Understanding carbon footprints

What a carbon footprint is

A carbon footprint is the total amount of greenhouse gases emitted, directly or indirectly, by something: a person, a household, a product or an event. As the name suggests, it tries to show in a number how big a trace an activity leaves behind. Despite the word carbon, it does not count carbon dioxide alone; it also counts other gases that drive warming, such as methane and nitrous oxide. This guide covers the concept and how to read it, and deliberately avoids quoting emission figures for specific activities or products, because those depend heavily on how the calculation is set up.

Combining several gases into one number

Different greenhouse gases trap different amounts of heat and stay in the atmosphere for different lengths of time. So each gas is given a factor reflecting how much it contributes to warming over a set period compared with the same mass of carbon dioxide; emissions are multiplied by that factor, converted to carbon dioxide terms, and added up. The total is called carbon dioxide equivalent. These factors are adjusted as new scientific assessments come out, and they also change depending on the time horizon chosen. In other words, the same activity can give slightly different results depending on which factors were used.

Direct and indirect emissions

Everyday emissions can be split into direct and indirect. Direct emissions come from fuel burned where you use it, such as petrol in your car or gas in your boiler. Indirect emissions happen elsewhere but because of your consumption; electricity at home is the classic example. Using electricity produces no smoke in your living room, but the power station that generates it does emit. Emissions from making and transporting the goods you buy and the food you eat also count as indirect. Taken broadly, indirect emissions often make up a larger part of a personal footprint than people expect.

A product's footprint follows its life cycle

A product's carbon footprint is usually calculated by following its whole life. Which stage dominates varies: appliances that use a lot of electricity over the years are dominated by use, while electronics and clothing are known to be dominated by manufacture. Deciding where the calculation starts and stops is called setting the boundary, and the same product gives a different number depending on whether you count only up to the factory gate or all the way through use and disposal.

• Materials: extraction and processing
• Manufacture: production in the factory
• Transport and retail: moving and selling
• Use: energy consumed while in use
• Disposal: throwing away and treatment

Why calculators give different numbers

Entering the same lifestyle into different online carbon calculators often gives quite different results. That is less because they are wrong than because their assumptions differ: what falls inside the boundary, what emission value is assigned to a unit of electricity, and what an average lifestyle looks like. The emission factor for electricity in particular varies widely by country and by year as the generation mix changes. Comparing numbers from different calculators side by side therefore tells you little.

• They include different items in scope
• They use different emission factors for power and fuel
• They assume different average lifestyles
• They use different base years and data sources

Common misconceptions

Carbon footprints produce quite a few counter-intuitive results. It is tempting to assume local produce always has lower emissions because it travels less, but transport's share of the total varies with the product and how it is grown, so that cannot be taken for granted. Comparisons like paper versus plastic bags or glass versus plastic bottles flip depending on how many times each is reused. It is also common to agonise over small purchases while leaving big items like home heating or how you travel untouched. And remember that paying for reduction projects elsewhere to offset emissions is not the same as actually cutting your own.

Using it in daily life

A carbon footprint is more useful for gauging where the big items are and tracking your own changes than for pinning down an exact absolute figure. Split your life into roughly home energy, travel, food and the things you buy, and compare which is largest within a single calculator. Start home energy from the electricity and fuel figures on your bills, and travel from distance driven and fuel economy. Measure again a few months later with the same calculator and settings, and whether the number went up or down becomes the meaningful information. The other guides in this topic, on standby power, efficiency labels, food waste and making things last, are all ways of reducing one of these items.

Reading labels and claims

Many countries have schemes for showing a product's carbon emissions or awarding certification marks, and Korea has a scheme that certifies and displays product carbon emissions too. Such labels are useful when comparing products of the same type. Advertising that simply shouts carbon neutral or eco-friendly, however, deserves scrutiny. If it does not say what scope was calculated, whether it reflects real reductions or offsets, and who verified it, treat the claim as a rough reference at most. Scheme names and criteria can change, so check official sources.