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IB Environmental Systems & Societies: 8 · Human populations and urban systems
Population dynamics, urban systems, urban air pollution. 3 subtopics, 16 key terms.
In this topic: 8.1 · 8.2 · 8.3
8.1Human populations
Key terms
- Crude birth rate (CBR)
- Births per 1000 people per year.
- Crude death rate (CDR)
- Deaths per 1000 people per year.
- Natural increase rate (NIR)
- (CBR − CDR) ÷ 10, as a %.
- Doubling time
- Years for a population to double ≈ 70 ÷ NIR (%).
- Total fertility rate (TFR)
- Average number of children per woman; about 2.1 is replacement level.
- Demographic transition model (DTM)
- Model of how birth and death rates change as countries develop.
DTM stages: 1 high stationary (high CBR and CDR), 2 early expanding (CDR falls), 3 late expanding (CBR falls), 4 low stationary, 5 possible decline.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Use the DTM as a model: say what it predicts, then explain why a specific country does not fit it exactly.
8.2Urban systems and urban planning
Key terms
- Urbanisation
- The increasing proportion of people living in urban areas.
- Urban system
- A city with inputs (food, water, energy, people) and outputs (waste, emissions, products).
- Urban sprawl
- Unplanned spreading of a city into surrounding land.
- Urban heat island
- Cities are warmer than surrounding rural areas.
- Sustainable city
- A city designed to reduce its environmental impact and improve quality of life.
Push factors (rural poverty, lack of services, conflict) and pull factors (jobs, services, education) drive rural–urban migration.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Model the city as a system: list inputs and outputs, then show how a strategy changes them.
8.3Urban air pollution
Key terms
- Primary pollutant
- Released directly (e.g. NO, CO, SO₂, particulates).
- Secondary pollutant
- Formed by reactions in the atmosphere (e.g. tropospheric ozone, nitric and sulfuric acid).
- Photochemical smog
- A haze formed when sunlight drives reactions between NOx and volatile organic compounds.
- Temperature inversion
- Warm air above cool air traps pollutants near the ground.
- Acid deposition
- Wet or dry deposition of acids formed from SO₂ and NOx.
Sources: vehicles, industry, power stations, domestic heating and cooking, biomass burning.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Distinguish primary from secondary pollutants and stratospheric (good) from tropospheric (harmful) ozone.
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