David S.G. Thomas

World atlas of desertification.

This text aims to summarize the state of scientific knowledge on the drylands of the globe. It explores the current stage of understanding of desertification as well as its extent and possible solutions. The book argues that desertification is one of the worlds most pressing environmental problems and that it is a global issue which is accelerating. This edition has been revised and expanded to include updated computer images of desertification, as well as fuller descriptions and explanations of the issues concerned. Using detailed data of the physical and chemical status of soil degradation provided by the Global Assessment of Soil Degradation, the book is fully referenced and covers topics including: desertification and global warming; monitoring on the ground and by remote sensing; vegetation and degradation; local action; cultural factors; financial issues; land reclaimations; the political economy and desertification; and desertification and refugees.

Remobilization of southern African desert dune systems by twenty-first century global warming

Although desert dunes cover 5 per cent of the global land surface and 30 per cent of Africa, the potential impacts of twenty-first century global warming on desert dune systems are not well understood. The inactive Sahel and southern African dune systems, which developed in multiple arid phases since the last interglacial period, are used today by pastoral and agricultural systems that could be disrupted if climate change alters twenty-first century dune dynamics. Empirical data and model simulations have established that the interplay between dune surface erodibility (determined by vegetation cover and moisture availability) and atmospheric erosivity (determined by wind energy) is critical for dunefield dynamics. This relationship between erodibility and erosivity is susceptible to climate-change impacts. Here we use simulations with three global climate models and a range of emission scenarios to assess the potential future activity of three Kalahari dunefields. We determine monthly values of dune activity by modifying and improving an established dune mobility index so that it can account for global climate model data outputs. We find that, regardless of the emission scenario used, significantly enhanced dune activity is simulated in the southern dunefield by 2039, and in the eastern and northern dunefields by 2069. By 2099 all dunefields are highly dynamic, from northern South Africa to Angola and Zambia. Our results suggest that dunefields are likely to be reactivated (the sand will become significantly exposed and move) as a consequence of twenty-first century climate warming.

Multiple episodes of aridity in southern Africa since the last interglacial period

There is generally a dearth of evidence of the nature of Quaternary climate change within desert systems, which has limited previous interpretations of past environmental change at low latitudes. The Last Glacial Maximum has previously been identified as the peak of Late Quaternary aridity, when desert systems expanded to five times their present extent, and low-latitude aridity has been described for previous glaciations. But little evidence has been derived directly for large desert basins, particularly southern Africa. Here we report new chronological (optical dating) evidence of arid episodes recorded in aeolian sediments from the Mega Kalahari sand sea. Episodic aeolian activity is recorded at the northeastern desert margin, whereas more sustained activity is evident from the southwestern desert core. Several significant arid events are apparent since the last interglacial period, with dune-building (arid) phases at ∼95–115, 41–46, 20–26 and 9–16 kyr before present. Existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast–southwest summer rainfall gradient, which are in turn related to sea surface temperatures in the southeastern Atlantic Ocean.

Late Quaternary environmental change in central southern Africa: new data, synthesis, issues and prospects

Much of the interior of central southern Africa is a sand sea, within which aeolian and lacustrine landforms and sediments of local and regional extent are preserved. Closed cave sites are restricted to very few locations, while fluvial systems traverse the margins of the interior. Until the early 1990s, chronologies of late Quaternary environmental and climatic changes developed for this region were based on only a limited number of proxy data sets, derived largely from lacustrine deposits and precipitates. In particular, directly determined ages from aeolian deposits, the most extensive suite of features in the region, were absent. The application of luminescence-dating techniques to dune sediments, and the development of further detailed chronologies from cave precipitates, is now providing a more comprehensive record for the last 50 ka, with some chronologies extending back a further 100 ka. We present and review these data, assess their contribution to enhanced understanding of late Quaternary environmental changes in the region, and for the first time assess them against corrected radiocarbon ages from lacustrine sites. It is concluded that there is now an enhanced understanding of the spatial and temporal complexity of climate changes affecting the region in the last glacial cycle, including a complex record of punctuated aridity, but that many issues, including data-set integration and forcing mechanism controls, are imperfectly understood.

Sedimentation and diagenesis of Chinese loess : Implications for the preservation of continuous, high-resolution climate records

Chinese loess has been extensively utilized to produce continuous and high-resolution climate records of the late Cenozoic. Such work assumes uninterrupted loess deposition and limited diagenesis. Here, closely spaced optically stimulated luminescence (OSL) dates are used to characterize the Holocene and Late Pleistocene sedimentation histories of three sites across a NW-SE transect of the Chinese Loess Plateau. The results suggest that sedimentation is episodic at subglacial-interglacial time scales, with rates rapidly varying within units and between sites. Unconformities, noneolian deposition, and mixing of sediments also appear to be common. Existing understanding of loess deposition therefore requires reexamination, while previous reconstructions of rapid climate change, not dated using absolute methods, should be regarded with caution. Loess deposits may still yield detailed climate records from specific high-sedimentation-rate strata, and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating. The rapid changes in sedimentation presented here indicate the East Asian Monsoon has the capacity to vary on millennial scales.

Evaluating Successful Livelihood Adaptation to Climate Variability and Change in Southern Africa

This paper examines the success of small-scale farming livelihoods in adapting to climate variability and change. We represent adaptation actions as choices within a response space that includes coping but also longer-term adaptation actions, and define success as those actions which promote system resilience, promote legitimate institutional change, and hence generate and sustain collective action. We explore data on social responses from four regions across South Africa and Mozambique facing a variety of climate risks. The analysis suggests that some collective adaptation actions enhance livelihood resilience to climate change and variability but others have negative spillover effects to other scales. Any assessment of successful adaptation is, however, constrained by the scale of analysis in terms of the temporal and spatial boundaries on the system being investigated. In addition, the diversity of mechanisms by which rural communities in southern Africa adapt to risks suggests that external interventions to assist adaptation will need to be sensitive to the location-specific nature of adaptation.

A record of rapid Holocene climate change preserved in hyrax middens from southwestern Africa

The discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central Namib Desert, Africa, is providing unprecedented insight into the region’s environmental history. High-resolution stable carbon and nitrogen isotope records spanning 0–11,700 cal (calibrated) yr B.P. indicate phases of relatively humid conditions from 8700–7500, 6900–6700, 5600–4900, and 4200–3500 cal yr B.P., with a period of marked aridity occurring from 3500 until ca. 300 cal yr B.P. Transitions between these phases appear to have occurred very rapidly, often within <200 years. Of particular importance are: (1) the observed relationship between regional aridifi cation and the decline in Northern Hemisphere insolation across the Holocene, and (2) the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing. Together, these elements call for a fundamental reexamination of the role of orbital forcing on tropical African systems, and a reconsideration of what factors drive climate change in the region. The quality and resolution of these data far surpass any other evidence available from the region, and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern Africa.

Environmental Change in the Kalahari: Integrated Land Degradation Studies for Nonequilibrium Dryland Environments

Recent decades have seen major intensification of cattle-based agricultural production in semiarid savanna ecosystems. In the Kalahari of Botswana, cattle production now occurs on privatized and fenced ranches. Patterns of ecological change, notably increased bush dominance, have been linked to increased cattle-grazing intensity, but it remains contentious whether these changes represent land degradation. Uncertainty in ecological understanding stems from the dynamic, “nonequilibrium” functioning of semiarid ecosystems. Given the inherent ecological variability of drylands, we argue that degradation assessments should be based, not on ecological observations alone, but on the study of long-term changes in pastoral production figures and on changes in the ecologically determining factors of soil water and soil nutrient availability. Provided here is a framework incorporating soil and ecological changes at a range of scales that can enable us to distinguish drought-induced fluctuations from long-term ecolog...

Optical dating of abrupt shifts in the late Pleistocene East Asian monsoon

Chinese loess is regarded as one of the most detailed and complete terrestrial archives of late Cenozoic climate change. However, high-resolution optically stimulated luminescence (OSL) dates presented here reveal that the suborbital chronological framework of Chinese loess used in many previous climate reconstructions requires reassessment. Chronological uncertainty of as much as 10–15 k.y. for the late Pleistocene is largely a result of the widespread use of nonradiometric dating techniques that fail to account for site-specific depositional conditions associated with loess emplacement and diagenesis. OSL-based age models that account for these processes are used to examine detailed records of past sedimentation, as well as grain size and magnetic susceptibility proxies for late Pleistocene East Asian monsoon variation. Abrupt shifts in monsoon proxies occur over 10 2 –10 3 yr time scales, potentially forced by a variety of factors and influenced by site location and site-specific changes in sedimentation.

Late Pleistocene wetting and drying in the NW Kalahari: an integrated study from the Tsodilo Hills, Botswana

Abstract The sediments and landforms at the Tsodilo Hills, in the northwestern Kalahari desert, provide an opportunity to directly investigate the late Quaternary wetting and drying of the region from evidence at a single site. Lacustrine carbonates, including incorporated molluscs and diatoms, a lake shoreline feature and stabalised linear dunes were investigated for their constituent palaeoenvironmental signals. Chronometric control is provided by calibrated 14 C , AMS and OSL dating. The evidence suggests that linear dune construction has not occurred since the Last Glacial Maximum, with particular development from 36 to 28 ka . Lake stands indicating wetter regional conditions than present occurred at 40– 32 ka , with more seasonal conditions from 36 ka , and at 27– 12 ka with a possible drying out at 22– 19 ka . Data are consistent with other independent studies from the region, and with recent evidence obtained from Atlantic cores off the coast of Namibia. It is concluded that careful consideration of multi-proxy data from a single location can assist in resolving discrepancies that arise from independent studies of lake, cave and dune records in the Kalahari.