This is a somewhat restated version of an earlier post seeking to understand the differences between Hansen Scenarios A and B. Rather than trying to clarify matters here, Gavin Schmidt posted over at Tim Lambert’s. In this morning’s post, I correctly identified that the difference between Scenarios A and B for periods up to the present pertained primarily to the handling of CFCs and had nothing to do with one being “exponential” and one being “linear”, as Gavin had stated at realclimate. However, I got wrongfooted somewhat in my interpretation of the data as archived at realclimate.
In Hansen et al 1988, they stated that they dealt with other CFCs and trace gases by doubling the effect of CFC11 and CFC12, a point that I noted in my post yesterday. They said:
Potential effects of several other trace gases are approximated by multiplying the CFC11 and CFC12 amounts by 2.
In my post yesterday, I incorrectly surmised that this would not be a substantial effect. When I analysed the data as archived at realclimate, I assumed that the effect of other trace gases would be counted by a separate line item in which they set the impact equal to the combined CFC11 and CFC12 impact. I did not consider the possibility that they would actually insert incorrect CFC11 and CFC12 values into their Schedule A physical inventories, making these values not directly reconcilable to physical measurements. However, this turns out to be what they’ve done. Schmidt snickered at my failing to consider the possibility of them altering their physical inventories to incorrect values. I see now what they’ve done and can follow through their reasoning, but I’d have been inclined to keep the Other Trace Gases as a distinct entry so that people can keep track of it. Climate scientists can be tricky accountants.
The more substantive issue is that this ad hoc handling of Other Trace Gases is what accounts for the near time differences between Scenarios A and B – a point that I made this morning, even if I didn’t quite connect to the doubling for OTGs.
Here is a diagram showing CO2 concentrations for Scenarios A,B and C (per realclimate version), as compared with observations to 2006 (GISS). Obviously, there is no material difference between Scenario A and Scenario B concentrations or, for that matter, with observations up to 2006 or so. CO2 concentrations start to diverge between the two scenarios in the next decade, but, for the near time analysis, calling one graph “exponential” and another “linear” is not really salient to the quantitative analysis (contra Schmidt’s characterization at realclimate.)
Figure 1. CO2 concentrations. A,B, C from realclimate; observed from NASA GISS.
Next methane, the second most important GHG. The left panel shows methane levels in Hansen et al 1988 as compared with present estimates; the right panel shows radiative forcing estimates. There is a slight difference between Scenario A and Scenario B up to 1998, when the Hansen-Michaels dispute occurred, but the difference is insufficient to yield the difference in Scenario A and B reported outcomes. A couple of interesting points though: the levels in Hansen et al 1988 reported for 1987 are noticeably higher than levels presently believed to have existed in 1987. Is this because of changing ways of measuring methane content? At present, I don’t know, but the differences are surprisingly high, as compared to CO2. As noted yesterday, methane concentrations have not increased nearly as much as Hansen projected, following a course parallel to Scenario C at a lower concentration. The forcing estimates obviously parallel the concentration estimates.
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There is negligible difference between Scenario A and Scenario B N2O.
The main action is in CFCs and, in particular, the Trace Gases other than CFC11 and CFC12. Here’s how Hansen et al 1988 described their CFC scenarios:
In Scenario A, … CCl3F (F-11) and CCl2F2 (F-12) emissions are from reported rates (Chemical Manufacturers Association (CMA) 1982] and assume 3% yr-1 increased emission in the future with atmospheric lifetimes for the gases of 75 and 150 years respectively. ,, In Scenario B… the annual growth of CFC11 and CFC12 is reduced from 3% yr-1 today to 2% yr-1 in 1990, 1% yr-1 in 2000 and 0 in 2010…. In Scenario C, CFC11 and CFC12 abundances are the same as scenarios A and B until 1990; thereafter CFC11 and CFC12 emissions decrease linearly to zero in 2000.
As noted above, they also said:
Potential effects of several other trace gases are approximated by multiplying the CFC11 and CFC12 amounts by 2.
The Figure below shows CFC11 and CFC12 concentrations, using the data from realclimate only pertaining to CFC11 and CFC12. In this calculation, the arbitrary doubling of CFC11 and CFC12 values in Scenario A (representing OTGs) has been backed out and will be shown separately under OTGs. Scenario A and B projections for CFC11 and CFC12 are identical ; and thus this is not a reason for Scenario A and B differences. Both scenarios over-estimated actual concentrations, which were like Scenario C.
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In my first try at this post yesterday, when I noted the doubling as an ad hoc method for Other Trace Gases, I hypothesized that the effect would not be material, but this does not prove to be the case. The figure below show how the primary near-time difference between Scenarios A and B arises. The solid black line shows the Scenario A radiative forcing (equal by assumption to the sum of Scenario A-B forcing from CFC11 and CFC12) – Hansen’s “Business as Usual” case; the dotted line (zero for all years) shows the Scenario B forcing (Hansen’s “more plausible” case). The red points show the radiative forcing for OTGs from realclimate; green lines are a first try at IPCC A1B and A2.
So where does this leave us? To clarify one point, I don’t have any problem with properly articulated scenarios as a way of analyzing projections. How else are you going to do it? With respect to projections of CO2 concentration made in the 1980s, the projections are pretty much on the money. But even if they were off to some degree, all it would do is defer or accelerate the date at which CO2 doubled somewhat. And I agree with concerned climate scientists that doubling CO2 is an unplanned real-life experiment that could have very serious consequences. If the impact is 2.5 deg C, then we should probably be glad that that’s all it is, as the results could have been much worse.
If Hansen’s 1988 model didn’t work very well, then that does not mean for me that some other model mightn’t work, even some other model by Hansen. Many readers here take much more hard-edged positions on these things than I believe to be justified.
Having said that, I’m also trying to figure out how one gets from point A to point B in all of this, a path made more difficult because the field is all too often characterized by sloppy practices.
The different handling of Other Trace Gases was material to Scenarios A and B outcomes, accounting for virtually the entire difference between scenarios. Scenario B hypothesized that there would be no contribution from the Other Trace Gases. Why did Hansen think in 1987 that this was “more plausible” than the doubling in Scenario A. With the benefit of hindsight, it looks like OTG forcing was about halfway between Scenarios B and Scenario A (before any counting from tropospheric ozone etc. which would move the results closer to Scenario A.
What then is the basis, if any, for concluding that Scenario B was the more “plausible”? I doubt whether Hansen was really thinking about differences between near-time Other Trace Gas concentrations when he said that Scenario B was “more plausible” for the near-time dispute with Michaels. The 1988 paper strongly suggests that he was thinking about “deep time” differences out past 2025 where exponential versus linear actually matters. However the term was “convenient” when Hansen re-visited the matter in his 1998 debate.
However, trying to put oneself in Hansen’s shoes back in 1988, there’s nothing to suggest that Hansen thought that it was “less realistic” to account for OTGs by doubling than to ignore them (Scenario B.) Of course, this ad hoc method appears to have over-estimated the impact of OTGs, but, once again, there’s nothing to suggest that Hansen in 1988 thought that it was less “realistic” to ignore the contribution of OTGs.
As to how Hansen’s model is faring, I need to do some more analysis. But it looks to me like forcings are coming in below even Scenario B projections. So I agree that it’s unfair for Hansen critics to compare Scenario A temperature results to actual outcomes as a test of the model mechanics. On the other hand, Hansen’s supporters have also been far too quick to claim vindication given the hodgepodge of GHG concentration results. If it’s unfair to blame the blame the model for differences between actual and projected if the GHG projections are wrong, then it i equally unfair to credit the model with “success” if it gets a “right” answer using wrong GHG projections. One would really have to re-run the 1988 model with observed GHG concentrations to make an assessment. Given that GISS have changed their models since 1988, GISS would presumably argue that the run is pointless, but the cost of doing the run doesn’t appear to be large and it seems like a reasonable exercise for someone to do. It would be interesting to obtain a listing of the 1988 model to that end.
Climate Audit 
