Evolution of (a) effective temperature T_eff and (b) temperature difference across TBL ΔT, luminosity at the outer edge of the TBL l₁, and planet surface luminosity L with time for a Uranus-sized (red) and a Neptune-sized (blue) ice giant with a thin conductive interface of Δm = 0.0015 M_E. Dashed lines in (a) are adiabatic models of the same composition and dotted lines give the equilibrium temperatures (see text for discussion).