Why does permafrost take thousands of years to reach thermal equilibrium?

Permafrost takes thousands of years to reach thermal equilibrium because it is a massive, deep-seated feature whose thickness is governed by a delicate thermal balance between two opposing forces: the cold temperatures of the atmosphere and the heat flowing upward from Earth’s interior.1

As permafrost grows downward, it must reach a state where the average amount of geothermal heat reaching the base of the frozen layer is equal to the heat lost to the atmosphere. Because this process involves the gradual penetration of cold through hundreds of feet of ground—and because the rate of this change is influenced by factors such as ground composition, ice content, and the geothermal gradient—the adjustment to a stable, equilibrium thickness is a slow, long-term process. Consequently, permafrost often reflects the conditions of past climates rather than the present one, and it requires millennia to fully stabilize in response to sustained environmental conditions.12