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The motivation for quantitative assessment of land degradation at a global scale is its recognition as an environmental issue of global societal implications. Yet, due to the non-robust definition of “land degradation” and to the paucity of field data, the five global assessments carried out and presented between 1977 and 2003 differ in the selection of measurable attributes of land degradation, in the quality of the data sets, and in their spatial coverage. This resulted in a plethora of degradation estimates ranging 15% to 63% of global degradation and 4% to 74% of dryland degradation. Of these, the figure of 70% degradation (for the drylands only, comprising 41% of global land) has been cited more than the others. Though likely to be overly exaggerated (because it stands for a combination of degradation degree of a land unit and its spatial extent within the mapping unit of which it is a part), this high estimate has apparently served well the globality notion of the dryland degradation syndrome, essential to rallying support for international development assistance under the UNCCD. This thirst for development assistance aimed at “combating desertification” attracted to the UNCCD some 70 non-dryland developing countries (compared to 93 developing dryland country Parties) which experience land degradation that is not included in global assessments of desertification, since only dryland degradation qualifies as “desertification”. The texts of the various assessments, including that of GLASOD as well as the UNCCD definition often trade off “desertification” with “susceptibility” to or “threat” of desertification. This suggests that an assessment of vulnerability to desertification rather than its actual occurrence are of higher credibility and utility for policy- and decision-making.

Though soil degradation featured highly in the currently available global degradation assessments, remotely-sensed vegetation attributes not only assess the most valued but threatened ecosystem service, but are also amenable for assessment at the global scale. However, caution is required when using this tool especially in drylands where productivity is tightly linked to rainfall variations. The monitoring required to meet the persistence criterion for qualifying desertification can be also used to detect current desertification trends, which are of relevance for policy-making even more than defining current desertification status. To discern changes of productivity due to state of the land from those due to rainfall features, the ratio of NPP to rainfall (RUE) could be useful were it not negatively correlated with rainfall itself. An alternative method for detecting degradation trends, the Residual NPP Trends (RESTREND) is currently under development. It is based on an analysis of the residuals of the productivity-rainfall relationship throughout a time period for each pixel in the explored region. A statistically significant negative regression of the residuals on time identifies a degradation trend, and the slope stands for its magnitude. To be reliable on a global scale such a remote-sensing approach would serve for guiding field observations required for its own verification.

There is insufficient data on the extent, severity and trend of land degradation in Africa. Through a four steps approach, which is based on the identifi- cation of a pillar layer (FAO problem soils map) and the combination of dynamic factors (human activities, livestock, climate) to determine risk factors, the current paper aims at providing quantitative data on the status and trend of land degradation at agro-ecological and main river basin levels.

The results revealed that: (i) “hot spots” of land degradation are largely predominant at continental level, compare to the “bright spots” of very low to low degradation; (ii) there is an increasing trend of severity and extend of land degradation from the humid zones of the Congo and Zambezi basins (24 to 29%) to the dry areas of the Nile, Niger and lake Chad basins (78 to 86%); (iii) the interrelation and cumulative effects of water and wind erosion are also increasing along these agro ecological zones.

The study also stressed on the high spatial variability in the extent and trend of degradation process according to the various agro-ecological zones and river basins. This variability could be strongly linked to soils behaviour and level of resistance, to the quality of their surrounding environment as well as to the impact of investments for conservation of natural resources and for better land care.

Land degradation is a universal problem; it has cumulative effects at regional and global scales. This paper, based on the 500-meter resolution satellite image (band 1∼7), which was acquired in 2003, and the correlative statistical data, examines the status of land degradation and desertification in the Asian Region. The perspective trends of land degradation are analyzed and indicated. The primary purpose of this study was to assess the major causes of land degradation and desertification in Asia, to suggest procedures and methods for combating desertification and mitigating the effects of land degradation and promote sustainable development.

Latin America has a rich reserve of genetic resources. About 40% of the known living species are present in this Region. The continent represents an important reserve of cultivated land and fresh water. About one third of the forest of the world lives in their important tropical and temperate biomes, much of them are in pristine condition. Despite its genetic richness, important deforestation has affected mainly coastal ecosystems, settlement of the most part of its population. Originally, this continent had 6.93 millions km of forests. At present only 3.66 of its original forest coverage remain. Present rate of forest loss is 15,000 km yr, that is to say, almost 3 ha per minute. About 45% of croplands in South America and 74% in Meso-America are degraded. The arid lands are threatened by desertification and very often by droughts. Both phenomena have high social costs pushing millions of people to move to cities, creating social pressure in urban areas. This is one of the sources of crime increase and political instability in many countries. At present, the tropical rain forest continues to be cleared, mainly using fire, to open lands for annual crops and pastures. There are some biomes like temperate forest in Chile and Argentina, the Mata Atlantica in Brazil, the dry subtropical forest of the Chaco, that have been reduced to small patches.

The main source of human pressure on the environment comes from unsound agricultural practices and interventions on natural ecosystems to extract goods and services. In addition, climate has been fluctuating forcing important landscape changes in the last thousand of years. Climatic trends are evident in extensive areas of the continent. Temperature in tropical Andes shows a significant warming of about 0.33°C per decade since the mid-1970s. Minimum temperature has increased as much as 2°C in some coastal areas. In the South Western Pacific coast, rainfall has shown a clear negative trend throughout the 20 century. A contrary trend has been observed in the Atlantic coast of Argentina and Southern Brazil. Climatic variability seems to be increasing, making more frequent extreme climatic events of drought and floods. In the overall continent a rapid reduction in the permanent ice bodies is observed, mainly Andean permafrost and glaciers, which moved upward their lower front about 300 m or more in a century. Main biomes of the continent are subjected to different natural and human drivers or pressure. Among the most threatened biomes are the Caatinga in Brazil, the Amazon rain forest, temperate forest of the Patagonia, the arid and semiarid forest and highlands of Puna.

Increasing climatic hazards are forcing farmers to opt low input agriculture in order to reduce economic risk. This leads to a marginal agriculture, associated with low yields and income, and consequently, social deterioration and very often, the primary cause of massive migrations. In some areas, farmers will never be able to adapt to these conditions at the required speed. Currently, prices of agricultural products are at the lower limit to support reductions on yields, so, farmers are in an extremely vulnerable condition.

The adoption of the EU Thematic Strategy for Soil Protection by the European Commission on 22 September 2006 has given formal recognition of the severity of the soil and land degradation processes within the European Union and its bordering countries. Available information suggests that, over recent decades, there has been a significant increase in soil degradation processes, and there is evidence that these processes will further increase if no action is taken. Soil degradation processes are driven or exacerbated by human activity. Climate change, together with individual extreme weather events, which are becoming more frequent, will also have negative effects on soil. Soil degradation processes occurring in the European Union include erosion, organic matter decline, compaction, salinisation, landslides, contamination, sealing and biodiversity decline. Effective soil protection policies can only be based on a detailed assessment of the costs of non-action, and the potential economic benefits from enhanced soil protection strategies in Europe. The total costs of soil degradation for erosion, organic matter decline, salinisation, landslides and contamination on the basis of available data, would be up to €38 billion annually for EU25. These estimates are necessarily wide ranging due to the lack of sufficient quantitative and qualitative data. The Soil Thematic Strategy of the European Union paves the way towards adequate measures in order to reverse the negative trends in soil and land degradation in Europe and will have also an extensive impact at the global scale by promoting similar actions in the framework of internationally binding agreements related to land degradation, like the UNCCD, UNFCCC and CBD.

The definition of land degradation in the United Nations Convention to Combat Desertification (UNCCD) gives explicit recognition to climatic variations as one of the major factors contributing to land degradation. In order to accurately assess sustainable land management practices, the climate resources and the risk of climate-related or induced natural disasters in a region must be known. Land surface is an important part of the climate system and changes of vegetation type can modify the characteristics of the regional atmospheric circulation and the large-scale external moisture fluxes. Following deforestation, surface evapotranspiration and sensible heat flux are related to the dynamic structure of the low-level atmosphere and these changes could influence the regional, and potentially, global-scale atmospheric circulation. Surface parameters such as soil moisture, forest coverage, transpiration and surface roughness may affect the formation of convective clouds and rainfall through their effect on boundary-layer growth. Land use and land cover changes influence carbon fluxes and GHG emissions which directly alter atmospheric composition and radiative forcing properties. Land degradation aggravates CO-induced climate change through the release of CO from cleared and dead vegetation and through the reduction of the carbon sequestration potential of degraded land.

Climate exerts a strong influence over dry land vegetation type, biomass and diversity. Precipitation and temperature determine the potential distribution of terrestrial vegetation and constitute principal factors in the genesis and evolution of soil. Precipitation also influences vegetation production, which in turn controls the spatial and temporal occurrence of grazing and favours nomadic lifestyle. The generally high temperatures and low precipitation in the dry lands lead to poor organic matter production and rapid oxidation. Low organic matter leads to poor aggregation and low aggregate stability leading to a high potential for wind and water erosion. The severity, frequency, and extent of erosion are likely to be altered by changes in rainfall amount and intensity and changes in wind. Impacts of extreme events such as droughts, sand and dust storms, floods, heat waves, wild fires etc., on land degradation are explained with suitable examples. Current advances in weather and climate science to deal more effectively with the impacts of different climatic parameters on land degradation are explained with suitable examples. Several activities promoted by WMO’s programmes around the world help promote a better understanding of the interactions between climate and land degradation through dedicated observations of the climate system; improvements in the application of agrometeorological methods and the proper assessment and management of water resources; advances in climate science and prediction; and promotion of capacity building in the application of meteorological and hydrological data and information in drought preparedness and management. The definition of land degradation adopted by UNCCD assigns a major importance to climatic factors contributing to land degradation, but there is no concerted effort at the global level to systematically monitor the impacts of different climatic factors on land degradation in different regions and for different classes of land degradation. Hence there is an urgent need to monitor the interactions between climate and land degradation. To better understand these interactions, it is also important to identify the sources and sinks of dryland carbon, aerosols and trace gases in drylands. This can be effectively done through regional climate monitoring networks. Such networks could also help enhance the application of seasonal climate forecasting for more effective dryland management.

The frequency of occurrence of climate extremes in temperature and precipitation is expected to increase during the next century (). Here we examine the impact of the climate extremes of heavy rainfall, drought, and high winds, on processes of land degradation, including floods, mass movements, soil erosion by both water and wind, and salinisation. Case studies are used to explore the impacts of individual events on land degradation, as well as the role of decadal-scale temporal and spatial variability in climate systems in driving extreme events. Predictions of future trends in the frequency and magnitude of extreme events, based on an ensemble of general circulation models and on regional climate models, are examined

The impact of some meteorological parameters on land degradation in Tanzania is analysed. Rainfall is responsible for floods in case if it is in excess and drought in case of deficit. In recent years, parts of Tanzania have experienced recurring droughts. The most devastating droughts were those of 1983–1984 and 1993–1994. According to Tanzania historical data, droughts occur every four years which affect over 3.6 million people. The most frequently hit areas are, central areas of Dodoma, Singida and some parts of Pwani, Shinyanga, Mwanza and Mara. Experience over the twenty-year period from 1980 to 2000 has shown that floods occurred 15 times, killing 54 people and affecting 800,000 people. Flood prone regions are Tanga, Mbeya, Pwani, Morogoro, Arusha, Rukwa, Iringa, Kigoma and Lindi. The impact of El-Niño rains is discussed and the probability of rainfall exceeding specific thresholds is analysed. Wind erosion is discussed. The impact of climate change and its relationship to land degradation is analysed. Finally the impact of solar radiation, temperature and evaporation is discussed. The paper concludes that climate and weather contribute significantly to land degradation in Tanzania. The paper recommends that there is a need to: make an inventory of national land resources; assess potentials and constraints in dryland farming; identify agricultural options to safely increase cropping intensity and yields; adopt more sustainable forms of land use, including contingency crop planning in the case of droughts; study the reasons behind poor land use; encourage pastoralists to reduce their herds of stocks and finally encourage the use of indigenous knowledge in land preservation.

The complexity of the notion ‘land’ and its scale features leads to many different definitions of land and land degradation. Among the components of land degradation are desertification, soil degradation and erosion. There is a wealth of literature claiming that land degradation is serious. These so-called doom papers are based on ‘hard’ facts from remote sensing, computer models and measurements. However, other papers have raised the question ‘how serious is land degradation?’ In any discussion of land degradation there are four spatial-temporal scales that should be distinguished: regional, watershed, field and point. At each scale, one may use different proxies for land degradation. When assessing land degradation, ‘experts’ tend to overestimate the phenomenon; they should take better account of its spatial and temporal dimensions. One of the main reasons they currently overestimate land degradation is that they underestimate the abilities of local farmers, many of whom have been able to modify their land management. In order to study land degradation at multiple scales it is also necessary to study rainfall at multiple scales. Rainfall can be analysed for land degradation at four different scales: from the ‘small’ annual scale to the ‘large’ minute scale. Besides scales one may also distinguish between average values and temporal and spatial variations. In this paper, one or more examples of rainfall data are presented for each scale and interpreted with respect to land degradation. At the annual scale, trend analysis and rainfall probabilities are important. The decadal (10-day) scale is especially suitable for calculating the varying lengths of the growing season. At the scale of one day, the size classes of showers, return period (design storm), hydrological and agronomic modelling and dry spell analysis are discussed. At the minute scale, erosivity in El Niño and La Niña years is important.

Rainfall meets land at the soil surface. Rainfall is divided over several pedohydrological components. Green water is that part of rainfall that is stored in the soil and available to plants. Land degradation decreases infiltration, waterholding capacity and transpiration, but enhances runoff and soil evaporation. These agrophysical processes decrease the Green Water Use Efficiency (GWUE; the ratio of transpiration to precipitation). Special attention is given to estimating the effects of land degradation on ‘computing available soil moisture’ in order to understand what farmers perceive as drought. Rain falling on the land may be intercepted by vegetation, run off the ground surface, or infiltrate into the soil; this is reflected in the rainwater balance. Infiltrating water may be stored in the root zone or drain below the root zone to groundwater and stream base flow, contributing what is nowadays called ‘blue water’. These processes are reflected in the infiltration water balance. The maximum amount of water stored in the root zone available for plant growth is a very important soil characteristic because it determines the potential survival of plants in a dry spell. Water stored in the root zone may be lost as evaporation from the soil surface into the atmosphere, or taken up by plants and lost as transpiration. This is reflected in the soil water balance. In drylands in sub-Saharan Africa the GWUE ranges from 5–15%, which is very low. In East Africa it may reach 20%, but in comparable climates in the USA the GWUE may be above 50%.

The concepts of land degradation mitigation are derived from the rainwater balance. After drawing a number of conclusions, it is suggested how we could improve our understanding of land degradation by improving the availability of rainfall data at multiple scales.

Forty one years of daily data (1960–2000) have been used to investigate the spatial and temporal distribution of wet and dry spells during the rainfall seasons of Tanzania. The frequency characteristics of wet and dry spells were based on the threshold of 1.0 mm of daily rainfall events. The observed frequency of wet and dry spells during the rainfall seasons over the period of study showed that at both bimodal and unimodal locations the frequency of occurrence of one-day wet and dry spell was highest at all locations then reduced smoothly as the length of the season progress. The analysis gave an indication that the longest run of wet spell of 25-days occurred during March - May rainfall season at Lushoto over the north eastern highlands on the bimodal regime. Similarly, longest run of wet spells of 28 days was also observed at Mbeya and Mahenge over the southwestern highlands on the unimodal regime. Longest dry spells run were noted over the semi arid parts of Tanzania including Dodoma, which registered the longest run of 249 days that occurred in 1999 and coincided to a cold El Niño-Southern Oscillation (ENSO) event.