Westerly and Laurentide ice sheet fluctuations during the last glacial maximum

The last glacial maximum (LGM) is widely acknowledged as the most recent cold period representing maximum global ice conditions. However, substantial warming is observed over Northern Hemisphere. Here, we show that the LGM climate shifted from very cold to fairly warm, followed by less cold conditions in the early Heinrich Stadial 1 (HS1) phases. Our synthesis of accurate AMS ¹⁴C dates refines the exact timing of Laurentide Ice Sheet (LIS) advances during the early LGM/HS1, constraining the chronology of the LIS decay during the late LGM. The summertime soil temperatures near ice fronts were found to increase by 1.3 °C from the early to late LGM and to decrease by 0.5 °C to the early HS1 phases, consistent with the cold-warm-cool climate patterns. The early/late LGM and early HS1 climates are found to be characterized by frequent cold/warm summers and cold winters since the world’s largest LIS began to decay.