The Earth surface temperature is set by the lapse rate (decrease of temperature with height) since the effective top of the atmosphere TOA temperature is determined to -18 C. The observed lapse rate is 6.5 C/km corresponding a TOA at a height of 5 km and an Earth surface temperature of 15 C with a temperature drop of 33 = 5 x 6.5 C.
The lapse rate of an atmosphere in equilibrium without convective motion and phase change (evaporation/condensation) may range from 0 (isothermal) to 9.8 C/km (adiabatic). In such an
atmosphere (without also radiation) there would be no heat transport from the Earth surface to TOA.
Suppose we now view the observed lapse rate of 6.5 C/km as being obtained from the adiabatic rate 9.8 by adding effects of convection/phase change transporting an observed 180 W/m2 from the Earth surface to TOA (with 60 out a total of observed 240 transported by radiation).
We would then view the reduction of the lapse rate from 9.8 to 6.5 as an effect of the heat transport from the Earth surface to TOA by convection/phase change: Increased heat transport coupled to increased convection/phase change, would then correspond to a further reduction of the lapse rate and thus correspond to global cooling.
Now, increased CO2 would require more heat to be transported by convection/phase change (under constant insolation of 240 W/m2), which with the above argument could cause global cooling.
We have thus presented a simplistic argument suggesting that climate sensitivity may very well be negative: more CO2 could cause global cooling. Is this argument correct? Maybe. At least it appears to be as plausible as any other simplistic argument floating around suggesting a climate sensitivity in the range 0 - 5 C. Maybe even more plausible, if the lapse rate is determined by thermodynamics rather than radiation.