Just as radiative forcing provides a simplified means of comparing the various factors that are believed to influence the climate system to one another, Global Warming Potentials (GWPs) are one type of simplified index based upon radiative properties that can be used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense. The formulation of GWPs, reasons for the choice of various time horizons, and the effects of clouds, scenarios, and many other factors upon GWP values were discussed in detail in IPCC (1994). That discussion will not be repeated here. Section 6.2 discusses the relationship between radiative forcing and climate response and describes recent studies that have supported the view that many different kinds of forcing agents (e.g., various greenhouse gases, sulphate aerosols, solar activity, etc.) yield similar globally averaged climate responses per Wm^-2 of forcing (albeit with different spatial patterns in some important cases). These parallels in global mean climate responses have motivated the use of simplified measures to estimate in an approximate fashion the relative effects of emissions of different gases on climate. The emphasis on relative rather than absolute effects (such as computed temperature change) avoids dependence upon any particular model of climate response.

The impact of greenhouse gas emissions upon the atmosphere is related not only to radiative properties, but also to the time-scale characterising the removal of the substance from the atmosphere. Radiative properties control the absorption of radiation per kilogram of gas present at any instant, but the lifetime (or adjustment time, see Chapter 4) controls how long an emitted kilogram is retained in the atmosphere and hence is able to influence the thermal budget. The climate system responds to changes in the thermal budget on time-scales ranging from the order of months to millennia depending upon processes within the atmosphere, ocean, cryosphere, etc.
GWPs are a measure of the relative radiative effect of a given substance compared to another, integrated over a chosen time horizon. The choice of the time horizon depends in part upon whether the user wishes to emphasise shorter-term processes (e.g., responses of cloud cover to surface temperature changes) or longer-term phenomena (such as sea level rise) that are linked to sustained alterations of the thermal budget (e.g., the slow transfer of heat between, for example, the atmosphere and ocean). In addition, if the speed of potential climate change is of greatest interest (rather than the eventual magnitude), then a focus on shorter time horizons can be useful (IPCC, l994; Skodvin and Fuglestvedt, 1997; Fuglestvedt et al., 2000; Smith and Wigley, 2000a,b).

As in previous reports, here we present GWPs for 20, 100, and 500 year time horizons. The most recent GWP evaluations are those of WMO (l999) and the SAR, and the results presented here are drawn in large part from those assessments, with updates for those cases where significantly different new laboratory or radiative transfer results have been published. The sources used for input variables for the GWP calculations are indicated in this section and in the headers and footnotes to the tables, where sources of new estimates since the SAR are identified.

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):

where TH is the time horizon over which the calculation is considered, a_x 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. Updated information since the SAR is presented for many gases in Section 6.3. Many important changes in these quantities were recently reviewed in WMO (l999) and will be briefly summarised here. In addition, some gases can indirectly affect radiative forcing, mainly through chemical processes. For example, tropospheric O₃ provides a significant radiative forcing of the climate system, but its production occurs indirectly, as a result of atmospheric chemistry following emissions of precursors such as NO_x, CO, and NMHCs (see Section 6.6 and Chapter 4). Indirect effects will be described below for a number of key gases.

It is important to distinguish between the integrated relative effect of an emitted kilogram of gas which is represented by a GWP and the actual radiative forcings for specific gas amounts presented, for example, in Section 6.3 and in Figure 6.6. GWPs are intended for use in studying relative rather than absolute impacts of emissions, and pertain to specific time horizons.

The radiative efficiencies ar and a_x are not necessarily constant over time. While the absorption of infrared radiation by many greenhouse gases varies linearly with their abundance, a few important ones display non-linear behaviour for current and likely future abundances (e.g., CO₂, CH₄, and N₂O). For those gases, the relative radiative forcing will depend upon abundance and hence upon the future scenario adopted. These issues were discussed in detail and some sensitivities to chosen scenarios were presented in IPCC (l994).

A key aspect of GWP calculations is the choice of the reference gas, taken here to be CO₂. In IPCC (l994), it was shown, for example, that a particular scenario for future growth of CO₂ (S650, see Chapter 3) would change the denominator of Equation 6.2 by as much as 15% compared to a calculation employing constant pre-industrial CO₂ mixing ratios.

The atmospheric response time of CO₂ is subject to substantial scientific uncertainties, due to limitations in our knowledge of key processes including its uptake by the biosphere and ocean. When CO₂ is used as the reference, the numerical values of the GWPs of all greenhouse gases can change substantially as research improves the understanding of the removal processes of CO₂. The removal function for CO₂ used for the GWPs presented here is based upon carbon cycle models such as those discussed in Chapter 3. The CO₂ radiative efficiency (a_r) used in this report has been updated since the SAR, as discussed in Section 6.3 (see below).

The lifetimes of non-CO₂ greenhouse gases are dependent largely on atmospheric photochemistry, which controls photo-lysis and related removal processes as discussed in Chapter 4. When the lifetime of the gas in question is comparable to the response time of CO₂ (nominally about 150 years, although it is clear that the removal of CO₂ cannot be adequately described by a single, simple exponential lifetime; see IPCC (l994) and the discussion below), the GWP is relatively insensitive to choice of time horizon, i.e., for N₂O. When the lifetime of the gas in question differs substantially from the response time of the reference gas, the GWP becomes sensitive to the choice of time horizon, which in turn implies a decision regarding the climate processes and impacts of interest, as noted above. For longer time horizons, those species that decay more rapidly than the reference gas display decreasing GWPs, with the slope of the decay being dependent mainly on the lifetime of the gas in question. Gases with lifetimes much longer than that of the reference gas (e.g., C2F6) display increasing GWPs over long time horizons (i.e., greater than a hundred years). We emphasise that the GWP is an integral from zero to the chosen time horizon; hence the values presented in the table for 25, 100, and 500 years are not additive.

A number of studies have suggested modified or different indices for evaluating relative future climate effects. Here we provide only an indication of the kinds of issues that are being considered in these alternative indices. Wuebbles and Calm (l997) emphasised the fact that some halocarbon substitutes used, for example, in refrigeration are less efficient than the CFCs they are replacing, so that more energy is consumed through their use and hence more CO₂ emitted per hour of operation. Evaluation of these technological factors would only be possible through detailed emission inventories coupled with scenarios, and are not included here. Some studies have argued for use of “discount rates” to reflect increasing uncertainty and changing policies with time (e.g., to account for the possibility that new technologies will emerge to solve problems; see for example Lashof and Ahuja, l990; Reilly et al., l999). Economic factors could be considered along with the technological ones mentioned above, adding another aspect to any scenario. These are not included here. Smith and Wigley (2000a,b), Fuglestvedt et al. (2000), and Reilly et al. (l999) have examined the relationship between GWPs and climate response using simple energy balance models. These studies emphasised the point made above regarding the links between choice of time horizon, lifetime of a particular substance, and the climate response of interest, noting for example that while the 100-year GWP for N₂O represented the model-calculated temperature responses (both instantaneous and integrated over time), and the calculated sea level rise to good accuracy, that for CH₄ (a short-lived gas with a lifetime of about 10 years) represented sea level rise far better than it did the instantaneous temperature change for that time horizon. For very short-lived gases, GWPs are often calculated using a ‘slab’ (continuous) rather than pulse emission. This approach involves the assumption of specific scenarios for the magnitudes of the slabs. For some very short-lived gases such as NMHCs, models must employ ‘slab’ emissions in GWP analyses since their impact depends critically on non-linear coupled chemical processes. Some examples are given in Sections 6.12.3.2 and 6.12.3.4.

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