Only a few estimates account for uncertainty in forcings other than from aerosols (e.g., Gregory et al., 2002a; Knutti et al., 2002, 2003); some other studies perform some sensitivity testing to assess the effect of forcing uncertainty not accounted for, for example, in natural forcing (e.g., Forest et al., 2006; see Table 9.1 for an overview). (ipcc.ch)
Consistent model biases among the simulations driven by a set of alternative forcings suggest that uncertainty in the forcing plays only a relatively minor role. (pubs.giss.nasa.gov)
It would clearly be inappropriate to regress surface temperature changes on forcing changes for the reasons you give, since relative uncertainty in forcing changes is much larger than that in temperature changes. (climateaudit.org)