Hard Evidence of CO2 Warming Inflated by Factor 20

The hard scientific evidence of the warming effect of atmospheric CO2 consists of radiation spectra computed by the atmospheric radiation model Modtran, predicting a "radiative forcing" of 3.7 W/m2 from doubling of the concentration of CO2 to 0.06% from preindustrial level of 0.03% (by volume)

To get perspective, let us use the online model of Modtran to compare the present 0.039% of CO2 with a typical value of 2% water vapor (by volume). We get the following OLR spectrum for a 1976 USA standard atmosphere with 1.7 ppm CH4, trop. ozone 28 ppb, strat ozone scale 1:

We see the effect of water vapor as the area between the blue to the red curve for wave numbers smaller than 550 and the effect of CO2 as the comparable area between 550 and 800, thus with an effect of 0.039% of CO2 comparable to that of 2% water vapor. More precisely, Modtran gives the following OLR numbers:

• CO2 = 390, water vapor 2%: 248 W/m2
• CO2 = 0, water vapor 0%: 337 W/m2
• CO2 = 390, water vapor 0%: 304 W/m2
• CO2 = 0, water vapor 2%: 273 W/m2

which shows an effect of 0.039% CO2 which is about half of that of 2% water vapor.

Modtran thus attributes a warming effect of the CO2 molecule which is at least 20 times more powerful than that of water vapor!!

We can now summarize the recent posts on Modtran as follows:

• Modtran is the main hard evidence of the warming effect of CO2.
• Modtran appears to inflate the warming effect of CO2 by factor more than 20.

PS1 In the above example the water vapor concentration was in fact 5% (water vapor scale = 2 in Modtran). Considering the more standard case of water vapor scale = 1 with thus 2.5% water vapor, we get the following numbers

• CO2 = 390, water vapor 2.5%: 259 W/m2
• CO2 = 0, water vapor 0%: 337 W/m2
• CO2 = 390, water vapor 0%: 304 W/m2
• CO2 = 0, water vapor 2%: 286 W/m2

which shows an effect of 0.039% CO2 which is again about half of that of 2.5% water vapor, with the same inflation factor of 20.

PS2 The comparison of 0.04% of CO2 vs 2% water vapor does not take into account the fact that the concentration of water vapor falls off with altitude while that of CO2 does not. If included this reduces water vapor to 0.5% that is about 10 times as much as CO2. The factor 20 then gets replaced by 5, which is still remarkable. Compare with a later post.