What a city footprint includes
A city’s carbon footprint is an estimate of the greenhouse gases associated with its activities, expressed in a common unit (carbon dioxide equivalent). There are different ways to draw the boundary. Some inventories count only emissions released within the city limits, such as from burning fuel in buildings and vehicles and from waste. Others also count emissions from producing the electricity the city imports, and some add consumption-based emissions, which include the goods and food residents buy no matter where they are made. The differences are large, so any figure should be accompanied by its method.
The main sources
Buildings. Heating, cooling, hot water, lighting and appliances are a major source in most cities. The size depends on climate, building age, the fuels used and how electricity is generated. Older, poorly insulated buildings that burn fossil fuels for heat tend to be among the highest per square foot.
Transport. Cars, trucks, buses and trains contribute, along with aviation in some inventories. Travel patterns depend on where people live relative to work and services, which ties emissions to land use.
Materials and construction. Producing cement, steel, glass and other building materials emits greenhouse gases before a building ever opens. These are known as embodied emissions. They are often left out of city inventories, yet they are significant in construction-heavy periods.
Waste. Landfills emit methane from decomposing organic waste, and waste treatment and transport add more.
Other. Industrial processes, water treatment and food supply contribute depending on the city.
How renewal can lower the footprint
Reusing existing buildings and land. Keeping a sound building avoids the embodied emissions of a new one. Building on already developed sites can reduce pressure to expand into undeveloped land, which tends to lock in longer travel distances.
Improving building performance. Retrofitting insulation, windows, heating systems and controls reduces operating emissions. This is often where renewal achieves its most reliable savings. For new buildings, high efficiency standards and electrification, where electricity is cleaner than fossil fuels, are central.
Locating near transit and services. Renewal that puts homes and jobs near frequent transit and daily services can reduce car travel. The effect is larger when parking is limited and walking and cycling routes are safe.
Reducing materials impact. Using lower-carbon materials, avoiding oversizing structures and reusing salvaged components can cut embodied emissions.
Adding trees and vegetation. Planting provides cooling and may store some carbon, though the quantities are usually small compared with a city’s total emissions. Their other benefits are usually the stronger reason to plant.
How renewal can raise it
Renewal is not automatically low-carbon.
- Demolition and new construction release embodied emissions that take years to recoup through operating savings, particularly if the old building was reasonably efficient.
- Car-oriented redevelopment, with large parking supplies and distant services, can increase driving.
- Larger floor areas per person, such as luxury housing replacing modest housing, can raise energy use even if the building is efficient.
- Displacement can push lower-income residents to outer areas with longer commutes, shifting emissions elsewhere in the metropolitan region.
- Gas-fired systems installed today may last decades and lock in emissions unless replaced.
A hypothetical comparison
Imagine two options for a district: retaining and upgrading an older building, or demolishing it and constructing a new, more efficient one. The new building might use less energy each year, but the emissions from producing its materials are released up front. Whether the new building eventually comes out ahead depends on how much energy it saves, how long it lasts and how clean the electricity supply becomes. No single answer applies; a lifecycle comparison for the specific case is the honest method.
Measuring and managing
Cities can improve results by estimating carbon impacts as part of project review, including embodied emissions, and by setting targets for operating performance. Tracking regional effects, such as changes in commuting, prevents narrow wins that merely shift emissions elsewhere. All estimates carry uncertainty, so ranges and clear assumptions are better than single precise figures.
Footprint reduction should be balanced with affordability and fairness, since measures that raise costs for lower-income households can face resistance and cause hardship.