Question

How could a decrease in seasonality (i.e warmer and cooler summers) lead to an advance of glacial ice on continents?

Answer 1

Warmer winters and cooler summers will lead to an advancement of glacial ice over the continents.

Rise in winter temperature will lead to non formation of snow and also due to increased temperature snow will melt but at a much lesser rate. On the other hands if the summer temperature decreases the amount of snow melting will decrease resulting it its accumulation. So, combined this will lead to greater accumulation of snow and in the advance of glacial ice.