Permafrost is a layer of permanently frozen soil, rock or sediment. It forms in areas where the mean annual air temperature remains below 0°C for several years. Permafrost may be as thick as 1500 m and can occur at depths of up to 2000 m. The upper limit of permafrost is known as the permafrost boundary.
Permafrost occurs on every continent except Australia and Antarctica. In the Northern Hemisphere, it is most extensive in Siberia, northern Canada and Alaska. In the Southern Hemisphere, it is found in parts of Patagonia and Tierra del Fuego. Global warming is causing permafrost to thaw in many areas, which can lead to flooding and other problems.
Permafrost is an important part of the global climate system because it contains large amounts of carbon dioxide and methane, which are greenhouse gases that contribute to climate change. When permafrost thaws, these gases are released into the atmosphere, further exacerbating climate change.