Carbon footprint: definition, calculation and units

• The carbon footprint is the total amount of greenhouse gases released directly or indirectly by an individual, an organisation, an event or a product, expressed in tonnes of CO₂-equivalent (t CO₂e) per year so that all gases are comparable.
• Primary (direct) footprint: fuel burnt on site — petrol in a car, gas for heating and cooking, diesel in machinery.
• Secondary (indirect) footprint: emissions of the power stations that supply the electricity, plus everything embedded in the goods and services consumed, the transport of food and the treatment of waste.
• Typical factors: petrol about 2.3 kg CO₂ per litre; diesel 2.7; natural gas about 0.19 kg per kWh of heat; grid electricity from 0.1 to 0.9 kg CO₂ per kWh depending on the fuel mix; a return flight London–Cairo roughly 0.5–0.7 t CO₂ per passenger.
• Global average footprint ≈ 4.7 t CO₂e per person; high-income countries 8–15 t; Egypt about 2.3–2.5 t per person, well below the world average, which is the ethical core of the climate-justice argument.
• Carbon trading and carbon tax: a cap is set and permits are bought and sold, or a price is placed on each tonne, so that reducing emissions becomes the cheaper option for a firm.

Worked calculation

A household burns 1,200 L of petrol a year in a car, uses 3,000 kWh of electricity from a grid emitting 0.45 kg CO₂/kWh, and 150 m³ of natural gas (about 0.2 kg CO₂/m³ × 10.5 kWh/m³, i.e. roughly 32 kg per m³). Direct and indirect emissions: 1,200 × 2.3 = 2,760 kg; 3,000 × 0.45 = 1,350 kg; 150 × 2.1 ≈ 315 kg (as heat) — total ≈ 4.4 t CO₂e a year.