Population Notes

Doubling time

• Time required for an exponentially growing amount to double
• Rule of 72:
  • % growth rate X doubling time = 72
  • so, if population grows at 2%, it takes 36 years to double

Changing population growth

• Doubling time in 500 AD - about 700 years, 0.10% growth rate
• Doubling time in 2000 - about 50 years, 1.5% growth rate

How does population growth work?

• Population growth rate depends on: Birth rate – death rate
• Replacement rate = how many children it takes to replace the parents
• Which is 2 plus a bit to make up for the children who die before reproducing
• So about 2.1 children per woman on average

Thinking about countries:

• We have to take into account immigration and emigration
• So population growth depends on
• Rate of natural increase (birth-death) PLUS
• Rate of net immigration (immigration-emigration)
• Most Western industrialized countries have RNI near zero or even negative
• So immigration is responsible for population increases (and emigration for decreases)

Relationships between population growth, per capita GDP, energy use

• Per capita GDP
  • GDP - domestic gross product: a measure of the income of a country
  • per capita: per person
• Population growth and GDP:
  • high population growth tends to happen in low GDP countries
    • agricultural economies where children help bring in income
    • wealthier, industrial countries - children are a cost, not an asset
  • negative population growth in moderately low GDP countries
    • poor economic growth leads young people to leave
    • lack of population growth means fewer workers, weaker economy
  • highest GDP tends to go with population growth rates near 1%
• GDP and energy use
  • the higher the GDP, the higher the energy use

So how do we manage ever increasing energy use?

• The most effective way to control population growth is to encourage industrialization
• Increased industrialization leads to higher GDP
• Higher GDP means greater energy use per person
• So the best way to control population also leads to increased energy use per person
• Do we have sufficient energy resources to support a fully industrialized world, even if the global population does level off and stabilize?

If you want to look at the interactive graphs we used in class, here's the link: www.gapminder.org/data

We looked at some of these graphs (you can make them yourself by choosing the variables you want to see on the drop-down menu on each axis; you can run the graphs through time by clicking the Play button):

• Children per woman v. income per person
• Children per woman v. under-five mortality rate
• Crude birth rate v. crude death rate
• Life expectancy at birth v. income per person
• CO2 map

Here are the graphs we used during this exercise: