Abstract Most research on the state dependence of climate sensitivity has focused on radiative feedbacks, with less attention given to radiative forcing. However, recent studies show that the carbon dioxide (CO 2 ) radiative forcing depends not only on the CO 2 concentration but also on the base state, particularly the stratospheric temperature profile. Hence, we here carry out atmosphere-only experiments with prescribed sea surface temperatures using Community Earth System Model, version 1, Large Ensemble (CESM1-LE), broadband radiative transfer calculations, and a one-dimensional radiative–convective equilibrium model to thoroughly investigate the dependence of effective radiative forcing (ERF) on varying levels of CO 2 forcing and base-state stratospheric temperatures from 1/16× to 16×CO 2 . Using both the CESM1-LE and a radiative–convective equilibrium model, we demonstrate that ERF strongly depends on the CO 2 value of the underlying base state, deviating significantly from a simple logarithmic relationship with CO 2 concentration. Specifically, doubling CO 2 from a base state of 8×CO 2 results in an ERF value that is 50% higher than doubling CO 2 from a 1/16×CO 2 state. By decomposing ERF into instantaneous radiative forcing (IRF) and radiative adjustments, we show that the IRF is largely responsible for the state dependence of ERF. We attribute the increase in IRF with CO 2 concentrations to the stratospheric cooling at 10 hPa. Furthermore, we find that the radiative a

Radiative forcing (or climate forcing) is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various Radiative forcing (or climate forcing) is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of greenhouse gases and aerosols, and changes in surface albedo and solar irradiance. In more technical terms, it is defined as "the change in the net, downward minus upward The atmospheric burden of greenhouse gases due to human activity has grown especially rapidly during the last several decades (since about year 1950). For carbon dioxide, the 50% increase (C/C0 = 1.5) realized as of year 2020 since 1750 corresponds to a cumulative radiative forcing change (delta F) of +2.17 W/m2. Assuming no change in the emissions growth path, a doubling of concentrations (C/C0 = 2) within the next several decades would correspond to a cumulative radiative forcing change (delta F) of +3.71 W/m2. The relationship between CO2 and radiative forcing is logarithmic at concentrations up to around eight times the current value. Constant concentration increases thus have a progressively smaller warming effect. However, the first-order approximation is inaccurate at higher concentrations and there is no saturation in the absorption of infrared radiation by CO2. Various mechanism behind the logarithmic scaling has been proposed but the spectrum distribution of the carbon dioxide seems to be essential, particularly a broadening in the relevant 15-μm band coming from a Fermi resonance present in the molecule. The IPCC summarized the current scientific consensus about radiative forcing changes as follows: "Human-caused radiative forcing of 2.72 [1.96 to 3.48] W/m2 in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GHG concentrations, partly reduced by cooling due to increased aerosol concentrations". Radiative forcing can be a useful way to compare the growing warming influence of different anthropogenic greenhouse gases over time. The radiative forcing of long-lived and well-mixed greenhouse gases have been increasing in earth's atmosphere since the industrial revolution. The table includes the direct forcing contributions from carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O); chlorofluorocarbons (CFCs) 12 and 11; and fifteen other halogenated gases. These data do not include the significant forcing contributions from shorter-lived and less-well-mixed gases or aerosols; including those indirect forcings from the decay of methane and some halogens. They also do not account for changes in land use or… T…