influenced by the settings and
circumstances (the environment) of the population
under study. Perhaps the population has reached
the carrying capacity (here denoted by K) of
its environment. The presence of K in this
equation means that population growth is
endogenous.
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With the parameters of Pt-1 1, r 1.3, and K
1000 we obtain the following figure for
population next year (Pt).
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Let us now look at the population experience of
the place that was the laboratory for all of
Malthuss writing (and hand wringing)England.I
use England because I am working on the economic
history of England starting in the 12th
century.And we are fortunate to have reasonably
good population data as far back as 1086 (though
there are long gaps in those early
years).First, we will see the early Medieval
picture (1250-1450), and then one for the full
sweep of recorded population data
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In the period between 1250 and 1300 Englands
population was showing a gradual increase.
Then, in 1315 there were very heavy rains
throughout the summer that destroyed virtually
the entire crop, and so 1315 and 1316 were
hungry timesmillions of people starved. It
is estimated that 10-15 percent of the population
died.
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The population began to recover around 1320 and
was on a general upward trajectory when the Black
Death struck (arrived) in the summer of
1348.This catastrophe wiped out 30-40 percent
of the population in the period of 12-18
months.The dead were everywhere. The plague
re-appeared again and again over the ensuing
centuries, though on a less dramatic scale. It
was not until 1550two hundred years after the
original outbreakthat Englands population began
to increase again. There was one more serious
instance around 1660.
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We see several things here.Malthus was writing
in the early stages of a quite rapid increase in
population after nearly four-five centuries of
population stagnation.The Industrial Revolution
began around 1740 following several decades of
marvelous weather, bounteous agricultural yields,
improved diets, and some considerable new income.
Family size fairly explodednot necessarily
because people were suddenly having more
children, though there was some of that. But
infant mortality plummeted with the advent of
better diets and improved health care.
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So Malthus was very worried that population would
outstrip food supply. But there was more going
onas there usually is.Specifically, with the
industrial revolution came an increase in
horrible living conditions for the poor. Read
Charles Dickens. These grim conditions were
also noticed by a German economist and historian
living in London by the name of Karl
Marx.There can be little mystery why Marx was
appalled at the reality of un-restrained
capitalism. It was barbaric and in-humane.
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England began to undertake poor reliefwhat we
would call welfare.And Malthus was not too
happy about the breeding habits of the poor who
then were making claims on the public treasury
for poor relief. He did not much care how many
children the rich had, since they were
comfortable enough to provide for them.It was
the poor who concerned Malthus. Does this
sound familiar?
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Current politics (welfare reform) aside, it
also puts a new interpretation on contemporary
fussing about population growth.Specifically,
as you now see, most of the population growth in
the world occurs in the poor countries, while the
rich countries are not having very many children.
And the ones we do have are, for the most
part, well taken care of.
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Latter-day Malthusians who complain about
population growth are quite ready to whine about
family size among the poor (in the tropics),
while overlooking the ostentatious and often
obscene consumption patterns of those of us in
the rich countries.The real threat to nature
over the long run is not their numbers (family
size). The serious threat is that they may
someday be wealthy enough to consume and waste
resources on a scale similar to our patterns of
consumption. If you want to worry about nature,
there is something to worry about.
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SUMMARY

• 1. World population will probably stabilize at
  8-9 billion
• 2. The faster we can help improve the living
  conditions (both economic and social) in the
  developing world, the quicker world population
  will stabilize
• 3. It is a mistake for the U.S. government to
  oppose family planning programs in the poor
  countries
• Market-capitalism will be undermined by the
  prospects of zero-population growth unless
  fundamental changes occur in how we think of
  economic relations. This follows from the
  inherent aspect of capitalism which is the
  continued production of stuff in order to
  continue economic growth. If populations
  stabilize, consumption stabilizes and this can be
  problematic if all policies are predicated on
  growth. Several western European countries
  (Italy, Germany, others?) are now struggling with
  this problem.