In the various disputes over Hansen et al 1988, Roger Pielke Jr and NASA apologist Eli Rabett (who has been said to be occasional NASA contractor Josh Halpern) have each attempted to disentangle the forcing projections implied by Hansen et al 1988 – Pielke here and Rabett here for CO2 and here for other gases.
Commenters on the previous thread have naturally puzzled over the actual differences in GHG projections between Scenario A, B and C and, in an effort to clarify matters, I’ve attempted to calculate GHG concentrations through to 2030 implementing the assumptions described in Hansen et al 1988. (Roger Jr posted up a chart similar to the ones below for CO2). In another post, I’ll apply these results to calculate radiative forcings. I’ve saved my collations of Hansen projected GHG concentrations for the three scenarios at:
The collation of these tables is shown in the script http://data.climateaudit.org/scripts/hansen/collation.hansen_ghg.txt. I’ll review the results some more when I get to radiative forcings, but the differences between major GHG concentrations in Scenario A and B are very slight and it’s a little puzzling how the differences arise between the two scenarios. I’ll look at each GHG contribution below.
One idiosyncrasy that you have to watch in Hansen’s descriptions is that he typically talks about growth rates for the increment , rather than growth rates expressed in terms of the quantity. Thus a 1.5% growth rate in the CO2 increment yields a much lower growth rate than a 1.5% growth rate (as an unwary reader might interpret). For CO2, he uses Keeling values to 1981, then:
A: 1.5% growth of annual increment after 1981. Figure B2 shows 15.6 ppmv in 1980s and opening increment set at 1.56 ppmv accordingly.
B: 1.5% increment growth rate to 1990;1% to 2000, 0.5% to 2010 and 0 after 2010. Thus constant 1.9 ppmv increase after 2010.
C: equals A, B through 1985, then 1.5 ppmv increase through 2000; then fixed at 368
Using my procedures, my scenario C leveled off at 369.5, so up to microscopic tuning, I’m confident that I got his procedures correct here (and for similar interpretations below.) The figure below compares CO2 concentrations in the three scenarios to observed concentrations. While Scenario A is “exponential” and Scenario B is “linear” (and NASA spokesman Gavin Schmidt places great weight on this distinction as an argument in favor of B), in the period to the present, there is actually negligible difference between the two versions – so the explanation of the difference between Scenario A and B temperature projections to 2007 does not lie with the difference between exponential and linear CO2 histories, contrary to the impression given by NASA spokesman Gavin Schmidt in his “spare time”. Observed values are already over Scenario C values. Policy measures to date appear to have had negligible impact on CO2 concentration increases – the Hansen et al 1988 CO2 projections were quite reasonable, other than a very slight displacement caused in the early 1990s by economic turmoil in Russia, and any discrepancy between model results and observed values cannot be plausibly attributed to CO2 concentration increases being a lot slower than anticipated.
The methane concentration projections presents a very different picture – something that Pat Michaels noted as long ago as his 1998 debate with Hansen. The rate of methane concentration increase declined sharply almost immediately after Hansen’s 1988 presentation and have been stable in the 21st century (NASA data shows 2006 values at the same level as 1999 values.) The leveling off has occurred below even Scenario C values. This has occasioned considerable puzzlement in the specialist community (noted in AR4). As an editorial comment, it seems to me that some people who are unsatisfied with explanations of carbon balance (i.e. certain “skeptics”) might divert some of their energy into mulling over CH4 concentrations and balances.
Once again, there is negligible difference between Scenarios A and B in terms of CH4 impact – so the explanation of lower Scenario B results has to lie elsewhere.
There’s another interesting issue in terms of methane. The figure below shows the A1B (solid) and A2 (dashed) scenarios for methane used by IPCC. While the increases are less than Hansen Scenarios A and B, Scenario A2 in particular shows a very strong continued increase in methane levels, and, even A1B shows continued increases through mid-century. Perhaps the recent stasis is itself an anomaly and methane concentration will resume its earlier increase, but the matter is puzzling.
AR4 doesn’t talk much about methane. The chapter 2 summary says (tautologically?):
Methane concentrations are not currently increasing in the atmosphere because growth rates decreased over the last two decades.
Later they say:
The reasons for the decrease in the atmospheric CH4 growth rate and the implications for future changes in its atmospheric burden are not understood (Prather et al., 2001) but are clearly related to changes in the imbalance between CH4 sources and sinks.
So while lower-than-expected methane increases might well account for one of the lower radiative forcing projections being more appropriate to evaluate Hansen’s model, this is a bit of a two-edged sword: given that substantial methane increases are included in IPCC projections without, according to IPCC, any understanding of the leveling off of methane concentrations, this indicates a substantial uncertainty in canonical IPCC models, an uncertainty that, to the extent that it is disclosed at all, is buried deep in the fine print.
The figure below compares observed N2O to Hansen A and B projections – Hansen doesn’t mention how C projections are done – together with IPCC A1B and A2 projections. Again there is negligible difference between Hansen Scenarios A and B up to 2007 – only a ppbv or so. Total radiative forcing from N2O is said in AR4 (Table 2.1) to be only 0.16 wm-2: so we’re getting into very slight impacts with the small changes here.
Again, the figure below makes the same comparison for CF12, the radiative forcing of which in 2005 was said by IPCC AR4 to be 0.17 wm-2, about the same as N2O. Again there is no material difference between Hansen Scenarios A and B through 2010. As with methane, CFC12 concentrations have leveled off slightly below Scenario C levels. IPCC A1B and A2 scenarios appear to be identical, showing a gradual decline through the 21st century. Update: Since I wrote this, I became aware of Hansen data archived, oddly enough at realclimate, which shows that CFC11 and CFC12 concentrations were doubled in Scenario A as a means of modeling Other CFCs and Traces Gases and this accounts for the main near time difference between Scenarios A and B.
CFC11 is said in AR4 to have about 1/3 the forcing of CFC12. As shown below, it has very similar features to CFC11 – negligible difference between Scenarios A and B; actual growth less than Scenario C; projected decline in concentration through the 21st century. Update: Since I wrote this, I became aware of Hansen data archived, oddly enough at realclimate, which shows that CFC11 and CFC12 concentrations were doubled in Scenario A as a means of modeling Other CFCs and Traces Gases and this accounts for the main near time difference between Scenarios A and B.
Other Trace Gases
AR4 Table 2.1 shows other CFCs as having 2005 radiative forcing of only 0.023 wm-2 (about 10% of the two main CFCs) with the total of all other Montreal Protocol gases, on a cursory inspection, looking to have about the same negligible impact as other CFCs. In Scenario A, the “other” trace gases are accounted for by doubling the impact of CFC11 and CFC12, while in Scenarios B and C, they are held to have no incremental impact. So there is a difference between Scenarios A and B here, but it doesn’t look like it should have much impact. Update: Notwithstanding the low forcings in AR4, Hansen et al 1988 accounted for these other Trace Gases by doubling CFC11 and CFC12 forcings, which themselves were estimates. This accounts for the main near time difference between Scenarios A and B.
Figure B2 of Appendix B of Hansen et al 1988 mentions “speculative” forcings, which, in addition to other CFCs, includes ozone and stratospheric water vapor. These “speculative” forcings occur in Scenario A, but are not included in Scenario B. Hansen says:
Figure B2 summarizes the estimated decadal increments to global greenhouse forcing. The forcings shown by dotted lines in Figure B2 are speculative; their effect was included in scenario A but was excluded in scenarios B and C.
The size of these “speculative” forcings is surprisingly large – being not dissimilar in wm-2 to methane or to observed CO2 increase, for that matter.
The final difference between Scenario A and Scenario B is that Scenario B assumed a major volcano in 1995 (and an actual major volcano, Pinatubo, occurred in 1992.) My understanding was that volcanic forcing was transitory and that such forcing could introduce a change over a year or two, but it would wear off, leaving things more or less as though it had never happened.
Differences between Scenarios A and B
As noted above, in my next post, I’ll consider differences in radiative forcing between these scenarios. As a preview, here is Figure 2 from Hansen et al 1988, which shows radiative forcing for CO2 and the total of CO2 and trace gases. As you can see, most of the difference between Scenario A and Scenario B arises from gases other than CO2. Also Hansen Figure 2 shows that a noticeable difference between Scenarios A and B had already arisen by 1987 – something that will need to be examined closely to see exactly which gases are contributing to it, as well as to the increasing differences between the two scenarios for gases other than CO2 by 2010. Right now, based on the review of GHG concentrations, it’s hard to see exactly what is accounting for the difference in radiative forcing. Update: As noted in a subsequent post, the handling of Other CFCs and Other Trace Gases accounts for the near time difference.