Finally, the 4 W m − 2 CO2 forcing avoids the uncertainty in the
exact magnitude of a doubled CO2 forcing [1,48] estimate
of 3.7 W m − 2 for doubled CO2, whereas Hansen et al. [66] obtain 4.1 W m − 2, as well as problems associated with the fact that a doubled CO2 forcing varies as the CO2 amount changes (the assumption that each CO2 doubling has the same forcing is meant to approximate the
effect of CO2 absorption line saturation, but actually the forcing per doubling increases as CO2 increases [66,85]-RRB-.