Using HITRAN 2000, the updated absorption line data of atmospheric molecules, and an exact line-by-line algorithm, we have calculated the radiative forcing (in turn the greenhouse effect) due to the changes in concentration of atmospheric carbon dioxide. Based on the comparison among radiative forcing resulted from various concentration of atmospheric CO₂, the saturation of greenhouse effect due to carbon dioxide and the spectral distribution of the saturation have been examined in the paper. The content and major conclusions of the paper are the follows:
(1) Six model atmospheres: Tropical Atmosphere (TRP), Middle Latitude Summer Atmosphere (MLS), Middle Latitude Winter Atmosphere (MLW), Subarctic Summer Atmosphere (SAS), Subarctic Winter Atmosphere (SAW), and US Standard Atmosphere (USSTD), are chosen to study the effects of surface temperature and temperature profile on the radiative forcing. The results show that, in general, the radiative forcing is larger when the surface temperature is higher. However, the radiative forcing depends not only on the surface temperature, but also the atmospheric temperature profile.
(2) The radiative forcing due to carbon dioxide includes both of the contributions from the short-wave and long-wave. The solar zenith angle, duration of daylight and the solar irradiance on the top of the atmosphere are calculated in order to compute the short-wave radiative forcing for each model atmosphere. Compared with the long-wave radiative forcing, the solar short-wave radiative forcing is very small.
(3) Effects of overlapping of the absorption bands on the radiative forcing have been examined for different model atmosphere. We found that the band overlapping has significant effect on the radiative forcing and it depends on the model atmosphere. The effect is the largest for the TRP atmosphere and smallest for the SAW atmosphere. 
(4) The saturation of greenhouse effect due to carbon dioxide and its spectral distribution are studied through calculating respectively the radiative forcing due to two times, four times and eight times of present concentration of atmospheric carbon dioxide. The results show that the greenhouse effect due to carbon dioxide is saturated in the center of CO₂ 15μm band, indeed, but it is never saturated and will not be saturated in the near future for the wings of 15μm band and the other bands.