Ozone radiative forcing estimates

Stevenson, D.S. Johnson, C.E. Collins, W.J. Derwent, R.G. and Edwards, J.M. (2000). Future estimates of tropospheric ozone radiative forcing and methane turnover The impact of climate change. Geophys. Res. Lett., 27(14), 2073-2076. 

Table 14.4 summarizes the estimated total direct radiative forcing calculated for the period from preindustrial times to 1992 for C02, CH4, N20, and O, (IPCC, 1996). The estimate for CH4 includes the effects due to its impacts on tropospheric ozone levels or on stratospheric water vapor, both of which are generated during the oxidation of methane. That shown for 03 is based on the assumption that its concentration increased from 25 to 50 ppb over the Northern Flemi-sphere. The total radiative forcing due to the increase in these four gases from preindustrial times to the present is estimated to be 2.57 W m 2. 

Haywood, J. M., M. D. Schwarzkopf, and V. Ramaswamy, Estimates of Radiative Forcing Due to Modeled Increases in Tropospheric Ozone, J. Geophys. Res., 103, 16999-17007 (1998c). 

In this paper estimates of radiative forcing due to changes in stratospheric as well as tropospheric ozone given in the literature are reviewed. The review is largely based on the WMO (1999) assessment. 

Estimates of radiative forcing due to tropospheric ozone are listed in Table 2. CTM calculations of changes in tropospheric ozone and the radiative forcing are based on a variety of assumptions. Most models, but not all, include temperature adjustment in the stratosphere and effects of clouds. Neglect of either of these effects has been estimated to lead to an overestimate of radiative forcing by approximately 10-25% (Berntsen et al., 1997). 

Several estimates of the radiative forcing due to changes in tropospheric ozone are based on 3D CTMs. The MOGUNT1A model was used by van Dorland et ah (1997), Berntsen et ah (1997) based their work on the Oslo 3D CTM 1 model, and Roelofs et ah (1997) used ozone changes predicted using the European Centre Hamburg Model version 4 coupled to a tropospheric chemistry model. Forster et ah (1996), on the other hand, used two different 2D CTM models to calculate the ozone increase since pre-industrial time, namely the Cambridge and the UK.MO models. One study was based on observations of ozone. Portmann et ah (1997) estimated tropical tropospheric ozone from ozonesonde profiles and ozone columns derived from satellite maps. 

Large uncertainties remain due to insufficient knowledge about the ozone distributions in the unperturbed as well as the present atmosphere. The ozone distribution is particularly uncertain in the tropics. The best estimate of WMO (1999) of global-mean radiative forcing since the mid-1800s is 0.35 Wm 2 with an uncertainty range +0.15 Wm 2, bracketing the results from a majority of studies. 

Figure 22.32 shows the IPCC (1995) summary of estimates of global-mean radiative forcing by greenhouse gases, stratospheric and tropospheric ozone, aerosols, and solar output. The height of the bar represents the IPCC best estimate as to the value of the forcing, and... 

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