Is a comprehensive database on Permafrost. Permafrost is defined as ground that remains continuously at or below 0°C or at least two consecutive years; some 24% of the land surface in the northern Hemisphere is classified as permafrost. In the Northern high latitudes, strong warming has been observed over the recent decades, and climate models project strong future warming. A projected decline in the extent of permafrost will have a major impact on the Earth system, affecting global climate through the mobilization of carbon and nitrogen stored in permafrost.