5. The table lists some of the main greenhouse gases and their concentrations in pre-industrial times and in 1994; atmospheric lifetimes; anthropogenic sources; and Global Warming Potential (GWP). GWP is an index defined as the cumulative radiative forcing between the present and some chosen time horizon caused by a unit mass of gas emitted now, expressed relative to a reference gas such as CO2, as is used here. GWP is an attempt to provide a simple measure of the relative radiative effects of different greenhouse gases. The future global warming commitment of a greenhouse gas can be calculated over a chosen time horizon (such as 100 years) by multiplying the appropriate GWP by the amount of gas emitted. The choice of time horizon will depend on policy considerations. There are several other points that need to be kept in mind when using GWPs: (i) the typical uncertainty value is +/-35%, not including the uncertainty in the CO2 reference; (ii) GWPs are based on the radiative forcing concept and are therefore difficult to apply to radiatively important constituents that are unevenly distributed in the atmosphere; and (iii) GWPs need to take into account any indirect effects of the emitted gases if they are to correctly reflect future warming potential.