8.2 Urban systems and urban planningIB Environmental Systems and Societies SL: Revision notes

An urban area is a built-up area with a high population density, buildings and infrastructure. It works as a system: inputs such as people, food, water, fuel and electricity flow in, and outputs such as waste, wastewater, heat and pollutants flow out. Its components (housing, transport, industry, green space) are linked by flows of matter and energy.

Urban areas also contain urban ecosystems: parks, gardens, rivers, roadside verges and the organisms that live in them, such as foxes, pigeons and street trees.

Urbanisation is the population shift from rural to urban areas. It is driven by rural-urban migration: push factors (low incomes, farm mechanisation, land shortage, drought, poor services) and pull factors (jobs, higher wages, schools, healthcare).

As a result more people now live in urban than rural systems. Globally about 30% of people were urban in 1950, about 56% in 2020, and the United Nations projects roughly two-thirds by 2050. The proportion is still increasing.

Suburbanisation is the movement of people from dense central urban areas to lower-density areas on the periphery. It is often linked to car ownership, cheaper land, bigger homes and the wish for green space. Effects include urban sprawl, longer commuting by car, and the loss of farmland or natural habitat.

The expansion of urban and suburban systems changes the environment.

• Habitat loss and fragmentation, reducing biodiversity.
• Impermeable surfaces increase surface runoff and flood risk and reduce infiltration.
• Urban heat island: dark surfaces and buildings absorb heat, so cities are warmer than the surrounding countryside.
• More energy use, waste and air pollution.
• Greater resource demand, especially for water and food.

Urban planning helps decide the best way to use land and buildings, for housing, industry, transport and green space. Modern planning may also consider the sustainability of the urban system, for example public transport, energy efficiency, waste reduction and green space.

Ecological urban planning is more holistic. It treats the city as an ecosystem and aims to understand the complex relationships between its biotic and abiotic components.

Example: Curitiba, Brazil: bus rapid transit, parks on flood-prone land, and a recycling scheme linked to food and transport tokens.