Permafrost is ground that remains at a temperature below freezing continuously for two or more years. It consists of ice holding soil, sand, and rocks and also contains large amounts of biomass as well as decomposed biomass that has been stored as methane and carbon dioxide. It can be the top layer on the ground, or it can be as deep as several miles below the surface. About eleven percent of the earth's surface is underlain by permafrost, as is about 15 percent of the northern hemisphere.
The thawing of the permafrost due to climate change is releasing ever-larger quantities of its stored greenhouse gases into the atmosphere, creating a feedback cycle that leads to further global warming.