a number of people are asking for information on the relative forcings in Scenarios A, B, and C. Gavin previously provided lots of scenario data and some plots to support an earlier blog post at Real Climate. However, I, and others in conversations at various blogs are interested in knowing how the volcanos fit in.
Here is a plot of the data I downloaded from from Real Climate
If I understand Gavin’s earlier post correctly, the scenario data I’ve plotted from his file does not include the “scenario” volcanos. Gavin’s intention was to explain the projections and not illustrate the unpredictable random forcing.
However, it’s quite evident that the volcanos have a significant effect on the climate predictions. So, I normalized historical forcing to set the zero point in 1958 and plotted the historical forcing data two ways:
- Summing all contributions in Gavin’s file (shown in green) and
- Summing all contributions except the stratospheric aerosols which I think are the portion related to volcanos (show in purple).
My only two observations are:
- If we don’t include volcanos, Hansen Scenario “C” tracks the forcing best since 1958.
- Compared using this baseline, and examining since 1958 would suggest that the reason Hansen et al. 1988 overpredicts forcing is a) the GCM is ok b) the actual man induced forcing was on the low side, c) unexpected volcanic eruptions slowed the temperature increase below that predicted by the GCM and d) the we need to know how big the “scenario C volcanic eruption” was are to confirm “C” is closest to reality as it occurred.
I’ve requested Scenario’s ABC with volcano data from Gavin. With luck, those still exist, and he will be able to find them.
Update: Note that Scenario ABC data at Real Climate were digitized, see Gavin’s response to my request.
Lucia,
Your graph of forcing stops at 2002. The divergance from the temperature predictions only becomes obvious after that point. I suspect that the actual forcings are larger than the scenario C forcings in the last 5 years which means any further temperature divergance cannot be explained by under estimates of the forcings.
Raven– Shoot! Yes my plot stops at 2003 because I only have volcano data for that period, and initially I plotted that to see the differences. I emailed back and forth with Gavin yesterday, and I know now to add the Hansen fictional volcanos to the ABC scenarios a bit better. I’ll do that later this week.
I’m afraid I’m a bit obsessed with my lumped parameter climate model right now. I’m calling it “planet Lucia” for now, because it’s such a simple model. As I suspected, the volcanos made a difference to the fit.
I’m double checking the calculations, some post-processing calcs so that I don’t announce a sensitivity and then have to take it all back because I forgot to multiply by 2 or something. But, the fit is pretty good.