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Remobilisation of southern African desert dune systems by 21st century global warming

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... These dunes are not considered active but formed under different environmental conditions, which were drier and/or windier and did not allow vegetation to establish (Ash & Wasson, 1983;Lancaster, 1988). Future climate change may reduce vegetation cover in some areas, resulting in sand transport and possible desert expansion (Knight, Thomas, & Wiggs, 2004;Thomas, Knight, & Wiggs, 2005;Wang, Yang, Dong, & Zhang, 2009;Yizhaq, Ashkenazy, & Tsoar, 2009). For both these scenarios, reconstructing past climate conditions, and predicting future landscape changes, a better understanding of the relationships between climate, vegetation cover and dune activity is required. ...
... In this study, we use the term 'active' to refer to dunes which have visible bare slip faces on their crests. These dunes would largely fall into the class of 'restricted dune activity' of Thomas et al. (2005). Three specific questions were identified relating to the main aim: ...
Article
The stable longitudinal dunes in the northern Simpson Desert, Australia, were observed in satellite imagery to become more active after vegetation cover was reduced by fire and drought. Subsequent rainfall events also resulted in significant vegetation regrowth and dune stabilisation. These switches between more active and stable conditions have not been previously described in the largely vegetated dune fields of central Australia. The observations, made on 12 dune sites, relied on high spatial resolution satellite imagery to observe dune crest activity, and seasonal Landsat fractional cover imagery to observe vegetation cover changes. The non‐photosynthetic vegetation (NPV) component of the fractional vegetation cover images revealed significant changes in hummock grass cover on the dunes between 1988‐2018, with a positive relationship with the 3‐year cumulative rainfall, disrupted by two periods of patchy burning. Only those sites that had burnt became active, and only after vegetation cover had remained low (NPV < 16%) during the ‘Millennium Drought’. There is no threshold in vegetation cover, below which dune crests become active, but active dune features require 4‐years of low NPV cover (<16%) to develop. The large rainfall event that ended the drought increased NPV cover, stabilising the dunes. Similar hummock grass covered dunes are present across large areas of the arid zone, and are likely to respond in similar ways, given that fire and drought are common occurrences in Australia.
... The fine dust flux produced by beach sand at Oceano (Fig. 2) tentatively supports the hypothesis that dust emissions from active sands might have been globally important sources of dust in past climates, and that emissions from active sands might 15 increase in the future because of the reactivation of inactive sand dunes (Mason et al., 2003;Thomas et al. 2005;Bhattachan et al., 2012). Many stable dunes have accumulated extensive clay-mineral coatings (Bowler, 1973;Gardner and Pye, 1981;Muhs et al., 1997), and, if activated, these dunes could produce substantial fine dust emissions through sandblasting-induced removal of clay-mineral coatings (Bullard et al., 2007;Swet et al., 2018). ...
... Nonetheless, our measurements support previous work indicating that clay-coating removal is likely a major emission process for active sands (Bullard et al., 2007). As such, dust emissions from active sands could increase because land-use changes and desertification might reactivate currently stable sand dunes, which can have accumulated extensive clay-mineral coatings (Thomas et al., 2005). Because this process is not accounted for in most current climate models, it could enhance future dust effects on climate beyond what current models simulate . ...
Article
Full-text available
Sand dunes and other active sands generally have a low content of fine grains and, therefore, are not considered to be major dust sources in climate models. However, recent remote sensing studies have indicated that a surprisingly large fraction of dust storms are generated from regions covered by sand dunes, leading these studies to propose that sand dunes might be globally-relevant sources of dust. To help understand the dust emission potential of sand dunes and other active sands, we present in situ field measurements of dust emission under natural saltation from a coastal sand sheet at Oceano Dunes in California. We find that saltation drives substantial dust emissions from this setting. Laboratory analyses of sand samples suggest that these emissions are produced by aeolian abrasion of feldspars and removal of coatings of clay minerals on sand grains. We further find that this emitted dust is substantially finer than dust emitted from non-sandy soils and dust observed in situ over North Africa. As such, dust emitted from the sand sheet, and potentially from other active sands affected by similar dust emission processes, could have potent impacts on climate, the hydrological cycle, and human health. These measurements thus support the hypothesis that considerable emissions of fine dust can be generated by the reactivation of inactive dunes with accumulated clay minerals. This might occur due to future land-use changes and desertification, and is not currently represented in most climate models.
... This sand now covers one-third of the Nation's land, forming dunes that move 40 m per year (Fig. 1a) across roads, gardens and grazing land, threatening hundreds of homes [1]. Many arid regions have similar problems: sand dunes encroach on solar panel installations in the Mojave Desert, and global warming has increased dune movement across farmland in Namibia and Angola [2]. As global warming continues, these areas will likely get warmer, drier [3], and sandier. ...
Preprint
A dry decade in the Navajo Nation has killed vegetation, dessicated soils, and released once-stable sand into the wind. This sand now covers one-third of the Nation's land, threatening roads, gardens and hundreds of homes. Many arid regions have similar problems: global warming has increased dune movement across farmland in Namibia and Angola, and the southwestern US. Current dune models, unfortunately, do not scale well enough to provide useful forecasts for the $\sim$5\% of land surfaces covered by mobile sand. We test the ability of two deep learning algorithms, a GAN and a CNN, to model the motion of sand dunes. The models are trained on simulated data from community-standard cellular automaton model of sand dunes. Preliminary results show the GAN producing reasonable forward predictions of dune migration at ten million times the speed of the existing model.
... The change to vegetated dunes typically takes place in concert with climate change, and a trend to higher rainfall, higher water tables, lower wind speeds, etc. (e.g. Thomas and Tsoar 1990;Swezey 2001;Thomas et al. 2005;Tsoar 2005;Singhvi et al. 2010). The original dune form may be pre- served largely intact as, for example, it appears to have been in some of the Australian linear dunes systems, or in the example in Figure 7.11b where the rapidity of vegetation colonisation is such to almost completely capture the transverse dune in place. ...
Chapter
This chapter examines dunes that are 'anchored' to an obstacle, whose geometric characteristics then control the shape of the dunes. There are many kinds and sizes of obstacle: plants, organic debris of various kinds, pebbles, boulders, or isolated hills. Nebkhas are discrete dunes formed by the trapping of sand, silt, clay, and coarse particles up to the size of small gravel, and/or snow by a plant. Shadow dunes are pyramidal through triangular to tear‐drop‐shaped dunes. They are formed downwind, or in the shelter of nebkhas, plants, or other obstacles. Lunettes are typically crescentic or arcuate shaped dunes formed on pan, playa, and lake margins, usually mimicking the plan shape of the adjacent palaeo or present shoreline, with the horns pointing upwind. Blowouts are erosional hollows typically found on vegetated to semi‐vegetated sediment deposits. Transgressive dunefields are commonly colonised and stabilised naturally by vegetation over time. Topography variably influences dune orientation and migration rate.
... Ecological and geomorphic systems in sand seas and dune fields are sensitive to climate change (Dong, 2002;Thomas et al., 2005) and, more recently, human activity. Changes in these systems also have many local environmental and economic impacts and even affect global climate through global biogeochemical cycles (Jickells et al., 2005). ...
Article
One significant change of terrestrial landscapes in response to past climate change has been the transformation between activity and stability of extensively distributed wind-blown sand dunes. The relations between the dynamics of the aeolian landscape and its drivers are not yet completely understood, however. Evidence of aeolian sand deposition during the Last Glacial Maximum (LGM) is scarce in many mid-latitude dune fields, whereas abundant evidence exists for aeolian sand accumulation during the deglaciation, i.e. after about 15 ka. Whether this contrast actually reflects changes in dune activity is still unclear, making paleoclimatic interpretation uncertain. Comprehensive field investigation and luminescence dating in the Mu Us dune field, north-central China, demonstrates that aeolian sands deposited during the LGM are preserved as fills in periglacial sand wedges and beneath loess deposits near the downwind dune field margin. The scarcity of LGM dune sand elsewhere in the dune field is interpreted as the result of intensive aeolian activity without substantial net sand accumulation. Increasing sand accumulation after 15 ka, reflected by much more extensive preservation, signals a change in sand supply relative to sand transportation through the dune field. Reduced wind strength and other environmental changes including regional permafrost degradation after 15 ka transformed the dune field state from net erosion to net accumulation; the dunes, however, remained largely mobile as they were in the LGM. Similar diverging patterns of dune sand accumulation and preservation before and after 15 ka in many mid-latitude dune fields imply broad climatic controls linked to the changes in high-northern-latitude forcing.
... Presently, some dune fields are observably in transi- tion [e.g., Tsoar and Blumberg, 2002;Ardon et al., 2009] (see Figure 1) or are still responding to climate shifts hundreds of years in the past [e.g., Hugenholtz and Wolfe, 2005b;Wolfe and Hugenholtz, 2009, Figure 1]. In the future, climate changes could result in consequential activity changes [e.g., Thomas et al., 2005]. Semi-arid regions most susceptible to changes in dune field activity also tend to be the most inhabited of dune lands; therefore, understanding the controls on dune field activity has implications for socio- economic adaptation. ...
Article
The activity of inland aeolian dune fields is typically related to the external forcing imposed by climate: active (bare) dunes are associated with windy and/or arid settings, and inactive (vegetated) dunes are associated with humid and/or calm environments. When a climate shifts the dune field reacts; however, the behavior, rate, and potential impact of diverse dune geomorphologies on these transitions are poorly understood. Here, we use a numerical model to systematically investigate the influence of dune field geomorphology (dune height, organization and collisions) on the time a dune field takes to stabilize. To generate diverse initial un-vegetated dune field geomorphologies under unidirectional winds, we varied pre-stabilization growth time and initial sediment thickness (termed equivalent sediment thickness: EST). Following dune field development from a flat bed, we introduced vegetation (simulating a climate shift) and transport-vegetation feedbacks slowly stabilized the dune fields. Qualitatively, very young and immature dune fields stabilized quickly, whereas older dune fields took longer. Dune fields with greater EST stabilized quicker than those with less EST. Larger dunes stabilized quicker because of low celerity, which facilitated higher vegetation growth rates. Extended stabilization times were associated with the extension of parabolic dunes. Dune-dune collisions resulted in premature stabilization; the frequency of collisions was related to dune spacing. Quantitatively comparing the distribution of deposition rates in a dune field to the deposition tolerance of vegetation provides a promising predictor of relative stabilization time. Dune fields with deposition rates dominantly above the deposition tolerance of vegetation advanced unimpeded and prolonged stabilization as parabolic dunes. Paleoenvironmental reconstructions or predictions of dune field activity should not assume that dune activity directly translates to climate, considerable lags to stabilizing climate shifts may exist in unidirectional dune forms.
... Climate change scenarios for that area project increases in aridity and number of exceptional climate events (e.g. Hulme et al. 2001; Thomas et al. 2005;Christensen et al. 2007) and respective bioclimatic models based on climate change scenarios predict reduced species richness (Midgley et al. 2002;. ...
Article
Question: How do species composition and abundance change in the medium term and can the detected fluctuations and trends be attributed to weather conditions or climate change? Location: Quartz fields of the Knersvlakte, Succulent Karoo, South Africa, a plant diversity hotspot. Methods: We monitored vascular plant composition and abundance of all taxa on 12 permanent plots (25 m²) over a period of 12 yr. With multiple regressions, we tested the effect of rainfall in different periods on species richness and abundance. Results: The plots showed significant inter-annual changes in population size, species richness and species composition. In general, plant populations showed a significant positive trend over the 12 yr analysed, while therophytes decreased significantly. Number of chamaephyte and geophyte individuals was positively influenced by rainfall of the preceding year, whereas therophytes were significantly positively influenced by rainfall of the same year. Conclusions: We conclude that the increase in inter-annual variability of rainfall has so far had no negative effects on the quartz field vegetation. However, observations of recent climatic trends and climate change projections suggest that there will be much stronger changes in rainfall and temperature patterns in the next decades. Continuous long-term monitoring of biodiversity is thus crucial to understand potential impacts of climate change in the Knersvlakte.
... increased coastal erosion, flooding). Despite abundant suitable contexts, however, again 'geomorphology' or related terms hardly appear, despite the fact that geomorphologists are now increasingly engaged with the climate change debate (e.g., Thomas et al., 2005;Lane et al., 2007). ...
Article
Earth Surface Processes and Landforms (ESPL) is the journal of the British Society for Geomorphology (BSG, incorporating the British Geomorphological Research Group). Along with other national and international geomorphological organizations, the BSG aims to promote research and education in pure and applied geomorphology through engagement with different academic and non-academic audiences. Through scientific outlets such as ESPL, the promotion of geomorphological research may be strong, but how successful is the promotion of geomorphological education? Notwithstanding valuable outreach activities by individuals, the contention of this commentary is that the discipline of geomorphology remains little known and little understood, certainly in relation to other academic disciplines, and especially outside university circles. In particular, ‘geomorphology’ and related terms (e.g. ‘geomorphological’, ‘geomorphologist’) are commonly absent from debates on important scientific issues, so that, rather than sharing centre ground with cognate disciplines, geomorphology’s contributions are well hidden at best, and effectively invisible at worst. Below, I offer examples of this geomorphological invisibility, and provide counter-examples of where there is greater – but nowhere near full – visibility. I also suggest reasons for this state of affairs, and consider whether it actually matters from the viewpoint of advancing geomorphology.
Chapter
The wind is an important agent of sediment movement and landform development in deserts and some coastal environments today. In line with major climate changes affecting earth history, the extent and locations of wind-shaped landscapes have changed over time. For the Quaternary period, these changes have commonly left a record in the landscapes of today: evidence of aeolian paleoenvironments in the form of sandy sediments and landscapes that are not experiencing aeolian activity today. This article analyzes the evidence for Quaternary desert Aeolian paleoenvironments: what that evidence is; how it can be interpreted in terms of extent and timing of occurrence; what the evidence means for understanding past climate changes; and examines key issues and debates in reconstructing past aeolian desert conditions. As techniques to interrogate Quaternary aeolian sediments have developed, especially optically stimulated luminescence dating, so simple models of climate change-desert expansion have been replaced by recognition of the complexity and multiple occurrence of aeolian system expansions and contractions. These in turn have led to debates about the climate signals that Quaternary aeolian sediments may represent, with simple assumptions of greater past aridity needing to be placed into explanations integrating changes in wind erosivity as well as surface erodibility. A further issue that challenges successful interpretation relates to record preservation. The chapter concludes by addressing these issues and the prospects for enhanced interpretations of aeolian paleoenvironmental data in the future.
Preprint
Full-text available
Barchan dune fields are a dominant landscape feature in SE Qatar and a key element of the peninsula’s geodiversity. The migration of barchan dunes is mainly controlled by dune size, wind pattern, vegetation cover and human impact. We investigate the variability of dune migration in Qatar over a time period of 50 years using high-resolution satellite and aerial imagery and explore its relation to the regional Shamal wind system, teleconnection patterns, and limitations in sand supply associated with the transgression of the Arabian Gulf. We detect strong size-dependent differences in migration rates of individual dunes as well as significant decadal variability on a dune-field scale, which was found to be correlating with the intensity of the North Atlantic Oscillation (NAO) and the Indian Summer Monsoon (ISM). High uncertainties associated with the extrapolation of migration rates back into the Holocene, however, do not permit to further specify the timing of the loss of sand supply and the onset of the mid-Holocene relative sea-level (RSL) highstand. For the youngest phase considered in this study (2006–2015), human impact is anticipated to have accelerated dune migration under a weakening Shamal regime through sand mining and excessive vehicle frequentation upwind of the core study area.
Article
Luminescence dating of desert dune sediments has generated many hundreds of ages, many used in reconstructions of Quaternary environmental changes, others in attempts to elucidate dune processes. Environmental and climatic interpretations of these records have proved problematic and it remains challenging to test hypotheses of the systematic response of dunefields to changes in external forcing in the past and to make predictions of the future. We use a method that quantifies dune sediment accumulation to interpret dune luminescence age datasets, rather than simply using the ages themselves as proxies of change. The Accumulation Intensity method allows periods of dune sediment accumulation, here over the timescale 102-105 years, to be identified from compilations of dated sand sea stratigraphic sequences. We apply this approach to two of the largest dune age datasets, from southern Africa and Australia, testing whether or not dunefield accumulation has co-varied in the Late Quaternary and whether systematic relationships to external drivers at global, hemispheric, regional and local scales can be identified.
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