The world's drylands supported about 850 million
people in 1984, of whom 230 million were on lands affected by
severe desertification. Worldwide it is estimated that millions of
hectares are losing their biological diversity each year, as human
and animal pressures have accelerated the removal of vegetation and
consequent soil erosion. Two fifths of Africa's non-desert land
risks being turned into desert, as does one-third of Asia's and
one-fifth of Latin America's.
Disturbance of the ecological system has
decreased the infiltration of rainwater, increased surface runoff,
lowered ground-water levels, and caused the drying up of surface
water and loss of topsoil and soil nutrients. Under these
conditions, drought will more quickly reduce food output and lead
to famine. However, political, economic and social factors are more
important than low rainfall in the process of desertification.
Besides rapid growth of human and animal populations and
detrimental land use practices, the cultivation of cash crops on
unsuitable rangelands has forced
herders and their cattle onto marginal
lands, thus accelerating soil degradation and outright
desertification.
Declines in soil fertility or even total losses
of land to agriculture are common in many parts of the world. Soil
erosion by wind and water is serious in the arid parts of North
Africa and the Middle East, parts of South Asia and South America.
The problem is caused in great measure by inappropriate land use
and cropping patterns. Substitution of traditional mixed cropping
(which includes plants or shrubs along with food crops) by
monoculture, and poor management of land and water have caused
significant soil erosion and other types of degradation.
Salinization affects extensive land areas in many countries in
North Africa, the Middle East, and Asia. About half the land under
irrigation is affected by secondary salinization and/or
alkalinization in varying degrees. Some 40 million hectares of
irrigated land are either waterlogged or suffer from excessive
salinity or both. Salinization may be removing as much land from
production as is added by irrigation. Irrigation has greatly
improved farm productivity in areas of uncertain or inadequate
rainfall and has been responsible for the adoption of high yielding
varieties in many developing countries. Yet inappropriate
irrigation has wasted water, polluted groundwater, and damaged the
productivity of millions of hectares. Excessive extraction of
groundwater for irrigation has depleted aquifers in Asia and
Africa. Acidification of peat soils occurs when the groundwater
table is lowered. It drastically reduces production of crops and
fish (in ponds) as, for example, along extensive coastal areas in
southeast Asia.