Considerable uncertainty exists about climate change responses to greenhouse gas emissions. This is due to incomplete understanding of 1) the interactive feedback mechanisms between clouds, oceans, and polar ice, which govern the responses; and 2) the size and nature of the sources and sinks of the greenhouse gases. "Radiative forcing" (a parameter used to perturb the heat balance in a simplified model of the earth-atmosphere system) involves less complexity and is used by atmospheric scientists to make assessments of the relative impacts of different gases.
Global warming potentials (GWPs) were developed to provide a simplified means of describing the relative ability of each greenhouse gas emission to affect future radiative forcing and thereby the global climate. GWPs relative to carbon dioxide were reported in the first international scientific assessment of the Intergovernmental Panel on Climate Change (IPCC, 1990) and have been updated in the supplement to that report (IPCC, 1992).
The extent to which a greenhouse gas directly contributes to radiative forcing--and hence to calculated global warming--depends on the quantity of gas emitted, the elapsed time before it is purged from the atmosphere, and the infrared energy absorption properties of the gas. The GWP parameter encompasses the latter two properties.
Carbon dioxide, CFCs, HCFCs, and HFCs are purged from the atmosphere at very different rates. The concentration of carbon dioxide in the atmosphere decays very slowly. CFCs are also removed very slowly with rates varying from a 55-year lifetime for CFC-11 (faster than carbon dioxide) to a 550-year lifetime CFC-115 (slower than carbon dioxide). Decay rates of HCFC and HFC concentrations also vary but their lifetimes are much less than those of carbon dioxide (see table). The decay of an HCFC/HFC compound with a mid-range lifetime (HFC-134a) is shown in the figure.
In order to evaluate contributions to future global warming, policy makers must reach a decision on the appropriate time span over which GWPs should be calculated. This time span is referred to as the integration time horizon (ITH); GWPs are calculated from the cumulative radiative forcing over a given ITH. GWPs for 20-, 100-, and 500-year ITHs were reported in the IPCC assessments.
In each case, the impact of a compound was evaluated relative to the impact of carbon dioxide over the selected time horizon. For a short-lived gas like HFC-134a, the concentration and, hence, the contribution to global warming, drops rapidly to zero. However, the contribution due to emissions of a long-lived gas like carbon dioxide persist for over 500 years.
If the ITH is set at 100 years, the entire potential impact of a short-lived compound, such as HFC-134a, would be counted; but a substantial part of the effect of carbon dioxide (the shaded area in the figure) would be excluded. At an ITH of 20 years, 72% of the potential effect of HFC-134a has been counted, while less than 10% of the effect of carbon dioxide is included. GWPs for several greenhouse gases are tabulated for 20, 100, and 500-year time horizons in the table below. The values quoted--like those used to construct the figure--are reported in the supplementary report to the 1990 IPCC Scientific Assessment.