Mechanisms leading to a warmer climate on high obliquity planets
We first find the high obliquity planets to be warmer than their low obliquity equivalents, even when the climate is very warm (1800 W/m2 will blow up the low obliquity experiment), indicating the previously proposed ice-albedo feedback may not be the full story.
Global annual mean surface temperature difference between the high and low obliquity, as insolation gradually increases (solid curve, left axis). Shown on the right axis are the high obliquity (dashed red) and low obliquity (dashed black) mean surface temperature.
Through a series of mechanism-denying experiments, we conclude that the relative warmness stems from the low cloud albedo under high obliquity, which in turn is because the high surface heat capacity makes cloud formation largely lag behind the substellar point migration. As shown below, the high obliquity is warmer than the low obliquity even without ice-albedo feedback, but the temperature difference vanishes or even reverses without cloud radiative effects or without the seasonal variation of insolation.
Annual mean latitudinal surface temperature profile under low obliquity (dark and light blue) and high obliquity (orange and red). Global mean annual mean surface temperature is marked by dashed curves for all cases, and the difference is highlighted by shadings. Shown are for (a) control experiments with all feedbacks on, (b) experiments without ice-albedo feedback, (c) experiments without ice-albedo feedback and without cloud radiation effects, and (d) experiments without ice-albedo feedback and without seasonal cycle (apply annual mean insolation). In (a), there are two equilibrium states for both of the high and low obliquity climate. Dark blue and orange denote the colder equilibrium states, and light blue and red denote the warmer states. In (b,c,d), there is only one equilibrium state, and they are plotted in dark blue and red curves.
Wanying Kang 