2. Radiative Forcing – its origin, evolution and formulation
2.4 Summary and challenges
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A future challenge with respect to the forcing concept is to quantify whether the efficacy is
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unity when adopting the current definition of ERF for all drivers of climate change and various
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models, and thus to understand the diversity among some previous results (Shine et al., 2003;
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Hansen et al., 2005; Shindell 2014; Marvel et al., 2015; Richardson et al., 2019).
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Further there is a need to better understand the rapid adjustment processes in climate models,
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both the degree of influence of diversity in IRF as indicated in Smith et al. (2018), but also
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dedicated process studies comparing GCMs with high resolution models with weaker degrees of
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parametrization, such as convection permitting models. There is a high potential for progress to
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be made using results from the ongoing CMIP6 model intercomparison project, which is
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supporting IPCC AR6 (Eyring et al., 2016); efforts have been made to ensure that more
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diagnostics are available to enable the drivers of ERF to be better quantified and hence for inter-
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model diversity to be better characterized. Such studies will aid the understanding of whether
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uncertainties in ERF of CO2 and other greenhouse gases are substantially larger than using RF,
1095
as was indicated in Myhre et al. (2013).
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Lastly, there is a need to develop methodologies to compare weak radiative perturbations,
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which will continue to need to be quantified using RF, with the major climate drivers which are
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increasingly being quantified from various model simulations using the ERF concept. It is
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possible that once the generic understanding of the rapid adjustments has improved, it can be
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applied to the weak forcings and enable ERF to be estimated from their RFs.
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1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121
Figure 2.1: Changes in top-of-atmosphere shortwave (∆S) and longwave (∆F↑) irradiances
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following a doubling of CO2 from 300 ppm, in a one-dimensional radiative-convective model.
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Wavy lines represent changes in convective fluxes, with all other lines radiative. The values in
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boxes show the net flux changes at the surface and for the atmosphere. (a) the instantaneous
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flux change, (b) the change after a few months and includes the effect of stratospheric
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temperature adjustment and other rapid adjustments represented in the model and (c) the flux
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changes when the system has come to equilibrium following a change in surface temperature
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(TS). Figure taken from Hansen et al. (1981).
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1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151
Figure 2.1: Changes in top-of-atmosphere shortwave (∆S) and longwave (∆F↑) irradiances
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following a doubling of CO2 from 300 ppm, in a one-dimensional radiative-convective model.
1153
Wavy lines represent changes in convective fluxes, with all other lines radiative. The values in
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boxes show the net flux changes at the surface and for the atmosphere. (a) the instantaneous flux
1155
change, (b) shows the change after a few months and includes the effect of stratospheric
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temperature adjustment and other rapid adjustments represented in the model and (c) shows the
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flux changes when the system has come to equilibrium following a change in surface
1158
temperature (TS). Figure taken from Hansen et al. (1981).
Figure 2.2
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Components of RF by emissions of gases, aerosols, or their precursors for the period 1750-
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2005. O3(T) and O3(S) indicate tropospheric and stratospheric ozone respectively. Figure from
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Forster et al. (2007).
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[Jpeg can be obtained from
1167 https://archive.ipcc.ch/report/graphics/images/Assessment%20Reports/AR4%20-%2 1168 WG1/Chapter%2002/fig-2-21.jpg]. 1169 1170 1171 1172 1173
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JPEG can be obtained from:
1177 https://archive.ipcc.ch/report/graphics/images/Assessment%20Reports/AR5%20- 1178 %20WG1/Chapter%2008/Fig8-01.jpg 1179 1180
Figure 2.3 Schematic comparing (a) instantaneous RF, (b) RF, which allows stratospheric
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temperature to adjust, (c) flux change when the surface temperature is fixed over the whole
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Earth (a method of calculating ERF), (d) the ERF calculated allowing atmospheric and land
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temperature to adjust while ocean conditions are fixed and (e) equilibrium response. (Figure
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taken from Myhre et al. 2013).
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Figure 2.4: Illustration of the change in the global energy balance (top of the
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atmosphere -TOA, atmosphere - Atmos., and surface) from a doubling of the CO2
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concentration for an instantaneous perturbation, instantaneous and rapid adjustment,
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and the climate system in new equilibrium. Changes in the energy fluxes of solar
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radiation (SW) is given in yellow bars, longwave (LW) in red bars, latent heat (LH) in
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blue bars and sensible heat (SH) in green bars. The net flux changes at TOA, Atmos.,
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and surface are given in numerical values in boxes. This figure can be considered as a
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modern day-version of Figure 2.1.
1201 1202
1204
Figure 2.5: Instantaneous radiative forcing (IRF), individual and total rapid adjustments, and
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effective radiative forcing (ERF) at the top of the atmosphere for a doubling of CO2 and a
1206
tenfold increase in the BC concentration. The total rapid adjustment is the sum of the individual
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terms of surface temperature change (only land), tropospheric temperature, stratospheric
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temperature, water vapor, surface albedo change, and clouds. The uncertainties are one standard
1209
deviation among the PDRMIP models. It is a coincidence that the IRF for CO2 and BC is almost
1210
identical. The figure is modified from Smith et al. (2018).
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