As noted in Harris et al. (l998) and Oltmans et al. (l998), the observed upward trends in surface O₃ in Europe and North America appear to be less steep in the past decade or two than in earlier periods (e.g., before about l980), perhaps because of control measures designed to reduce emissions of O₃ precursors and mitigate urban pollution problems. Non-linear chemical feedbacks may also contribute to damping the recent past and future trends (Wang and Jacob, l998). Theoretical studies using chemistry/transport models have attempted to prescribe likely future emissions of precursors and predict future tropospheric O₃ abundances. The largest future increases in O₃ forcings may occur in Asia in association with projected population growth and future development (van Dorland et al., l997; Brasseur et al., l998). Chalita et al. (1996) calculated a globally averaged radiative forcing from pre-industrial times to 2050 of 0.43 Wm^-2. The models of van Dorland et al. (l997) and Brasseur et al. (l998) suggest a higher globally averaged total radiative forcing from pre-industrial times to 2050 of 0.66 Wm^-2 and 0.63 Wm^-2, respectively, while that of Stevenson et al. (l998) yields a forcing of 0.48 Wm^-2 in 2100. As the analysis presented above shows, these differences are likely to be due to modelled differences in the latitudinal distributions or magnitudes of the projected O₃ change due for example to different emission inventories or model processes such as transport, and much less likely to be due to differences in radiative codes.

In addition to the direct forcings caused by injection of radiatively active gases to the atmosphere, some compounds or processes can also modify the radiative balance through indirect effects relating to chemical transformation or change in the distribution of radiatively active species. As previously indicated (IPCC, 1992, 1994; SAR), the tropospheric chemical processes determining the indirect greenhouse effects are highly complex and not fully understood. The uncertainties connected with estimates of the indirect effects are larger than the uncertainties of those connected to estimates of the direct effects. Because of the central role that O₃ and OH play in tropospheric chemistry, the chemistry of CH₄, CO, NMHC, and NO_x is strongly intertwined, making the interpretation of the effects associated with emission changes rather complex. It should be noted that indirect effects involving OH feedback on the lifetime of well-mixed greenhouse gases, and tropospheric O₃ concentration changes since the pre-industrial are implicitly accounted for in Sections 6.3 and 6.5.

Increased penetration of ultraviolet radiation into the troposphere as a result of stratospheric ozone depletion leads to changes in the photodissociation rates of some key chemical species. One of the primary species affected by possible changes in photodissociation rates is the hydroxyl radical OH, which regulates the tropospheric lifetime of a large number of trace gases such as CH₄, CO, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), NO_x, and to a lesser extent, sulphur dioxide (SO₂) (see Chapter 4). The impact of stratospheric O₃ changes on the fate of tropo-spheric species has been discussed by Ma and van Weele (2000), Fuglestvedt et al. (1994), Bekki et al. (1994); and Fuglestvedt et al. (1995); on the basis of two-dimensional model simulations, and by Madronich and Granier (1994), Granier et al. (1996), Van Dop and Krol (1996) and Krol et al. (1998) using three-dimensional models. These studies and the updated calculations presented by WMO (1999) estimated that a 1% decrease in global total O₃ leads to a global increase in O₃ photolysis rate of 1.4%, resulting in a 0.7 to 0.8% increase in global OH. Changes in photolysis rates from reduced stratospheric O₃ also have the potential to alter tropospheric O₃ production and destruction rates. Based on stratospheric O₃ evolution over the period 1980 to 1996, Myhre et al. (2000) have calculated a reduction in tropospheric O₃ associated with increased ultra-violet penetration and a corresponding negative radiative forcing reaching -0.01 Wm^-2 over that period of time. Another indirect impact of stratospheric O₃ depletion on climate forcing has been proposed by Toumi et al. (1994, 1995). The hydroxyl radical oxidises SO₂ to gaseous sulphuric acid, which is a source of H₂SO₄ particles. Changes in H₂SO₄ formation resulting from OH changes might affect the number of particles which act as condensation nuclei. Rodhe and Crutzen (1995) challenged whether this mechanism was of importance.

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