Measuring population change
Big idea: Population change is determined by births, deaths, and migration. Understanding these rates helps us predict future population trends and their environmental implications.
Key population terms
- Crude birth rate (CBR): Number of births per 1,000 population per year
- Crude death rate (CDR): Number of deaths per 1,000 population per year
- Natural increase rate: (CBR - CDR) ÷ 10 = annual % change (excluding migration)
- Total fertility rate (TFR): Average children per woman; replacement level is ~2.1
- Doubling time: Years to double population = 70 ÷ growth rate (%)
Current global trends
- World population: ~8 billion (2023); projected 9.7 billion by 2050
- Growth rate: ~1% per year (down from 2% peak in 1960s)
- Uneven distribution: 60% live in Asia; fastest growth in sub-Saharan Africa
- Urbanisation: >55% now live in cities; projected 68% by 2050
Population growth rate has slowed, but absolute numbers keep rising because we are adding 1% to an already large base (~80 million people/year).
Exam tip: When describing population data, state the TREND (increasing/decreasing), the RATE of change, and any REGIONAL differences.
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Population pyramids and structure
Big idea: Population pyramids reveal a populations past, present, and likely future — showing whether its growing, stable, or declining.
Reading population pyramids
- X-axis: Population size (often as % or absolute numbers)
- Y-axis: Age groups (usually 5-year cohorts)
- Left side: Males; Right side: Females
- Base width: Indicates birth rate
- Top width: Indicates life expectancy
- Bulges/indentations: Show baby booms, wars, migration, or disease events
Types of population structure
Expansive (youthful)
- Wide base, narrow top
- High birth rate, low life expectancy
- Rapid population growth
- Common in LICs (e.g., Niger, Uganda)
- High dependency ratio (young)
Constrictive (ageing)
- Narrow base, wider middle/top
- Low birth rate, high life expectancy
- Slow growth or decline
- Common in HICs (e.g., Japan, Germany)
- High dependency ratio (elderly)
Dependency ratio indicates economic pressure. Both very young AND very old populations have high dependency ratios.
Exam tip: When interpreting pyramids, DESCRIBE what you see (wide base, narrow top) before EXPLAINING what it means (high birth rate, low life expectancy).
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IB-style question — Population dynamics [2]
The town of Marisol has a population of 48,000. Births = 1,440 per year, deaths = 720 per year, immigration = 200, emigration = 80. Calculate the natural increase rate (%) and outline whether the population is growing. [2]
How to answer it, step by step
- Natural increase rate
• Natural increase = births − deaths = 1,440 − 720 = 720
• Rate = (720 ÷ 48,000) × 100 = 1.5% - Is it growing?
• Natural increase is positive (more births than deaths)
• Net migration (+120) adds further, so population grows
Final answer
Examiner tip: natural increase uses only births minus deaths; migration is a separate factor — don't mix them in the rate.
IB-style question — Doubling time [1]
Marisol's population grows at 3.5% per year. Calculate how many years it will take for the population to double in size. [1]
How to answer it, step by step
- Apply the rule of 70
• Doubling time = 70 ÷ growth rate (%)
• = 70 ÷ 3.5 = 20 years - State the answer
• The population will double in about 20 years
• Higher growth rate = shorter doubling time
Final answer
Examiner tip: always use 70 ÷ percentage growth rate; show the division to earn the calculation mark.