In depth: Carbon footprints

Carbon footprints are talked about a lot, but do we really understand what they mean? Rachel Dunne explains why carbon footprints may not be as simple as they seem.

‘The carbon footprint of milk is 1.39kgCO2eq…’[1] is a seemingly simple statement which in fact requires a number of important questions to be answered before its meaning is clear. Here I’d like to run you through a few of these questions, and hopefully therefore convey just how much is hidden in a carbon footprint.

Footprints and a tyre track in the sand.
Photo by Yann Allegre on Unsplash

1 - What is the functional unit?

The functional unit associated with a carbon footprint is the amount of the relevant product that causes the stated greenhouse gas emissions over its life cycle. For example, the above carbon footprint of 1.39kgCO2eq for milk has a functional unit of 1kg milk, meaning that producing 1kg of milk emits 1.39kgCO2eq.

Functional units allow the useful comparison of products. The obvious functional unit is 1kg by mass of product, e.g. 1kg beef might be compared to 1kg chicken in terms of their carbon footprints. There are other options, however. For example, in the case of beef and chicken you might compare them per kg protein. This would mean that the functional unit would be 1kg protein’s worth of the meat instead of just the simple 1kg of mass. If chicken contains 10% protein and beef 20% (made up figures!) then this would require comparing the carbon footprint of 10kg chicken to that of 5kg beef.

2 - What are the system boundaries?

The system boundaries of a carbon footprint calculation define the processes in a product's life cycle that are included. Therefore the system boundaries can make a big difference to the calculated carbon footprint of a product. If a process in the product’s life cycle that emits a lot of greenhouse gases is not included in the system boundaries this could lead to a considerable under-estimation of its carbon footprint.

It is particularly important when two products are to be compared that the system boundaries used for each product are as similar as possible. Often we consider either cradle-to-gate or cradle-to-grave system boundaries. The former only considers the life cycle of a product up to factory gate, while the latter additionally considers its sale, use and disposal as well as any associated transportation.

3 - Which allocation method was used?

The allocation method used in the calculation of a carbon footprint is the rule used to apportion greenhouse gas emissions between multiple outputs of the same process or set of processes. For example, the rearing of dairy cattle may result in both milk and beef being produced. The greenhouse gas emissions of the rearing, slaughtering etc… can be calculated, but how is this allocated between the final milk and beef produced? Options include allocation according to the mass of each product produced, allocation according to the energy content of each product (if they are food or fuel), or allocation according to the prices at which the products will be sold at. The allocation method used in the calculation of a carbon footprint can significantly impact the result. This means that in particular when comparing products it is important to use the same allocation method in both cases.

4 - And the rest?

The more information that is known about the calculations behind a stated carbon footprint the better. Further details to seek might firstly include the database used for any background data. A number of Life Cycle Assessment (LCA) databases exist such as EcoInvent and Agri-Footprint, though most have to be paid for. Free options include Agribalyse and ELCD.

It is also useful to know the impact assessment method used to calculate a carbon footprint. A given impact assessment method may use different global warming potentials (GWPs) for certain greenhouse gases. GWPs are the conversion factors from kg of gas emitted to kg CO2 equivalent emitted. There is still considerable scientific debate around their values. It depends in part on how far into the future you are looking as some of these other greenhouse gases are much less long-lived than others and therefore will only have their warming effect for a limited amount of time before they react to form a different chemical. The GWPs used in the calculation of a carbon footprint will affect the result, potentially changing how two products compare if they emit differing amounts of various greenhouse gases.

What does this mean for me?

It is important for civil servants working on the climate crisis to understand that carbon footprint statements are not as simple as they first appear. Although phrases like ‘high carbon footprint’ are in common parlance, they don’t hold a clear meaning for most people, even when quantified in kg CO2 equivalent. If possible, it is best to aid understanding by comparing one carbon footprint to that of another well-known or comparable item, for example ‘the carbon footprint of oat milk is considerably lower than that of cow’s milk...’[2] as this has a clear, actionable message.


[1] Clune, S., Crossin, E. and Verghese, K. (2016). Systematic review of greenhouse gas emissions from different fresh food categories. Journal of Cleaner Production 140(2) pp.766-783. [2] Carbon Cloud. Assessment of Carbon Footprint of Oatly products by CarbonCloud. Available at: [Accessed 24th August 2019].