This method uses the “Temperature Offset” input to Modtran to bring Modified Condition to  the initial or Standard Condition.

PROCEDURE

Set “Locality” = “1976 Standard Atmosphere” and “No Clouds or Rain” or “Stratus/Strato CU base 0.66 km top 2 km”.

• Use remaining default conditions for Standard run,
• Run Modtran for Standard, record Upward IR Heat Flux (Flux)
• For Modified Condition (1st Adjustment), change “CO2 (ppm)” or other variable to your choice and run Modtran  and record Flux.
• Compute Forcing by subracting Flux of Modified Condition from Flux of Standard Condition.
• Estimate “Offset Temperature” from “0” to (0.223 * Forcing).
• Adjust (by trial & error) the Ground Offset T to closely match the Flux of Standard (2nd Adjustment).  This offset temperature the δT.

NOTE: “Climate Sensitivity” for CO2 is Forcing from 400 ppm CO2 to 800 ppm.  Current (April 2021) CO2 concentration is 420 ppm.