Absolute and Relative Global Warming Potentials

In Eqs. (S) and (T), [gas] and [ref] represent the time-dependent concentrations of the gas of interest and the reference gas, respectively, which are assumed to decay with characteristic lifetimes or response times after the instantaneous injection of the pulse, and ax (units of W m-2 per ppb or ppm) is the radiative forcing of the gas or reference per unit increase in their atmospheric concentrations. The value of ax is assumed to be time-independent. 

There may, however, be some cancellation of errors. For example, the concentration of atmospheric C02 ([ref], in Eq. (T)) depends in a nonlinear fashion on the amount of total dissolved inorganic carbon in the ocean surface layer because of the equilibria with water (see Chapter 8.B) so that relatively less atmospheric C02 can be taken up by the oceans as its atmospheric concentrations increase. This would leave relatively more C02 in the atmosphere, increasing its greenhouse effect. On the other hand, since the strongest infrared absorption bands of C02 are already saturated (vide supra), the radiative forcing (at-(), in Eq. (T)) decreases as its concentrations increase. 

Caldeira and Kasting (1993) show that these two factors largely cancel each other so that using C02 as the reference gas is still useful. 

For the alternatives and proposed replacements for CFCs, CFC-11 has been used in some cases as the reference compound. In interpreting the GWPs, the reader should take note of which compound has been used as the reference. 

Another index has been proposed as well, a forcing equivalent index (FEI) (Wigley, 1998), defined as 

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