A	B	C	D	E	F	G	H	I
		old: http://geoflop.uchicago.edu/forecast/docs/Projects/modtran.html new: http://forecast.uchicago.edu/Projects/modtran_form.html           even newer: http://climatemodels.uchicago.edu/modtran                     alternate: http://lorelei.uchicago.edu/modtran/						old: http://geoflop.uchicago.edu/forecast/docs/Projects/full_spectrum.html new: http://forecast.uchicago.edu/Projects/full_spectrum.html
		MODTRAN tropical atmosphere						NCAR radiation code
		clear sky, const H2O pressure		clear sky, const rel humidity		cumulus 0.66km-2.7km, const rel humidity		insolation 500, const rel hum 70%, CH4 1.8 ppm, N2O 0.3 ppm, low cloud 30%, high cloud 25%
CO2 (ppm)	description	Iout at 299.7 K (ground level) (new=old)	Temp offset from 299.7K @Iout=287.655 (new=old)	Temp offset from 299.7K @Iout=287.655 (old)	Temp offset from 299.7K @Iout=287.655 (new)	Temp offset from 299.7K @Iout=260.714 (old)	Temp offset from 299.7K @Iout=260.714 (new)	Temp (K)
0	no CO2	318.396	-7.47	-12.40	-8.30	-8.87	-7.32	250.6
19.6	half of current warming	302.225	-3.74	-6.12	-4.12	-4.48	-3.66	260.4
285	est. pre-industrial	289.131	-0.40	-0.66	-0.44	-0.46	-0.37	278.3
300	est. 1900	288.880	-0.34	-0.56	-0.37	-0.39	-0.31	278.4
305	est. 1930	288.817	-0.32	-0.53	-0.35	-0.36	-0.30	278.4
310	est. 1950	288.723	-0.30	-0.49	-0.32	-0.34	-0.28	278.5
315	1958, first Mauna Loa meas.	288.660	-0.28	-0.46	-0.30	-0.32	-0.26	278.5
392	12/2011 (Mauna Loa)	287.655	0.00	0.00	0.00	0.00	0.00	279.1
570	est. doubling vs. pre-indust.	285.928	0.48	0.79	0.52	0.54	0.44	280.1
Note #1:  The "old" MODTRAN calculations were done with the University of Chicago's web-based MODTRAN interface at http://geoflop.uchicago.edu/forecast/docs/Projects/modtran.html.  That link no longer works; the page appears to have moved to http://forecast.uchicago.edu/Projects/modtran_form.html, which strangely generates different results (shown as "new"). I don't know why they are different. Columns E and G were calculated with the old page; columns F and H were calculated with the new page. UPDATE: They now have an even newer version at http://climatemodels.uchicago.edu/modtran but I've not yet repeated the calculations using that version. Beware of an ideosyncracy of that user interface: at first glance there doesn't appear to be any way to select between constant water vapor concentration and constant relative humidity. The option is still there, it's just hidden. It's the "Holding fixed" option, which only appears if you set the Temperature offset to non-zero.
Note #2:  Comparing column E or F ("constant relative humidity") to column D ("constant H2O pressure") illustrates the magnitude of the "water vapor feedback." The "constant relative humidity" numbers include the water vapor feedback, the "constant H2O pressure" numbers do not. The feedback works like this: additional CO2 (a greenouse gas) blocks the exit of some additional IR, which warms the earth's surface and near-surface air. The warmer air holds additional water vapor, which is, itself, a greenhouse gas, thus amplifying the warming from the CO2. The magnitude of the amplification can be seen by comparing the temperature differences in columns D and E/F. If column E is correct, then the amplification is roughly 0.79 / 0.48 = 1.65, i.e., the water vapor increases warming by about 65% (not taking into account negative feedbacks from increased evaporation: increased water-cycle cooling, and perhaps increased cloudiness). But if column F is correct, then the amplification is only 0.52 / 0.48 = 1.08, or 0.96 / 0.88 = 1.09, i.e., the water vapor increases warming by only about 8% to 9%.
Note #3:  This page is an exported Excel 2003 spreadsheet, which can be loaded directly by Microsoft Excel or LibreOffice Calc.
Contact: Dave Burton