Why does a rise in ambient temperature cause a base-voltage-biased grounded emitter stage to saturate?
3) fixed Vbe    and the collector current Ic increases by a factor of 10 for a 30∘C rise in temperature.

If 10x Ic occurs from 30'C rise so 8'C rise implies 8'C/30'C *10= 2.667x rise in current. 

Given  Vc=Vcc/2  Re=0 thus  saturation is twice the current at Vce=0 , 8'C is sufficient.