IB DP Environmental Systems and Societies – Human populations, urban systems and urban air pollution Study Guide

This study guide teaches Topic 8, Human populations and urban systems, for IB Diploma Programme Environmental Systems and Societies (ESS). It is aligned to the International Baccalaureate Diploma Programme Subject Brief: Environmental systems and societies, first assessment 2026, and covers syllabus sections 8.1 (human populations), 8.2 (urban systems and urban planning) and 8.3 (urban air pollution). The topic is studied at both SL and HL; the brief gives it 9 teaching hours at SL and 15 at HL, so HL students study some parts in extra depth. It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The brief gives Topic 8 Human populations and urban systems 9 teaching hours at SL and 15 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.

When you have worked through it, fix the key facts with the revision notes and test yourself with the practice questions. The IB DP ESS course hub and the printable syllabus checklist show where this topic sits in the course.

What this unit covers

Syllabus section	What you must be able to do	Level
8.1 Human populations	Calculate and interpret demographic rates; read population pyramids; apply and critique the demographic transition model; explain how population policies and growth affect the environment	SL and HL
8.2 Urban systems and urban planning	Describe a city as a system with inputs, outputs and storages; explain why urbanisation happens; evaluate urban planning for sustainability	SL and HL
8.3 Urban air pollution	Name primary and secondary pollutants; explain how photochemical smog and thermal inversions form; evaluate management strategies	SL and HL

The public subject brief lists only the topic name and hours, so check the detailed wording in your school’s copy of the subject guide. Keep Topic 1 in view: cities are open systems, and every management choice involves perspectives. The Topic 1 Foundations study guide covers that vocabulary.

8.1 Human populations

Demographic measures

You need these definitions word for word.

• Crude birth rate (CBR): live births per 1,000 people per year.
• Crude death rate (CDR): deaths per 1,000 people per year.
• Natural increase rate (NIR): (CBR − CDR) ÷ 10, given as a percentage. It ignores migration.
• Doubling time: the years a population takes to double at a constant growth rate. Estimate it with the rule of 70: doubling time ≈ 70 ÷ growth rate (%).
• Total fertility rate (TFR): the average number of children a woman would have in her lifetime at current age-specific birth rates. About 2.1 is replacement level where child mortality is low.
• Dependency ratio: (people aged 0–14 + people aged 65 and over) ÷ people aged 15–64 × 100.

Worked example 1: rates and doubling time

Country R (fictional) has 15.0 million people. In one year there are 345,000 births and 105,000 deaths. Find the CBR, CDR, NIR and doubling time.

CBR = 345 000 ÷ 15 000 000 × 1000 = 23.0 per 1000
CDR = 105 000 ÷ 15 000 000 × 1000 = 7.0 per 1000
NIR = (23.0 − 7.0) ÷ 10 = 1.6 %
Doubling time ≈ 70 ÷ 1.6 = 43.75 ≈ 44 years

State the assumption: the growth rate stays at 1.6% and migration is zero, so the figure is only an estimate.

Population pyramids

A pyramid shows the percentage (or number) of males and females in each age band.

• Wide base, narrow top (expanding): high birth rate, high or falling death rate, young population.
• Straight sides (stationary): low birth and death rates, slow growth.
• Narrow base, wide middle and top (contracting): birth rate below replacement, ageing population.

Read the shape as a record of the past: a notch may show a war, an epidemic or a strict birth policy. Quote figures from the axis when you describe one.

A young population carries population momentum: even if TFR falls to replacement, the large number of young people entering child-bearing age keeps births high for decades.

The demographic transition model (DTM)

The DTM describes how birth and death rates change as a society develops.

Stage	Birth rate	Death rate	Growth
1 High stationary	High	High, fluctuating	Very slow
2 Early expanding	High	Falling fast (better food, water, sanitation, healthcare)	Rapid
3 Late expanding	Falling (contraception, education of women, urban living, lower infant mortality)	Low	Slowing
4 Low stationary	Low	Low	Very slow
5 Declining (proposed)	Below death rate	Low, may rise as population ages	Negative

Limitations are a common evaluation point. The model is based on the history of European countries. It assumes that falling death rates are followed by falling birth rates, which may not happen where culture, religion or policy keep fertility high. It ignores migration. It gives no timescale, and countries today often move through the stages far faster than Europe did. It says nothing about why stage 5 happens.

Population and resources

Two contrasting views help in evaluation. The Malthusian view says population grows faster than food supply, so growth will be limited by famine, disease or conflict. The Boserup view says population pressure drives innovation, such as new farming methods, so food supply rises to meet demand. Real outcomes depend on technology, trade and consumption.

The environmental impact of a population is not set by numbers alone. The IPAT relationship is one model: Impact = Population × Affluence × Technology. A small, high-consuming population can have a larger ecological footprint than a large, low-consuming one.

Population policies

• Pro-natalist policies raise birth rates: child benefit payments, paid parental leave, subsidised childcare, tax breaks for larger families.
• Anti-natalist policies lower birth rates: free contraception, family planning services, education and employment for women, incentives for smaller families.

8.2 Urban systems and urban planning

The city as a system

A city is an open system. It exchanges both energy and matter with its surroundings.

• Inputs: food, water, energy (fuel and electricity), building materials, manufactured goods, people (migrants and commuters).
• Storages: buildings, infrastructure, people, stocks of goods.
• Outputs: solid waste, sewage, air pollutants, greenhouse gases, waste heat, manufactured goods and services, people leaving.

A city produces very little of its own food or energy, so its ecological footprint is much larger than its own land area. That is the systems link to Topic 1.

Urbanisation

Urbanisation is the increase in the proportion of a population living in urban areas. It results from rural–urban migration and from natural increase within cities.

• Push factors (rural): few jobs, mechanised farming, low incomes, poor services, land degradation, drought.
• Pull factors (urban): jobs, higher wages, schools, hospitals, perceived opportunity.

Suburbanisation moves people to the city edge; counter-urbanisation moves them out to rural areas and small towns. Rapid urban growth without planning can produce informal settlements with poor water, sanitation and air quality.

Environmental effects of urban areas

• Land-use change: loss of habitat and farmland at the urban fringe (urban sprawl).
• Impermeable surfaces: faster runoff and a higher flood risk.
• Urban heat island: cities are warmer than the surrounding countryside, especially at night. Causes include dark surfaces that absorb solar radiation and release heat slowly, waste heat from vehicles, buildings and air conditioning, less vegetation and so less evapotranspiration, and tall buildings that trap heat and reduce wind speed.
• Concentrated demand for water and energy, and concentrated waste and pollution.

Worked example 2: projecting urban growth

A city (fictional) has 2.4 million people and grows at 3.5% per year. Estimate its population in 20 years.

Population = 2.4 × 1.035²⁰
           = 2.4 × 1.990
           = 4.78 million (3 s.f.)
Check with rule of 70: 70 ÷ 3.5 = 20 years to double → about 4.8 million

The methods agree: housing, water, transport and waste capacity must roughly double in 20 years.

Sustainable urban planning

Urban planning shapes how land is used, so it controls much of a city’s footprint. Strategies include:

• Compact, mixed-use development: homes, jobs and services close together so people walk, cycle or use transit. This reduces car use and sprawl.
• Brownfield before greenfield: redeveloping old industrial land protects farmland and habitats.
• Public transport and active travel: rail, bus rapid transit, cycle lanes, pedestrian zones.
• Green infrastructure: parks, street trees, green roofs and walls. These cool the city, absorb rainwater, support biodiversity and trap some particulates.
• Energy and water efficiency: building standards, district heating, rainwater harvesting, water recycling.
• Waste systems: separation, recycling and composting at source.

Evaluate each strategy by cost, who benefits and who pays. Compact housing can raise land prices and push poorer residents out; transit needs large public investment. Name the stakeholders and their perspectives.

8.3 Urban air pollution

Primary and secondary pollutants

• Primary pollutants are emitted directly: carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO₂), particulate matter (PM), and volatile organic compounds (VOCs). Main sources are road vehicles, power stations, industry and burning of fuels for cooking and heating.
• Secondary pollutants form when primary pollutants react in the atmosphere. The key examples are tropospheric (ground-level) ozone and peroxyacyl nitrates (PANs).

Photochemical smog

Photochemical smog forms on sunny, still days in cities with heavy traffic.

1. Vehicles emit nitrogen monoxide and VOCs during the morning rush hour. Nitrogen monoxide is oxidised to nitrogen dioxide (NO₂).
2. Sunlight splits NO₂: NO₂ → NO + O.
3. The free oxygen atom joins oxygen: O + O₂ → O₃ (ozone).
4. VOCs react with the nitrogen oxides to form PANs and more ozone.

So NO₂ usually peaks in the morning and ozone peaks in the afternoon, when sunlight is strongest. Ozone is an irritant. It harms lungs and eyes, damages crops by reducing photosynthesis, and weakens materials such as rubber.

Thermal inversions

Normally air temperature falls with height, so warm polluted air rises and disperses. In a thermal (temperature) inversion, a layer of warm air sits above cooler air near the ground. The cool air cannot rise through it, so pollutants are trapped close to the surface. Inversions are common on calm, clear nights and in cities in valleys or basins surrounded by hills. Smog episodes are worst when an inversion lasts several days.

Managing urban air pollution

Use the three levels of pollution management.

Level	Examples for urban air
Alter the human activity	Public transport, cycling, compact planning, switching to renewable electricity, congestion charges, car-free days
Control release of the pollutant	Catalytic converters, particulate filters, flue-gas scrubbers, emission standards, low-emission zones
Clean up / restore	Street trees and green walls that trap particulates, air quality alerts that tell vulnerable people to stay indoors

Altering human activity tackles the cause but takes years and needs public support. Control acts faster but needs enforcement. Clean-up is weakest for air, because pollutants disperse.

Worked example 3: judging a low-emission zone

A city (fictional) introduces a low-emission zone. Mean annual roadside NO₂ falls from 58 µg/m³ to 41 µg/m³.

Percentage change = (41 − 58) ÷ 58 × 100 = −29.3 %

Then evaluate. The fall is large, but one year’s data may reflect weather. Traffic may have moved to roads outside the zone. Ozone, a secondary pollutant, may not fall in step with NO₂. Poorer drivers with older cars pay most. A strong conclusion weighs these against the health benefit.

Common errors

• Giving NIR per 1,000 instead of as a percentage, or forgetting that NIR ignores migration.
• Using the rule of 70 without stating the assumption of a constant growth rate.
• Calling ozone a primary pollutant. Ground-level ozone is secondary.
• Mixing up tropospheric ozone (a harmful pollutant) with stratospheric ozone (which protects life from UV).
• Saying a thermal inversion is “hot air trapping pollution”. The point is that warm air above cool air stops the cool, polluted air rising.
• Listing DTM stages without explaining the causes of each change in birth or death rate.
• Evaluating a strategy with only advantages. Add cost, enforcement, who is affected and the timescale.

Where to go next

Recap with the revision notes, then try the practice questions. The ESS exam preparation guide and ESS syllabus guide show how this topic is assessed.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.