Why is some present-day permafrost a remnant of colder past climates?

Some present-day permafrost is considered a remnant of colder past climates because it is not in equilibrium with the current climate at the sites where it is measured.1

Permafrost typically forms and persists in environments where the mean annual air temperature is 0 °C (32 °F) or colder. However, significant portions of existing permafrost, particularly in the subarctic, likely formed during the cold glacial periods of the past 100,000 years. Because the climate has warmed since the late 19th century, this ground remains frozen only because of the thermal legacy of those colder historical conditions, rather than the current atmospheric temperature.21

This phenomenon is also evident in subsea permafrost. During the height of the last glacial epoch, approximately 20,000 years ago, large areas of the continental shelf were exposed to polar climates, allowing deep permafrost to develop. As sea levels rose over the last 10,000 years, this frozen landscape was submerged, creating "relict" permafrost that is now degrading due to the warming influence of seawater and geothermal heat.1