The phrase
"less insolation" means that there is a decrease in the amount of solar energy or sunlight received in an area.
Full definition
When there was
less insolation during the summer months, the average temperature would be slightly lower and some of the ice in this region could survive and build up — eventually producing an ice sheet.
Implication 2) The many weeks where the SP has
less insolation than PHX has a significant effect on temperature differences between the two places.