Temps de lecture :7 minutes
The world’s drylands support over a billion people, and their numbers are growing faster than the global average. In 2004 the UN figure for annual global population growth was 1.14 per cent. But for Africa, which is 40 per cent dryland, annual growth is 2.4 per cent. Dry Afghanistan has the highest annual population growth rate in the world at 4.77 per cent. Mongolia, dominated by cold drylands, has a yearly growth rate of 1.45 per cent. Conservation International estimates population growth in designated dryland biodiversity hotspots is 1.8 per cent each year.
This rapid growth, combined with likely climate change, is increasingly pressurising dryland resources, often degrading land and threatening people’s livelihoods.
How people use drylands resources
People are poorly adapted to drylands, where rainfall is scarce and temperatures are often high. Yet societies have survived here for millennia. Over 6,000 years ago, people used the constantly-flowing Euphrates and Tigris rivers to irrigate crops in dry Mesopotamia.  Elsewhere in the Middle East and North Africa, local groundwater supported oasis agriculture in harsh environments.
More recently, people have often extracted dryland resources for use in non-dryland regions. The Roman Empire relied on salt from North Africa’s drylands. Later, through the seventeenth to nineteenth centuries, gum Arabic was traded between the south western Sahara and Europe. Today, much of the world depends on oil from the Middle East and uranium mined in the Namib Desert.
Alternatively, they have lived as small populations moving from place to place as conditions dictate. Mongolian Yak herders traditionally live extensively, following the rains and new grazing, while the San (or bushmen) of the Kalahari in southern Africa follow migrating wildlife and the shifting availability of fruits and roots.
Both colonial and post-colonial development has disrupted such extensive lifestyles in many dryland regions. In Kenya, British colonial rule restricted the movements of Masai herders by creating wildlife reserves and export-centred agriculture. In some places, including parts of Australia, Botswana and the western United States, extensive pastoralism has been commercialised within fenced farms. Elsewhere in Africa, agricultural systems have been ‘imported’ from wetter environments. Only 20 per cent of Kenya has suitable soils and rainfall for stable crop production, yet settled agriculture has soared in the past three decades. This generates crops to fulfil regional, or even global, demands, but risks failure, not least because the crops need water at set times of the year.
In remote parts of Mongolia, nomadic pastoralism is persisting. But even here the pumped wells, built and maintained with Soviet support, are collapsing, restricting livestock movements and increasing the pressure on those remaining and on seasonal rivers. Successful dryland cultivation relies on irrigation from perennial rivers like the Colorado, Indus or Nile, or from water harvested from underground aquifers with open galleries or qanats etc (see Improving access to water in deserts and drylands).
Urbanisation, expanding agriculture and land degradation
Wherever people use drylands, they affect local ecosystems. Rapid urbanisation is a particular problem. Many dryland towns and cities, in both developed and developing countries, have expanded dramatically in the last 50 years. Phoenix in the United States has grown from about 500,000 people in 1960 to 1.5 million today. Bejing, Dubai, Karachi , Santiago and Tehran are all in drylands. Dryland urban populations rose 4 – 16 per cent in the last forty years, compared with approximately 3.2 per cent for the rest of the world. [2, 3]
Elsewhere, growing dryland populations and their agriculture can severely degrade land, often called ‘desertification’ — a somewhat contested and complex term.  For example, in Africa, research indicates that almost 25 per cent, or some 320 million hectares, of drylands (excluding hyperarid regions where cultivation is impossible) are affected by soil degradation associated with agriculture, with 5 – 8 per cent severely degraded. 
Land degradation is not simply rapid erosion. It encompasses other factors that decrease land productivity. For example, nutrients can be depleted when land is intensely cultivated, when fallow periods are shortened or abandoned, or when no fertilisers are applied. This is especially severe in sub-Saharan Africa, partly because of expanding cultivation and the high cost of fertilisers. Salinisation is an additional threat where mineral-rich water (as groundwater often is), high temperatures and strong evaporation combine to create salty soils. In Pakistan, salinisation has reduced irrigated crop productivity by 25 per cent. 
Varying climate, political instability or new and inappropriate farming techniques can all exacerbate land degradation. For example, land degradation and famine in the Sahel during the 1960s and 70s was caused partly by drought, but also by the population growth, political conflicts and corruption that affected movements of food and people and by the expansion of Western-type agriculture during the preceding very wet 1950s. Similarly, rapid farming expansion followed by a decade of severe droughts led to the dust bowl years of the 1920s and 30s in dryland North America. The dust bowl problems did however generate one tangible benefit – a realisation of the need to promote soil conservation techniques and appropriate land management.
All forms of land degradation make agriculture less sustainable. In many dryland areas, pastoral systems that avoided lasting pressures on land have been replaced by settled agriculture using the same land area year-round. Good grazing can be lost as fast growing invasive annual grasses replace more nutritious and productive perennial species. In its most extreme form, shrub cover increases because deeper-rooting species out-compete shallow rooting grasses. This process, known as bush encroachment, has been reported in Africa, Australia and the western United States. 
Water quality and quantity is declining
Expanding and intensified agriculture increases pressures on fixed groundwater resources, such as wells and boreholes. For example, 30 per cent of irrigation water in the United States comes from the Ogalala Aquifer, which stretches from South Dakota to northern Texas. Here, water is being extracted dramatically faster than it recharges. In one part of Oklahoma, the Ogalala water table had fallen over four metres by 1995.  The Kalahari basin of southern Africa is experiencing similar problems because of increased sedentary livestock production. Research indicates water is being recharged at no more than 1mm per annum in the centre of the basin, while extraction has lowered the water table over a metre in some places in recent years. 
As water tables fall, salinity increases and water quality declines. This is especially true in expanding urban areas, where wastewater can also affect water quality. Desalinisation technologies can remove salt from sea-water (or brackish water), but are expensive, and produce highly concentrated and potentially damaging waste. But for countries like those on the Arabian Peninsula, desalinisation is essential to gaining potable water, and advances in technology using solar energy are expected to reduce its costs. Still, the viability of desalinisation as an effective and economic means of providing water to dryland urban areas is hotly contested — some argue that treating low quality local water sources, recycling or water transfers are more realistic solutions. 
Changing lifestyles, often driven by policies at both national and international levels have degraded drylands in many developing countries. Dryland farmers in Africa earn money by growing crops for Western supermarkets, yet this may be risky in the long term. Relying on a single production system in an uncertain and variable environment makes households less resilient to problems such as droughts and economic changes.
Global climate change also threatens dryland livelihoods. We can expect significant changes in ecosystems, water availability and dryland conditions in the coming decades, and subtropical drylands, especially those in Africa, are particularly vulnerable.  Dryland communities will need to adapt to the changing conditions (see ‘Adapting to climate change: why and how‘).
In the past, indigenous techniques and local knowledge, based on understanding the environment, have helped communities to cope with varying climate. But these techniques have increasingly been replaced by inappropriate imported technologies. For example, some researchers suggest that displacing indigenous land use systems from Kenyan hill slopes during colonial rule disrupted soil conservation techniques that had prevented soil erosion for many generations.  Many development agencies now recognise the benefits of long-gained local knowledge, for example the UN Convention to Combat Desertification explicitly acknowledges the value of indigenous knowledge, and promotes a bottom-up approach that includes non-governmental communities in its Conference of the Parties.
Diversifying livelihoods is another way of coping with environmental (and economic) uncertainty, as it makes people less vulnerable when one activity or system fails. In the Beitbridge district of Zimbabwe, dryland farmers have switched from maize, which is difficult to grow in persistent drought conditions, to drought-tolerant and pest-resistant sorghum, and are supplementing this with wetland rehabilitation, goat herding and seed nurseries.  Part of the Beitbridge success has been establishing a community institution that can receive, manage and use external funds without intermediaries. Implementing policies, or providing subsidies, that support community-led institutions like this may help diversification, but care must be taken not to polarise activities (as happened in Botswana, see Box 1).
Dryland areas need strategies that will address future uncertainties while meeting people’s immediate needs, especially in poorer regions. Direct interventions may be needed, but the knowledge and abilities of indigenous people must not be ignored. Centralised interventions can fail to recognise the cultural and environmental diversities within dryland areas. At a time when our altering climate may be driving environmental changes faster and further than we have previously known, the capacity to adapt to changes through local knowledge and experience may be crucial.
 Jacobson T. and Adams, R.,M. Salt and silt in ancient Mesopotamian agriculture. Science 128: 125-1258. (1958)
 White R. and Nackoney, J. Drylands, People, and Ecosystem Goods and Services: A Web-Based Geospatial Analysis. World Resources Institute. Available at http://forests.wri.org/pubs_description.cfm?PubID=3813 (2003)
 Cooke, R.U., Brunsden, D., Doornkamp J.C. et al. Urban Geomorphology in Drylands, pp324. Oxford University Press, Oxford (1982)
 Thomas D.S.G. and Middleton N.J. Desertification: Exploding the myth, pp194. Wiley, Chichester (1994)
 Middleton N. and Thomas D.S.G., eds. World Atlas of Desertification 2nd edition. UN Environment programme P/Edward Arnold, London (1997)
 Dougill A.J., Heathwaite A.L. and Thomas, D.S.G. Soil water movement and nutrient cycling in semi-arid rangeland: Vegetation change and system resilience. Hydrological Processes 12, 443-459 (1998)
 Oklahoma Agricultural Statistics Services, Oklahoma Department of Agriculture.
 Thomas, D.S.G. The environmental impact of groundwater exploitation in African grasslands. Proceedings of Dubai 2002 International Conference on water resources. Balkema, Rotterdam (2003)
 Cooley H., Glieck, P.H. and Wolff, G. Desalination, with a grain of salt, 100pp. Pacific Institute, Oakland (2006)
 De Wit M. and Stankiewicz, J. Changes in surface water supply across Africa with predicted climate change. Science 311, 1917-1921 (2006)
 Tiffen M., Mortimore M., Gichuki, F. More People, Less Erosion. Environmental recovery in Kenya. Wiley, Chichester (1994)
 Chigwada J. Case study 6: Zimbabwe climate proofing infrastructure and diversifying livelihoods in Zimbabwe. Institute of Development Studies Bulletin, 36, 103-116 (2004)
People, deserts and drylands in the developing world
SciDev, octobre 2006