How did ice age climates contribute to the formation of modern permafrost?

Permafrost is thought to have first emerged during the late Pliocene Epoch approximately three million years ago, coinciding with the onset of glacial conditions. During the subsequent Pleistocene Epoch, the Earth system cycled between glacial and interglacial regimes, with most existing subarctic permafrost forming during the cold glacial periods of the past 100,000 years.12

During these glacial epochs, particularly at the height of glaciation about 20,000 years ago, polar climates exposed vast areas of the continental shelf to freezing temperatures for thousands of years, allowing permafrost to form to depths exceeding 700 metres. In regions surrounding ice sheets, mean annual temperatures were significantly lower—often 12 to 20 °C colder than today—creating extensive periglacial zones where permafrost developed. These environments were characterized by intense freeze-thaw activity, which, through the expansion of freezing water, acted as a powerful agent of physical weathering and shaped the landscape. While much of this permafrost has since degraded or melted due to subsequent warming, it remains a defining feature of high-latitude geology.134