Drylands are defined by their scarcity of water. They are zones where precipitation is counterbalanced by evaporation from surfaces and transpiration by plants (evapotranspiration). UNEP defines drylands as tropical and temperate areas with an aridity index of less than 0.65. The drylands can be further classified into four sub-types: dry sub-humid lands, semi-arid lands, arid lands, and hyper-arid lands. Some authorities consider Hyper-arid lands as deserts (UNCCD) although a number of the world’s deserts include both hyper arid and arid climate zones. The UNCCD excludes hyper-arid zones from its definition of drylands.
Drylands cover 41.3% of the earth’s land surface, including 15% of Latin America, 66% of Africa, 40% of Asia and 24% of Europe. Worldwide there is a significantly greater proportion of drylands in developing countries (72%), and the proportion increases with aridity: almost 100% of all Hyper Arid lands are in the developing world. Nevertheless, the United States, Australia and several countries in Southern Europe also contain significant dryland areas.
Drylands are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated links between climate, soil, and vegetation.
Dryland biodiversity
The livelihoods of millions of people in developing countries depend highly on dryland biodiversity to ensure their food security and their well-being. Dryland inhabitants’ life style provides global environmental benefits which contribute to halt climate change, such as carbon sequestration and species conservation. Dryland biodiversity is equally of central importance as to ensuring sustainable development along with providing significant global economic values through the provision of ecosystem services and biodiversity products. The UN Conference on Sustainable Development Rio+20 held in Brazil in June 2012 stressed the intrinsic value of biological diversity and recognized the severity of global biodiversity loss and degradation of ecosystems.