The Makgadikgadi Basin in central Botswana is a key region for understanding Stone Age archaeology and human evolution. However, paleoenvironmental reconstructions have so far offered only fragmentary insights into past hydroclimatic and environmental conditions. We present the first environmental record from Sowa Pan, paleolake Makgadikgadi, covering the last 82-20 ka (Marine Isotope Stages 5-2). Sedimentological and biomarker lipid analyses reveal four distinct lake phases, reflecting major hydrological shifts and sediment provenance. From 82 to 75 ka, the lake was shallow, with lowest lake levels temporally coinciding with Middle Stone Age archaeological material (silcrete lithics, mainly unifacial and bifacial points) on the western lakebed. Between 75 -58 ka, the lake refilled, likely fed by river inflow sourced from the Angolan highlands. A second low stand likely occurred around 58-56 ka. From 52 to 37 ka, geochemical proxies indicating a change in lake inflow with less influence of the Kwando catchment. During this time, the lake had high primary productivity and biomarkers for terrestrial herbivores are recorded in the sediments. After an accumulatory hiatus, the youngest unit (ca. 21 ka) marks the formation of the Sowa Spit, characterized by high sand input and minimal organic preservation. This record refines the chronology of lake-level changes and reveals that desiccation phases provided ecologically viable landscapes for human activity. It also identifies shifting sediment and nutrient sources that shaped lake dynamics during the late Pleistocene.

Grand anti-desertification schemes often fail when trees die and funding dries up — yet one project has broken the mould. Grand anti-desertification schemes often fail when trees die and funding dries up — yet one project has broken the mould.

Fire is often cited as a major disturbance that can induce increased aeolian activity on vegetated sand dunes due to the removal of protective plant cover. This study investigated the spatial and temporal dimensions of burning as an agent of disturbance in partially vegetated linear dunes in the southwest Kalahari. The region is often omitted as a burn-prone area in continental and global burned area studies due to its aridity and proximity to intense temperate zone fires. The MODIS burned area product was used to create an inventory of burned area from 2000 to 2023 to identify trends in burned area extent and occurrence. In addition, MODIS-derived Soil Adjusted Total Vegetation Index (SATVI) was used to calculate vegetation recovery post-burn. 13,310 km2 of the southwest Kalahari dune field burned over the 24-year study period; 11.1 % of the total area. The land management regime significantly impacted on fire frequency (p < 0.001) and size (p < 0.001), with more burns occurring on privately owned land but larger burns within the Kgalagadi National Park. It also significantly affected burn duration (p < 0.001), with the National Park having longer lasting burns. Vegetation recovery is swift after burning, with cover returning to control levels within about two years of being burned. This study highlights how the temporal and spatial disturbance effect of the fire is limited by land management in the study region and rapid vegetation recovery post-burn.

The lower Mesopotamia and Kuwait exposed to human pressures and environmental changes and described as one of the main natural sources of dust worldwide, influencing nearby cities. The challenges of undertaking field studies in the region, due to access difficulty and source location uncertainty, have hindered identification of the specific geomorphic units that act as significant dust sources. The capability of high-resolution satellite data to pinpoint small-scale dust-emitting units now enables precise surface identification, which is crucial for understanding the spatial variability and controls on dust emissions. Launching Sentinel 2 A-B in 2015 and 2017 and Landsat 7-9 in 1999, 2013 and 2021 means the possibility of systematic high-resolution monitoring every 3-4 days at 10-15 m2 (pixel size), is now possible. This paper aims to identify dust-emitting units in lower Mesopotamia and Kuwait (2013-2023) within a regional context, based on a systematic analysis of I) MODIS\VIIRS data associated with wind data to identify the regional dust sources and dynamics, II) Sentinel-2\Landsat 7-9 data to pinpoint the exact dust sources and their geomorphic units, and III) MODIS Aerosol Optical Depth (AOD) to identify the relative magnitude of dust emission from each identified unit. The dominant dust-emitting geomorphic units, primarily located in southern Mesopotamia and along the northern coastline of Kuwait, include inland sabkhas and barchan sand dunes in Al-Batha, generating 62% (number of events = 446) of dust events, the intertidal zone of Subiya (11%, n = 76), coastal sabkhas and muddy flats in Bubiyan Island (10%, n = 68), the estuarine environment of Al-Faw (9%, n = 61). These units, many with crusted or moisture-affected surfaces, were previously generalized or overlooked in regional assessments. Seasonal trends indicate that dust emissions peak in June and July, driven by prevailing wind speeds exceeding 6.5 m/s and AOD values reaching up to 0.6. Collectively, these units cover approximately 2.5% (2743 km2) of the total research region (111,042 Km2) and amounted to about 651 dust events over 11 years, offering a more refined and spatially explicit understanding of dust sources at high spatio-temporal resolution.

Vegetated sand dunes are not typically considered to be significant sources of dust because of coarse grain sizes and high vegetation cover. Yet, plumes of dust have been observed from vegetated dune fields, and wind tunnel experiments show the potential for interdunes to emit dust. The partially vegetated southwest Kalahari is one such dune field that is vulnerable to climate-driven land degradation and vegetation cover change and has been posited as a potential future dust source. In this study, we monitor the post-fire de-vegetated state of 11 interdunes to investigate the dust emission potential and erodibility controls on emission. Findings suggest that there is little emission of dust despite a fine-grain component (up to 30% of resident grains <62.5 mu m) and a lack of vegetation, even with high velocity wind events (>7 m s(-1)) post-fire. Erosive wind events were less frequent compared to other dust-producing regions, with wind speeds generally under 7 m s(-1). Five high wind speed events over 7 m s(-1) were recorded in September 2022, the month immediately following burning, but even then, only one event had a concurrent increase in aerosol concentration. This is indicative of other erodibility factors limiting dust emission. These include high (>50%) burned debris cover in the immediate post-fire period and evidence of biological soil crusts that survived burning. Both protect the surface after fire whilst the natural vegetation cover recovers. Combined, the general low wind speeds, high initial surface cover, and the protective effect of biocrusts result in a low probability of the southwest Kalahari emitting dust post-fire. However, fine-grain sizes and low vegetation cover under drought and high grazing may lead to conditions conducive for dust emission.

Understanding dune patterning on a landscape scale provides key insights into the controls shaping dune fields and requires robust methods that capture morphological patterns in a geomorphologically meaningful manner. This study presents a data-driven approach for the creation of dune pattern classification schemes and their mapping. We apply dimensionality reduction and clustering to 198 randomly distributed dune crest tiles to derive morphometric-based classification schemes for the planimetric dune patterns across the south-western Kalahari dune field. A convolutional neural network is trained on two newly generated and a foundational classification scheme to map pattern distribution across the study area. Maps based on the new schemes presented here reproduced some broad spatial trends from previous work, like the shift from straight, parallel ridges in the north to dendritic patterns in the south. We further characterise the defining morphometrics and morphometric signature of classification schemes and pattern types, finding that orientation range is the dominant discriminating morphometric. Based on model uncertainty, we locate transitional zones between patterns in the study area. We argue that these are the most geomorphologically meaningful product of the classification: their recurrence across schemes suggests that they may reflect changes in boundary conditions that have shaped dune morphology.

The removal of stabilising vegetation from sand dunes by fire has been widely linked to increases in aeolian sediment transport and dune movement. However, substantial gaps exist in our knowledge of whether burned dunes in arid environments have the potential to emit dust. To explore relationships between fire and dust emission on partially vegetated sand dunes in the Namibian Kalahari Desert, 180 measurements of wind erosion threshold and dust flux were carried out using a portable wind tunnel (Portable In-Situ Wind Erosion Laboratory or PI-SWERL). Data were analysed to compare erosion thresholds and dust emission flux on adjacent burned and unburned sites. The data suggest that both burned and unburned dune crests, flanks, and interdunes have a low potential for dust emission. Whilst there was no significant difference in dust emission flux between burned and unburned control surfaces (Kruskal-Wallis, p > 0.05), there was evidence of significantly higher erosion thresholds on burned surfaces (T-test, p < 0.01). Where the surface had been disturbed, resulting in the removal of the typically present biological soil crusts (biocrust), our data suggest that dust emission fluxes are, on average, 8-13 times higher those of undisturbed surfaces. The analysis reveals that even when burned and devoid of vegetation, the Kalahari linear dune system is sediment-availability limited. This finding indicates the importance of ground surface characteristics, such as biocrusts, in preventing dust emission from the Kalahari dune field.

Air quality monitoring in most Pacific Island Countries, Territories, and States (PICTS) is minimal, with notable exceptions in Hawai'i and New Caledonia. However, air quality issues are increasingly significant in the region. Existing data on air quality, particularly regarding PM2.5 and PM10, are limited, with studies focusing on Fiji and New Caledonia. Our research provides the first continuous and comparative air quality monitoring in urban and peri-urban areas of Fiji and the Solomon Islands, and it is the first assessment since the introduction of the 2021 World Health Organization (WHO) Air Quality Guidelines (AQG). This study assesses health risks and air pollution trends to inform governmental recommendations. We collected PM2.5, PM10, and weather data from Honiara, Solomon Islands (February 2020-August 2023), and Suva, Fiji (April 2021-August 2023). In Honiara, PM2.5 levels exceeded WHO AQG on 75% of days in urban areas and 51% in peri-urban areas, while PM10 levels surpassed guidelines on 2% of days in both areas. In Suva, urban areas had a 10% exceedance of PM2.5 guidelines, compared to 13% in peri-urban areas. Annual PM2.5 averages exceeded WHO guidelines every year, with levels in Suva and Honiara exceeding guidelines by 2-4 times. PM10 levels were 1.5 times higher than WHO AQG in urban Honiara and 1.2 times higher in peri-urban areas. These findings highlight the urgent need for governmental action to establish robust air quality standards and long-term monitoring programs in Fiji and the Solomon Islands to mitigate health risks from poor air quality.

The impacts of climate change are being felt in weaker economies, especially among the rural poor dependent on rainfed agriculture and ecosystem-based livelihoods. Using mixed approaches to strengthen the methodological toolkit, the Sustainable Livelihood Framework and Livelihood Vulnerability Indicators (LVIs) in the context of the Intergovernmental Panel Climate Change (IPCC) were drawn to quantitatively assess vulnerability to climate change in two agro-ecological zones (AEZs) of Zambia. Livelihood capital was profiled and computed to understand how adaptive capacity was shaped within and between AEZs by household size, level of education, gender, age, and migration status. Many variations were observed. While both zones recorded positive LVI-IPCC values suggesting high vulnerability, AEZ IIa was relatively less vulnerable. Gender, age, and household size appeared to play major roles in shaping household capital. Female and child-headed households were most vulnerable, just like locals and smaller households were. It is concluded that there was low adaptive capacity to climate change in the zones examined. As it stands, although future vulnerability is uncertain, energy needs are likely to continue driving deforestation and environmental degradation, affecting future climate in turn. Deforestation rates continue to exponentially increase with nationwide energy shortages resulting from droughts linked to the El Niño-Southern Oscillation (ENSO) events. Similarly, poor infrastructure, late farming input delivery, market failure, and access challenges are highly likely to continue to weaken the climate change adaptation potential of ecosystem-based livelihoods.

Loess-paleosol sequences have been used in Asia to study climate and environmental changes during the Quaternary. The scarcity of age control datasets and proxy indices analysis data for Asian loess has limited our understanding of loess depositional processes and the reconstruction of paleoclimatic changes from loess-paleosol records. In this study, we present a dataset that includes 1785 quartz optically stimulated luminescence ages and 1038 K-feldspar post-infrared infrared stimulated luminescence ages from 128 loess-paleosol sequences located in different regions of Asia. We generate 38 high-resolution age-depth models of loess records based on the provided datasets. We provide data on 12,365 grain size records, 14,964 magnetic susceptibility records, 2204 CaCO3 content records, and 3326 color reflection records. This dataset contains the most detailed and accurate chronologies and proxy index data for loess records in Asia yet published. It provides fundamental data for understanding the spatial-temporal variations in loess depositional processes and climatic changes across the continent during the mid-late Quaternary.

Multiple parabolic sand dune fields formed in Arctic Sweden after the last deglaciation, facilitated by an abundance of loose glaciofluvial sediment, limited vegetation cover and strong winds. Following initial stabilisation, these dunes underwent repeated reworking after fire events, as evidenced by the presence of buried soils, charcoal layers and redeposited sands in the dune stratigraphy. These reworking events may be driven by wider climate forcing; however, to date, no chronological framework exists for this activity in Sweden. As such, here, we apply quartz optically stimulated luminescence (OSL) dating of Arctic Swedish sand dunes using two dunes at the sites of Vastakielinen and Jorggastat. Resultant double-SAR (single aliquot regenerative dose) quartz OSL ages are in good agreement with independent ages provided by C-14 dating of charcoal fragments recovered from charcoal layers within the dunes, and we conclude that the chosen protocol is generally well suited for dating aeolian reworking of dune sediments in Arctic Sweden. While feldspar contamination limits precise age assignment for initial dune movement, our results nonetheless suggest repeated and long-lasting aeolian activity in Arctic Sweden throughout the Holocene and, although there are differences in detail, further suggest some general trends in terms of dune stability and reworking over Arctic Fennoscandia.

Sand patches are one of the early stages of aeolian bedforms. They form onnon-erodible surfaces in both desert and coastal environments. Their initiation is associated with thechange of saltation transport law on rigid and granular beds [1]. Here wepresent a two-dimensional model that couples these surface-dependenttransport laws with the feedback of the bed elevation on the wind flow.Analysing the spatio-temporal evolution of an initial very flat sandpatch, we emphasise the central role of the input flux as well as thelengthscale over which occurs the transition between the two transportlaws. We also show that, for adjusted parameters of the model, we are ableto reproduce the growth and the propagation of these small metre-scalebedforms over time, in quantitative comparison with field measurements. [1] P. Delorme, J.M. Nield, G.F.S. Wiggs, M.C. Baddock, N.R. Bristow, J.Best, K.T. Christensen and P. Claudin, Field evidence for the initiationof isolated aeolian sand patches, Geophys. Res. Lett. 50 , e2022GL101553(2023).

The formation and evolution of the cryosphere on the Tibetan Plateau is of great significance in understanding the Earth's carbon and climatic system. Periglacial loess deposits in southeastern Tibet offer a means to constrain this history as they contain critical information on glacial grinding and frost shattering processes in high-altitude mountain regions through time, which yield lithogenic fractions of largely loess silts and sands. Based on combined analyses of lithogenic magnetic properties and mineralogical composition, here we find that increasing high mountain production and supply of fresh detrital components since the mid-Pleistocene climate transition (MPT, 1.2-0.7 Ma) led to a substantial increase of the lithogenic susceptibility and decrease in chemical weathering intensity of periglacial loess in southeastern Tibet. The agreement of these findings with similar results from eolian loess on the northern margins of the Tibetan Plateau suggests a plateau-wide glacier expansion during the MPT. Enhanced glacial erosion and freeze-thaw activities occurred in the high-altitude mountain regions of the plateau during the MPT, thereby providing vast amounts of fresh detritus for the formation of loess deposits. High and constant values of lithogenic magnetic parameters in the loess deposits after 0.7-0.6 Ma further indicate that the Middle Pleistocene is a critical period for the establishment of modern-like glacial and periglacial landforms on the Tibetan Plateau. Our results further suggest that enhanced Quaternary glaciation in SE Tibet occurred earlier than in the north, which we attribute to climate cooling combined with regional seasonal snowline lowering. As such, a close relationship exists between global climate changes, development of pan-Tibetan glaciations, and large-scale dust emission.

Sediments of the Lena River represent an important environmental archive for understanding the Quaternary history of North-Eastern Siberia. However, at present, the structure, origin and age of the Lena River terraces are poorly known. This article presents results of lithofacies analysis and absolute dating of the Ust'-Buotama section exposing the fourth (Bestyakh) terrace of the Lena River. We report the first quartz and K-feldspar luminescence ages, the reliability of which was argued by age relations and standard tests. Three stratigraphic units have been recognized in the section (depths from the top): lacustrine-alluvial deposits (85-120 m) of the Mavrinka Formation; aeolian sand deposits of the Dolkuma Formation (23-85 m), and Holocene aeolian dune sediments (0-23 m). The resulting chronology suggests that the sediments of the Mavrinka Formation were deposited no later than 300 ka (MIS 9 or later). Deposition of the Dolkuma Formation occurred from -30 ka to -15 ka (late MIS 3 - late MIS 2). Holocene aeolian dune formed during initial Neoglacial cooling post climatic optimum (c. 5.5 cal ka BP). More extensive Late Holocene dune sediments which formed -400 years ago are coeval with Little Ice Age (11th-19th centuries).

Water clarity on the inshore Great Barrier Reef (GBR) is greatly influenced by terrestrial runoff of suspended particulate matter (SPM). Catchment sediment tracing studies often do not extend into the marine environment, preventing the analysis of preferential marine transport. This study employs novel collection and sediment tracing techniques to examine the transport of the terrigenous 'mineral' component of plume SPM within the GBR lagoon for two flood events. Utilising geochemical, radionuclide and clay mineral analysis, we trace terrigenous mineral sediments > 100 km from the river mouth. We show that the SPM geochemistry is highly influenced by particle-size fractionation, desorption, and dilution within the plume, rendering traditional tracing methods unviable. However, the ratios of rare earth elements (REE) to thorium (Th) provide stable tracers of mineral SPM transported across the catchment to marine continuum and allow the identification of discrete catchment sources for each flood event. Plume sediment radionuclides are also stable and consistent with sub-surface erosion sources.

A thin (20-80 cm), patchy layer of silt-rich sediment occurs at the surface throughout Svartedalen, a nature reserve 30 km north of Gothenburg, Sweden. This surface silt mantles a bedrock-dominated, fracture-valley landscape. Using data from grain-size analysis, OSL dating and detrital-zircon U-Pb dating, we argue that the silt is loess sourced from glacial sediment that was eroded from local bedrock. The sediment has a grain-size distribution typical of wind-blown silt and is especially similar to thin deposits of loess overlying coarser material. OSL ages on five samples range from 1 to 8 ka, although analysis of equivalent dose distributions of one may suggest an age as old as 11 ka. The dates may represent true depositional ages and represent several Holocene eolian events. However, we consider as more likely that the loess was deposited during deglaciation, and quartz-grain signals have been partially reset during bioturbation. U-Pb ages on 273 zircon grains from the loess show prominent peaks at 1.6 and 1.8 Ga, as well as smaller numbers of grains from 1.0 to 1.6 Ga. These ages match dates from the Idefjord Terrane which comprises the bedrock of the study area. We argue that during ice-margin retreat, debris in the glacier was dominated by locally derived debris. This glacial sediment was left in thin patches uplands and particularly in large ice-marginal deltas. These deposits served as the proximal source for the loess. The presence of thin loess in Svartedalen suggests loess to be common in soils of southwest Sweden.

Fluvial sediments preserve evidence of past hydroclimatic regimes and are important but potentially under-utilised indicators of palaeoenvironmental change in drylands. This paper presents a new chronological data-base of 603 ages from late Quaternary fluvial sediments at 70 sites in southern Africa's western dryland regions. This synthesis assesses the spatial and temporal trends in both the palaeoenvironmental record and the sup-porting data that underpins these interpretations. We find that topographic and climatic characteristics of river catchments influence the nature of fluvial archive formation and preservation. Channel confinement, legacies of research, and sampling strategies in specific river basins are found to control the sedimentary context and length of documented record available for palaeoenvironmental reconstruction. The age of dated deposits and associ-ated magnitude of chronological uncertainties influence the temporal range and resolution of climate and fluvial variability that can be discerned. Where calendar year uncertainties are small enough, predominantly dating to the last millennium, individual flood events and multi-year episodes within flash flood deposits can be linked to synoptic climate patterns, yet when uncertainties are larger, interpretations are restricted to understanding multidecadal to centennial-scale phases of flow regimes. This paper collates a scattered literature to detail the current state of southwestern Africa's fluvial record and highlights opportunities for future research. We consider controls on fluvial archive interpretation that are pertinent to wider dryland contexts and argue that fluvial records provide useful information for inclusion in regional palaeoenvironmental syntheses. Following a filtering process, quality-assessed chronologies are compiled to establish the combined fluvial history of 22 rivers to consider the response of fluvial systems to hydroclimatic drivers. At the Last Glacial Maximum (LGM; 24-18 ka), records detail a regionally consistent signal of intermittent to perennial flow under relatively humid conditions in what are currently ephemeral systems. The mid Holocene (MH; 8-4 ka) is also characterised by the occurrence of sustained river flow and higher discharge events than present, but with greater spatial variability. Sedimentary archives display evidence of both intermittent flow and extreme flood events within ephemeral regimes, and two rivers, the Hoanib and Molopo, are characterised by reduced fluvial activity. Within the last millennium, flash flood regimes within an arid to hyper-arid climate were established with coherence in the timing of flood frequency variations recorded between catchments.

The influence of natural factors such as bioturbation or sediment movement caused by wind and water is a perennial concern for Stone Age site selection and subsequent interpretation. This paper discusses the spatial artefact distribution of five recently excavated, open-air exposed Middle Stone Age (MSA) sites in Ntwetwe Pan, Botswana. The finds comprise lithic assemblages dominated by MSA points, manufactured in a variety of silcretes. The sites were examined following the assumption that archaeological sites are the product of a combination of natural and cultural factors, occurring both during and after artefacts are deposited. The results indicate that some of these exposed pan floor sites do preserve cultural artefact distribution patterns, and that the level of post-depositional disturbance varies locally. Refitting was an important tool of analysis, especially on the largest site, MAK33, where it was possible to identify working areas that focussed on different modes of lithic manufacture. In combination with a chaîne opératoire analysis of lithic production stages, it was then possible to map movement of artefacts across the site. We argue that the spatial organization of open-air sites may preserve behavioural records that are not present at caves and rock shelters, and provide a view into the short-term, single-use locations that likely formed the basis of MSA occupation patterns.

Much archaeological research is conducted within the environments, locational and cultural, that ar-chaeologists are familiar and comfortable with. But that which is marginal and difficult today, and that which is central and convenient, may not have been so in the past. To answer new and pressing questions about human evolution, less familiar and less comfortable environments therefore require systematic and prolonged multidisciplinary investigation. With a focus on the extensive Makgadikgadi salt pans of the Middle Kalahari Desert, Botswana, six papers represent the output of systematic investigations and excavations of, predominantly, Middle Stone Age (MSA) archaeology. This multidisciplinary research is insightful for revealing extensive open-air MSA sites throughout the region, that in turn are part of a wider landscape with abundant Stone Age finds (Coulson et al., 2022). Within western Makgadikgadi, five open-air sites are systematically excavated (Staurset et al., 2022a,b) and are given detailed and robust environmental context and Late Quaternary chronology (Burrough et al., 2022), documenting systematic MSA resource use of the dry basin floor (Nash et al., 2022). Together, this work frames a new understanding MSA adaptation to dry environmental conditions in the heart of the Kalahari (Thomas et al., 2022). (c) 2023 Published by Elsevier Ltd.

There is considerable uncertainty over the nature and causes of Holocene dust activity in the Arctic, and its links to climatic changes. Loess deposits act as near-source archives of dust deposition and provide a means to address these uncertainties. Here we develop a fully independent age model for a loess core taken in Adventdalen, Svalbard, based on 136 quartz luminescence ages taken at 2 cm intervals. This represents the most detailed luminescence dating analysis undertaken to date on a sedimentary archive. Extensive laboratory tests and stratigraphic consistency indicate that the quartz luminescence ages are reliable. Together with grain size and cryostratigraphic analyses, as well as stratigraphic investigation of an adjacent loess exposure, the exceptional detail of the chronology combined with Bayesian age modelling uniquely allows changes in loess accumulation rates, particle size and permafrost develop-ment to be reconstructed over the last 3000 yrs on 101-103 yr timescales. The results show that loess deposition was mostly continuous over this interval, albeit with a short period of reworking or non-aeolian sedimentation during the Little Ice Age. Permafrost development in the loess is dominantly syngenetic, with ice contents increasing with depth. There is considerable variability in loess mass accumulation rate over the late Holocene, with peaks occurring during the last 250 yrs, as well as at 750-900, 1050-1200, 1400-1600, 1900-2450 and possibly 2700-3000 a. These peaks generally coincide with increased coarse silt deposition, potentially suggesting a link with greater wind activity. However, the peaks also seem to coincide with possible warm phases on Svalbard, which would rather imply that temperature-driven sediment availability in glaciofluvial source areas is the main control on dustiness in Adventdalen. In any case, the rates of loess deposition in the Adventdalen core are exceptionally high globally (up to 0.35 cm yr-1/2900 g m-2 yr-1 sedimentation and mass accumulation rates, respectively), and if representative of wider patterns across Svalbard, may suggest that the archipelago is a more important high latitude dust source than previously realised. (c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).