A model, based on modern spectroscopic data, for calculation of radiative fluxes in the Earth’s cloudless atmosphere is presented. This model ensures high speed and accuracy of the simulation. Mass calculations are performed and impact of total atmospheric water vapor content on the radiative forcing of carbon dioxide and methane due to the increase in CO2 and CH4 concentrations over the past 50 years is estimated for mid-latitudes summer conditions (using the Lower Volga region as an example). It is shown that the CO2 radiative forcing is an order of magnitude greater than the CH4 radiative forcing. It is found that at the upper and lower boundaries of the atmosphere, the absolute value of the CO2 radiative forcing decreases when the water vapor content increases in the atmosphere. The relation between the CH4 radiative forcing in the atmosphere and the total column water vapor is statistically less significant than for the CO2 radiative forcing due to the fact that the intensity of the vibrational-rotational bands of CH4 is small in comparison with the intensities of the absorption bands of CO2, as well as the weaker continual absorption of water vapor overlapping with the CH4 spectra.