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Soc 663
Fall 2003
Lecture September 30
-Summary of previous lecture: agricultural revolution and homeostatic balance;
-The Irish case: an example of feedback mechanisms
I. The great divide: preindustrial population growth and industrial population growth
Unquestionably, the period that follows (approximately) 1750-1800 was one that is
unique in the history of human populations. The uniqueness can be found in the
following features:
a. End of population crisis as a regulatory mechanism
b. Large increases in the rates of increase of the populations;
c. Increase in production/consumption of energy
d. Increased rates of contacts/communications
{Graphs No 1 from Cohen, pp.61}
{Graph No2 from Coale, article in Science}
a. End of population crises
Although it is unclear whether crises per se were an efficient regulatory mechanisms,
there is no question that preindustrial population were subject to many of them, produced
by different sources, some endogenous and some exogenous. Disappearance of crises
illustrates control over nature and environment. It means the apparent dissolution of
population ceilings.
b. Large increase in the rates of population growth
In Europe the transition was triggered by a reduction of mortality rates around 1750 and
then more rapidly after 1850. This led to increased r (as high as .025) for at least a
century. It was only after 1870 or so that fertility began to fall in tandem, thus leading to
lower r.
In the rest of the world everything occurs later and more suddenly: mortality begins to
decline only after 1940 (public health revolution) and fertility rates do not begin to fall
until after 1970. The result is a large increase in r (as large as .040) among the largest
population in the world.
c. Urbanization
The industrial revolution leads to large concentration of population in urban centers. The
XXth century is the era of large megalopolis. Currently there are many cities with more
than 5 million inhabitants and very high density.
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{Refer to figure from Cohen pp.101}
d. Increased communications
At the same time that population has grown, the world has become a smaller place. Not
just because it is more highly densely populated but because of improvements in
communications. These lead to more efficient transactions, larger flows of information
but also to increased contacts and exposure to one environment to the actions of agents in
other environments (degradation plus emergence of diseases.
>>>Remarks:
1. To emphasize the uniqueness of the population growth regime that follows after 17501800, note the following:
-the world population doubled once in the last 40 years and about 3 times in the last 200
years. Yet, in the period between 10,000 years ago and today the population changed
from about 6 million people to about 6 billion. And increase of 1,000 or about 10
doublings since 210 is about 1024. That is, in about 10,000 years we have had 10
doublings but about 1/3 of them have occurred in the last 1/50 th of the period. And one
has occurred in the last 40/10000 or 1/500 th of the period
-If the population of the world had been growing at the same rate as it grew in the last 50
years (r~.02) then the starting point of human population should have be located in 1930.
2. There is great diversity. Although the trends described above are a fairly good
description of trends, continents and countries within continents display a great deal of
variability. Just in the period 1950-2000 as shown in the following graph there is great
diversity
{Graph No 3 Cohen pp. 55}
>>>>
II. Where are we going? Population in the XXIst century.
Unfortunately we cannot say for sure where we will be 50 years from now, much less 100
years from now. But we could revise population projections and forecasts and pass
judgment on what the most likely situation will be.
The distinction between a population projection and forecast
A population projection cannot be wrong since it simple says what the population will be
if it follows a particular pattern of mortality, fertility and migration. For example, the UN
produces three different projections: a low, an intermediate and a high one. Each of them
embody different assumptions about fertility and mortality leading to high, medium and
low numbers of total populations
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A population forecast implies to attach a probability that trends in births, deaths and
migration will follow a certain paths. Thus, a forecast can be right or wrong.
What have past projection told us?
According to Beyond Six Billions, errors have varied but quite a bit and are in general
associated with the horizon of the projections: the longer it is the larger the error.
They have also varied substantially by region
{Graph # 4 and 5 from pp.39 and 43 from Beyond 6 Billion}
In general, past projections have underestimated the decline in fertility (particularly in
China between 1960 and 1980 and later in China, India and Bangladesh and Brazil. We
have also underestimated the progress in mortality.
There are two errors that fall are neither of the above:
=We were not able to forecast HIV/AIDS (morality increase; fertility decrease)
=We were not able to forecast the breakup of the Soviet Union and
its aftermath (mortality increase)
What do current projection tell us?
In the next figure we have assessments from various agencies. If we take the median of
these projections we fall right on one of the UN projection (medium scenario)
{Graphs No 6, No 7 pp. 18, pp.20 from Beyond 6 Billion}
Main assumptions:
Mortality: continued increases in life expectancy to about 85.5 years
Fertility: continued decline in TFR to levels of about 2.1. (replacement)
1950….2.5B
2000…..6B
2050…..9B
r~.0175 (between 1950 and 2000)
r~.0081 (between 2000 and 2050)
>>>>>>Remarks:
a. How long does it take to add a billion people?
Billion No 1………….1800 (thus, it took about 130,000 years!)
Billion No 2………….1925 ………..125 years
Billion No3…………..1960………….25 years
Billion No 4………….1974………….14 years
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Billion No 5…………..1987…………13 years
Billion No 6………….1999………….12 years
b. Sensitivity:
Projected numbers are very sensitive to fertility assumptions and virtually not sensitive at
all to assumptions about mortality. Indeed if one assumes that TFR remains slightly
above replacement we would end up with 10.6B instead of 9B in 1950. In, instead, it
remained slightly below, the total population in 2050 would be 7.5B
c. Age distribution: One implication of these projections is that the age distribution will
become older; more so in Europe and North America than elsewhere with proportions
over 65 rising to about 20%
How reliable are projections?
Suppose that NRR went down to 1.0 and that mortality did not change at all. Then the
population growth will appear as in the Figure. About slightly less than 2/3 of the growth
expected to occur by 2050 will occur anyway. The population could be at least 7.9B.
This is simply the result of the momentum of past population growth
{Graph # 8 from Beyond 6 Billion}
We can err on a number of things:
= Fertility with no state intervention? Will Italians stand still while their population
declines by 1/3 in the next 50 years?
=Mortality can increase: can poverty, spread of diseases, environmental contamination
and global warming have catastrophic effects?
Probabilistic forecasts
What about if we adopt a probabilistic view? We can adopt the forecasts made by IIASA
and observe that a 80% confidence interval for 2050 includes the values 7.3B-10.4B
whereas for 2100 we have 5.6B-12.1B, that is there is a possibility of population decline!
{Graph No 9 from Lutz et l in Nature, pp.544}