Most cities still run on a straight line: pull resources in at one end, push waste out at the other. Two models try to bend that line. The circular economy asks how outputs can become inputs again. Doughnut economics asks whether the city meets everyone’s needs without breaking Earth’s limits. They answer different questions, and the best answers use both.
📚 What you need to know
Linear metabolism cities take, use and dispose. Resources enter once and leave as waste.
Circular metabolism cities recycle, reuse and repair, so outputs are fed back in as inputs.
The circular economy rests on minimising waste, using resources efficiently, and closing the loop.
Doughnut economics combines a social foundation (inner limit) with an ecological ceiling (outer limit).
Named examples: Amsterdam’s circular neighbourhood and its 2020 doughnut strategy; San Francisco’s zero-waste programme; Brussels using the doughnut for city planning.
Both models have real limitations, and saying what they are is where the higher marks sit.
Linear against circular metabolism
The word “metabolism” is deliberate. Just like an organism, a city takes in materials and energy, transforms them, and excretes what it cannot use. The question is whether anything gets returned to the start.
The two diagrams have identical boxes. Adding one return path changes both the size of the input and the size of the output.
Feature
Linear metabolism
Circular metabolism
Approach
Take, use, dispose
Reduce, reuse, repair, recycle
Inputs
Large, mostly non-renewable and imported
Smaller, more renewable and more local
Outputs
High waste, pollution and emissions
Much reduced; waste treated as a resource
Waste destination
Landfill or incineration
Recycling, composting, energy recovery
Long-term outlook
Depends on continued extraction elsewhere
Far less dependent on imported raw materials
The circular economy
The circular economy applies that loop to whole systems of production and consumption. It rests on three ideas.
Minimise waste. Products and materials are reused, repaired, recycled or repurposed instead of discarded.
Use resources efficiently. Design things to last, to be repaired easily, and to be taken apart at the end of their life.
Close the loop. Reduce dependence on new raw materials, most of which are imported over long distances.
Recycling is the last resort inside the loop, not the first choice. Reducing and reusing come first because they avoid the energy cost of reprocessing.
Cities doing it
Amsterdam, Netherlands. The Buiksloterham district is a pilot circular neighbourhood: buildings constructed from recycled materials, and residents and businesses sharing resources such as tools and transport to cut consumption.
San Francisco, USA. A zero-waste strategy diverting around 80% of waste from landfill and incineration, using compulsory composting and recycling and construction waste repurposed into new buildings.
WORKED EXAMPLE
A city produces 1.2 million tonnes of waste a year and diverts 80% from landfill. Calculate the mass diverted and the mass landfilled, then state one limitation of using diversion rate as a sustainability measure.
Step 1: mass diverted1.2 × 0.80 = 0.96 million tonnes0.96 Mt divertedStep 2: mass landfilled1.2 − 0.96 = 0.24 million tonnes0.24 Mt to landfillStep 3: the limitation
Diversion rate says nothing about how much waste is produced in the first place. A city generating twice as much waste but recycling 80% of it still sends more to landfill in absolute terms.
Reduction sits above recycling in the hierarchy, so a falling total is a better sign than a rising diversion rate.
Doughnut economics in a city
You met the doughnut in 8.1 as a global framework. Cities have started using it as a planning tool, because a city is a scale where decisions actually get made.
The social foundation becomes concrete: is there enough affordable housing, are people healthy, can everyone reach a school and a job?
The ecological ceiling becomes local and global at once: the city’s own emissions, water use and land conversion, plus the footprint of everything it imports.
The safe and just space between them is the target.
Amsterdam adopted the doughnut as a blueprint for urban sustainability in 2020, combining emissions and energy reduction with affordable housing and transport, expanded green roof programmes, and support for urban farming to reduce dependence on imported food. Brussels uses the same model to guide green infrastructure and social equity, cutting emissions through electric public transport and cycling infrastructure while increasing access to affordable housing and public services.
Why the doughnut suits cities particularly well. National governments argue about targets; city governments own the housing, the buses, the waste contracts and the planning rules. The doughnut gives them a way to check that a decision helping one boundary is not quietly breaking the other.
Being honest about the limits
Examiners want criticism as well as description. Both models have genuine weaknesses.
Model
Strengths
Limitations
Circular economy
Practical and measurable; cuts waste, imports and costs; already working in real cities
Recycling itself uses energy; some materials degrade each cycle; efficiency savings can be cancelled out by rising consumption
Doughnut economics
Puts social needs and ecological limits in one picture; prevents fixing one at the other’s expense
Hard to measure precisely; offers no detailed policy; a city’s footprint mostly falls outside its own boundary
That last point is the sharpest criticism of city-level sustainability. A city can hit every target inside its boundary while the food, steel, concrete and electronics it consumes cause deforestation and emissions on the other side of the world. Boundaries in accounting are not boundaries in reality.
WORKED EXAMPLE
Compare the circular economy model and the doughnut economics model as tools for planning a sustainable city.
Step 1: what each one is for
The circular economy is about materials and waste. The doughnut is about limits, both social and ecological.
Step 2: scope
Circular is narrower but easier to act on: collection rounds, building codes, repair schemes. The doughnut is broader but vaguer, since it names goals rather than methods.
Step 3: measurement
Circular progress can be measured in tonnes diverted. Doughnut progress needs many indicators, several of which are contested.
Step 4: how they fit together
The doughnut sets the destination; the circular economy is one of the routes.
Complementary rather than competingAmsterdam uses both at once, which is the neatest evidence you can cite for this conclusion.
💡 Exam tip
Practise drawing linear and circular metabolism side by side. It is fast and it demonstrates the whole idea.
Use the word metabolism. It is the syllabus term and it signals you know the model.
Keep two circular economy cities ready: Amsterdam and San Francisco.
Quote San Francisco’s roughly 80% diversion rate. A specific figure lifts an answer.
Always give a limitation. Both models are frameworks, not solutions.
Say the two models are complementary if asked to compare. That is the strongest conclusion available.
⚠ Common mix-ups
Treating the circular economy as just recycling. Reduce, reuse and repair come first and matter more.
Claiming a circular city has zero waste. No loop is perfect; some material is always lost.
Confusing the doughnut’s two boundaries. Inner is the social foundation, outer is the ecological ceiling.
Saying a linear city is simply badly run. Almost every city is still mostly linear; the models describe a direction of travel.
Forgetting that recycling costs energy. Where that energy comes from decides whether recycling is actually a gain.
Judging a city only on what happens inside its boundary. Its imports carry a footprint that the accounts often miss.
Up next: Green Architecture — the same ideas at the scale of a single building, and the traditional designs that got there first.
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