Global Drylands: A UN system-wide response
What and where are the drylands? This chapter defines and maps the drylands, showing linkages to poverty and the achievement of the Millenium Development Goals. It explains and characterizes their unique ecosystem services and introduces the functions of dryland systems. Drylands communities are not victims to their environment. Instead, they demonstrate high levels of adaptation in their livelihoods and are closely linked to markets and trade dynamics. The drylands already provide many important ecosystem services to the world which are currently under-valued. Drylands are land areas with one overriding characteristic: they receive relatively low overall amounts of precipitation in the form of rainfall or snow. Although conceptually easy to grasp, drylands are difficult to define precisely. This report uses a broad definition in which drylands are land areas with an aridity index of less than 0.65. The aridity index is a measure of the ratio between average annual precipitation and total annual potential evapotranspiration. Drylands can be further subdivided (see Table 1) into: hyper-arid deserts (<0.5 index of aridity), arid (0.05–0.20 index of aridity), semi-arid (0.20–0.50 index of aridity), and dry sub-humid (0.50–0.65 index of aridity). Arid ecosystems are annual grasslands suitable mainly for grazing animals, except where interrupted by rivers or lakes. Semi-arid ecosystems are thorny savannahs with annual and perennial grass species, which may be cleared for farming and livestock and carry the highest population densities of the drylands. Subhumid ecosystems are broad-leaved savannah woodlands with higher, denser tree canopies and perennial grasses. The drylands also contain some large, irrigated areas along perennial rivers, which are farmed intensively. Almost all drylands experience high rainfall variability within seasons (it is often concentrated in short rainy seasons), between years and in longer-term cycles. Combined with low average rainfall, this variability is a high risk to non-irrigated agricultural enterprises.
According to the Millennium Ecosystem Assessment (MA), drylands cover about 40% of the world’s land area. They occur on every continent, and span a diversity of cultures and landscapes. This report focuses on the drylands of developing countries as they are most strongly associated with the objectives of UN dryland intervention.
1.1
People, land and water
Drylands are home to just over 40% of the human population of both Africa and Asia, and to between 25–30% of the rest of the world’s population, or around two billion people (Reynolds et al. 2007a), 90% of whom live in developing countries (UNEP 2007). All four of the so-called ‘BRIC’3 countries (Brazil, Russia, India and China) contain some drylands, as do other large, rapidly developing countries such as Mexico and South Africa. There are numerous dryland areas in Europe, particularly around the Mediterranean and Central Asia. But a major focus of international concern is dryland populations in poor countries, notably in Africa. Drylands are rural and urban: they are home to around one billion people who rely directly on dryland ecosystem services for their livelihoods, but they also support some of the world’s biggest cities including those located in drylands such as Cairo, Mexico City and New Delhi.
3 Shorthand term for Brazil, Russia, India and China–all large, rapidly developing economies.
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Chapter 1: Introducing the drylands