Schematic overview of a typical terrestrial and shallow-marine permafrost landscape during summer and winter. Permafrost is defined as ground that remains continuously at or below 0°C for at least two consecutive years; some 24% of the land surface in the northern hemisphere is classified as permafrost. This schematic figure (summer) pictures a terrestrial and shallow marine permafrost system. A permafrost landscape is characterized by its large heterogeneity with morphological permafrost-related features such as polygonal patterned ground with underlying ice wedges, thaw ponds, thermokarst lakes, and wetland areas. During winter, the terrestrial landscape is covered with snow, and water bodies and the ocean are typically covered with ice.The last pictures shows schematically the fluxes (not scaled) that occur between the terrestrial and marine environment and atmosphere.