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.
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).
Know your predicted grade
Take timed mock exams and get detailed feedback on every answer. See exactly where you're losing marks.
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.