He walks through how scientists connected the dots between massive meltwater pulses and the return of arctic conditions in Northern Europe. Around 14,600 years ago, a sudden influx of ice-cold meltwater from the Arctic interrupted the ocean conveyor that normally delivers heat to the North Atlantic. When that circulation stalled, temperatures plummeted, and Dryas octopetala, a cold-adapted plant, returned to regions it had previously abandoned. The climate didn't stay frozen, though. Milankovitch cycles, the slow orbital shifts that drive long-term warming and cooling, eventually overpowered the meltwater effect. As warming resumed, Dryas migrated back north, following the cold. It's a textbook example of how abrupt climate shifts can temporarily override longer astronomical cycles, and how the geologic record preserves both signals if you know where to look.