Concerning the discussion, which was lately going on (a bit misplaced) in this thread:

I sent now an email to Gunnar Myrhe, Frode Stordal and Atul Jain asking about the possibly not included UV spectral data in the calculation of the radiative forcing of methane and its possible imact. Here is my email (second part in the next section)



Dear people

By looking at various climate phenomena I got the impression that the global waring potential of methane might eventually be vastly higher then as the current estimations seem to indicate. With vastly I currently imagine something at least a factor 10, which would put methane on a similar if not higher level as a cause for global warming as CO2.

Although this is still rather speculative, I was looking for possible components which could lead to such different estimations. I got the impression that a possible cause could eventually be the radiative forcing due to higher energy spectral lines in the methane absorption spectrum. I basically don't know anything about the radiative forcing models (narrow bad broadband models etc.) but I tried to figure out, which spectral data entered which models and got the impression that e.g. UV data is not taken into account. Here a brief summary, why I got this impresssion:

In the supplement to AR5 https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/supplementary/WG1AR5_Ch08SM_FINAL.pdf page 7 sectin 8.SM.3 it is written: "The formulae used to calculate the radiative forcings (RFs) from carbon dioxide (CO2), CH4 and nitrous oxide (N2O) are taken from Myhre et al. (1998) Table 3 as in Third Assessment Report (TAR) and Fourth Assessment Report (AR4). They are listed here for convenience."

That formula for CH4 is listed in table 8.SM.1. It depends onvalues M, M_0, N, N_0 measured in ppm. Where its not said what these quantities are, but luckily there is a copy of the paper Myhre, G., E. J. Highwood, K. P. Shine, and F. Stordal, 1998: New estimates of radiative forcing due to well mixed greenhouse gases. Geophys. Res. Lett., 25, 2715–2718 on Gunnar Myhre's (probably old) postdoc homepage, that is the article is at the URL:

http://folk.uio.no/gunnarmy/paper/myhre_grl98.pdf

As a side remark: There is a typo in the formula under Notes that is in the formula for f(M,N) it should be (MN)^(0.75) and not as written there: (MN)0.75


In the article Myrrhe et al 1998 it is written (modulo typing errors, pasting doesn't work):

>"The LBL model is used to calculate optical depth (Edwards1992) and radiative fluxes are calculated as in the work of Myrhe and Stordal (1997). The NBM is the 10 cm^(-1) narrow band radiative transfer scheme of Shine (1991). In this study, the scheme is used with spectral band data from HITRAN-1996 except for CFC 11 which uses the average crosssection from Christides et al [1997] and CFC-12 which is from HITRAN-92. The BBM includes about 50 gases about all the trace gases of importance for modelling of the terrestial radiation. This model is also described in Myrhe and Stordal [1997]. The LBL model and the BBM use spectrocopic data for the halocarbons from the HITRAN96 database and for the other WMGG as in Myrhe and Stordal (1997)."


Table 2 lists for CH4 the NBM and the NBM altered model and the BBM model. I don't know though wether this listing is exhaustive. That is wether this list reflects all models used for determining the methane radiative forcing. Let's assume this is so.


So then according to table 2 and the above paragraph in [Myrhe et al 1998] the data which enters the determination of the radiative forcing of methane is for the NBM scheme the data from HITRAN-96 and for the BBM the data about WMGG (well mixed greenhouse gases) are in Myrhe and Stordal [1997] at

http://folk.uio.no/gunnarmy/paper/myhre_jgr97.pdf

In Myrhe and Stordal (1997) the Broadband model (BBM) which seems to be meant, seems to be the Oslo Broadband Infrared Model (OBIR) it uses an "emissivity formulation based on Ramanathan (1976) for calculating radiative fluxes in the entire terrestial infrared spectral region". According to section 2.1. it seems to use as spectral data for CH4:

>"For CH4 and N2O an approach based on Donner and Ramanathan (1980) is taken."

L. Donner and V. Ramanathan, 1980: Methane and Nitrous Oxide: Their Effects on the Terrestrial Climate. J. Atmos. Sci., 37, 119–124. is here:
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281980%29037%3C0119%3AMANOTE%3E2.0.CO%3B2


There it is written that on p.119 (modulo typing errors, again this can't be pasted):

> "The present analysis includes the 1306 cm^-1 band of CH4 and the 589 and 1285 cm^-1 bands of N2O. Both these species have several other bands between 2000 and 6000 cm^-1 (Cess and Chen 1975; Kunitomo et al 1977) whose contributions to terrestial climate are negligible."

This sounds as if for the BBM model the used spectral methane data was the 1306 cm^-1 band, finally other bands which could have been included are not mentioned. ?


On page 3 in Table 2 at https://www.cfa.harvard.edu/hitran/Download/HITRAN96.pdf I found for the spectral range of HITRAN96 the range:

>0-6185 cm^-1.


Concluding for the calculations of the radiative forcing of methane which entered AR5 it seems only spectral data in the range from 0-6185 cm^-1 was taken into account. Is that correct?


I was thus looking for research which might include higher frequency bands but so far found only this overview, which has somewhat similar results, but also
doesn't seem to be taking UV spectral data into account, but eventually infrared bands: