ΔT is the raising of earth’s average surface temperature.  Two methods are used: statistical analysis of historical data and the Modtran computer program.

Statistical Analysis of Historical Data
For temperatures , only the data from the UAH Satellite program is used, which began in 1979.  Atmospheric CO2 is from the NOAA’s Moana Loa (Hawaii) observatory.

ModTran Computer Program
This computation is performed using the Modtran computer program by first computing the initial upward Infrared Radiative Flux (Flux) for the initial Standard or Current Conditions and then computing Flux for the conditions in question by modifying the input parameters of Modtran.  There are two subsections to this procedure: first is instructions on using Modtran, second is instructions to compute Flux change (ΔF) between the initial conditions and the modified conditions.

SEE: Using MODTRAN