Before carrying out a GHG inventory, understanding the different ways one can measure carbon emissions is crucial.
Indeed, when exploring data and existing methodologies, papers and reports several different terms are used and can quickly become confusing: depending on the study, different terms can be used to describe the same emissions or the same term can be used to describe different emissions.
Hence, it is important to briefly define each of these terms: carbon (C), carbon dioxide (CO2), carbon dioxide equivalent (CO2e) and greenhouse gas (GHG).
GHG
A GHG is a gas in the atmosphere that absorbs heat (“absorbs and emits radiant energy within the thermal infrared range”) causing the greenhouse effect (a natural process trapping heat close to the Earth).
The main GHG are water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3).
Some GHGs are natural (e.g. water vapour) but others result from human activities and cause an additional greenhouse effect and climate change. Hence, when mentioning GHG emissions, are only taken into account GHGs emitted by human activities that cause climate change and that are defined in the Kyoto protocol.
Kyoto Protocol - Gas
Carbon dioxide, methane, nitrous oxide, fluorinated gases (hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6))
CO2
Carbon dioxide is the most common GHG that is emitted by human activities, both in terms of quantity released and impact on climate change.
For example, it is estimated, in the US, in 2019, that about 80% of GHG emissions from human activities are CO2.
In some studies and databases, CO2 can be misused to refer to all GHG or to CO2e emissions, causing confusion.
CO2e
Carbon dioxide equivalent is a common unit used to describe and compare all GHGs. It quantifies, using the gases’ global warming potential (GWP) for a certain time horizon, the impact of the different GHGs described in the Kyoto Protocol compared to the impact of CO2.
Global warming potential (GWP) values relative to CO2 (source: Greenhouse Gas Protocol)
To compare different studies and databases referring to GHG emissions, one has to be sure that the same GHGs are included.
Carbon
Carbon is a chemical element with the atomic number 6.
It is the most confusing and ambiguous term because it can be used in several different ways. Indeed, carbon can refer to CO2, CO2e or GHGs but also the amount of carbon in a molecule (e.g. 1 kg of CO2 contains 0.27 kg of carbon).