How to Use Method B

This method uses the Temperature Offset input to Modtran to either:

Bring Baseline Condition to equal Modified Condition


Bring Modified Condition to  the Baseline Condition.


  • Locality = 1976 Standard Atmosphere with Standard Cirrus Model for cloud cover.
  • Use default conditions for Baseline run.
  • Run Modtran for Baseline, record Upward IR Heat Flux (Flux)
  • For Modified Condition (1st Adjustment), change “CO2 (ppm)” to your choice and run Modtran for Change in CO2 ppm
  • Adjust (by trial & error) the Ground Offset T to match the Flux of Baseline (2nd Adjustment).  This yields the 1st δT.
  • Using Offset T from 2nd Adjustment, compute ratio of increased H2O vapor pressure (δP/P) to Baseline using the formula:
    ( δP/P = 0.064 * δT, for ground temps around 15C )
    See H2O feedback calculation  for development of this equation.
  • Run Modtran for 2nd Adjustment’s CO2 ppm level and change Water Vapor Scale to 1 + δP/P

The following are examples of the Modtran Panels for Baseline, Modified and Final Conditions (Baseline H ≈ Modified H):

Baseline & 800 ppm no adjustment

800 ppm 2nd & 3rd Adjustments

Note that 2nd Adjustment ground temperature is 289.03K, which is 0.83C higher than 400 ppm baseline case (288.2K).  Also 3rd Adjustment ground temperature is 289.23K which is 0.20 C higher than 2nd Adjustment ground temperature, this being the result of H2O vapor feedback.  Thus the temperature increase for doubling 400 ppm CO2, including H2O vapor feedback is stated in the Ground Offset T for the 3rd Adjustment: 1.03C