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Human Population
Envi 201 Spring 2004
Key Concepts
Exponential Growth – Rule of 70 Demographic transition Age structure and population
momentum Causes of fertility decline Links between fertility decisions and
environment (Dasgupta article)
Exponential Growth
Quantity grows by the same relative amount (g) each year
P(next year) = P(this year)* (1+g)
At time t, P = P0 x (1+g)t
Exponential Growth
Quantity grows by the same relative amount each year
Doubling time d is constant
Rule of 70:
Doubling time x growth rate = 70 Examples:
r, growth rate, % per year
d, doubling time, years
10 7
1 70
2 35
World Population Growth
World Population Grew Slowly Through 1945…Then accelerated…Then slowed
Trends in Population Growth WorldwidePopulation Increase and Growth Rate, Five-Year Periods
7987
82 79 77 75 7369
0
10
20
30
40
50
60
70
80
90
1980-1985
1985-1990
1990-1995
1995-2000
2000-2005
2005-2010
2010-2015
2015-2020
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Net population added per year Annual population growth rate
Mill
ions
Per
cen
t in
crea
se p
er y
ear
Source: United Nations, World Population Prospects: The 2002 Revision (medium scenario), 2003.
Growth Rates have Peaked…
0%
1%
2%
3%
4%
5%
6%
1950-60 1960-70 1970-80 1980-90 1990-2000
Alaska Population: Average Annual Rates of Growth
(due to births, deaths, and net migration)
The Demographic Transition
Stage I: high birthrates and death rates Stage II: continued high birthrates,
declining death rates Stage III: falling birthrates and death
rates, eventually stabilizing
Replacement Rate would be 2 children per woman
Population Momentum
Lots of young people Although each woman may have total
fertility = 2 children, There are many more future mothers
than current mothers
Native Age Structure
-1500 -1000 -500 0 500 1000 1500
Age
4
9
14
19
24
29
34
39
44
49
54
59
64
69
74
79
84
89
Male
Female
Does Alaska have Population Momentum?
Replacement Rate would be 2 children per woman
Testing the Relationship:
Bangladesh fertility dropped from 7.0 to 3.8 between 1970 and 1998
Bangladesh income per capita in 1998 is supposed to be $2200 in 1980$….
Latest HDR says $1602 year 2000$ Fertility has dropped faster than curve
predicts