Desertification and development: Some broader contexts

https://doi.org/10.1016/j.jaridenv.2021.104575Get rights and content

Highlights

  • Desertification is discussed in the context of a mixed subsistence/cash economy.

  • A model estimates labour surplus as a function of climate, fertiliser and population.

  • The model is applied for both arable and pastoral economies.

  • Labour surplus provides for re-investment in the land or improving infrastructure.

  • Can arid lands be sustainably managed in an urbanising world?

Abstract

The dominant direct physical processes responsible for desertification are water erosion, wind erosion and salinization. Other threats that degrade the soil include loss of biodiversity, loss of soil organic matter, fire, changing water resources, soil compaction, soil sealing and contamination. Soil management inevitably combines human and physical effects. Climate, which is the most important driver of the physical systems, is now being rapidly modified by human action, and at a scale which is much coarser than any local remedial action. In a model of near-subsistence systems, productivity is limited by climate and available labour, with some options for additional inputs through improved seed, fertilizer or tillage equipment. Optimum solutions in a particular environment depend on both climate and access to markets. Agricultural surpluses, if any, allow investment in infrastructure – some of it directly supporting agriculture through irrigation and market systems, some less directly useful through, for example, warfare or pyramid building. Today some traditional drivers of desertification, based on subsistence agriculture and grazing, may have become less relevant, as land, particularly in the global South, is developed for intensive irrigated farming, and populations move into mega-cities. The dominant drivers may become soil sealing around cities and transfers of urban and irrigation water. In semi-arid areas this will lead to competition for the best land – for urban expansion and agricultural land with irrigation potential. Desertification then becomes an issue increasingly focussed on abandoned marginal land, maintaining biodiversity, managing regional water resources and controlling erosion in the face of global climate change.

Introduction

Desertification is defined by the United Nations Convention to Combat Desertification (UNCCD; Mainguet, 1999; Martello, 2004) as ‘land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities’. The dominant direct physical processes responsible are water erosion, wind erosion and salinization, but a number other threats that degrade the soil have also been identified (EU, 1999). These include loss of biodiversity, loss of soil organic matter, fire, changing water resources, soil compaction, soil sealing and contamination (Gregory et al., 2015). Management of both the soil and the broader environment are strongly influenced by human activity, so that it is almost impossible to disentangle human and physical effects. Even climate, which is the most important driver of the physical systems, is now being modified by human action, and at a scale which is much coarser than any local remedial action.

Desertification may be compartmentalised and analysed in terms of the three inter-dependent components included in the UNCCD definition, soil, water and people. The soil component refers to the land that is potentially degraded. In semi arid areas water is the key to the survival or degradation of the land; and human actors determine the balance between degradation and recovery of the land as they attempt to support themselves. The definition of desertification is related primarily to degradation of the soil, by water and wind erosion, by loss of organic matter and by salinization, and is relevant for both cultivation and grazing. These forms of degradation can be partially or completely offset by management practices, such as fallow rotation in shifting cultivation, by various forms of terracing or bund construction and through the application of manure or fertilizer (Morgan et al., 1994; Rose, 1994; Panagos et al., 2016). Water is also critical to desertification and its mitigation or prevention. The retention of scarce water is critical to crop production in semi-arid environments, either through irrigation or water harvesting (Critchely and Gowing, 2012) that control its spatial and temporal allocation for effective cultivation, and for the annual recovery of pasture and natural watering holes However, intense rains on poor soils can also increase soil erosion and lead to unwanted flooding. At a broader scale, water retention in headwater areas may increase water scarcity downstream.

Desertification and the potential for recovery can be seen as due to the interlocking of human and physical responses. Hessel et al., 2014, Puigdefabregas, 1995 Central to this, at a local scale, is the tension between sustainability, productivity and degradation. On the physical side, overuse of the land leads to degradation and abandonment. If degradation has not become irreversible, then the land slowly recovers, in a process of natural re-wilding, and may once again support sustainable, and perhaps more intensive, agriculture, risking a return to overuse and degradation. On the human side, improved public health has increased population pressure on the land, leading to over-exploitation of the land and forcing migration toward the cities and overseas. Money transfers from migrants then becomes essential components of an economy that may then sustain rural livelihoods.

The management of threatened lands has two components that cannot easily be separated. First it can be seen as a contribution towards maintenance of non-human ecosystems that are threatened by global climate change. Second it can be considered as ultimately, for the benefit of people, providing food and income for subsistence, and maintaining traditional ways of life. Some population is needed to till the land, and obtain fertilizer or other additional support needed, but any additional population needs to be fed, and is a drain on resources unless productive in other ways. Any food surplus can generate needed additional income by selling it directly, or through income earned (Nicholls, 1963; Onakuse, 2012). In many cases, additional income is also provided through remittances from out-migrants to the city or overseas. Where climatic conditions are marginal, the role of welfare may also be critical to continuing survival.

Combatting desertification is seen as one component in a broader view of managing our environment for the benefit of both the ecosystem and people in it. By doing do, global soil and food resources are maintained. Soil is a threatened and, on a human time scale, irreplaceable resource. ‘Tolerable’ rates of erosion may balance replacement of the ‘A’ horizon, but not its organic content, and, only very exceptionally, balance geological rates of weathering (Verheijen et al., 2009). By maintaining a healthy ecosystem that returns moisture to the atmosphere through transpiration, atmospheric moisture is re-cycled, allowing weather systems to convey moist air masses farther into continental interiors.

Not only can erosion be slowed by efficient agriculture that conserves the soil, but good practice fosters biodiversity and improves food security (Baiphethi and Jacobs, 2009). Effectively combatting desertification also helps to sustain rural life, slowing urban growth and maintaining national and cultural identities. One important component is to make best use of scarce water, constraining intensive irrigation and helping to minimise trans-national conflict. Effective action against desertification is unlikely to be effective unless there is clarity about the primary objectives so that soil conservation, for example, on its own, will provide only local and short-lived mitigation.

Some current literature (Puigdefabregas, 1998; Hellden, 2008) treats the balance between environmental resources and people as a predator-prey relationship. This analysis however ignores the potential for additional people to generate additional cash or other resources that support communities and is able to improve the quality of agricultural production through, for example. improved seed, fertiliser, machinery or wells. Surpluses may be redistributed within a single household, or at community or higher level. Where re-distribution is at the level of the state, this provides a mechanism for political control, as occurred in early Mesopotamian societies (Frangipane, 2018). Focusing here on more individual enterprise, the use of surpluses may represent transition from true subsistence farming to a more cash-based society (Alexandri et al., 2015; Kostov and Lingard, 2004). The challenge is to maintain both the environment and the cultural links between the farmer and the land in an economically viable way.

In near-subsistence systems, productivity is limited by climate and available labour, with some options for additional inputs through improved seed, fertilizer or tillage equipment that may be considered as alternative uses of labour, to earn the costs incurred. In a given physical environment, critical farmer decisions include the choice of cereal crop to grow, the level of fertiliser application and the balance between alternative uses for labour (to till the land or earn money that will buy fertiliser or equipment) Optimum solutions, maximising the number of families supported, lead to hunter-gatherer, subsistence or intensive farming, according to climate, its reliability and access to markets. Here we present a simple model that maps labour surplus as a function of available labour and fertilizer inputs in the context of near-subsistence cereal farming. Potential cereal production is estimated here from annual (wet season) rainfall and fertilizer input using a modified Michaelis and Menten, 1913 equation, based on data presented by Harmsen (2000) for crop yields in Syria. Although the equation for potential yield that is used here, and set out below, is quantitative in form, it is used primarily to exemplify the following observed qualitative behaviours.

  • 1.

    Response to annual rainfall is very limited below about 100 mm, rising more steeply thereafter, and reaching an upper limit of around 10 tonnes/Ha above annual values of 1000 mm (Doorenbos and Kassam, 1979).

  • 2.

    The addition of nitrogen fertilizer further reduces yields at low annual rainfall, but strongly and progressively increases yields at higher rainfalls (Cantero-Martinez et al., 2003).

  • 3.

    It is recognised that soils contain a low background level of available nitrogen that is supplemented by fertilizer additions.

These principles have been combined in this equation for potential yield.(Y0YP)nY=(N0N+NB)nN+{R0R[1exp(R5(N+NB))]}nRwhere YP = potential grain yield in T/Ha (1 T/year is assumed sufficient to support 5 people), R = annual rainfall in mm, N = Nitrogen fertilizer application rate (kg/Ha) and Y0, R0, N0, NB, nY, nN, nR are constants, assigned these values:

  • Y0 = 10 T/Ha: maximum upper yield

  • N0 = 100 kg/Ha: maximum useful nitrogen application rate

  • NB = 5 kg/Ha: background soil nitrogen level

  • R0 = 1000 mm/yr: maximum effective rainfall for yield increase

  • nY = 1: nN = 1: exponents of linear yield response to fertilisation

  • nR = 2: exponent provides threshold response at low rainfalls

The form of this expression gives a response that is dominated by the scarcer resource (water or nitrogen). Fig. 1 shows the response to rainfall and fertilizer inputs. In (a) it can be seen that for the expression in equation (1), production always increases with rainfall for a given fertilizer input, but (b) shows how higher fertiliser inputs raise the threshold rainfall required to obtain acceptable yields while offering the potential for greater yields in more humid environments. Increased fertilizer input has a negative impact on yields under arid climates, but an increasingly beneficial effect in wetter areas. These relationships have been developed to provide a quantitative illustration of what are essentially qualitative relationships. They have been generated with wheat in mind. The underlying physiological responses are common to other grain crops although each differs in detail, and, perhaps as importantly, in its suitability in areas of more saline soils or greater inter-annual variability of rainfall.

Potential yield is finally modified by labour available to cultivate the crop. It is assumed that an adequate labour force L is required to achieve maximum yields, and final yield YF is here related to the potential yield YP byYF=YP[1-exp(-L/L0)]where L0 = 200/km2.

These expressions have been used to estimate the labour surplus, if any, where a crop is grown in an environment defined by its annual (wet season) rainfall, with a known fertilizer input and supporting a known population density. The potential labour surplus S, providing resources to develop non-agricultural business or infrastructure, is then calculated asS=PLFwhere P is the resident population (per km2), L is the labour force needed to produce enough grain to feed the population. F is the labour force needed to earn enough to pay for the fertilizer applied.

To calculate the labour needed L, we solve the equationP/5=YP(1exp(L/L0)]

In this expression, the left hand side represents the yield needed to support the population and the right hand side is the actual production, where a labour force of L is engaged in cultivation.

Rearranging equation (4),L=L0 ln[1P/(5YP)]with solutions valid where L ≤ P.

Exploration of this model illustrates how, in less arid climates, there is scope for considerable investment in fertilizer and technical innovation, supporting large agricultural or labour surpluses, whereas in semi-arid climates there is little opportunity for improvement and minimal surplus. Fig. 2 shows that, in low rainfall areas, there may be only very restricted possibilities to create labour surpluses, and these are achieved with low fertilizer inputs. In contrast, wetter areas benefit from increased fertilization and are able to generate large labour surpluses, in many cases more than 50% of the total population. Fig. 3 shows the strength of this dependence on adequate rainfall to generate a viable production base. The diagram is drawn for a modest fertilizer input (14 kg/Ha), and shows a weak dependence on this value, with the threshold raised as additional fertilizer is applied.

Herding cattle or sheep as a primary food source has advantages in arid conditions where grain yields are no longer sufficient to justify tillage. Perhaps 50% of the world's livestock is supported in this way (Puigdefabregas, 1998). If sufficient grazing areas and watering locations are available, then sufficient fodder is always available if there is unrestricted nomadic or seasonal herd movement, but overgrazing can reduce the carrying capacity of the land, and adequate grazing must be available within range of water (Accatino et al., 2017; Tietjen and Jeltsch, 2007). To support a pastoral economy, there must be sufficient rainfall to support enough animals to feed the population. If that condition is satisfied then the number of herders required is a more or less constant proportion of the population, generating a labour surplus for other activities. In this way the surplus, if any, can be put in com parable terms to those for cereal cultivation above.

The first step in analysing the optimal management is to select the appropriate intensity of grazing. Fig. 1c shows a modelled example of the relationship between grazing pressure, expressed as percentage of biomass grazed and carrying capacity, based on applying the PESERA model (Kirkby et al, 2008) with climate data based on Cyprus. Similar relationships are found in observed data (eg. Miao et al., 2015). Under any given climatic conditions, increased grazing leads to reductions in biomass that soon outweigh the nutritional gains, giving a clear optimum for sustainable grazing pressure and carrying capacity, at a level which both allows pasture to be maintained and recover and brings benefits in limiting runoff and erosion.

With increasing rainfall, the optimum carrying capacity increases, more or less linearly. At a rainfall of 200 mm, it is assumed that optimal grazing will support a sheep population of about 6 per Ha and a human population of 20 per km2. Where population density is lower than this, approximately half the population is required to support herding, leaving the other half as a surplus labour pool. Combining the models for pastoral and arable subsistence, the available labour surplus for both activities is shown in Fig. 3. The choice of dominant agricultural activity is determined by the greater calculated labour surplus. It can be seen that pastoralism is only the more viable alternative in the most arid conditions and where population density is too low to farm the land. It should be borne in mind that weather and other conditions change from year to year, but that changes in farming style generally take place only slowly.

Agricultural surpluses have, since the beginnings of civilisation, allowed investment in infrastructure – some of it directly supporting agriculture through irrigation and market systems, some less directly useful through, for example, warfare or pyramid building. Exploitation of human or material resources has also provided surpluses that support industrial development in ways that may no longer be sustainable in the face of opportunities for mass migration. Today some traditional drivers of desertification may no longer be relevant, as land, particularly in the global South, is grabbed for intensive irrigated farming, and populations move into mega-cities. As labour surpluses become more important, there is also a drift away from subsistence to a more cash dominated economy. The dominant drivers may soon become soil sealing around cities and transfers of urban and irrigation water. In semi-arid areas this will lead to competition for the best land – for urban expansion and farmland with irrigation potential. Control of desertification is then increasingly focussed on the management of abandoned marginal land to maintain biodiversity for conservation and recreation. Abandoned land then also becomes critical for managing regional water resources and for controlling erosion which will become more severe as increasingly variable climates lead to greater frequency of fires and more intense storms.

Climate change and global heating are expected to increase aridity and make rainfall less effective in many areas, increasing abandonment as farmland becomes less productive and threatening nomadic herding. Coastal inundation and migration from marginal regions toward cities can only add to the potential risks. These trends are already apparent due to population growth and technological changes but are exacerbated and interact with desertification processes.

At progressively coarser scales, socio-economic factors become increasingly important. The model described above shows that one of the key drivers of desertification is population pressure, where the land can no longer support the farming population depending on it (Geist and Lambin, 2004). Although desertification and desertion are quite distinct concepts, there is no doubt that desertification leads to desertion and abandonment of the most marginal land. Where land is irreversibly degraded by erosion, with gullying of steep slopes, natural regeneration and recovery may be impossible, but, in most cases, the natural vegetation regenerates on a decadal time scale, and there has been an observed greening of much abandoned upland. By making full use of available rainfall, this greening may, however, reduce water availability to the water courses, transmitting the risk of desertification to areas downstream (Garcia-Ruiz et al., 2011).

Desertion of the land, together with the perception of greater prosperity, drives migration, initially to cities and, indirectly, to other regions and countries (Requier-Desjardins, 2006; Vidal-Macua et al., 2018). With an increasing risk of widespread desertification and increasing levels of information, these migration pressures have, in the past, and will, increasingly in the future, lead to potential conflict for scarce resources. Water, and land where it can be used, may be the critical resource. The equitable partition of river flows between headwater steepland source areas and downstream irrigable plains will become ever more contentious as marginal areas become increasingly degraded and abandoned.

Turning to the positive, the potential population surpluses shown in the model above provide the basis for development. Fig. 4 illustrates how these surpluses can, under favorable conditions, fuel regional or national investment in infrastructure and enterprise that can benefit agriculture and help to mitigate desertification. However, this potential may not be fully realised if government or other power structures siphon off too much of the wealth generated instead of investing it in developing infrastructure.

Surplus labour can, most directly, be applied to enhance agricultural production, through investment in improved seeds, adequate fertilizer and appropriate mechanisation. Such investments can enhance the population surplus that can provide earnings to allow cultivation of additional and/or more marginal land and to increase the prosperity of agricultural communities. More broadly, earnings increase GDP and provide governments with a tax base for improving rural infrastructure through, for example, roads, education, welfare, housing, energy and internet provision. Such improvements further improve production, providing access to markets and business opportunities. Under ideal circumstances, rarely realised in full, there is scope for a virtuous cycle with a positive feedback between agriculture, wealth creation and provision of infrastructure.

The potential for positive development may be constrained by both internal and external factors. Perhaps the key internal factor is population growth, with growing numbers that outstrip the ability of cities to accommodate them. Little resource is then available to support rural livelihoods, and farming communities have little incentive to remain on the land. Development is also squeezed by external factors. Some of these are built into the geography of each area, with differing access to non-renewable mineral and energy resources, and constraints of access to external markets dependent on transport systems and access to ports.

Section snippets

Discussion

It is a truism to say that there are complicated two-way links between desertification and development. Agricultural surplus is needed to support infra-structure, which can, in turn, further support agriculture, and this positive feedback can only take off if the required population are there, and the climate is sufficiently reliable to maintain accumulated surpluses from year to year.

Ideally there is then a benign positive feedback, in which labour surpluses support the improvement of health,

CRediT authorship contribution statement

Mike Kirkby: Conceptualization, Software, Writing – original draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (31)

  • The Medalus Project: Mediterranean Desertification and Land Use: Manual on Key Indicators of Desertification and Mapping Environmentally Sensitive Areas to Desertification

    (1999)
  • M. Frangipane

    From a subsistence economy to the production of wealth in ancient formative societies: a political economy perspective

    Econ. Politic.

    (2018)
  • H.J. Geist et al.

    Dynamic causal patterns of desertification

    Bioscience

    (2004)
  • A.S. Gregory et al.

    A review of the impacts of degradation threats on soil properties in the UK

    Soil Use Manag.

    (2015)
  • U. Hellden

    LU-CDM, A Conceptual Model of Desertification

    (2008)
  • Cited by (0)

    View full text