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1A major topic in geomorphology is the impact of global change on geomorphic systems. Drylands tend to cover approximately half of the Earth’s land surface, but represent only 20% of its population. Understanding how dryland areas are likely to respond to climate change is therefore of great societal and economic significance. The main objective of this special issue is to review recent research results that increase our knowledge of semi-arid and arid geo-ecosystem responses to climate change. Contributing authors do so through case studies focusing on the late Pleistocene, Holocene and recent past on three continents.

2The geomorphic response to global change is what most directly impacts on landscapes and humans. In arid and semi-arid areas of south-western Morocco, A. Weisrock, L. Wahl, A. Ouammou, and L. Chakir show that it is less the long-term trends in mean rainfall than the cumulative geomorphic response to rain storms and drought through processes such as floods, wind erosion, dune migration and dust storms that affect drainage networks and landscapes. Late Pleistocene and Holocene wadi deposits provide a long term perspective on geomorphic responses to climatic change. Both older and longer-lasting episodes, such as OIS 3 (ca. 55–25 ka), and shorter episodes such as the “Neolithic optimum” and the Little Ice Age, are documented.

3The impacts of global change differ from one landscape to the next. However, late Pleistocene records in the arid zone of southern Australia yield results comparable to those of Morocco. Reconstructed processes of dust transport and deposition have allowed M. Williams, N. Nitschke and C. Chor to establish that a mean annual temperature decrease of 6 °C occurred between 35 and 17 ka. Nevertheless, lake levels still remained high during this dry period, because of the correlative decrease in evaporation rates.

4Some changes in the landscape reflect the impact of past global changes, while others reflect more local or regional phenomena. In the case of northern Syria (J.-B. Rigot), the study of Holocene and historical terrace deposits reveals a mitigation of erosional/depositional processes compared to those observed in adjacent Mediterranean areas during the same period. The effects of topography, but also land-use and slope terracing experiments, on the geomorphic response to environmental change are highlighted as discriminating factors between this north-Syrian environment and the broader east-Mediterranean record.

5As demonstrated by the desert-margin loess sequences of the Loess Plateau, the north-western deserts of China appear to have formed in middle Cenozoic times. These are therefore very ancient arid environments (Yang Xiaoping), but there is also abundant evidence of climatic fluctuations during the late Quaternary. The geomorphological link between two extant generations of sand seas in this region is not yet clearly established. However, a new explanation for the formation and conservation of the megadunes of the Badain Jaran Desert is that groundwater helped in maintaining the dunes. During wetter epochs, permanent lakes in the inter-dune basins promoted calcareous cementation of the sands. This stabilised the megadunes.

6Some fundamentals of desert hydrology are outlined by F. Joly. This is completed by an up-dated review of Dryland Rivers, Hydrology and Geomorphology of semi-arid channels, edited by L.J. Bull and M.J. Kirkby, in which modern concepts in the fluvial geomorphology of arid and semi-arid zones are developed (A. Weisrock).

7I would like to acknowledge the wide range of advice I have received and tried to incorporate into the different papers of this issue. I also thank our colleagues J.-L. Ballais and J. Riser, in Aix-en-Provence, who instigated this special issue, as well as the editorial committee: J.-C. Thouret, J. Raffy, Y. Gunnell, E. Anthony, A. Héquette and reviewers of Géomorphologie : relief, processus, environnement, for all of their unseen but efficient contributions.

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André Weisrock, « Geomorphology of dry lands, deserts and their margins : morphological responses to climate changes in dry lands », Géomorphologie : relief, processus, environnement [En ligne], vol. 12 - n° 4 | 2006, mis en ligne le 01 octobre 2008, consulté le 30 avril 2017. URL : http://geomorphologie.revues.org/46

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André Weisrock

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