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
Carbon dioxide	CO2	1
Methane	CH4	28
Nitrous oxide	N2O	265
Halon-1301	CBrF3	6.29
Halon-1211	CBrClF2	1.75
Halon-2402	CBrF2CBrF2	1.47
Carbon tetrachloride	CCl4	1.73
Methyl bromide	CH3Br	2
Methyl chloroform	CH3CCl3	160
Sulfur hexafluoride	SF6	23.5
Nitrogen trifluoride	NF3	16.1
CFC-11	CCl3F	4.66
CFC-12	CCl2F2	10.2
CFC-13	CClF3	13.9
CFC-113	CCl2FClF2	5.82
CFC-114	CClF2CClF2	8.59
CFC-115	CClF2CF3	7.67

Table 1. Global warming potential of some of the main Greenhouse Gases (values ​​for a 100-year horizon) (Source: IPCC, 2015).

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).