lecture 2: population growth
TRANSCRIPT
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Population Growth
Nan Astone, PhDJohns Hopkins University
3
Objectives of the Lecture
At the end of this lecture and after reading the texts that accompany it, students will be able to:−
Explain the demographic balancing equation
Define natural increase and net migration−
Define population momentum
Interpret a population pyramid
Section A
The Demographic Balancing Equation
5
Demographic Balancing Equation
Pt2 = Pt1 + (B – D) + (I – O)−
Where …
Pt2 = population at time 2Pt1 = population at time 1B = birthsD = deathsI = in migrantsO = out migrants
6
Demographic Balancing Equation
Pt2 = Pt1 + (B – D) + (I – O)−
(B – D) is referred to as natural increase
7
For Most of History, Natural Increase Was 0
Births = deaths until about 1000 CEThen, gradually as the ability to store food and trade increased, births began to exceed deaths−
Famines and epidemics were periodic setbacks
8Sources: Population Reference Bureau; United Nations. (1998). World population projections to 2100.
World Population Growth through History
9Sources: First and second billion: Population Reference Bureau; Third through ninth billion: United Nations. (2005). World population prospects: the 2004 revision (medium scenario).
Number of Years to Add Each Billion
10Source: United Nations. (2005). World population prospects: the 2004 revision.
Natural increase
Rates of Birth, Death, and Natural Increase
Rates of birth, death, and natural increase per 1,000 population
11
Time
Stage 1 Stage 2 Stage 3 Stage 4
Naturalincrease
Birth rate
Death rate
Note: Natural increase is produced from the excess of births over deaths
The Classic Stages of “Demographic Transition”
12
Brief Digression
“The” demographic transition—nonsenseMany demographic transitions−
Fertility decline
−
Mortality decline−
Hurricane Katrina
−
The occupation of the Americas−
The Great Influenza
13
Demographic Balancing Equation
Pt2 = Pt1 + (B – D) + (I – O)−
(I – O) is referred to as net migration
14
Globally, Net Migration Is Always 0
Globally, net migration is always 0At least, most of us think so“The truth is out there!”
15
In Sum
For most of history, natural increase was 0Gradually, between 1000 CE and 1800 (or so), natural increase slowly increased as death rates went down and births began to gradually exceed deathsFrom 1800 onward, escalating sharply after 1950, births began toexceed deaths by a great deal, resulting in dramatic population growthAnd then, fertility began to decline
Section B
Population Momentum
17
Objectives of the Lecture
At the end of this lecture and after reading the texts that accompany it, students will be able to:−
Explain the demographic balancing equation
Define natural increase and net migration−
Define population momentum
Interpret a population pyramid
18Source: United Nations. (2005). World population prospects: The 2004 revision.
Average Number of Children per Woman
19
Once Fertility Reaches Replacement
So, will population be stable once fertility falls (or rises) toreplacement?Population momentum−
Population can continue to grow (or decline) even after natural increase has reached 0, because the number of women of childbearing age is very large (or small) because of past fertility rates
20
Number of Births and Fertility Rate for the U.S. 1960 to 2001
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
20011998199519921989198619831980197719741971196819651962
Year
Number of Births
0
20
40
60
80
100
120
140
Births Per 1000 Women 15-49
number of births fertility rates
21
Population of Ireland, 1986 to 2016
22
Population of Ireland, 1986
23
Population of Ireland, 1996
24
Population of Ireland, 2006
25
Population of Ireland, 2016
26
Population of Ireland, 2026
27
Population of Ireland, 2036
28
Population of Ireland, 2046
29
Why Is This Important?
Just because fertility rates have fallen for any given population doesn’t necessarily mean that the distribution of the population by age is going to remain the same