The GWP has been defined as the ratio of the time-integrated

radiative forcing from the instantaneous release of 1 kg of a trace

substance relative to that of 1 kg of a reference gas (IPCC, l990):

---formula---

where TH is the time horizon over which the calculation is

considered, ax is the radiative efficiency due to a unit increase

in atmospheric abundance of the substance in question (i.e.,

Wm−2 kg−1), [x(t)] is the time-dependent decay in abundance of

the instantaneous release of the substance, and the corresponding

quantities for the reference gas are in the denominator. The GWP

of any substance therefore expresses the integrated forcing of a

pulse (of given small mass) of that substance relative to the

integrated forcing of a pulse (of the same mass) of the reference

gas over some time horizon. The numerator of Equation 6.2 is the

absolute (rather than relative) GWP of a given substance, referred

to as the AGWP. The GWPs of various greenhouse gases can then

be easily compared to determine which will cause the greatest

integrated radiative forcing over the time horizon of interest. The

direct relative radiative forcings per ppbv are derived from

infrared radiative transfer models based on laboratory measurements

of the molecular properties of each substance and considering

the molecular weights.