Understanding Carbon Footprints Revision Notes | GCSE AQA Higher Chemistry

1. What Is a Carbon Footprint?

Every time you charge your phone, eat a burger, or buy a t-shirt, you're adding invisible greenhouse gases to the atmosphere. But how much exactly? That's where carbon footprints come in — they put a number on your environmental impact.

Notice the key phrase 'full life cycle'. This doesn't mean the greenhouse gases produced when you use something. It means everything — from digging the raw materials out of the ground, through manufacturing, transport, use, and finally disposal or recycling.

Think of a plastic water bottle. Its carbon footprint includes the oil extraction for the plastic, the energy to run the factory, the fuel for the lorry that delivered it to the shop, and the energy to recycle it. Most students think it is about what happens when you drink from it — that's the trap.

2. Why We Measure in CO₂ Equivalent

Carbon footprints include greenhouse gases such as carbon dioxide and methane over the full life cycle.

Scientists compare the greenhouse gases emitted over the full life cycle of a product or service.

Worked example: Converting Greenhouse Gases to CO₂ Equivalent

A factory emits carbon dioxide and methane per day. Describe why both gases matter when considering the carbon footprint.

1. CO₂ emissions = 2 tonnes Methane emissions = 0.5 tonnes Methane warming factor = 25 — Always list what you know first
2. Methane emissions should be included alongside carbon dioxide. — Multiply by the warming factor
3. Total emissions are the combined greenhouse gases produced. — CO₂ is already in the right units

Answer: The total carbon footprint per day

3. Life Cycle Assessment — The Full Picture

To calculate a carbon footprint properly, you need to track emissions at every stage. This process is called life cycle assessment (LCA). Let's follow a simple product — a cotton t-shirt — from cradle to grave.

Stage 1: Raw materials — Growing cotton requires fertilisers (which produce nitrous oxide), tractors (burning diesel), and irrigation pumps (using electricity).

Stage 2: Manufacturing — Spinning cotton into thread, weaving fabric, dyeing, and cutting all require energy. The factory might burn coal or gas for power.

Stage 3: Transport — Shipping from Bangladesh to the UK burns heavy fuel oil. Then lorries distribute to shops using diesel.

Stage 4: Use phase — You wash the t-shirt dozens of times. Each wash uses hot water (heated by gas) and electricity for the washing machine.

Stage 5: End of life — If it goes to landfill, it decomposes and produces methane. If incinerated, it releases CO₂.

4. Comparing Carbon Footprints

Carbon footprints help us make better choices, but you need to compare like with like. The carbon footprint of driving 10 miles isn't comparable to eating a steak — different functions, different units.

Here's where it gets interesting. A plastic bag has a carbon footprint of about 6g CO₂ equivalent. A cotton bag? Around 170g. But the cotton bag can be reused hundreds of times, while the plastic bag is single-use.

Worked example: Comparing Bags Over Their Lifetime

A plastic bag is used once then thrown away. A cotton bag can be reused many times. Which has the lower carbon footprint per use?

1. Plastic bag = 6g per use — Single use means no division needed
2. Cotton bag = 170g ÷ 200 uses = 0.85g per use — Spread the manufacturing impact over all uses
3. Plastic: 6g per use Cotton: 0.85g per use — Cotton bag wins if you actually reuse it 200 times

Answer: Cotton bag has a lower carbon footprint per use if it is reused enough times.

The trap here is assuming 'natural' always means lower carbon footprint. Cotton production is very energy-intensive. The cotton bag only wins if you actually reuse it enough times.

5. Reducing Carbon Footprints

Understanding carbon footprints is pointless unless you can reduce them. The biggest impact comes from targeting the largest sources first. For most people in the UK, that's heating homes, transport, and food.

Transport: A return flight to Spain produces about 0.6 tonnes CO₂ equivalent per person. That's the same as driving 2,500 miles in an average car. Taking the train to Paris instead of flying cuts emissions by 90%.

Food: Beef has a carbon footprint of about 60kg CO₂ equivalent per kg of meat. Chicken is 6kg per kg. Plant proteins like lentils are under 1kg per kg. The difference comes mainly from methane emissions from cows.

Energy: Heating your home with gas produces about 0.2kg CO₂ per kWh. Using electricity from the grid averages 0.23kg CO₂ per kWh (but falling as more renewables come online). Heat pumps can cut emissions by 70%.

6. Exam Technique and Common Mistakes

Carbon footprint questions appear in both chemistry and physics papers. Examiners test whether you understand the full life cycle concept and can work with CO₂ equivalent units.

Exam language: Use 'carbon footprint' for the measurement and 'greenhouse gas emissions' when discussing the actual gases. Don't say 'carbon emissions' — that's technically just carbon, not CO₂.

Worked example: Exam-Style Carbon Footprint Calculation

A factory produces carbon dioxide and methane each year. Describe why both gases matter when discussing its carbon footprint. [3 marks]

1. Methane emissions should be included in the carbon footprint. — [1 mark] for correct conversion factor
2. Total emissions are the combined greenhouse gases produced. — [1 mark] for correct addition
3. Annual carbon footprint = the total greenhouse gases emitted over the year — [1 mark] for correct units and final answer

Answer: 1000 tonnes CO₂ equivalent per year