The unknowns of the radiative forcing of doubling CO2

Although definite numbers, e.g., 4 W m-2 or so in the IPCC reports, are commonly given for the radiative forcing (RF) of every doubling of atmospheric CO2 concentration, there remains much unknown and uncertainty about the quantification. For instance, why the CO2 RF scales logarithmically with its concentration has yet to be fully explained. Conventional wisdom resorts to the spectrally averaged atmospheric absorptivity in relation to the spectroscopic line features (line strength and shape). We show, however, such logarithmic dependency cannot be accounted for by spectroscopy alone but concerns radiative transfer (the integral nature of infrared radiative transfer solution) as well. On the other hand, the value of CO2 forcing depends on other atmospheric and surface conditions such as temperature, cloud distribution and water vapor concentration. In the scenario experiments, even when the CO2 concentrations are identically prescribed in all the climate models, the value of radiative forcing (measured by W m-2) may still vary among the models. It is important to recognize this forcing uncertainty when analyzing the feedback and climate sensitivity in these models. Generally, the non-uniform distribution of the CO2 forcing increases the latitudinal gradient of the top-of-the-atmosphere net radiation and thus accounts for the increase in poleward energy transport in a warmer climate driven by CO2 increase.