According to PAS 2080:2016 Carbon
Management in Infrastructure, greenhouse gases (GHGs) are the gaseous
constituents of the atmosphere, both natural and anthropogenic, that absorb and
emit radiation at specific wavelengths within the spectrum of infrared
radiation emitted by the Earth’s surface, the atmosphere, and clouds. The six main greenhouse gases (UNFCC Kyoto
Protocol) include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O),
hydrofluorocarbons (HFCs), perluorocarbons (PCFs) and sulphur hexafluoride
(SF₆).

The term ’emissions factor’ characterises the amount of
greenhouse gases emitted relative to a unit of activity, expressed as ‘carbon dioxide equivalent’ (written
variously as; CO2e, CO2eq CO₂eq etc). This is the unit for comparing the
radiative forcing of a greenhouse gas to carbon dioxide and is calculated using the mass of a given GHG
multiplied by its global warming potential, a factor describing the
radiative forcing impact of one mass-based unit of a given greenhouse gas
relative to an equivalent unit of CO₂ over a given period of time.

Throughout PAS 2080 and other documents, the term ‘carbon’ (e.g. operational
carbon, embodied carbon, capital carbon, user carbon) is often used as
sort-hand of GHSs which may be misleading if out of context.

An example of the use of CO2e is for conventional
Portland cement (CEMI). This is made by heating limestone and clay and then grinding
the resulting material, known as clinker, into a fine powder. The process of
heating and decomposing the limestone releases about 840kg CO2e for every tonne of cement produced (MPACement Fact Sheet No.18). This is partly down to them chemical process as well as the
fuel used in heating the limestone. This value can be reduced by using a Portland cement with a lower clinker proportion whether at the cement plant or added at a concrete plant.

‘Embodied carbon’ is
a common term used and relates to the CO2e built into the fabric of, for
example, a structure. This relates to the amount of carbon emitted during the
construction through extraction of raw materials, the manufacturing and
refinement of materials, transportation, installation and disposal of old
supplies. In contrast ‘operational carbon’ is the amount of CO2e emitted once
the building is in use.