This study investigates the distinct climate factors that can switch the surface mass balance of the Laurentide Ice Sheet from positive (growth phase) to negative (decay phase). Given two stages of the glacial cycle with the same ice volume, what causes one ice volume to grow and the other to decay? We analyze the relative roles of insolation forcing, North Atlantic sea-ice extent, atmospheric circulation changes, and temperature–precipitation covariability in shaping LIS evolution. By contrasting these two regimes, we aim to identify the conditions under which the LIS is most sensitive to external forcing versus internal climate variability, providing constraints on ice–climate feedbacks relevant to past and future ice-sheet evolution. 