Population Growth. Nan Astone, PhD Johns Hopkins University

Transcription

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2 Population Growth Nan Astone, PhD Johns 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 3

4 Section A The Demographic Balancing Equation

5 Demographic Balancing Equation P t2 = P t1 + (B D) + (I O) Where P t2 = population at time 2 P t1 = population at time 1 B = births D = deaths I = in migrants O = out migrants 5

6 Demographic Balancing Equation P t2 = P t1 + (B D) + (I O) (B D) is referred to as natural increase 6

7 For Most of History, Natural Increase Was 0 Births = deaths until about 1000 CE Then, gradually as the ability to store food and trade increased, births began to exceed deaths Famines and epidemics were periodic setbacks 7

8 World Population Growth through History Sources: Population Reference Bureau; United Nations. (1998). World population projections to

9 Number of Years to Add Each Billion Sources: First and second billion: Population Reference Bureau; Third through ninth billion: United Nations. (2005). World population prospects: the 2004 revision (medium scenario). 9

10 Rates of Birth, Death, and Natural Increase Rates of birth, death, and natural increase per 1,000 population Natural increase Source: United Nations. (2005). World population prospects: the 2004 revision. 10

11 The Classic Stages of Demographic Transition Stage 1 Stage 2 Stage 3 Stage 4 Birth rate Natural increase Death rate Time Note: Natural increase is produced from the excess of births over deaths 11

12 Brief Digression The demographic transition nonsense Many demographic transitions Fertility decline Mortality decline Hurricane Katrina The occupation of the Americas The Great Influenza 12

13 Demographic Balancing Equation P t2 = P t1 + (B D) + (I O) (I O) is referred to as net migration 13

14 Globally, Net Migration Is Always 0 Globally, net migration is always 0 At least, most of us think so The truth is out there! 14

15 In Sum For most of history, natural increase was 0 Gradually, between 1000 CE and 1800 (or so), natural increase slowly increased as death rates went down and births began to gradually exceed deaths From 1800 onward, escalating sharply after 1950, births began to exceed deaths by a great deal, resulting in dramatic population growth And then, fertility began to decline 15

16 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 17

18 Average Number of Children per Woman Source: United Nations. (2005). World population prospects: The 2004 revision. 18

19 Once Fertility Reaches Replacement So, will population be stable once fertility falls (or rises) to replacement? 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 19

20 Number of Births and Fertility Rate for the U.S to Births Per 1000 Women Number of Births Year number of births fertility rates 20

21 Population of Ireland, 1986 to

22 Population of Ireland,

23 Population of Ireland,

24 Population of Ireland,

25 Population of Ireland,

26 Population of Ireland,

27 Population of Ireland,

28 Population of Ireland,

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 29