Estimates of Variations in CO2 Radiative Forcing in the Last Century and in the Future

Abstract

Based on the analysis of the radiative transfer equation in the integral form, we investigate the physical causes of changes in the Earth’s outgoing thermal radiation (OTR) during an increase in the atmospheric CO2 content. Increased CO2 content can lead to both an increase and decrease in various components of the OTR depending on the optical density in a spectral interval. We calculated and analyzed the spectra of OTR fluxes in the range 3–30 µm for five climatic atmospheric models and different CO2 content in the atmosphere, from pre-industrial times (280 ppm) to the present day (407 ppm) and for forecasted values of 800 and 1200 ppm. Seasonal and spatial variations in the CO2 radiative forcing (RF) are currently about 20% and 40%, respectively. The 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 CO2 absorption bands, caused by the upward displacement of radiating layers in the troposphere. We estimate the RF to be in the range (0.94–1.62) W/m2 due to the increase in CO2 content from preindustrial to present values.