Paraglacial and paraperiglacial landsystems: concepts, temporal scales and spatial distribution
Résumé
The Pleistocene Earth history has been characterized by major climatic fluctuations. During glacial periods, ice may have covered around 30 per cent of the Earth surface compared to approximately 10 per cent nowadays. With global change, polar environments and other montainous glacial environments of the world are presently undergoing the most important changes since the end of the Last Glacial Maximum and are experiencing paraglacial and paraperiglacial geomorphological readjustments. Paraglacial and paraperiglacial landsystems consist of several subsystems including gravitational, fluvial, coastal, aeolian and lacustrine environments. Paraglacial and paraperiglacial landsystems can be analysed as open and complex landsystems characterized by energy, water and sediment fluxes and exchange with surrounding environments, especially with glacial and periglacial landsystems as inputs. Those cascading landsystems are likely to react to climate change because they rely on an ice-cold water stock (glacier and permafrost) that developed during a previous cold sequence (glaciation). The response of paraglacial and paraperiglacial systems to climatic forcing takes place over a long time span ranging from an immediate reaction to several millennia. The spatial limits of paraglacial and paraperiglacial landsystems are inherently dependant on the time scale over which the system is analyzed. During the Pleistocene, glaciations widely affected the high latitudes and the high altitudes of the Earth and were followed by inherited paraglacial sequences. Glacier forelands in Arctic and alpine areas experience paraglacial processes with the present warming. The expected global warming for the twenty-first century will result in significant impacts on present glacier areas in mountains and could result in the appearance of new areas for paraglacial dynamics. In permafrost terrain, landscapes underwent a similar paraperiglacial geomorphological adjustment in mountainous, continental and coastal areas, with permafrost thaw-degradation and thermokarst processes.