Revised 23 August 2021



SECTION SUMMARY


  • Population dynamics is the study of changes in population size by tracking births, deaths, and net migration


  • The size of the world population in 10,000 BC is estimated to have been 2.4 million people.


  • The average annual growth rate of our human population between 10,000 BC to 1000 AD was very slow at 0.04% per year.


  • The number of years that it takes for an exponentially growing population to double in size, or doubling time, is approximately 70 divided by the annual growth rate as a percentage.


  • During the 20th century our world population doubled within one lifetime and there are now strong signals that our world population has exceeded the carrying capacity of our planet Earth.


  • A larger human population will consume more resources than a smaller population. 


  • Continued growth in human populations exacerbates the impact of climate change at the very same time that fossil fuels are needed to enable a transition to renewable energy. 


  • Continued increases in population during a transition from fossil fuels to renewable energy can but only result in a Sisyphus like undermining of efforts for a smooth transition. 


  • The crude birth rate (b) is the ratio of births over one year per 1,000 people within a population.


  • The crude death rate (d) is the total deaths at all ages per 1,000 people per year.


  • With improvements in medical care and nutrition in the 20th Century, there was a simultaneous sharp decline in the crude death rate and sharp increase in the crude birth rate until 1965 which resulted in a population boom. 


  • From 1965 onwards both the crude birth and death rates have declined in tandem with the crude birth rate declining more rapidly.


  • The high proportion of deaths due to communicable, maternal, neonatal, and nutritional diseases in the low Socio-Demographic Index (SDI) countries in 2016 is of major concern, especially when this category of deaths is preventable with better medical care and nutrition.


  • Lack of sufficient medical care and nutrition reflects the inequity in income and educational attainment between countries. 


  • The infant mortality rate is the number of infants dying before reaching one year of age per 1,000 female live births in a given year.


  • World infant mortality has declined substantially in middle-income and low-income countries since 1960.  Populations of countries with high infant mortality rates tend to grow faster than those countries with low infant mortality rates


  • The growth rate (r), the crude birth rate less the crude death rate (b-d), describes the increase of births over deaths per 1,000 people per year within a population.


  • A necessary condition of Zero Population Growth (ZPG) is that the average crude birth rate from generation to generation equals the average crude death rate.


  • When a population is growing rapidly, it is more illuminating to focus on changes in the growth rate rather than the population level.


  • The world population grew from 3.0 billion people in 1960 to 7.5 billion people in 2017. From 1960 until 2017 the growth rate had been almost linear rather than exponential. The reason why is provided by examining the change in the growth rate.


  • The size of the populations in the middle-income countries in 1960 and subsequent annual growth rates have dominated growth in the world population.


  • There are substantial differences in the growth rates of countries within income groups and especially in the low-income group of countries.


  • When forecasting the future size of a population, the changing age composition of the population must be taken into account.


  • A rapid growth population has a triangular spear shape age-group profile which is typical of a country with a high crude birth rate.


  • A Zero Population Growth (ZPG) population has a cylindrical bullet shape age-group profile. 


  • A transition of the world population to a ZPG population can be described as taking place over 4 stages.


  • In a ZPG population there are more adults over the age of 65 than children under the age of 15.


  • In a transition from a growth population to a ZPG population, the ratio of the number of dependents under the age of 15 and over the age of 65 to the number of working age from 15 to 64 changes.


  • An example of a major change in fertility is the population of China which adopted a one child per family policy. This policy was discontinued in 2016 when China adopted a two-child policy. 


  • By 2015 Chinese woman had 1.62 children on average compared to the world average of 2.45 children, 


  • The one-child policy had a dramatic and lasting impact on the age composition of the Chinese population.


  • In a transition from a growth to a ZPG population, there would be a period of continued momentum of growth in the population called population momentum.


  • The age composition of the population will change as the crude birth rate declines to match and equal the crude death rate.


  • If crude births rates were to equal crude death rates and maintained, then the population would continue to grow due to a bulge of females moving up the age composition stack of cohorts into the child-bearing rank of cohorts between the ages of 15 to 44.


  • In 1971, Demographer Nathan Keyfitz calculated that if the less developed countries with high birth rates were to achieve a replacement fertility rate overnight, then their populations would continue to increase until they stabilised at about 1.6 times their then present size. If a replacement fertility rate took 30 years to achieve, then their final populations would be 2.5 times their then present size. 


  • The crude birth rate does not compensate for changes in age composition. A better indicator of birth trends is the general fertility rate, the number of births per 1,000 women per year between the ages of 15 and 44.


  • A more meaningful fertility indicator is the average completed family size or total fertility. 


  • At the replacement fertility level, each family on average would comprise of 2.11 children. The extra 0.11 child on average compensates for those children who do not survive to reproduction age and for those women who do not have children by virtue of choice or infertility. 


  • World total fertility increased to a maximum of 5.07 children per family in 1964 and then declined monotonically to 2.44 children per family in 2016.


  • The rate of decline in total fertility since 1993 has slowed down. 


  • Continued growth in the population of the middle-income countries will dominate future growth in the world population before the year 2100.


  • Average life expectancy at birth is the average number of years that a cohort of new-born infants would live if prevailing patterns of mortality at the time of birth were to stay the same throughout life.


  • The average life expectancy at birth and remaining average life expectancies at each age are estimated using a life table which is based on age-specific crude death rates per 1,000 people of the same age.


  • A population with a high crude death rate has a corresponding low average life expectancy.


  • The world average life expectancy increased monotonically from 52.6 years in 1960 to 72.0 years in 2016. At the same time the world crude death rate decreased from 17.74 to 7.75 deaths per 1,000 people per year.


  • Females tend to live longer than males and the average life expectancy of the world female population in 1960 was 54.6 years compared to 50.7 years for males. These average life expectancies improved to 74.3 and 69.6 years respectively by 2016.


  • The crude death rates in low-income countries tends to be higher than that in high-income countries and this is reflected in the lower average life expectancy.


  • The age dependency ratio is the ratio of the number of dependents in a population younger than 15 or older than 64 to that of the working-age population aged between 15 to 64.


  • The age dependency ratio is given as the proportion of dependents per 100 working-age people in the population.


  • A decline in the age dependency ratio has the potential to alleviate the burden of the working-age population to support those who are dependents.


  • The transition from a rapid growth world population prior to 1971 to a world population with a declining growth rate has resulted in a substantial decrease in the world age dependency ratio from 77.0% in 1967 to 54.0% in 2014.


  • If the world population continues to fully transition to a ZPG population and human mortality in all countries improves to the same level as that of the New Zealand population in 2016, then the ratio of adults over the age of 65 in all countries to those of working age would eventually undergo an almost three-fold increase. The ratio of children under the age of 15 to those of working age would simultaneously undergo an almost 25% reduction.


  • Whether a reversal in the age dependency ratio to previous levels of the early 1960s would actually increase the burden of care for dependents requires close examination.


  • Net migration is the net total of migrants over time - the total number of immigrants less the total number of which include both citizens and non-citizens.


  • The reasons for emigration are various including the wish for greater work opportunities and lifestyle of high-income countries, the need to flee from war-stricken zones to countries of safety, and the consequences of climate change which has resulted in climate change refugees.

 

  • Urban population refers to the percentage of people living in urban areas as defined by national statistical offices.


  • Prior to 1960 the group of high-income countries were already highly urbanised (63.8%) and urbanisation continued to increase to 81.5% by 2017.


  • Populations in the group of low-income countries were primarily rural (12.8% urbanised) in 1960 and urbanisation increased to 32.4% by 2017.


  • Population density is the mid-year population divided by land area in square kilometres.


  • The low-income countries had the lowest population density in 1960 compared to the middle-income countries (12.0 vs 28.0 people per sq.km.) and these population densities increased to 51.5 and 73.8 people per sq. km. respectively.


  • The degree of both increases in population density follow that of increases in population growth.


  • The population density of high-income countries was mid-way between that of low-income and middle-income countries (21.0 people per sq. km.) and finished up at 33.0 people per sq. km. due to a much lower increase in population over 57 years.


  • A significant statistic is the ratio of people to cultivatable land as this is a better indicator of a country’s ability to be self-sufficient in food production.


  • Several organisations have estimated projections of the world’s population. All projections agree that the world population in the future will be bigger though increasing in size less rapidly, will be more urban, and will be an older population.


  • The United Nations World Population Prospects 2017 Revision states that “with a certainty of 95 per cent, the size of the global population will stand between 8.4 and 8.7 billion in 2030, between 9.4 and 10.2 billion in 2050”
  • Projections are not predictions and are based on many assumptions (Cohen, 2002).


  • Whatever the future holds in store for us, continued growth in the world population before 2100 is inevitable unless dire consequences of climate change, inadequate access to energy be it fossil fuels or renewable energy, political strife, and over-reaching the carrying capacity of a local environment take over.

 

  • All that current and future generations can do during a transition to ZPG is to be better prepared and become more resilient to change.