6.15.2.4 Aerosol indirect effect
The indirect radiative forcing was calculated by the LLNL/Umich model (Chuang
et al., 2000a), the GISS model (Koch et al., 1999), and the Max Planck/Dalhousie
(Lohmann et al., 1999a,b, 2000) model. The LLNL/Umich model uses the mechanistic
formulation for the determination of droplet concentration described by Chuang
et al. (1997) but has been updated to include interactive dust and sea salt.
This model provides an estimate of the “first” indirect effect. The
GISS model used an empirical formulation for relating droplet concentrations
and aerosol concentrations and also provides an estimate of the “first”
indirect effect. The MaxPlanck/Dalhousie model used the mechanistic formulation
of Chuang et al. (1997) as described in Lohmann et al. (1999a,b, 2000) and includes
both the “first” and “second” indirect effects. In addition,
the MaxPlanck/Dalhousie model used monthly averaged dust and sea salt fields
from the LLNL/Umich model. Further details of the models are provided in Chapter 5. The indirect radiative forcing scenarios are summarised in Table
6.16, and represent the radiative forcing from the year 2000 due to sulphate
and carbonaceous aerosols. Some simulations suggest that changes in the concentrations
of natural aerosols due to changes in the climate could also contribute to a
negative aerosol indirect effect (see Chapter 5).
As for the aerosol direct effect, the uncertainty associated with these estimates
is higher than for the estimates for the present day. Considering the very low
LOSU of the indirect aerosol effect and the fact that no best estimate was recommended
for the present day (Sections 6.8 and 6.13),
the numbers provided in Table 6.16 give only a rough indication
of how the forcings could change between the present and the future (according
to the SRES scenarios) in three different models. Relative to 2000, the change
in the direct plus indirect aerosol radiative forcing is projected to be smaller
in magnitude than that of CO_{2}.
Table 6.16: Indirect aerosol radiative forcing
(Wm^{2}) estimated by different models for the IPCC SRES scenarios
described in Chapter 5. The sulphate burdens in each
case are given in Chapter 5, Table
5.14. These estimates are from the average difference in cloud forcing
between two simulations. The numbers represent the radiative forcing from
2000. No numbers are given for the year 2000 as no best estimate of the
radiative forcing is suggested in Section 6.8. 


LLNL/Umich

GISS

Max Planck/Dalhousie


A2 2030 
0.24

0.29

0.05

A2 2100 
0.47

0.36

0.32

B1 2100 
+0.10




