Fig. 4: Results of sensitivity experiments.

a Incoming solar radiation in the experiments of changing obliquity (0°, 23.3°, and 40°). Surface temperatures in the experiments of (b) changing obliquity, (c) changing ocean heat transport (OHT; 100%, 40% and 0% of pre-industrial level), (d) turning off cloud radiative effects, and (e) changing planetary rotation rate (4, 1, and 1/10 of modern rate (Ω_e)). f Air temperatures at 500 hPa in the experiments of changing planetary rotation rate. In each group, the CO₂ concentration is the same except the dashed lines. The black dashed line in (b) is an experiment in which the obliquity is 23.3°, but the CO₂ concentration is increased from 1120 (solid lines) to 2240 ppmv (dashed line). The red dashed lines in (e, f) is for the experiment of the rotation rate being Ω_e/10, but CO₂ concentration is increased from 280 (solid lines) to 1120 ppmv (dashed line). The vertical dotted lines in (e) indicate the corresponding latitudes where the surface temperature difference between the equator and the specific latitude is 8 °C. All the sensitivity experiments are performed using a slab-ocean model; the varying obliquity experiments (a, b) are under an aqua-planet configuration with no lands, and the other experiments are with modern Earth’s land-sea configuration (see “Methods”).