CO2e stands for "carbon dioxide equivalent" and is a unit used to measure and compare emissions from greenhouse gases based on how severely they contribute to global warming and climate change. It is a metric measure used to compare the emissions from various greenhouse gases on the basis of their global-warming potential (GWP), by converting amounts of other gases to the equivalent amount of carbon dioxide with the same global warming potential. CO2e is a very useful term because it allows "bundles" of greenhouse gases to be expressed as a single number and allows different bundles of greenhouse gases to be easily compared. The main gases measured using CO2e include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3) . Carbon dioxide is the most common greenhouse gas emitted by human activities and is responsible for about 80% of all human-caused greenhouse gas emissions.