What Is A Carbon Footprint?

by L.J. Martin

In the past couple of years our vocabulary has been bombarded with environmental buzzphrases, with "carbon footprint" being top of the list. But what does carbon footprint actually mean?

There are various definitions of carbon footprint, but the most common one is that your carbon footprint is the net amount of fossil carbon dioxide that you are responsible for emitting into the atmosphere.

The term "carbon footprint" can be controversial, as it is not a proper scientific term and has been used by various people to suit their own purposes. There are several problems with this term which often lead to confusion.

Fossil versus organic carbon

Burning fossil fuels releases fossil carbon into the atmosphere as CO₂. Breathing also releases CO₂, but this is organic carbon which does not produce a net rise in atmospheric carbon levels, as it is simply passing around the carbon cycle. The confusion between fossil and organic carbon has in the past led to strange claims about carbon footprints, for instance that a car releases less CO₂ than a group of cyclists travelling the same distance at a certain speed. This may be scientifically correct, but in the context of global warming it is utterly meaningless, as only fossil carbon emissions increase the net amount of carbon in the atmosphere, while the release of CO₂ from organic sources (such as cyclists) is irrelevant. Only fossil emissions of carbon dioxide should be included in a carbon footprint.

What about methane?

The difference between fossil and organic carbon emissions is simple, but in the context of global warming, something else must be taken into account. Methane (CH₄) is the natural gas which is piped to homes for heating and cooking. However, methane is also released from organic sources, such as cows and rice paddies. The problem here is that methane is far more effective as a greenhouse gas than carbon dioxide. In other words, releasing organic carbon in the form of CH₄ can be as bad as releasing fossil carbon as CO₂. Methane breaks down naturally in the atmosphere to form water and CO₂, with most of the methane consumed in around a decade, but in the meantime it has acted as a highly effective greenhouse gas. So should organic methane release be included in your carbon footprint? How much weight should be given to it relative to fossil carbon dioxide? As carbon footprint is not a well-defined term, people tend to make their own decisions on this point.

Whose footprint does this cover?

Another problem with the term carbon footprint is that there can be some dispute as to whose carbon footprint a certain source of CO₂ should be added to. For instance, a sports stadium might take various steps to reduce its fossil CO₂ emissions, such as installing wind turbines and solar electricity generation. It may then claim that it has a tiny carbon footprint. However, some people will then point out that if the CO₂ emissions of all the spectators driving to an event are taken into account, the stadium has a very large carbon footprint. The stadium might counter that the fans should be responsible for the carbon emissions on their drive to the stadium. So should this be added to the stadium's carbon footprint, or the fans', or both? This is a genuine concern for businesses such as stadiums and supermarkets, as there are proposals to tax them on their CO₂ emissions.

Conclusions

Carbon footprint is a phrase that we will be hearing more and more, but unfortunately there can be competing definitions, which means that most people are vague on its meaning. When you hear somebody making claims about their carbon footprint, you should take a moment to consider what they mean by the term, and whether they are truly becoming more carbon neutral.