An urban area functions as an open system because matter and energy cross its boundary. It receives inputs, contains storages, processes resources through flows and transformations, and releases outputs. This is core SL and HL content in Subtopic 8.2, Urban systems and urban planning.
A systems approach treats the city as a set of interacting environmental and social components. The urban boundary may be administrative, such as a municipal boundary, or functional, such as the area connected by commuting and resource flows.
| System component | Urban examples |
|---|---|
| Inputs | Food, freshwater, energy, construction materials, people, information and finance |
| Storages | Population, buildings, roads, reservoirs, goods and green spaces |
| Transfers | Movement of electricity, water, food, people and waste without changing their form |
| Transformations | Fuel combustion, food consumption, construction, sewage treatment and recycling |
| Outputs | Solid waste, wastewater, air pollutants, greenhouse gases, products and information |
These components are interdependent. For example, population growth increases demand for housing, transport, water and energy. This can expand infrastructure but also increase waste, pollution and pressure on surrounding ecosystems.
Urban systems also contain feedback loops. Congestion may encourage investment in public transport, which can reduce car use and stabilize traffic through negative feedback. Alternatively, road expansion may encourage more driving and suburbanization, amplifying congestion and land conversion through positive feedback.
Curitiba, Brazil, illustrates urban planning that changes system flows: its bus rapid transit network concentrates passenger movement along planned corridors. However, urban decisions reflect different perspectives, and environmental justice must be considered because access to transport, clean air, green space and safe housing is often unequal.
Exam technique: For an “explain” question, identify the inputs, storages, processes and outputs, then show causal links between them. In a systems diagram, use rectangular boxes for storages and directional arrows for flows; include both environmental and societal components rather than describing only physical infrastructure.