How do mean annual air temperature and geothermal heat control permafrost thickness?

Permafrost thickness is determined by the thermal balance between heat flowing from Earth’s interior and heat lost to the atmosphere.1

The Role of Temperature and Heat Flow

• Mean Annual Air Temperature: When the mean annual air temperature is below 0 °C (32 °F), ground that freezes in winter does not fully thaw in summer, allowing permafrost to form and grow downward. As the climate cools or warms, the temperature of the permafrost shifts, causing the base of the permafrost to rise or decline accordingly.1
• Geothermal Heat: Heat from Earth’s interior flows toward the surface, governed by the geothermal gradient—typically an increase of 1 °C for every 30 to 60 metres of depth. Permafrost reaches an equilibrium thickness when the geothermal heat reaching its base equals the heat lost to the atmosphere.1

Factors Influencing Thickness

Even with identical air temperatures, permafrost thickness varies based on ground properties. Areas with higher ground conductivity and a lower geothermal gradient will support thicker permafrost, as these conditions more efficiently dissipate internal heat.2