Permafrost temperatures have increased during the last 20–30 years in almost all areas of the Northern Hemisphere. An increase in the depth of the active layer above the permafrost, which thaws in the summer, is less certain. Further increases in air temperatures predicted for the 21st century are projected to initiate widespread permafrost thawing in the subarctic and in mountain regions in both hemispheres. Widespread thawing of permafrost will speed up the decomposition of organic material previously held frozen in permafrost, emitting large amounts of greenhouse gases into the atmosphere. Thawing of ice-rich permafrost may also have serious consequences for ecosystems and infrastructure, and in mountain regions, may reduce the stability of slopes and increase the danger of rock falls and landslides