What is a carbon footprint?

Carbon footprint has become a popular term nowadays but what does it actually mean?

Footprints carved in wood. Photograph: Reinhard Krause/Reuters

Carbon footprint is a horribly abused phrase, so it's worth spelling out exactly what it means.When talking about climate change, footprint is a metaphor for the total impact that something has. And carbon is a shorthand for all the different greenhouse gases that contribute to global warming. The term carbon footprint, therefore, is a shorthand to describe the best estimate that we can get of the full climate change impact of something. That something could be anything – an activity, an item, a lifestyle, a company, a country or even the whole world.

CO2? What's that?

Man-made climate change, or global warming, is caused by the release of certain types of gas into the atmosphere. The dominant man-made greenhouse gas is carbon dioxide (CO2), which is emitted whenever we burn fossil fuels in homes, factories or power stations. But other greenhouse gases are also important. Methane (CH4), for example, which is emitted mainly by agriculture and landfill sites, is 25 times more potent per kilogram than CO2. Even more potent but emitted in smaller quantities are nitrous oxide (N2O), which is about 300 times more potent than carbon dioxide and released mainly from industrial processes and farming, and refrigerant gases, which are typically several thousand times more potent than CO2.

In the UK, the total impact on the climate breaks down like this: carbon dioxide (86%), methane (7%), nitrous oxide (6%) and refrigerant gases (1%). Given that a single item or activity can cause multiple different greenhouse gases to be emitted, each in different quantities, a carbon footprint if written out in full could get pretty confusing. To avoid this, the convention is to express a carbon footprint in terms of carbon dioxide equivalent or CO2e. This means the total climate change impact of all the greenhouse gases caused by an item or activity rolled into one and expressed in terms of the amount of carbon dioxide that would have the same impact.

Beware carbon toe-prints

The most common abuse of the phrase carbon footprint is to miss out some or even most of the emissions caused, whatever activity or item is being discussed. For example, many online carbon calculator websites will tell you that your carbon footprint is a certain size based purely on your home energy and personal travel habits, while ignoring all of the goods and services you purchase.

Similarly, a magazine publisher might claim to have measured its carbon footprint but in doing so looked only at its office and cars while ignoring the much greater emissions caused by the printing house that produces the magazines themselves. These kinds of carbon footprint are actually more like carbon 'toe-prints' – they don't give the full picture.

The carbon footprint, as I have defined it, is the climate change metric that we need to be looking at. The dilemma is that it is also impossible to pin down accurately. We don't stand a hope of being able to understand how the impact of our bananas compares with the impact of all the other things we might buy instead unless we have some way of taking into account the farming, the transport, the storage and the processes that feed into those stages. So how should we deal with a situation in which the thing we need to understand is impossibly complex?