The Sahara: A desert of change

The Sahara Desert has a changing surface area depending on the rainfall occurring on the Sahel region south of this desert. Climatic parameters (precipitation and temperature) were obtained for many meteorological stations distributed within the Sahara and Sahel countries. The bulk of the Sahara Desert is dry and hot. Wind direction was extracted from the orientation of sand dunes of major sand seas as inferred from 30 Landsat satellite images (60 m spatial resolution). Sand dunes mostly belong to the longitudinal type and follow a southeast direction at the eastern Sahara and a southwestern trend at the majority of the desert. Topographic variations were analyzed using digital elevation models (1 km spatial resolution) revealing that the Sahara is generally a plateau desert except for some mountainous spots occurring at its northwest and middle parts. The surface area change of this desert was estimated using the Normalized Difference Vegetation Index (NDVI) product from 480 satellite images acquired from the Advanced Very High Resolution Radiometer (AVHRR) (8 km spatial resolution) and covering the entire Africa between 1984 and 2003. Image processing applied included a robust unsupervised classification scheme. Results showed that the Sahara Desert witnessed two opposite episodes of expansion and contraction. The period 1984-1993 was a cycle of expansion and the period 1994-2003 was a cycle of contraction. Maximum desert area was observed during 1984 (10.35 million km²) and minimum area was during 1999 (9.15 million km²). The average area of the Sahara Desert between 1984 and 2003 was 9.87 million km².