A B C D E F G
old: http://geoflop.uchicago.edu/forecast/docs/Projects/modtran.html
new: http://forecast.uchicago.edu/Projects/modtran_form.html        
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) Temp offset from 299.7K @Iout=287.655 Temp offset from 299.7K @Iout=287.655 Temp offset from 299.7K @Iout=260.714 Temp (K)
   0 no CO2 318.396 -7.47 -12.40 -8.87 250.6
    19.6 half of current warming 302.225 -3.74 -6.12 -4.48 260.4
285 est. pre-industrial 289.131 -0.40 -0.66 -0.46 278.3
300 est. 1900 288.880 -0.34 -0.56 -0.39 278.4
305 est. 1930 288.817 -0.32 -0.53 -0.36 278.4
310 est. 1950 288.723 -0.30 -0.49 -0.34 278.5
315 1958, first Mauna Loa meas. 288.660 -0.28 -0.46 -0.32 278.5
392 12/2011 (Mauna Loa) 287.655   0.00   0.00   0.00 279.1
570 est. doubling vs. pre-indust. 285.928   0.48   0.79   0.54 280.1
Note:  Comparing column E ("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; it is roughly 0.79 / 0.48 = 1.65 (i.e., the water vapor increases warming by about 65%).  However, this does not take into account negative feedbacks from increased evaporation: increased water-cycle cooling, and perhaps increased cloudiness, so that 65% is an upper bound. The real-world amplification of CO2's warming by H2O is almost certainly less than that.
Note #2 (1/5/2014):  The 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; I'll investigate why.

Note #3:  This page is an exported Excel 2003 spreadsheet, which can be loaded directly by Microsoft Excel or LibreOffice Calc.

Contact: Dave Burton