A GUIDE TO GLOBAL WARMING POTENTIALS (GWPS)

In order to quantitatively compare the greenhouse effect of different greenhouse gases a global warming potential (GWP) index has been used which is based on the ratio of the radiative forcing of an equal emission of two different gases, integrated either over all time or up to an arbitrarily determined time horizon. The GWP index is analogous to the ozone depleting potential (ODP) index. However, the GWP index is subject to major conceptual difficulties arising from the fact that the atmospheric lifespan for part of the emitted CO2 is, for all practical purposes, infinite. In addition, there are major uncertainties in the atmospheric lifespans and indirect heating effects of the important greenhouse gases, which are reviewed here. An alternative GWP index is proposed which explicitly takes into account the duration of capital investments in the energy sector and is less sensitive to uncertainties in atmospheric lifespans and radiative heating than the usual GWP index for time horizons longer than the lifespan of the capital investment. The effect of the GWP index proposed here, compared with previous indices, is to shift attention away from short lived gases such as methane and toward CO2.