The baseline assumption of a ‘fast duration that is 24 days shorter than the summer period with sea ice extent <30%’ (as in SIE bears) is contrasted against a scenario where ‘fasting begins as soon as extent <30%’. a, Demographic impact hindcasts as in Fig. 3; solid black line: baseline scenario, dot-dashed black line: early fast initiation; b, Demographic impact forecasts: as in Fig. 4, but now for the early fast initiation scenario; c, Difference between the baseline and early fast scenarios for the projected crossing of the first (-20%) impact threshold (that is, the difference between panel b & Fig. 4). Magenta: cub recruitment; blue: adult male survival; red: adult female survival. Crosses indicate cases where no impacts are predicted within the modelled timeframe for either scenario; asterisks mark cases where impacts occur with early fasting but not in the baseline, with the bar showing the minimum difference between the two scenarios in these cases. Red background: DIE subpopulations; blue background: CIE subpopulations.