A commonly-used pressure index relevant for climate change is total greenhouse gas emissions. The index is the weighted summation of five gases that contribute to global warming: carbon dioxide (CO2), methane (CH4), nitrogen oxides (NOx), chlorofluorocarbons (CFCs) and halons. The theoretical foundation for the weights used in the index is strong, since the weights are based on the Global Warming Potential (GWP) of each gas. One tonne of CO2, for instance, has a GWP of 1, while the other gases are known to be larger contributors to global warming, per unit weight. Multiplying the annual emissions of each gas by its GWP, transforms the emissions into CO2 equivalents (Ceq), and then these amounts are summed across the gases to produce the index.