January 13, 2020
A ? for my climate friends. The MODTRAN calculator gives the following TOA changes from a doubling of CO2:
200 ppmv -> 400 ppmv = 3.3 W/m2
400 ppmv -> 800 ppmv = 3.3 W/m2
The canonical IPCC change from a doubling is 3.7 W/m2.
Anyone know why?Roy Spencer of the University of Alabama Huntsville replies:
Are you sure it's at TOA? Also, the 3.7 value is after the stratosphere has come into radiative re-adjustment, which I've never really understood. I suppose there is also a dependence on what atmospheric temperature profile (and humidity) you are using.
Willis Eschenbach replies:
It gets more curious. Mosher pointed me to a canonical paper by Myhre et al. I've stuck it in my dropbox, link below. Mosh said that the true number was based on a line-by-line (LBL) model, rather than a narrow-band model (NBM) or a broad-band model (BBM).
But in fact, it's not based on any of them. Instead, they use an "altered" narrow-band model. They say:
"The 'altered NBM' has been adjusted for the effects of
solar absorption by CO2, decay of the gases in the stratosphere, CFC-12 absorption band strength from HITRAN96, based on results from the BBM."
That's the model that gives the 3.7 W/m2 for a doubling.
In addition, they give the following figures for a doubling of CO2 in clear air:
LBL 4.96 W/m2
NBM 5.05 W/m2
BBM 5.08 W/m2
But the MODTRAN model says 3.3 W/m2, as noted above, for clear air ...
Mosh says it's because it is an older version (MODTRAN 3 V1.3), but that doesn't seem like it would give an error of that size.
That's the good thing about "settled science", there are always so many more questions ...
PS—The figures in the paper are for the increase of CO2 from 280 ppmv (presumed pre-industrial)
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