Carbon dioxide equivalent (CO₂.eq) is a measure used to compare and combine the warming effect of emissions from different greenhouse gases, using single measure of impact. This is done on the basis of a conversion factor known as the Global Warming Potential (GWP), which is the ratio of the total energy trapped by a unit of greenhouse gas (e.g. a tonne of methane) over a specific period of time (normally 100 years), to that trapped by carbon dioxide over the same time period.