Carbon footprints and how to cut them

A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event. Methane and other gases are converted to the amount of carbon dioxide with the same warming effect.

Ways to reduce it:

• Use renewable energy (solar, wind) or nuclear power instead of fossil fuels.
• Use less energy: insulation, efficient lighting and heating, turning things off.
• Carbon capture and storage: catch carbon dioxide at power stations and store it underground.
• Carbon taxes and licences make high-emission choices cost more.
• Plant trees, travel less, and reuse and recycle.

Why it is hard: new technology costs money, countries do not always agree, and people may not want to change their lifestyle.

1. A home uses 3000 kWh of electricity a year at 0.25 kg of carbon dioxide per kWh
2. footprint = 3000 × 0.25 = 750 kg = 0.750 tonnes
3. LEDs save 400 kWh: 3000 − 400 = 2600 kWh; solar then supplies 25%, so 2600 × 0.75 = 1950 kWh from the grid
4. new footprint = 1950 × 0.25 = 487.5 kg = 0.488 tonnes (3 s.f.)

For a shared car journey, divide the car's emissions between the people in it before you compare with a train.

1. (Higher) Burning 32 kg of methane: CH₄ + 2O₂ → CO₂ + 2H₂O
2. Mr: CH₄ = 16, CO₂ = 44; 16 kg of methane gives 44 kg of carbon dioxide
3. mass of carbon dioxide = 32 × 44 ÷ 16 = 88 kg

Check which way round the ratio goes: a fuel gives a larger mass of carbon dioxide than its own mass, because oxygen from the air is added.