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. (planet3.org)
Of course they have no data that actually shows less insolation. (jennifermarohasy.com)
Implication 2) The many weeks where the SP has less insolation than PHX has a significant effect on temperature differences between the two places. (wattsupwiththat.com)