Estimates of variations in CO<sub>2</sub> radiative forcing in the last century and in future
Based on the analysis of the radiative transfer equation in the integral form, we investigate the physical causes of changes in the Earth outgoing thermal radiation (OTR) due to increasing atmospheric CO<sub>2</sub> content. CO<sub>2</sub> concentration growth can lead to both increase and decrease in various components of the OTR depending on the optical density of a spectral interval. We calculate and analyze the spectra of OTR fluxes in the range 3-30 µm for five climate atmospheric models and various concentrations of carbon dioxide from pre-industrial (280 ppm) to present (407 ppm) and projected in the future values (800 and 1200 ppm). Seasonal and spatial variations in CO<sub>2</sub> radiative forcing (RF) currently reach about 20% and 40%, respectively. Comparison of OTR measurements with different instruments in different years (for example, SI-1 in 1977 and 1979 and IKFS-2 in 2015-2017) demonstrates a decrease in the OTR values in the CO<sub>2</sub> absorption bands caused by the upward displacement of radiating layers in the troposphere. We estimate the RF due to the growth of CO<sub>2</sub> concentrations from pre-industrial to present values to be in the range -(0.94-1.62) W/m<sup>2</sup>.