Thinking about the footprint
Cities occupy a small share of the earth’s land surface but draw on resources from far beyond their borders. The idea of an urban footprint is a way of describing that reach: the land, water, energy and materials that sustain a city’s residents and activities, wherever they are located. It also covers what leaves the city, such as waste, heat and emissions.
Because the footprint is spread out, a city can look green inside its boundaries while its demands have large effects elsewhere. Conversely, a city with a big total footprint may still be efficient per person. Looking at both totals and per-person figures gives a fuller picture. Footprint estimates depend on method and assumptions, so numbers from different sources are rarely directly comparable.
Resource use
Energy. Cities use energy for buildings, transport, industry and services. How much depends on climate, building quality, income, the fuel mix of the local power supply and travel patterns. Growth in cities often raises total energy demand even if per-person use falls.
Water. Urban water systems collect, treat and distribute water, then collect and treat it again as wastewater. Many cities depend on distant reservoirs or groundwater. Leaky pipes, high outdoor use and drought can create shortages, which is why water mains and demand management receive attention.
Materials. Building and maintaining cities uses large amounts of sand, gravel, cement, steel and timber. Producing these materials has its own environmental costs, and construction and demolition generate a large part of the waste in many places.
Food. Urban residents typically rely on food produced elsewhere. Transport, refrigeration and packaging add to its footprint, though the share of total emissions from transport varies by product.
Land conversion
Urban expansion turns farmland, forest, wetland and other open land into built surfaces. This reduces habitat and farmland and changes how water moves across the landscape. The scale of land taken depends on how the city grows: outward at low density, or upward and inward at higher density. A growth pattern involving many separate, spread-out developments usually converts more land per person than compact ones. These themes are developed in other articles about sprawl effects and about local ecological harm; this overview focuses on how the pieces fit together.
Where cities can be efficient
There is a real case that cities can reduce environmental pressure per person, under the right conditions.
- Shorter trips and shared transport. When destinations are close together, more trips can be made on foot, by bicycle or by public transit. Dense, mixed areas often see lower per-person vehicle use, though results depend on the quality of transit and the local street network. Planning around transit-oriented development rests on this idea.
- Smaller dwellings and shared walls. Apartments and attached homes generally need less energy to heat and cool per household than detached houses of similar quality, because less surface area is exposed to the weather.
- Shared infrastructure. Spreading the cost of pipes, wires and roads over more users can lower the length of network per person, a theme in work on density and public costs. Some options, such as district energy, become feasible only at higher densities.
- Economies of scale in services. Waste collection, recycling, water treatment and public transit can operate more efficiently where users are concentrated.
- Land sparing. If more people live compactly, more land elsewhere may remain undeveloped, though this is not automatic and depends on policy and market conditions.
These advantages are potential, not certain. They are realized only when cities provide good alternatives to driving, maintain efficient buildings and clean energy, and keep housing affordable enough that people can in fact live in compact areas.
Where efficiency falls short
Density does not guarantee benefits. Several caveats apply.
- Income effects. Wealthier urban households often consume more goods, travel more and have larger footprints than poorer ones, even when they live compactly. The consumption that cities enable can outweigh efficiency gains.
- Local concentration. Dense areas can intensify air pollution, noise and heat at the neighborhood level, which is relevant to people living there.
- Lock-in. Infrastructure and buildings last for decades. Choices about street layout and building standards made during rapid growth are difficult to change later.
- Quality of the grid. If electricity comes mainly from fossil fuels, efficient buildings reduce but do not eliminate emissions.
- Displaced impact. Production that serves urban consumers may take place elsewhere, outside the accounting of the city itself.
Measures commonly discussed
Without implying guaranteed outcomes, common measures include:
- improving building energy efficiency through codes and retrofits
- expanding public transit, walking and cycling options
- reducing waste through collection, reuse and recycling systems, including approaches to municipal waste
- integrating trees, wetlands, parks and other forms of green infrastructure
- planning growth to use previously developed land where practical
- addressing climate-related emissions, as in work on the urban carbon footprint
Each of these has costs, requires political support and may affect different groups differently. For example, improving a district can raise housing costs, so the benefits and burdens of environmental projects deserve attention along with the environmental metrics.
A balanced reading
The environmental effects of urbanization are neither uniformly harmful nor uniformly beneficial. Cities concentrate demand and impact, yet they also provide the possibility of efficient shared systems. Whether that possibility is realized depends on specific choices about land, buildings, transport and energy, and on the economic conditions around them. Evidence continues to develop, and local circumstances often matter more than general rules.