Authors: Jantiene E.M. Baartman, Godert W.J. van Lynden, Mark S. Reed, Coen J. Ritsema and Rudi Hessel

Executive summary

Introduction
Desertification can be seen as a specific type of land degradation, occurring mainly, but not exclusively, in dryland regions. It is, for several reasons, almost impossible to give an accurate description of the severity and extent of desertification in the world. Despite extensive research, lack of good information on extent and severity of land degradation in drylands still hampers attempts to determine its significance (Dregne, 2002). It is generally accepted that a variety of both natural (climate; biophysical characteristics) and human-induced (land use; socio-economic) factors play a role in he occurrence of land degradation. Also, most scientists agree that participation of local stakeholders (e.g. farmers, local government etc.) is of key importance in the development and implementation of possible solutions. However, often the effects of solutions are not as successful as expected and new, alternative land use and management strategies need to be developed with the experiences of older strategies in mind. The review is strongly focused on the Mediterranean area – as is the DESIRE project but its scope is not exclusively limited to it.

This review: gives an overview of existing knowledge on desertification from published results of former projects and research; assesses the evidence of desertification; and indicates gaps in the existing knowledge.

Definition
Desertification is defined in this review as ‘land degradation in arid, semi-arid and dry sub-humid areas resulting form various factors, including climatic fluctuations and human activities’ (UNCCD, 2007), which is the most widely used definition.

Synthesis of existing and past projects

As has been indicated, many projects have been and continue to be carried out that are related to desertification. A synthesis of these projects is made, based on two types of criteria:

type or aim of the project (Table 1; databases, networks, programmes and projects; the latter are subdivided according to their aim or starting point); and the objectives of the project (policy or management oriented, improvement of knowledge, practical activities or techniques, identifying problems, identifying solutions, (use of) indicators, monitoring desertification and other).

A standardized description of 34 past projects and 14 ongoing projects is given in Appendix II of the report.

Table 1: Desertification projects organized in categories regarding type of project and aim

Evolution of desertification in the Mediterranean

To understand the origin and evolution of desertification, it is necessary to look at past environmental changes and processes in the Mediterranean. By doing so, the full length of timescales over which land degradation occurs can be defined and, as a consequence, the notion of reversibility of degradation can be put into a better long-term context (Wainwright, 2004).

The history of desertification in the Mediterranean follows the course of evolution of two groups of causes, i.e. natural and anthropogenic, and their interactions (Sciortino, 2001). Natural events acting on the environment were dominant until ~5000 BC (i.e. climatic fluctuations: glaciations, the Younger Dryas and the Climatic Optimum). Afterwards, human influence increased until the present (Grove, 1996; Quézel, 1999). It is important to realise, as Puigdefábregas and Mendizabal (1998) underline, that desertification as an outcome of climatic and social driving forces operating synergetically is not a new phenomenon in the Mediterranean region.

Human impact on Mediterranean landscape modification occurred very early, but the early impact on the environment was insignificant. Some incentives for human land use were the use of fire, the start of agriculture and pastoral livestock husbandry and the cultivation of fruit trees. These led to increased erosion and forest degradation. During Roman influence, population and cities grew, adding to the pressure on the land. After the decline of the Roman Empire, diseases and (religious) wars resulted in abandonment and extensive land use changes. The latest phase of human influence is the technological phase including mechanization, urban growth and tourism.

Primary drivers of desertification

The causes of dryland degradation are widely discussed in the literature but remain controversial (Thomas, 1997; Lambin et al., 2001; Reynolds and Stafford Smith, 2002; Geist and Lambin, 2004). However, most authors (e.g. Turner et al., 1995; Puigdefábregas, 1998; Geist and Lambin, 2004) agree that there is not one single factor that causes desertification or land degradation.

Both biophysical and socio-economic factors should be considered, even jointly, as they interact and reinforce each other to induce transition trigger events (Fig. 1; Turner et al, 1995; Puigdefábregas, 1998). Lambin et al., in 2001, conclude that various human-environment conditions react to and reshape the impacts of drivers differently, leading to specific pathways of land-use change and desertification.

Processes and consequences of desertification in the Mediterranean

As the concept of desertification is very broad, many environmental problems can be attributed to desertification (Martinez-Fernandez and Esteve, 2005). Here, we focus on the problems experienced in the Mediterranean area. A division is made in biophysical and socio-economic processes. However, it is not always possible to strictly divide problems in these categories, as interactions and feedback play a role.

The division should therefore not be seen as strict, but rather as a way of structuring occurring problems. Socio-economic and political factors include urbanization; competition for scarce water and unsustainable water management; abandonment; and policies. Biophysical processes include erosion; salinization; land use and vegetation change (agricultural change; overgrazing and overexploitation; deforestation); forest fires; flooding; sedimentation and siltation; and loss of biodiversity. This shows that the problems of desertification are more of a biophysical nature, while their causes can be both bio-physical and socio-economic or political. This is an important conclusion as it shows (once again) that when trying to solve or avoid desertification problems, not only bio-physical aspects should be assessed. As the biophysical problems are more visible, this could easily lead to the assumption that solutions should also be sought in that area. However, this synthesis of causes and problems shows that desertification is a complex issue and as such, simple solutions will not work.

Indicators: mapping and monitoring desertification

Mapping desertification prone areas is not only needed for developing a more thorough scientific understanding of the dynamic processes and driving forces, it also forms an important requirement for the drafting and implementation of development plans and policy decisions about the sustainable use of Mediterranean land resources (Hill et al., 1995; Lacaze et al., 1996).

However, caution should be taken in using maps of desertification risk, which have been used as if they were maps of actual desertification (Thomas, 1997). Also, the environmental, social and economic complexities of land degradation make accurate assessment a difficult challenge, especially in dynamic semi-arid environments. As direct monitoring and/or mapping of desertification is rather complicated, desertification is usually assessed by using indicators. Being dynamic processes, monitoring the indicators of desertification and land degradation should be a continuous activity and evaluation of the results should be done frequently. Techniques to assess these indicators include expert knowledge, land user perspectives, remote sensing and fieldwork. Of these, remote sensing is robust and fairly accurate, but remains restricted to a physical state assessment. With expert opinion, the socio-economic aspect can be included.

Land degradation indicators contain simplified, synthetic information on the state and tendency of complex processes such as desertification. Categories of indicators that play a role in desertification are ecological, economic, social and institutional. Until now, scientists have not reached consensus about a standard set of indicators to use in monitoring desertification (Pinet et al., 2006). Such consensus is probably not possible or even desirable as conditions and processes leading to desertification show such (spatial) variability that it is impossible to monitor desertification in any place without a set of site-specific indicators. As a result, many indices have been proposed to describe the susceptibility of drylands to desertification (Pinet et al., 2006; and e.g. Tongway and Hindley, 2000). Three projects that worked on indicator systems for desertification are DESERTLINKS (DIS4ME), MedAction and INDEX.

Modelling desertification

In most projects concerning desertification, modelling is included in at least one of the research stages. Here, a subdivision is made between biophysical and socio-economic modelling.

The biophysical models are classified based on themes that are relevant to desertification:
- Climate (i.e. modelling climatic variability and climate change (GCMs))
- Land surface – atmosphere exchange: Soil Vegetation Atmosphere Transfer models (SVATs)
- Land surface models
- Vegetation models
- Erosion and hydrological models

Increasingly sophisticated models are being used to represent the kinds of highly complex environmental, economic and social systems found in drylands susceptible to desertification. Social models such as agent-based models (ABMs) have begun being used in environmental disciplines to describe and predict the way people (‘social agents’ or ‘stakeholders’) are likely to behave in response to different stimuli given various decision-rules (Gilbert and Troitzsch, 1999, Janssen, 2002). However, these models tend to treat the environment as a static system (Matthews, 2006). In order to better approximate feedbacks and more accurately represent the complexity of real-life systems, dynamic models can be integrated from different disciplines. In this way it is possible to predict how people may respond to environmental change, and how their responses in turn are likely to influence their environment.

It is clear that it is impossible to comprehensively model desertification. However, much work has been done to model the various components and processes of desertification, both socio-economic and biophysical aspects. Also, various spatial scales are assessed in various projects, from plot and hillslope scale to European scale (e.g. PESERA). Expected progress in modelling includes improvement in up/downscaling, better DEMs, greater emphasis on physical reality than empiricism and importantly parameterisation and validation; on modelling the ecology of semi-arid vegetation and continued integration between physical, biological and socio-economic models, providing decision support against scenarios for environmental change (Mulligan, 2004).

Solutions

Next to the many projects and research on the processes, causes and extent of desertification, many solutions to desertification related problems (e.g. land degradation) have been proposed. As desertification problems are complex (e.g. Thomas, 1997), so are solutions (WOCAT, 2007). Reynolds et al. (2007) defined five lessons learned about sustainable development in the drylands, all of which show the complexity of the problem:

(1) Integrated approaches are needed; (2) Short term measures cannot solve slowly evolving conditions; (3) Dryland systems have nonlinear processes; (4) Cross-scale interactions must be anticipated; and (5) Greater value must be placed on local environmental knowledge.Even where solutions and remedial actions have been successful, they may not be simply transferable from one location to another, due to differences in the physical environment but also because cultural differences may make the components of the necessary actions unacceptable or difficult to apply (Thomas, 1997). Biophysical solutions include water conservation or harvesting, erosion reduction, grazing management, salinization and wildfire control.
While bio-physical solutions are important at the field level, these need to be embedded in an enabling environment. WOCAT (2007) defines this as the “Approach”: “the ways and means of support that help introduce, implement, adapt and apply SWC technologies on the ground”. This includes a variety of factors such as training and extension, markets, socio-cultural issues, participation, credit facilities, legislative and political issues, etc. Some important elements of socio-economic solutions are incentives; participation; training and extension; land tenure and land use rights; and research.

References

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• Geist, H.J., Lambin, E.F., 2004. Dynamic causal patterns of desertification. BioScience 54, 817 – 829.
• Gilbert, N., Troitzsch, K.G., 1999. Simulation for the social scientist, Milton Keynes, Open University Press.
• Grove, A.T., 1996. The historical context: Before 1850. In: Brandt, C.J., Thornes, J.B. (eds.), Mediterranean desertification and land use. John Wiley & Sons, Chichester, England, 554 pp.
• Hill, J., Sommer, S., Mehl, W., Megier, J., 1995. Use of earth observation satellite data for land degradation mapping and monitoring in Mediterranean ecosystems: towards a satellite-observatory. Environmental Monitoring and Assessment 37, 143 – 158.
• Janssen, M.A., 2002. Introduction. In: Ma, J. (ed.) Complexity and Ecosystem Management: the theory and practice of multi-agent systems. Cheltenham, Edward Elgar Publishing Ltd.
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• Lambin, E.F. et al., 2001. The causes of land-use and land-cover change: moving beyond the myths. Global environmental change 11, 261 – 269.
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• Thomas, D.S.G., 1997. Science and the desertification debate. Journal of Arid Environments 37: 599 – 608.
• Tongway, D., Hindley, N., 2000. Assessing and monitoring desertification with soil indicators. In: Arnalds, O., Archer, S. (Eds.). Rangeland desertification. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 89 – 98.
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• UNCCD, 2007. United Nations Convention to Combat Desertification. Official website of the United Nations Convention to Combat Desertification, http://www.unccd.int/.
• Wainwright, J., 2004. A review of European Union funded research into the history and evolution of Mediterranean desertification. Advances in Environmental Monitoring and Modelling, Special Issue Vol. 1, No. 4, 1 – 87.
• WOCAT, 2007: Where the land is greener – Case studies and analysis of soil and water conservation initiatives worldwide. Editors: H.P. Liniger and W.R.S. Critchley. Stämpfli AG, Bern, Switserland

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	Desertification and land degradation: origins, processes and solutions. A literature review
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