Permafrost and climate change

Permafrost is ground that remains at or below 0°C for at least two consecutive years. It varies in thickness from centimetres to ~1,500 m, and presently underlies ~16–21 x 106 km2 of Earth’s land surface, seafloor and subglacial terrain. Its upper metres are commonly rich in ground ice and underlie a layer of ground that freezes in winter and thaws in summer (the active layer). Permafrost is a geological product of climate and microclimate, and therefore its properties and distribution change as climate and land surface features change, though with a response that is delayed with depth and time. Direct observations of permafrost temperature and active-layer thickness (ALT) in recent decades indicate that permafrost has warmed and the active layer has thickened in many cold regions. Indirect observations of permafrost—via land surface conditions—indicate that some ice-rich permafrost has thawed in its upper centimetres to metres. These changes in permafrost conditions have been caused primarily by atmospheric warming. Their impacts on built environments and natural environments include damage to infrastructure, terrain disturbance, changes in water courses and ecosystems, and release of nutrients and disease vectors. The main conclusions from this review of recent changes to permafrost conditions draw attention to (1) the complexity of feedbacks in the permafrost system; (2) the financial costs to residents, industry and governments; (3) the continued trends in permafrost warming and active-layer deepening that are likely during the 21st century; and (4) some potential permafrost problems that may occur in coming decades.