• Radiative forcing continues to be a useful tool to estimate, to a first order, the relative climate impacts (viz., relative global mean surface temperature responses) due to radiatively induced perturbations. The practical appeal of the radiative forcing concept is due, in the main, to the assumption that there exists a general relationship between the global mean forcing and the global mean equilibrium surface temperature response (i.e., the global mean climate sensitivity parameter, ) which is similar for all the different types of forcings. Model investigations of responses to many of the relevant forcings indicate an approximate near invariance of l (to about 25%). There is some evidence from model studies, however, that can be substantially different for certain forcing types. Reiterating the IPCC WGI Second Assessment Report (IPCC, 1996a) (hereafter SAR), the global mean forcing estimates are not necessarily indicators of the detailed aspects of the potential climate responses (e.g., regional climate change).
• The simple formulae used by the IPCC to calculate the radiative forcing due to well-mixed greenhouse gases have been improved, leading to a slight change in the forcing estimates. Compared to the use of the earlier expressions, the improved formulae, for fixed changes in gas concentrations, decrease the carbon dioxide (CO₂) and nitrous oxide (N₂O) radiative forcing by 15%, increase the CFC-11 and CFC-12 radiative forcing by 10 to 15%, while yielding no change in the case of methane (CH₄). Using the new expressions, the radiative forcing due to the increases in the well-mixed greenhouse gases from the pre-industrial (1750) to present time (1998) is now estimated to be +2.43 Wm^-2 (comprising CO₂ (1.46 Wm^-2), CH₄ (0.48 Wm^-2), N₂O (0.15 Wm^-2) and halocarbons (halogen-containing compounds) (0.34 Wm^-2)), with an uncertainty¹ of 10% and a high level of scientific understanding (LOSU).
• The forcing due to the loss of stratospheric ozone (O₃) between 1979 and 1997 is estimated to be -0.15 Wm^-2 (range: -0.05 to -0.25 Wm^-2). The magnitude is slightly larger than in the SAR owing to the longer period now considered. Incomplete knowledge of the O₃ losses near the tropopause continues to be the main source of uncertainty. The LOSU of this forcing is assigned a medium rank.
• The global average radiative forcing due to increases in tropospheric O₃ since pre-industrial times is estimated to be +0.35 ± 0.15 Wm^-2. This estimate is consistent with the SAR estimate, but is based on a much wider range of model studies and a single analysis that is constrained by observations; there are uncertainties because of the inter-model differences, the limited information on pre-industrial O₃ distributions, and the limited data that are available to evaluate the model trends for modern (post-1960) conditions. A rank of medium is assigned for the LOSU of this forcing.
• The changes in tropospheric O₃ are mainly driven by increased emissions of CH₄, carbon monoxide (CO), non-methane hydrocarbons (NMHCs) and nitrogen oxides (NO_x), but the specific contributions of each are not yet well quantified. Tropospheric and stratospheric photochemical processes lead to other indirect radiative forcings through, for instance, changes in the hydroxyl radical (OH) distribution and increase in stratospheric water vapour concentrations.
  
• Models have been used to estimate the direct radiative forcing for five distinct aerosol species of anthropogenic origin. The global, annual mean radiative forcing is estimated as -0.4 Wm^-2 (�0.2 to �0.8 Wm^-2) for sulphate aerosols; -0.2 Wm^-2 (�0.07 to �0.6 Wm^-2) for biomass burning aerosols; -0.10 Wm^-2 (�0.03 to �0.30 Wm^-2) for fossil fuel organic carbon aerosols; +0.2 Wm^-2 (+0.1 to +0.4 Wm^-2) for fossil fuel black carbon aerosols; and in the range -0.6 to +0.4 Wm^-2 for mineral dust aerosols. The LOSU for sulphate aerosols is low while for biomass burning, fossil fuel organic carbon, fossil fuel black carbon, and mineral dust aerosols the LOSU is very low.
• Models have been used to estimate the �first� indirect effect of anthropogenic sulphate and carbonaceous aerosols (namely, a reduction in the cloud droplet size at constant liquid water content) as applicable in the context of liquid clouds, yielding global mean radiative forcings ranging from -0.3 to -1.8 Wm^-2. Because of the large uncertainties in aerosol and cloud processes and their parametrizations in general circulation models (GCMs), the potentially incomplete knowledge of the radiative effect of black carbon in clouds, and the possibility that the forcings for individual aerosol types may not be additive, a range of radiative forcing from 0 to -2 Wm^-2 is adopted considering all aerosol types, with no best estimate. The LOSU for this forcing is very low.
• The �second� indirect effect of aerosols (a decrease in the precipitation efficiency, increase in cloud water content and cloud lifetime) is another potentially important mechanism for climate change. It is difficult to define and quantify in the context of current radiative forcing of climate change evaluations and current model simulations. No estimate is therefore given. Nevertheless, present GCM calculations suggest that the radiative flux perturbation associated with the second aerosol indirect effect is of the same sign and could be of similar magnitude compared to the first effect.
• Aerosol levels in the stratosphere have now fallen to well below the peak values seen in 1991 to 1993 in the wake of the Mt. Pinatubo eruption, and are comparable to the low values seen in about 1979, a quiescent period for volcanic activity. Although episodic in nature and transient in duration, stratospheric aerosols from explosive volcanic eruptions can exert a significant influence on the time history of radiative forcing of climate.
• Owing to an increase in land-surface albedo during snow cover in deforested mid-latitude areas, changes in land use are estimated to yield a forcing of -0.2 Wm^-2 (range: 0 to -0.4 Wm^-2). However, the LOSU is very low and there have been much less intensive investigations compared with other anthropogenic forcings.
• Radiative forcing due to changes in total solar irradiance (TSI) is estimated to be +0.3 ± 0.2 Wm^-2 for the period 1750 to the present. The wide range given, and the very low LOSU, are largely due to uncertainties in past values of TSI. Satellite observations, which now extend for two decades, are of sufficient precision to show variations in TSI over the solar 11-year activity cycle of about 0.08%. Variations over longer periods may have been larger but the techniques used to reconstruct historical values of TSI from proxy observations (e.g., sunspots) have not been adequately verified. Solar radiation varies more substantially in the ultraviolet region and GCM studies suggest that inclusion of spectrally resolved solar irradiance variations and solar-induced stratospheric O₃ changes may improve the realism of model simulations of the impact of solar variability on climate. Other mechanisms for the amplification of solar effects on climate, such as enhancement of the Earth�s electric field causing electrofreezing of cloud particles, may exist but do not yet have a rigorous theoretical or observational basis.
• Radiative forcings and Global Warming Potentials (GWPs) are presented for an expanded set of gases. New categories of gases in the radiative forcing set include fluorinated organic molecules, many of which are ethers that may be considered as halocarbon substitutes. Some of the GWPs have larger uncertainties than others, particularly for those gases where detailed laboratory data on lifetimes are not yet available. The direct GWPs have been calculated relative to CO₂ using an improved calculation of the CO₂ radiative forcing, the SAR response function for a CO₂ pulse, and new estimates for the radiative forcing and lifetimes for a number of gases. As a consequence of changes in the radiative forcing for CO₂ and CFC-11, the revised GWPs are typically 20% higher than listed in the SAR. Indirect GWPs are also discussed for some new gases, including CO. The direct GWPs for those species whose lifetimes are well characterised are estimated to be accurate (relative to one another) to within ±35%, but the indirect GWPs are less certain.
• The geographical distributions of each of the forcing mechanisms vary considerably. While well-mixed greenhouse gases exert a significant radiative forcing everywhere on the globe, the forcings due to the short-lived species (e.g., direct and indirect aerosol effects, tropospheric and stratospheric O₃) are not global in extent and can be highly spatially inhomogeneous. Furthermore, different radiative forcing mechanisms lead to differences in the partitioning of the perturbation between the atmosphere and surface. While the Northern to Southern Hemisphere ratio of the solar and well-mixed greenhouse gas forcings is very nearly 1, that for the fossil fuel generated sulphate and carbonaceous aerosols and tropospheric O₃ is substantially greater than 1 (i.e., primarily in the Northern Hemisphere), and that for stratospheric O₃ and biomass burning aerosol is less than 1 (i.e., primarily in the Southern Hemisphere).
• The global mean radiative forcing evolution comprises of a steadily increasing contribution due to the well-mixed greenhouse gases. Other greenhouse gas contributions are due to stratospheric O₃ from the late 1970s to the present, and tropospheric O₃ whose precise evolution over the past century is uncertain. The evolution of the direct aerosol forcing due to sulphates parallels approximately the secular changes in the sulphur emissions, but it is more difficult to estimate the temporal evolution due to the other aerosol components, while estimates for the indirect forcings are even more problematic. The temporal evolution estimates indicate that the net natural forcing (solar plus stratospheric aerosols from volcanic eruptions) has been negative over the past two and possibly even the past four decades. In contrast, the positive forcing by well-mixed greenhouse gases has increased rapidly over the past four decades.
• Estimates of the global mean radiative forcing due to different future scenarios (up to 2100) of the emissions of trace gases and aerosols have been performed (Nakic´enovic´ et al., 2000; see also Chapters 3, 4 and 5). Although there is a large variation in the estimates from the different scenarios, the results indicate that the forcing (evaluated relative to pre-industrial times, 1750) due to the trace gases taken together is projected to increase, with the fraction of the total due to CO₂ becoming even greater than for the present day. The direct aerosol (sulphate, black and organic carbon components taken together) radiative forcing (evaluated relative to the present day, 2000) varies in sign for the different scenarios. The direct aerosol effects are estimated to be substantially smaller in magnitude than that of CO₂. No estimates are made for the spatial aspects of the future forcings. Relative to 2000, the change in the direct plus indirect aerosol radiative forcing is projected to be smaller in magnitude than that of CO₂.

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