Thaw

Spring Melt/Thaw
The process whereby warm temperatures melt winter snow and ice.

Though a thin layer may thaw during summer months, the majority of the permafrost in a given location will remain frozen until the local climate dramatically changes, as from continental drift toward the equator or global warming.

As temperatures keep getting warmer, permafrost will continue to thaw. For example, the map on the right shows how permafrost in northwestern Alaska could change by the year 2100.
Why does it matter?

The upper boundary of permafrost, usually coincident with the lower limit of seasonal thaw. See also permafrost (1.).
permanent charge ...

cryogenic lake a lake occupying a thaw basin in a region of permanently frozen ground
cryogenic meromixis meromixis caused, in part, by the vertical contrast in salinity that comes about from the freezing-out effect of surface ice ...

See also: Water, Environment, Soil, Gas, Air