Greenhouse gases in addition to carbon dioxide must be considered to project future climate changes. Concentrations of other gases are frequently expressed in terms of radiative equivalence to carbon dioxide. The ongoing and projected buildup of greenhouse gases should be described in a summarizing index, which is useful for explaining certain aspects of current and future greenhouse climate changes, especially to a nontechnical audience.
The proposed Greenhouse Gas Index (GGI) is defined as the percentage of the preindustrial concentration of carbon dioxide (index value 100), plus the change in other long-lived radiatively active trace gases from preindustrial levels, converted to carbon dioxide radiative equivalents. The common "doubled carbon dioxide" model scenario would have an index of 200, with the actual carbon dioxide increase around 70% and the remaining 30% increase based on increases in methane, nitrous oxide, CFCs, and tropospheric ozone (which actually has a relatively short lifetime). The index is linear, based on concentrations, and does not include indirect effects from aerosols or stratospheric ozone depletion.
Using IPCC 1995 radiative equivalents, the GGI for 1998 is about 150.4, based on carbon dioxide concentration of 365 ppmv (131.3% of the preindustrial 278 ppmv), methane increasing to 1.8 from 0.7 ppmv (equivalent to 7.9% carbon dioxide increase based on methane having 20 times the radiative effect of carbon dioxide per molecule), nitrous oxide increasing to 0.315 from 0.275 ppmv (2.9% equivalent carbon dioxide increase, 200 times relative radiative effect), CFCs increasing to .000823 from 0 ppmv (3.6% equivalent carbon dioxide increase, average 12000 times relative effect), and tropospheric ozone increasing to 0.04 from 0.025 ppmv (4.7% carbon dioxide increase, 900 times relative effect).
The GGI is useful only in the limited area of summarizing the buildup of greenhouse gases. Models must still be driven by the individual greenhouse components, with nonlinear radiative effects accounted for. The index does not project model outputs such as changes in temperature or precipitation, either globally or regionally, and it does not account for lags in response to radiative forcing. However, the index should be helpful in clarifying communication about the future potential for greenhouse-induced climate change and for creating awareness among the general public that such climate changes are already beginning to occur.