التصحر أسبابه و تأثيراته و طرق مكافحته

الملخص EN

Desertification is the persistent degradation of dry land ecosystems due to human activities and variations in climate.

It is widely recognized that desertification is a serious threat to arid and semiarid environments which cover40 % of the global land surface.

While climate oscillations have historically had pronounced effects upon desertification, activities of man during the Holocene have had the most pronounced impacts to induce desertification in the most 10 thousand years.

Dry land occupies nearly half of Earth's land area, home to a third of the human population in 2000.

Across the world, desertification affects the livelihoods of millions of people who rely on benefits that dry land ecosystems provide.

The proximate causes of desertification are dominated by agricultural intensification.

Unintended consequences of the so- called Green Revolution, begun in the 1970s has been massive overgrazing, monocropping ,excessive tillage practices in agriculture that place soils more vulnerable to wind and surface runoff scouring, salt buildup of irrigated lands ,over drafting of groundwater , utilization of lands unsuitable for arable practices and deforestation.

Desertification is caused by a combination of social, political, economic, and natural factors which vary from region to another.

Policies that can lead to an unsustainable use of resources and lack of infrastructure are major contributors to land degradation .Local adaptation and conservation practices can mitigate some losses of dry land services, but it will be difficult to reverse losses in term of biodiversity and in the provision of food and water which in linked to biodiversity.

Desertification assessment has shifted from simple appraisals of inter annual movement of desert boundaries to complex multivariate field surveys, to practical methodologies based on indicators of ecosystem functioning, such as rain use efficiency.

Future challenges for properly assessing desertification are the lack of reference situations against which actual desertification could be compared, and the difficulties that appear when desertification operates through structural rather than functional ecosystem change.

The exceptional growth of remote sensing tools and the extraordinary development of ecosystem ecology during the last two decades represent a unique opportunity to properly assess desertification all over arid the arid and semiarid world at virtually any reasonable spatial scale.