Desertification is the degradation of land

Land degradation

Land degradation is a process in which the value of the biophysical environment is affected by one or more combination of human-induced processes acting upon the land....

 in drylands

Drylands

Drylands is a Miles Franklin Award winning novel by Australian author Thea Astley. This novel shared the award with Benang by Kim Scott.-Awards:...

. Caused by a variety of factors, such as climate change

Climate change

Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

 and human activities, desertification is one of the most significant global environmental problems.
Considerable controversy exists over the proper definition of the term "desertification" for which Helmut Geist (2005) has identified more than 100 formal definitions. The most widely accepted of these is that of the United Nations Convention to Combat Desertification

United Nations Convention to Combat Desertification

The United Nations Convention to Combat Desertification in Those Countries Experiencing Serious Drought and/or Desertification, Particularly in Africa is a Convention to combat desertification and mitigate the effects of drought through national action programs that incorporate long-term strategies...

 which defines it as "land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities"

The earliest known discussion of the topic arose soon after the French colonization of West Africa

West Africa

West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...

, when the Comité d'Etudes commissioned a study on desséchement progressif to explore the prehistoric expansion of the Sahara Desert.
The world's great deserts were formed by natural processes interacting over long intervals of time.