In addi­tion to carbon diox­ide (CO₂), other green­house gases such as methane, nitrous oxide or hydro­flu­o­ro­car­bons (HFCs) also contribute to global warm­ing. The CO₂ equiv­a­lent (CO₂e) indi­cates how great the impact of a gas is during a certain period of time, compared to the same amount of CO₂. The refer­ence base is there­fore always carbon dioxide.