Alongside the most important human-caused greenhouse gas, carbon dioxide (CO₂), there are other greenhouse gases such as methane (CH₄) and nitrous oxide (N₂0). These gases do not have the same influence on the greenhouse effect and stay in the atmosphere for different amounts of time. For instance, methane’s effect on the climate is 28 times more severe than CO₂, but it doesn’t stay in the atmosphere as long. In order to make the various greenhouse gases comparable with one another, the expert panel of the UN, the IPCC, has defined an index called “global warming potential” (GWP). This index expresses the warming effect of a certain amount of a greenhouse gas over a set period of time (usually 100 years) in comparison to CO₂. In this way, greenhouse gases can be calculated as CO₂ equivalents. CO₂ equivalents are abbreviated as CO₂e.
Source and others.: IPCC 2007/2013