Why is permafrost especially deep in some unglaciated parts of Siberia?

Permafrost in parts of Siberia reaches exceptional depths—in some cases exceeding 2,000 feet (and up to 1,450 meters or 4,760 feet in specific reports)—primarily because these regions remained unglaciated during the extreme cold of the ice ages.12

While permafrost thickness is generally controlled by the thermal balance between geothermal heat from Earth’s interior and the cold atmosphere, the historical climate of a region plays a critical role in its current state. In areas that were not covered by glaciers, the ground was exposed to the intense, prolonged cold of glacial epochs for thousands of years, allowing the permafrost to penetrate to much greater depths than in areas where insulating ice sheets were present.314

Consequently, much of the deep permafrost observed today is considered a relict feature—a product of a much colder former climate that has not yet reached equilibrium with modern surface temperatures. In addition to this historical context, local variations in thickness are influenced by the geothermal gradient and the thermal conductivity of the underlying rock and soil.435