You say emissions, I say concentrations .....

and together we say forcings. I see that even I have been caught up in confusing emissions and concentrations in the various scenerios described by Hansen et al. To clear this up, Hansen et al., made a forecast of what the mixing ratios would be for CO2 and methane. They did not predict emissions. They predicted emissions for the CFCs. Building on previous work of Weiss (J. Geophys. Res. 86 (1981) 7185, they predicted emission scenerios for N2O, although this is somewhat hidden in the description.

However, the real point of all this is that it is possible to produce simple, accurate predictions of future forcing over a 15 year period, and perhaps longer. On the other hand emission scenerios are easier to link to social and economic policy decisions.