Chapter 2. The world’s drylands

Definition of drylands

Depending on definitions, about 47 percent of the surface of the earth can be classified as dryland (UNEP, 1992). Although there is no clear boundary, drylands are considered to be areas where average rainfall is less than the potential moisture losses through evaporation and transpiration. According to the World Atlas of Desertification (UNEP, 1992), drylands have a ratio of average annual precipitation (P) to potential evapotranspiration (PET) of less than 0.65.

Where the water deficit prevails throughout the year, drylands are classified as extremely arid or hyperarid, whereas when it occurs for most of the year they are arid and semi-arid regions. Aridity is assessed on the basis of climate variables (so-called aridity index), or according to FAO on the basis of how many days the water balance allows plant growth (growing season). The aridity index uses the P/PET to classify drylands into hyperarid, arid, semi-arid and dry subhumid (Table 2).

The negative balance between precipitation and evapotranspiration results in a short growing season for crops (usually less than 120 d). For CS purposes, drylands are also considered to include arid, semi-arid and dry subhumid areas. Hyperarid regions are not considered as there is no crop growth unless under irrigation.

Droughts are characteristic of drylands and can be defined as periods (1 - 2 years) where the rainfall is below the average. Droughts that persist for a decade or more are called desiccation, which can have disastrous consequences for land productivity and vegetation loss. Drought preparedness and risk mitigation are essential for the proper management of dryland areas. Populations living in these regions have been developing strategies to cope with them. These measures include: strengthening indigenous strategies to cope with drought; supporting the development and adoption of resource management practices that will protect and improve productivity, thereby increasing the resilience of agricultural systems; reducing fluctuations in prices of livestock and grains during drought periods through expanding market size and reducing transaction costs; developing a set of warning indicators; and setting aside drought grazing reserves or strategic water reserves (Øygard, Vedeld and Aune, 1999).

Land degradation in drylands

Desertification results from the degradation of the natural ecosystems in drylands and constitutes a major global problem (UNEP, 1992). It is defined by the CCD as “Land use degradation in arid, semi-arid and dry humid areas resulting from various factors, including climatic variation and human activities”. The degradation can be:

TABLE 2
Dryland categories according to FAO (1993) classification and extension (UNEP, 1992)

Bha = 10⁹ ha.

The main consequences of land degradation are: the chemical degradation of the soil; loss of vegetation cover; loss of topsoil infiltration capacity; reduction in soil water storage; loss of SOM, fertility and structure; loss of soil resilience; loss of natural regeneration; and lowering of the water table. Soil degradation affects about onefifth of arid zones, mostly on semi-arid margins where cultivation take place. Land degradation may have a significant impact on climate. The loss of plant cover can alter the surface energy balance. Atmospheric dusts from deserts modifies the scattering and absorption of solar radiation (Kassas, 1999). Although uncertainty exists with regard to the causes of climate change and global warming and the possible consequences, there is agreement that some impacts are probable. For example, temperature increases will affect evapotranspiration, which will be most significant in places where the climate is hot. Predictions about the quantity and distribution patterns of rainfall in these regions are uncertain, but the Intergovernmental Panel on Climate Change indicated that semi-arid regions are among those most likely to experience increased climate stress (IPCC; 1990). Furthermore, climate change may have unpredictable and perhaps extreme consequences with respect to the frequency and intensity of precipitation and temperature variability for semi-arid regions.

Table 3 indicates the extension of degraded lands according to cause. One of the problems of assessing the extent of desertification and the measures to prevent it, is the lack of reliable and easily measured land quality indicators. The Land Degradation Assessment in Drylands project, initiated by FAO, focuses on the development of a detailed methodology for the assessment of land degradation in an area that covers as much as half of the global land surface (FAO, 2002a, 2003).

Several estimates exist for the extent of desertification. According to the Global Assessment of Human and Induced Soil Degradation methodology, the land area affected by desertification is 1 140 000 000 ha, which are similar to the UNEP estimates (Table 4).

According to UNEP (1991a), when rangelands with vegetation degraded are included (2 576 000 000 ha), the percentage of degraded lands of the drylands is 69.5 percent (5 172 000 000 ha). According to Oldeman and Van Lynden (1998), the degraded areas for light, moderate and severe degradation are 489 000 000, 509 000 000 and 139 000 000 ha respectively.

TABLE 3
Degraded lands per continent

Source: UNEP (1997).

Estimates of rates of current desertification vary considerably mainly because of the lack of quantitative criteria for defining degradation. UNEP (1991a) distinguished between land degradation and vegetation degradation. The degradation of vegetation in rangelands can take place with or without soil degradation. UNEP (1991) estimates the annual rate of desertification to be 5 800 000 ha or 0.13 percent of the dryland in mid-latitudes (Table 5). However, although desertification is a problem in drylands, drylands have a high degree of resilience to human interventions. Dryland populations have developed welladapted and efficient resource management practices. Therefore, the participation of dryland communities is crucial to improving dryland management. If the policies and practices of donors are to succeed, they must be based on the knowledge, experiences, aspirations, priorities and decisions of the people living in drylands.

TABLE 4
GLASOD estimates of desertification (excluding hyper dry areas)

Bha = 10⁹ ha.
Sources: 1. UNEP (1991b). 2. Oldeman and Van Lynden (1998).

TABLE 5
Rates of land degradation in mid-latitudes drylands

Mha = 10⁶ ha.
Source: (UNEP, 1991a).

Desertification can be prevented through a proper management of the land to ensure sustainable development of its resources. In 1994, the United Nations agreed on the CCD by developing specific country action plans. Strategies for desertification control include: establishment and protection of vegetation cover to protect soils from erosion, controlled grazing; improved water conservation by residue management and mulching to help decrease water losses by runoff and evaporation; supplemental irrigation; soil fertility management which enhances biomass productivity; increased water use efficiency; and improved soil quality; improved farming systems that include crop rotations; fallowing; agroforestry; and grazing management (Lal, 2001b). All these strategies increase CS in soils.

Depending on land-use, desertification is manifested in different ways:

Desertification affects more than 100 developed and developing countries in all continents (UNEP, 1997). Some 200 million people are believed to be affected directly by desertification and more than 1 000 000 000 people at risk. The future sustainability of dryland ecosystems and the livelihoods of people living in them depend directly on the actions taken for land-use management. These activities should include soil and water conservation for improved land-use management practices and farming systems, taking into account health, social and economic issues when developing strategies and policies to improve land management.

Distribution of drylands

Most arid land areas of the world occur between the latitudes of 20° and 35°. The main semi-arid areas occur on each side of the arid zone and include Mediterranean-type and monsoonal-type climates. Mediterranean climates are characterized by cold wet winter and dry hot summers whereas monsoonal-type climates have hot wet summers and warm dry winters. Another type of dryland is the cold desert, which generally occurs in high-altitude continental areas

Drylands occupy 47.2 percent of the world’s land area, or 6 310 000 000 ha across four continents: Africa (2 000 000 000 ha), Asia (2 000 000 000 ha), Oceania (680 000 000 ha), North America (760 000 000 ha), South America (56 000 000 ha) and Europe (300 000 000 ha) (UNEP, 1992) in more than 110 countries (Figure 4). About 2 000 000 000 people live in drylands (UNEP, 1997), in many cases in poor conditions. The hyperarid zones extend mostly across the Saharan, Arabian and Gobi deserts and have only localized population around valleys such as the Nile Valley and the Nile Delta. The arid zones cover about 15 percent of the land surface. The annual rainfall in these areas is up to 200 mm in winter-rainfall areas and 300 mm in summerrainfall areas. Interannual variability is 50 - 100 percent. Africa and Asia have the largest extension of arid zones, they account for almost four-firths of hyperarid and arid zones in the world (Table 6).

Semi-arid zones are more extensive and occur in all the continents, and cover up to 18 percent of the land surface. They have highly seasonal rainfall regimes and a mean rainfall of up to 500 mm in winter-rainfall areas and up to 800 mm in summer-rainfall areas. With an interannual variability of 25 - 50 percent, grazing and cultivation are both vulnerable, and population distribution depends heavily upon water availability.

Soil and vegetation of drylands

As discussed above, the soils of drylands are characterized by frequent water stress, low organic matter content and low nutrient content, particularly nitrogen (N) (Skujins, 1991). Although dryland vary considerably, they are mostly Aridisols (2 120 000 000 ha) and Entisols (2 330 000 000 ha). Other soils include: Alfisols (380 000 000 ha), Mollisols (800 000 000 ha), Vertisols (210 000 000 ha) and others (470 000 000 ha) (Dregne, 1976) (Figure 5). Whatever their type, soils are the basic resource of drylands as they provide the medium in which plants grow, and their properties, such as texture and waterholding capacity, determine the proportion of rainfall available for plant growth. Low organic matter content, low germination and high seedling mortality are the main causes of very low plant productivity.

TABLE 6
The global dryland areas by continent

Mha = 10⁶ ha.
Source: FAO (2002a).

FIGURE 4 Distribution of drylands in the world

Source: FAO (2002a).

FIGURE 5. Major soil types of drylands

Source: World Soil Resources Map, FAO/EC/ISRIC, 2003

The vegetation supported by these soils ranges from barren or sparsely vegetated desert to grasslands, shrublands and savannahs, croplands and dry woodlands. Forest vegetation is usually poor, and is at low density with species adapted to arid soils and with a high water-use efficiency. Perennial vegetation varies considerably and tends to be sparse and patchy. Plants that have adapted to drylands survive irregular rainfall, high solar radiation and drought periods. Plants protect the soil surface from wind and water erosion. Removal or loss of vegetation cover results in an increased risk of soil erosion and degradation.

TABLE 7
Typical crops under rainfed conditions

FAO, 1993.

TABLE 8
Percentage land uses in arid regions in 1980

Source: Heathcote (1983).

The predominant land uses of the drylands are pastoralism and subsistence food production (Figure 6). Cereals produced in drylands include wheat, barley, sorghum and millet and pulses such as chickpea, lentils, peas and groundnuts (Table 7). Less important are oil crops (rape and lindseed) and a wide range of fruits, vegetables, herbs and spices. Pastoralism is widespread and highly mobile (Table 8). Food production is mainly from smallholding rainfed systems for subsistence or local consumption and markets. Natural woodlands are used for fuel wood, and efforts are ongoing to extend the forested areas for fuel and for CS. Chapter 3 describes the farming systems of drylands in detail.

The major constraint on agricultural development is low and highly variable rainfall and the consequent high risk for agriculture and animal husbandry. Traditional systems of rainfed cropping have evolved for thousands of years. Several general strategies have been developed to cope with low and erratic rainfall. Rainfed agriculture is generally practised in areas with a reasonable amount of rain and where soils are relatively deep. Drier regions are generally used for livestock grazing, with regular seasonal movements. Normally, several crops are sown to reduce the risk of total crop failure. Varieties that are resistant or adapted to drought are used. Long fallows are used to prevent stress on the land. During the fallow periods, soils are protected by a vegetation cover that provides nutrient and organic matter to the soils. Many pastoralists and sedentary farmers work together by exchanging crops and meat.

Characteristics of drylands that affect carbon sequestration

Dryland environments are characterized by a set of features that affect their capacity to sequester C. The main characteristic of drylands is lack of water. This constrains plant productivity severely and therefore affects the accumulation of C in soils. The problem is aggravated because rainfall is not only low but also generally erratic. Therefore, good management of the little available water is essential. In addition, the SOC pool tends to decrease exponentially with temperature (Lal, 2002a). Consequently, soils of drylands contain small amounts of C (between 1 percent and less than 0.5 percent) (Lal, 2002b). The SOC pool of soils generally increases with the addition of biomass to soils when the pool has been depleted as a consequence of land uses (Rasmussen and Collins, 1991; Paustian, Collins and Paul, 1997; Powlson, Smith and Coleman, 1998, Lal, 2001a). Soils in drylands are prone to degradation and desertification, which lead to dramatic reductions in the SOC pool. A good overview of the extent of land degradation in different dryland regions of the world is given in Dregne (2002). However, there are also some aspects of dryland soils that work in favour of CS in arid regions. Dry soils are less likely to lose C than wet soils (Glenn et al., 1992) as lack of water limits soil mineralization and therefore the flux of C to the atmosphere. Consequently, the residence time of C in drylands soils is long, sometimes even longer than in forest soils. The issue of permanence of C sequestered is an important one in the formulation of CS projects. Although the rate at which C can be sequestered in these regions is low, it may be cost-effective, particularly taking into account all the side-benefits resulting for soil improvement and restoration. Soil-quality improvement as a consequence of increased soil C will have an important social and economic impact on the livelihood of people living in these areas. Moreover, given the large extent of drylands, there is a great potential for CS. The potential offered by drylands to sequester C is large, not only because of the large extent, but because historically, soils in drylands have lost significant amounts of C and are far from saturation. Because of all of these characteristics, any strategy to re-establish SOM in these regions is particularly interesting (Box 1).

FIGURE 6 Major farming systems in the drylands ( arid, semi-arid and dry sub-humid) according to FAO, 2003

Source: Farming Systems and Poverty, FAO/World Bank, 2001

Desertification and carbon sequestration

The effects of desertification on soil quality include:

All of these effects accentuate the emission of CO₂ to the atmosphere. Lal (2001c) estimated the C loss as a result of desertification. Assuming a C loss of 8 - 12 Mg C/ha (Swift et al., 1994) on a land area of 1 020 000 000 ha (UNEP, 1991a), the total historic C loss would amount to 8 - 12 Pg C. Similarly, vegetation degradation has led to a C loss of 4 - 6 Mg C/ha on 2 600 000 000 ha, adding up to 10 - 16 Pg C. The total C loss as a consequence of desertification may be 18 - 28 Pg C. Assuming that two-thirds of the C lost (18 - 28 Pg) can be resequestered (IPCC, 1996) through soil and vegetation restoration, the potential of C sequestration through desertification control is 12 - 18 Pg C (Lal, 2001c). These estimates provide an idea about the loss of C as a result of desertification and the potential for CS through the restoration of soils in drylands.

Opportunities for improved land management as well as increasing CS should be developed in these areas. Agricultural systems contribute to carbon emissions through the use of fossil fuels in farm operations and through practices that result in loss of organic matter in soils. On the other hand, farming systems can offset carbon losses when accumulating organic matter in the soil, or when aboveground woody biomass is increased, which then acts either as a permanent sink or used as an energy source that substitutes fossil fuel. The potential for global benefits, as well as local benefits, to be obtained from increased CS in drylands should be an additional incentive for stronger support for reforestation and agriculture in drylands.

Although drylands have been studied (Heathcote, 1983; Thomas, 1997a, 1997b), the impact of desertification on the global carbon cycle and the potential impact of desertification control on CS in dryland ecosystems have not been widely investigated. There are few case studies, and little information. Consequently, there is little scientific evidence on the impact of desertification on carbon emission to the atmosphere. The aim here is to assess the state of knowledge, and the potential of different measures to increase CS.