Summary Permafrost underlies most of the Arctic land mass. The two main climatic parameters that determine the presence or absence of permafrost are air temperature and precipitation (especially in the form of snow). No species is dependent on permafrost, and on a circumarctic scale no ecosystems are restricted by the existence of permafrost because both tundra ecosystems and boreal forest scan occur in the presence or absence of permafrost. One catastrophic impact of changing vegetation on permafrost is likely to be an increased frequency of fires. Wildfire is a major disturbance in the Arctic tundra and boreal forests, which has a significant impact on soil hydrology, carbon cycling, and permafrost dynamics. Thawing permafrost is associated with gradual or episodic disturbance (slumping) of the land surface that can affect large areas. Periglacial processes change land surfaces, which can in turn affect ecosystem function.