I had left the “No Clouds or Rain” choice selected, thinking that the biggest change in CO2 would be in clear-sky conditions.
So I figured “well, perhaps clouds or rain increase the absorption when CO2 doubles” …
So I thought, “well, perhaps I’m looking at the wrong region of the Earth”.
The other latitude bands available in MODTRAN are Midlatitude Summer and Winter, and Subarctic Summer and Winter.
I took nominal CO2 values to represent the CO2 concentration in 1850 (285 ppmv), default (375 ppmv), doubling of 1850 value (570 ppmv) and doubling of the present value (750 ppmv).
I figured that would give me two doublings, and let me see if the increases were linear with the logarithm of the number of doublings.
Guest Post by Willis Eschenbach There’s an online calculator called MODTRAN that calculates the absorption of longwave (“greenhouse”) radiation for various greenhouse gases (“GHGs”), and shows their resulting effect.
It does this on a “line-by-line” basis, meaning it examines each interval of the longwave spectrum for each greenhouse gas at each altitude, and calculates the resulting absorption by each species given the concentration and the partial pressure of that species. The jagged red line at the top right shows what is not absorbed by the atmosphere.
Figure 1 shows the calculation results for the default values of CO2, ozone, and methane. Colored lines in the background are theoretical “no-absorption” curves for various temperatures.
The big “bite” out of the middle section of the jagged red line is mainly water vapor absorption, although it overlaps with the CO2 absorption bands in parts of it.
The graph at the lower right shows the pressure, temperature, and the concentration of H20, Ozone, CO2, and CH4 at various elevations.