3.1.2.4 - Desertification, risk and changing extent

Desertification is degradation of drylands

Desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting from climatic variations and human activities. It involves reduced biological or economic productivity and complexity, for example soil erosion, loss of vegetation or salinisation. It does not require sand dunes to advance over a village. Drought is a period of substantially below-normal rainfall with adverse effects; a drought may contribute to degradation, but the two terms are not interchangeable.

The Sahel is a semi-arid transition south of the Sahara, with seasonal farming and grazing landscapes. Degradation there concerns soil, water, cover and livelihoods, rather than treating the whole region as a hyper-arid dune desert. Repeated evidence of declining function matters: a bare field just after harvest or a temporary drought response is insufficient on its own to establish a lasting change. This distinction follows UNCCD Article 1.

Desert extent over the last 10,000 years

The present Sahara has not had an unchanging boundary. During the African Humid Period, stronger monsoon rainfall supported lakes and vegetation across areas now arid. Long-term orbital changes altered seasonal insolation and monsoon strength; vegetation and lake feedbacks influenced the response. Later drying expanded arid landscapes, but timing and abruptness differed between regions.

Armitage, Bristow and Drake's 2015 study dated deposits and shorelines of Mega-Chad. Their reconstruction places a high lake stand around 11.5–5 thousand years ago, with a major fall around 5 thousand years ago; the final drying of the Bodélé sub-basin occurred much later, around 1 thousand years ago. These are basin-scale proxy estimates, not a single date when the entire Sahara became desert. Ages and sediment records have uncertainty, and other archives show different regional responses.

Natural climatic aridification over millennia should therefore be distinguished from contemporary loss of dryland function through climate and land use. A former lake bed can now supply dust, linking inherited surfaces to present processes. See the primary Mega-Chad study.

Climate and land use can reinforce each other

Drought reduces plant growth and soil protection. Warming can increase evaporative demand, making the same rainfall less effective for plant growth. Intense rain on poorly protected ground can increase runoff and erosion even if annual rainfall does not decline. Climate change can therefore alter more than desert rainfall totals.