What is a carbon footprint?
The term carbon footprint crops up a lot these days. What exactly does it actually mean?
• The Guardian's quick carbon calculator
• The Guardian's quick carbon calculator
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 contribue 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.
CO2e? 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.
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.
Direct versus indirect emissions
Much of the confusion around footprints comes down to the distinction between 'direct' and 'indirect' emissions. The true carbon footprint of a plastic toy, for example, includes not only direct emissions resulting from the manufacturing process and the transportation of the toy to the shop: it also includes a whole host of indirect emissions, such as those caused by the extraction and processing of the oil used to make the plastic in the first place. These are just a few of the processes involved. If you think about it, tracing back all the things that have to happen to make that toy leads to an infinite number of pathways, most of which are infinitesimally small. To make the point clearly, let's try following just one of those pathways. The staff in the offices of the plastic factory used paper clips made of steel. Within the footprint of that steel is a small allocation to take account of the maintenance of a digger in the iron mine that the steel originally came from … and so on for ever. The carbon footprint of the plastic toy includes the lot, so working it out accurately is no easy task.
To give another example, the true carbon footprint of driving a car includes not only the emissions that come out of the exhaust pipe, but also all the emissions that take place when oil is extracted, shipped, refined into fuel and transported to the petrol station, not to mention the substantial emissions caused by producing and maintaining the car.
The essential but impossible measure
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?
One common response is to give up and measure something easier, even if that means losing most of what you are interested in off the radar. The illusionist Derren Brown refers to one of his core techniques as the misdirection of attention: by focusing his audience on something irrelevant he can make them miss the bit that matters. Examples include an airport waxing lyrical about the energy efficiency of its buildings without mentioning the flights themselves.
The same thing can happen by accident. If you settle for a toe-print,
there is a very good chance it will misdirect your attention away from
the big deals. An alternative response to the dilemma, and the approach that this book is all about, is to do the best job you can, despite the difficulties, of understanding the whole picture. My work is about making the most realistic estimates that are possible and practical, and being honest about the uncertainty.
there is a very good chance it will misdirect your attention away from
the big deals. An alternative response to the dilemma, and the approach that this book is all about, is to do the best job you can, despite the difficulties, of understanding the whole picture. My work is about making the most realistic estimates that are possible and practical, and being honest about the uncertainty.
• This article is drawn from How Bad Are Bananas? The Carbon Footprint of Everything by Mike Berners-Lee
Natalie Bhinder
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