One of the most obvious ways to understand our emissions - and ultimately the impact on climate change they lead to - is to be more aware of the amount of GHG that our daily activities represent. The CF of a specific product or activity measures the amount of GHG emissions that can be assigned to that consumption or activity. As in any other environmental impact assessment that we generate, it is necessary to consider two aspects to make it more complete: on the one hand, what dimension of the impact does it refer to and, on the other, which phases of the product's life it affects.
In the case of CF, the GHG emissions produced by an activity are calculated. The most prominent GHGs are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). To have a common scale of emissions and to simplify its analysis, the CF is usually expressed in mass (grams, kilograms, tons ...) of CO2 equivalent (CO2e), which quantifies the total emissions from the Global Warming Potential (GWP) (the term radiative forcing is also used) of each GHG in relation to that of CO2, which is used as a reference. In short, the GWP indicates the capacity of a certain gas to increase the greenhouse effect compared to that of CO2, considering a certain period of time (generally 100 years). For example, according to the latest report from the Climate Change Panel (IPCC), methane has 28 times more GWP than CO2 (Myhre et al., 2013). The following table indicates the GCP of the main GHGs for a 100-year horizon (Source: IPCC, 2015):
|COMMON OR INDUSTRIAL DENOMINATION||CHEMICAL FORMULA||GLOBAL WARMING POTENTIAL|
Therefore, the CF of any activity or product would be the result of the sum of the emissions of all the GHGs used, to obtain them, multiplied by their corresponding GWP, all expressed in kg of CO2. For food consumption products, CF is usually expressed in kg of emissions per kg of product mass (for example: 1.2 kg of CO2e per kg of bread consumed). For emissions related to transport, they can be expressed as CO2e emissions per km travelled, while for those related to land use, they can be indicated in emissions per surface area (CO2e emitted per km2 deforested).