More than one fifth of the area of India is occupied by Thar Desert, Africa by Sahara Desert, China by Gobi desert, South America By Atacama desert. The arid and semi arid lands constitute about two fifth of India where an estimated 200 million people are living. Deserts arid and semiarid regions of the world receive an average of less than 350 millimeter rain every year. These cover about 43 percent of earth’s landsurface.
Basically, a desert is a landscape form or region that receives little precipitation (rain or snow) generally less than 250 mm per year. The exact definition of a desert is sometimes disputed and so various types of arid zones have been defined.
Deserts often have common features. Soil from deserts is mostly composed of rocky surfaces called regs. Only a minority of desert surfaces have sand dunes called ergs and stony or hamada surfaces.
Rocky, exposures of terrain are is the more typical. This shows poor soil development and sparseness of vegetation. Some parts of deserts may be salt-covered flats. A major factor in shaping deserts is the wind (Eolian). This can be seen in sand dunes but also windblown rocks.
The dry conditions of deserts, can be good places for human artifacts and fossils to be preserved.
Deserts are likely to remain a significant part of the landscape of many continents in the coming years. There is some concern that some areas are becoming desertified, turned into deserts, due to human activities. This is a major concern because it means that people who used to live in area where they could grow food can now no longer.
In some desert areas, particularly Australia, there is concern that some animals are becoming extinct due to feral animals.
About 4500 years ago,the Harappan civilization flourished in regions now occupied by Thar desert. Rhinos and Elephants roamed in the forests that once grew there. Large scale deforestation and climatic changes gradually converted the region into a desert. The Rajasthan desert is one of the most densely populated deserts of the world. The density of population is 48 persons per square kilometer. Rapid increase in live stock population and increased cultivation resulting in ploughing more and more increased vulnerability of fertile soil if any left to soil erosion.
The deserts of the world show signs of expansion with time. Sahara desert is spreading southward as much as 50 km per year in some places. It has engulfed about 6500,000 square kilometers of arable land during the last century. The situation is not different in South America, West Asia, China and other places where desert occur. Many of the regions now occupied by deserts were once fertile. For about six centuries, North Africa was one of the principal granaries of the ancient Roman empire. The sand dunes of Sahara covers the ruins of great cities once flourished there.