Geology
140 - Environmental Geology |
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**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%

- high population growth tends
to happen in low GDP countries
- 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?

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