During our discussions of the differences between Hansen Scenarios A and B – during which the role of CFCs in Scenario A gradually became clearer – the absence of a graph clearly showing the allocation of radiative forcing between GHGs stood out rather starkly to me. When Gavin Schmidt re-visited the topic in May 2007, he only showed total forcing both in his graphic here (see below)and in the data set http://www.realclimate.org/data/H88_scenarios_eff.dat .
Figure 1. Forcing totals form Schmidt (2007)
Schmidt summarized the differences as follows:
The details varied for each scenario, but the net effect of all the changes was that Scenario A assumed exponential growth in forcings, Scenario B was roughly a linear increase in forcings, and Scenario C was similar to B, but had close to constant forcings from 2000 onwards. Scenario B and C had an ‘El Chichon’ sized volcanic eruption in 1995.
While it is true that the total forcing in Scenario A is “exponential” and the forcing in Scenario B is “linear”, the graphic below shows my estimates of how the forcing breaks down between GHGs within each of the three scenarios using contemporary or near-contemporary “simplified expressions”.
Obviously one point sticks out like a sore thumb: Scenario A increases are dominated by CFC greenhouse effect. In Scenario A, the CFC contribution to the Earth’s greenhouse effect becomes nearly double the CO2 contribution during the projection period. This is not mentioned either in Hansen et al 1988 or in Schmidt (realclimate, 2007).
The allocation between GHGs also clarifies why Scenario B is “linear” and Scenario A “exponential”. In Scenario B, the main forcing occurs from CO2 increase (not CFC increase). The “simplified expression” for the relationship between CO2 concentration and temperature change is logarithmic; thus, even though CO2 growth is (modestly) exponential in Scenario B, the composite of the exponential increase in GHC concentration and a logarithmic expression relating CO2 concentration to forcing yields linear growth in forcing.
On the other hand, the simplified expression relating CFC concentration to forcing is linear. Thus the exponential growth in CFC concentration in Scenario A (and the CFC growth rate is hypothesized to be much stronger than for CO2), combined with a linear relationship leads to an exponential growth in total forcing – driven primarily by CFCs.
The GHG concentrations used for the above calculations are taken from the file posted at realclimate on Dec 22, 2007 ( http://www.realclimate.org/data/H88_scenarios.dat ). Something close to these concentrations could be calculated from the verbal descriptions of Hansen et al 1988 (as I had done here [...] prior to becoming aware of this file at realclimate).
Hansen et al 1988 contained a series of “Simplified Expressions” relating forcing (in delta-T) to GHG concentrations for each of the GHGs. Usual current practice is to express forcing as wm-2; IPCC 1990 (p 52) said that a factor of 3.35 was needed to convert the Hansen equations to wm-2 and this has been applied here. (For the CFC11 and CFC12 equations, which could be checked directly, this conversion factor reconciles the Hansen expression to the IPCC 1990 expression.) I was unable to get the Hansen et al 1988 expressions for CH4 and N2O forcing to yield sensible results and for these two gases, I used the expression in IPCC 1990 to convert GHG concentration to radiative forcing. If anyone can get the Hansen et al 1988 expressions for CH4 and N2O to work, I’d be interested. I spent a fair bit of time on this before abandoning the effort and going with the IPCC 1990 expressions.
In all cases, I’ve used the “pre-industrial” concentrations used by NOAA in their calculations. In some Hansen calculations, a 1958 base is used (and these differences can be readily reconciled at least to a first approximation.)
Hansen et al 1988 said that “resource limitations” ultimately checked the expansion of Scenario A. While this is true for CO2, I’m a bit dubous that resource limitations come into play in connection with CFC emissions. Obviously CFC emissions have not increased anywhere like Hansen Scenario A. I can’t comment on whether this is due to Montreal Protocols or other factors, but “resource limitations” seem highly unlikely as a limiting factor for CFC growth history.
Second, while Hansen et al 1988 disclosed that they doubled the CFC11 and CFC12 contributions to account for minor CFCs, this seems like a pretty aggressive accounting, especially in a context of very strong CFC11 and CFC12 growth. Without an illustration of the allocation of forcing by GHG, an innocent reader of this article could easily assume that the doubling was a simple and reasonable way to deal with a minor effect and immaterial to the results. If the assumption is material (as it is), then the treatment and analysis of this assumption seems far too casual.
Third, one wonders how much subsequent controversy might have been avoided if Hansen et al had clearly shown and discussed the allocation between GHG in the clear form shown above. Here is how Hansen et al 1988 Figure 2 showed the results:
In my opinion, this graphic does not clearly show that CFC contribution to Hansen’s greenhouse effect in Scenario A becomes double that of CO2. Aside from the graphic, the running text does not clearly state that CFC greenhouse contributions exceed CO2 contributions during the projection period. Had there been a clearer graphic together with an explicit recognition of CFC contribution, people would have been able to look past Hansen’s unfortunate description of Scenario A as “Business As Usual” in his 1988 testimony and see that it was really an implausible upper bracket scenario, just as Scenario C was an implausible lower bracket scenario, and place no weight on Hansen’s “Business as Usual” label. In passing, Hansen’s 1987 testimony , not previously discussed here, provided the following further information on Hansen’s views on the respective merits of these scenarios:
Scenario A assumes that CO2 emissions will grow 1.5% per year and that CFC emission will grow 1.5% per year. Scenario B assumes constant future emissions. If populations increase, Scenario B requires emissions per capita to decrease. Scenario C has drastic cuts in emissions by the year 2000, with CFC emissions eliminated entirely and other trace gas emissions reduced to a level where they just balance their sinks. These scenarios are designed to cover a very broad range of cases. If I were forced to choose one as the most plausible, I would say Scenario B. My guess is that the world is now probably following a course that will take it somewhere between A and B. (p. 51)
If one is trying to evaluate Hansen’s skill as a forecaster of GHG concentrations, I think that this is probably the most reasonable basis – thus, some sort of weighted average of A and B, with somewhat more weight on B would seem appropriate.
Fourth, one has to distinguish between Hansen’s abilities as a forecaster of future GHG concentrations and the skill of the model, with Hansen himself obviously placing more weight on his role as modeler than as a GHG forecaster. To the extent that “somewhere between A and B” represents Hansen’s GHG forecast, in that GHG increases appear to have been closer to B than “somewhere between A and B”, it is more reasonable to use B to assess the model performance. (It would be more reasonable still for NASA to re-run the 1988 model with observed results.)
Fifth, Hansen argued vehemently that the skill of his results should not be assessed on Scenario A results. Fair enough. The difference between Scenario A and Scenario B points to the need to look carefully at GHG concentration projections in forecasts, which in 2007 are the IPCC SRES projections. The evaluation of the IPCC SRES projections becomes an important and perhaps under-appreciated activity. If one is prepared to agree with Hansen’s position that he should not be assessed on Scenario A (and I, for one, am prepared to agree on this), then it points to the need for caution in using publicizing results from today’s version of Scenario A GHG (e.g. perhaps IPCC A2), a point raised by Chip Knappenburger at RC.
Sixth, none of these calculations deal with feedbacks. So the sort of numbers that result from these calculations are in the range of 1.2 deg C for doubling CO2, depending on the precise radiative-convective model. In this case, the “simplified expressions” are based on the Lacis et al 1981 radiative-convective model and not from GCMs. Similar results are obtained with other radiative-convective models and someone seeking to dispute the results would need to show some systemic form of over-estimation in the radiative-convective models.
Seventh, Hansen et al 1998 , not cited in Schmidt (2007), contains an interesting and reasonable discussion of the 1988 scenarios ten years later and I’ll review this discussion some time in a subsequent post.