Carbon footprints and waste management

Overview

Carbon footprint, difference between production-based and consumption-based carbon accounting for a city and main sources of a city's carbon emissions.

What you need to know

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Questions from this subtopic

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1. 1

   What is a carbon footprint?

   Model answer

   A carbon footprint is the total quantity of greenhouse gases emitted directly and indirectly by an individual, household, organisation, product or settlement over a stated period, expressed in tonnes of carbon dioxide equivalent so that methane, nitrous oxide and other gases can be added to carbon dioxide on a common basis. Because it is an accounting measure, everything depends on what is included, so any figure must state its boundary and its period.

2. 2

   Distinguish clearly between a carbon footprint and an ecological footprint.

   Model answer

   They measure different things in different units. A carbon footprint measures greenhouse gas emissions only, in tonnes of carbon dioxide equivalent. An ecological footprint measures total demand on biologically productive area, in global hectares, and covers cropland, grazing land, fishing grounds, forest products and built land as well as a carbon component, which is the forest area notionally needed to absorb the carbon dioxide emitted. So carbon is one component of the ecological footprint, and because it is the largest single component for most high-income populations the two measures usually move together. They are not interchangeable, and a policy can improve one without improving the other: switching electricity generation from coal to nuclear or renewables cuts the carbon footprint sharply while barely changing the land needed to feed the city, whereas reducing meat consumption cuts the land component far more than it cuts emissions in proportion.

3. 3

   What is the difference between production-based and consumption-based carbon accounting for a city?

   Model answer

   Production-based accounting counts the emissions released inside the city boundary: heating and cooking, road traffic, local industry, waste treatment. Consumption-based accounting counts the emissions caused by what residents consume wherever those emissions occur: the electricity generated at a distant power station, the emissions embodied in imported food, clothing, vehicles and building materials, and the residents' aviation and shipping. For a wealthy service-based city the consumption-based figure is typically much larger than the production-based one, because manufacturing has moved elsewhere. Cities usually report the production-based figure because it is easier to measure and covers what they can directly control, but the consumption figure is the honest measure of their contribution.

4. 4

   What are the main sources of a city's carbon emissions?

   Model answer

   Buildings, through heating, hot water, cooling and lighting, which is usually the largest single source in a temperate climate; road transport; electricity generation attributed to the city's consumption; industry where it remains; waste, particularly methane released from decomposing organic material in landfill; and construction, through the embodied emissions of cement, steel, aluminium and bricks in new building. Aviation and shipping associated with the city's residents and trade are large but frequently excluded from official totals.

5. 5

   Explain how a city can reduce its carbon footprint.

   Model answer

   By retrofitting the existing building stock with insulation, better glazing and airtightness, and requiring high standards in new build; by replacing fossil-fuel heating with heat pumps, district heating and combined heat and power; by decarbonising the electricity supplied, and generating some locally from rooftop solar and energy from waste; by shifting travel from cars to walking, cycling and public transport, and electrifying the vehicles that remain; by planning at densities and mixes of use that shorten trips; by diverting organic waste from landfill to composting and anaerobic digestion and capturing landfill gas; by reusing existing buildings rather than demolishing and rebuilding, which saves embodied carbon; and by planting and protecting trees, which is useful for cooling and amenity though small in carbon terms relative to the other measures.

6. 6

   Why might a city's measured carbon footprint fall without global emissions falling?

   Model answer

   Because of where the emissions are counted. If heavy industry closes or relocates abroad and the goods are then imported, the city's production-based emissions fall while the emissions themselves continue somewhere else; this is carbon leakage. The same applies when electricity is imported from outside the boundary, or when residents fly more and aviation is excluded from the inventory. This is precisely why consumption-based accounting matters, and why a claim that a city has decarbonised should be checked against what it now imports rather than only against what it emits within its own boundary.

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