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Abstract

The 4.2 ka event at the Mid- to Late-Holocene transition is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. The event is most clearly manifested in the Mediterranean and Middle East as a regional dry anomaly beginning abruptly at 4.26 kyr BP and extending until 3.97 kyr BP. Yet the impacts of this regional drought are often extended to other regions and sometimes globally. In particular, the nature and spatial extent of the 4.2 ka event in the tropics have not been established. Here, we present a new stalagmite stable isotope record from Anjohikely, northwest Madagascar. Growing between 5.22 and 2.00 kyr BP, stalagmite AK1 shows a hiatus between 4.31 and 3.93 kyr BP (±40 and ± 35 yrs), replicating a hiatus in another stalagmite from nearby Anjohibe, and therefore indicating a significant drying at the Mid- to Late-Holocene transition. This result is the opposite to wet conditions at the 8.2 ka event, suggesting fundamentally different forcing mechanisms. Dry conditions are also recorded in sediment cores in Lake Malawi, Lake Masoko and the Tatos Basin on Mauritius, also in the southeast African monsoon domain. However, no notable event is recorded at the northern (equatorial East Africa) and eastern (Rodrigues) peripheries of the monsoon domain, while a wet event is recorded in sediment cores at Lake Muzi and Mkhuze Delta to the south. The spatial pattern is largely consistent with the modern rainfall anomaly pattern associated a with weak Mozambique Channel Trough and a northerly austral summer Intertropical Convergence Zone position. Within age error, the observed peak climate anomalies overlap with the 4.2 ka event. However regional hydrological change consistently begins earlier than a 4.26 kyr BP event onset. Gradual hydrological change frequently begins around 4.5 kyr BP, raising doubt as to whether any coherent regional hydrological change is merely coincident with the 4.2 ka event or part of a global climatic anomaly.

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... In recent years, a number of paleoclimate records have been published using mainly Malagasy speleothems but also lacustrine sediments (Teixeira et al., 2021), many of which provide higher resolution, better dated records than previously available. The majority of the speleothem records are Holocene and come from caves in northwestern Madagascar (Brook et al., 1999;Burns et al., 2016;Duan et al., 2021;Scroxton et al., 2017Scroxton et al., , 2023aScroxton et al., , 2023bVoarintsoa et al., 2017Voarintsoa et al., , 2019Wang et al., 2019). Voarintsoa et al. (2017) and Wang et al. (2019) published studies investigating climate in northwestern Madagascar throughout much of the Holocene. ...
... Recent results from speleothems from across Madagascar have produced high resolution, well-dated paleoclimate reconstructions, with most of these studies using speleothems from Anjohibe (Burns et al., 2016;Duan et al., 2021;Voarintsoa et al., 2017Voarintsoa et al., , 2019Wang et al., 2019) or from nearby Anjohikely (Scroxton et al., 2023a). The temporal coverage of several of these studies and our AB13 record partially overlap. ...
... One measure of the reliability of speleothem climate records is replication of the isotopic time series with other speleothems from the same approximate location (Dorale and Liu, 2009). In Figure 5 we compare our record to three previously published stalagmite records from NW Madagascar with varying temporal resolutions and dating accuracies: ANJ94-2 (Railsback et al., 2020) and ANJ94-5 (Wang et al., 2019) from Anjohibe and AK1 from Anjohikely ("little cave" in Malagasy), which is located 2.3 km southwest of Anjohibe (Scroxton et al., 2023a). ...
Article
We present a continuous high-resolution precisely dated multiproxy record of hydroclimate variability at Anjohibe cave in northwestern Madagascar using speleothem AB13. The record spans from ~4484 y BP to ~2863 y BP. Stalagmite δ ¹⁸ O, δ ¹³ C and Sr/Ca ratios show very similar changes in hydroclimate. The mechanism controlling Sr/Ca changes, however, from prior calcite precipitation to degree of dolomite dissolution at about 4 ky BP. Our record is also in good agreement with previously published speleothem records from the same area. This agreement and multiproxy consensus indicate that AB13 provides a robust record of hydroclimate variability, including a continuous record of hydroclimate variability across the 4.2 ka event. This 4.2 ka event in Madagascar is marked by two distinct periods of drying between ~3900 y BP to 4300 y BP. A dry 4.2 ka event at this Southern Hemisphere site helps limit possible mechanisms for the event, indicating that a meridional shift to the south in the ITCZ is not responsible for the 4.2 ka event. In addition, the 4.2 ka event does not stand out as a unique dry period in our record. The longest and driest period of the record lasted ~300 years with peak dryness at ~3000 y BP. Our record differs significantly from a speleothem record from Rodrigues Island, located ~1800 km to the east of our study area in Madagascar suggesting different climatological controls on northwest Madagascar and more oceanic sites to the east.
... In SE Africa, south of 25 • S, there is a shift from a wet to an arid phase after 3 ka corresponding to changes in Ilaika Marais δD 23 (Miller et al., 2019;Chevalier and Chase, 2015) while an opposite trend is observed between 20 and 25 • S (Miller et al., 2020). In the tropical African archives an abrupt drying is evident near 4.0 ka, which has been connected to seasonal precipitation changes and followed by lower δD values after 3.0 ka (Marchant and Hooghiemstra, 2004;Tierney et al., 2008Tierney et al., , 2011Berke et al., 2012;Scroxton et al., 2023). ...
... The dipole pattern based on the data from Tanzania, Kenya, the Mozambique Channel and Madagascar from the LGM to the late Holocene could thus be explained by the variability of MCT that could act together with ITCZ and MH Scroxton et al., 2023). In modern climate, strong (weak) MCT years result in wet (dry) conditions in Madagascar and coast of Mozambique and drier (wetter) mainly in Tanzania and Kenya ( Fig. 9) . ...
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Large uncertainties still exist about the long-term mechanisms influencing the hydroclimate variability of southeast Africa where proxy data and model simulations indicate rainfall dipoles between subtropical and tropical areas. The topography of Madagascar, located off the southeastern coast of Africa, modulates these dipoles while its climate is influenced by the position of the Intertropical Convergence Zone (ITCZ) and the Subtropical High as well as the sea surface temperature (SST) of SW Indian Ocean. The island can thus be considered a key location for the understanding of the tropical SE African climatic variability and the interplay between atmospheric patterns. However, the scarcity of continuous records from Madagascar has made the evolution of regional late Quaternary climate and its driving mechanisms difficult to assess. Here, we present a 26-kyr record of the deuterium/hydrogen isotope ratio (δD) of biomarkers (n-alkanes) from the central eastern part of the island at Antananarivo at around 1250 m a.s.l. Preliminary summary pollen data are also presented as a comparison. The δD profiles of aquatic plant and terrestrial plant-derived n-alkanes generally exhibit similar trends implying that they all record changes in the isotope composition of source water, namely meteoric water that recharges soil and lake waters. In this tropical region, the δD variability of precipitation is mainly influenced by the amount effect reflecting the intensity of precipitation associated with the monsoon. We observe: (i) stable and wet conditions during the Last Glacial Maximum, (ii) drier conditions from 18.5 to 15 ka (ka before present) during the Heinrich Stadial 1 (iii) high humidity after 15 ka culminating at the Younger Dryas (YD), (iv) drier conditions from 11.7 ka to 8.2 ka, (v) a return to humid climate until 2.8 ka, and (vi) an arid phase followed by increased wetness after 0.9 ka, although the record is likely influenced by human-induced vegetation changes the last 1.2 ka. This climate signal is similar to other records from the Mozambique Channel but opposite to records from the East African mainland and the subtropical southern Africa, especially between 20 and 25°S. Although there is a good correspondence of our record with insolation- driven migrations of ITCZ during the LGM and the early Holocene, the dipoles are largely consistent with the modern rainfall anomaly and are best explained by the interlinked effects of the SST changes and the variability of the Mozambique Channel Trough.
... Finally, temporal data can help clarify if there were changes over time in the degree to which hippos relied on C 4 plants. Mid-Holocene drying is recorded in stalagmites from northwestern Madagascar and broadly across the Indian Ocean (Scroxton et al., 2023). Lake sediments from the high-elevation western slope of the CH (ca. ...
... For the CH, we were able to evaluate isotopic differences for Holocene individuals from mid-elevation sites that lived before and after a drying event beginning ca. 4300 years ago (Razafimanantsoa, 2021;Scroxton et al., 2023). Sample size for dated hippos is small. ...
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Societal Impact Statement Today, expansive C 4 grassy biomes exist across central, western, and northern Madagascar. Some researchers have argued that the island's now‐extinct pygmy hippopotamuses belonged to a megaherbivore grazing guild that maintained these grasslands prior to human arrival. However, the chemistry of hippo bones indicates that C 4 grasses were only a minor part of hippo diet. This, in turn, suggests that C 4 grasses were present but not widespread when hippos were alive and that grasses expanded only after Malagasy people shifted from hunting and foraging to agropastoralism approximately 1000 years ago. These results have important implications for environmental reconstructions and biodiversity management. Summary Extinct hippopotamuses ( Hippopotamus spp.) were part of Madagascar's megaherbivore guild. Stable carbon (δ ¹³ C) and nitrogen (δ ¹⁵ N) isotopes in radiometrically dated bone collagen track spatial and temporal variation in diet and habitat. If hippos helped maintain C 4 grassy biomes, then they should have regularly consumed C 4 grasses, which have high δ ¹³ C values. However, if expansive C 4 grassy biomes are anthropogenic, then forests would have been more extensive in the past, and hippos would have predominantly consumed C 3 plants with low δ ¹³ C values. Nitrogen isotopes can clarify foraging habitat (moist or dry). We assessed δ ¹³ C and δ ¹⁵ N values for hippos from different ecoregions of Madagascar and compared these with data for extinct herbivorous lemurs from the same ecoregions. We further explored the effects of wet/dry transitions on isotopic trends for hippos from the central highlands and spiny thicket ecoregions. Carbon isotopes suggest (1) limited C 4 consumption by hippos in the central highlands, dry deciduous forest, and succulent woodland ecoregions; and (2) moderate consumption of C 4 resources in the spiny thicket. Nitrogen data indicate that hippos foraged in wetter habitats than sympatric lemurs in all regions. Malagasy hippos did not regularly graze C 4 grasses in dry, open habitats, even in regions blanketed by C 4 grassy biomes today. Malagasy grasses are adapted to grazing and fire, but these are likely ancient adaptations that accompanied grasses when they initially spread to Madagascar. C 4 grassy biomes were spatially limited in extent in the past and only expanded after the Late Holocene introduction of domesticated ungulates.
... In this study we focus on paleohydroclimate records from the tropical Indian Ocean basin (Fig. 1). We choose this region because: 1) there has been a recent spate of publications of high resolution paleoclimate records in the region, including new speleothem records from western India (Kathayat et al., 2017), Sumatra (Wurtzel et al., 2018), Rodrigues Island , and Madagascar (Wang et al., 2019;Scroxton et al., 2023), and a high resolution planktonic foraminifera record from a sediment core from the Arabian Sea (Giesche et al., 2019). 2) The tropical Indian Ocean basin contains a recognised, temporally proximal, climate anomaly at 4.0 kyr BP that should be distinguishable to the 4.2 ka event in high-resolution, precisely dated records (Denniston et al., 2013). ...
... Specifically, speleothem records had to have near-continual coverage and two UeTh ages between 5.0 and 3.0 kyr BP, and have better than 15-year sample resolution. The chosen records were from northern Australia, Oman, northwest India, Rodrigues, Sumatra and Madagascar (Denniston et al., 2013;Fleitmann et al., 2007;Kathayat et al., 2017;Li et al., 2018;Wurtzel et al., 2018;Scroxton et al., 2023). We also added a speleothem record from Borneo as an outgroup sample . ...
Article
The spatial pattern of Holocene climate anomalies is crucial to determining the mechanisms of change, distinguishing between unforced and forced climate variability, and understanding potential impacts on past and future human societies. The 4.2 ka event is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Yet outside the data-rich Northern Hemisphere mid-latitudes, the global pattern of climate anomalies is uncertain. In this study we investigate the spatial and temporal variability of the tropical Indian Ocean hydroclimate at the Mid- to Late-Holocene transition. We conducted Monte-Carlo principal component analysis, considering full age uncertainty, on ten high-resolution, precisely dated paleohydroclimate records from around the tropical Indian Ocean basin, all growing continuously or almost continuously between 5 and 3 kyr BP. The results indicate the dominant mode of variability in the region was a drying between 3.97 kyr BP (±0.08 kyr standard error) and 3.76 kyr BP (±0.07 kyr standard error) with dry conditions lasting for an additional 300 years in some records, and a permanent change in others. This drying in PC1, which we interpret as a proxy of summer monsoon variability, fits with a previously recognised tropic wide change in hydroclimate around 4.0 kyr BP. An abrupt event from 4.2 to 3.9 kyr BP is seen locally in individual records but lacks regional coherence. A lack of apparent 4.2 ka event in tropical Indian Ocean hydroclimate has ramifications for climate variability in the Indus valley, and for the Harappan civilization. Through a comparison of existing Indian subcontinent paleoclimate records, upstream climatic variability in the Indian Summer Monsoon and winter Westerly Disturbances source regions, and modern climatology, we present the “Double Drying hypothesis”. A winter rainfall drying between 4.2 and 3.9 kyr BP was followed by a summer rainfall drying between 3.97 kyr BP and at least 3.4 kyr BP. The Double Drying hypothesis provides more detailed climatic context for the Harappan civilization, resolves the cropping paradox, and fits the spatial-temporal pattern of urban abandonment. The consequences for the new Mid- to Late-Holocene Global Boundary Stratotype Section and Point in a stalagmite from Meghalaya are explored.
... Identification of a climate event in a particular area is important to characterize the spatial extent of that climate event, regardless of whether it is significantly different from centennial-scale variability. For instance, while both the 8.2 and 4.2 ka events are present in speleothems from Madagascar, they do not stand out against the backdrop of natural variability (Dawson et al., 2024;Scroxton et al., 2023a;Williams et al., 2023). That is, while they had an "impact," they are not "significant." ...
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Speleothem oxygen isotope records offer unique insights into Asian Monsoon evolution, with their precise chronologies used to identify abrupt climatic events. However, individual records are sometimes used to draw broad conclusions about global climate, without considering the dynamical context in which they exist. We present a robust framework for assessing the regional significance, and hence the potential global significance, of paleoclimate events, using the proposed Meghalayan age onset (associated with the “4.2 ka event”) as a case study. Analyzing 14 well‐dated speleothem oxygen isotope records from the SISAL v3 database and recent literature, we investigate the regional coherency of rapid shifts in Asian paleohydrology, which is the regional center of action for the proposed event, over the Holocene. Three robust methods fail to detect spatially coherent variability consistent with a 4.2 ka event across Asia, either because none exists or because it is of insufficient magnitude. In contrast, the 8.2 ka event is expressed in most records that resolve it. The absence of a clear isotopic excursion across this data set suggests that the “4.2 ka megadrought” was not global, with important implications for archeology and geochronology. This casts doubt on the proposal that the 4.2 ka event marks the onset of a new geologic age. We do, however, observe support for a gradual isotopic enrichment between 3.9 and 3.6 ka, followed by partial recovery—consistent with the “Double Drying” hypothesis and possibly related to changes in El Niño‐Southern Oscillation variability.
... According to the WFP, the changing weather systems in the Indian Ocean have interrupted the normal circulation of moisture air from the sea that brought rainfall to the island, usually in November. The disappearance of the rainy season has had severe consequences, with dried out lands and locust proliferation obliterating any possibility for the local communities to grow crops, access clean water, keep cattle, or simply preserve their traditional ways of life (Russo et al. 2023;Scroxton et al. 2023). ...
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This chapter presents a ‘wicked deadlock’ between human insecurity, underdevelopment, and unsustainability against the background of a more turbulent ocean environment, which can be best illustrated with the ongoing seaborne challenges on the island of Madagascar, especially those engendered by sea-originated nature hazards and man-caused maritime disruptions. It first investigates what human security means in respect of Madagascar’s position as both one of the least-developed island nations and the frontier of ocean sustainability. Then it examines how the worsening of human insecurity on the island has been accelerated recently in light of climatic and socio-ecological changes from the sea. The human security emergencies the island is confronting might have rooted partly in its oceanographic peculiarities, but they warn the world of a possible failing scenario of the sustainability-security nexus, which might have larger implications that go beyond Madagascar’s coastlines and reach to other oceanic communities.
... ka event" (Supplementary Fig. S2). Interestingly, dry conditions in both the Southern and Northern Hemisphere tropics 63 suggest that the mechanism for the drying is not simply the migration of the ITCZ, and variability in the expression and timing of these events in speleothem records of the Indian Ocean Basin suggest the "4.2 ka event" is more nuanced than the 8.2 ka event 24,64 . ...
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The Malagasy Summer Monsoon is an important part of the larger Indian Ocean and tropical monsoon region. As the effects of global warming play out, changes to precipitation in Madagascar will have important ramifications for the Malagasy people. To help understand how precipitation responds to climate changes we present a long-term Holocene speleothem record from Anjohibe, part of the Andranoboka cave system in northwestern Madagascar. To date, it is the most complete Holocene record from this region and sheds light on the nature of millennial and centennial precipitation changes in this region. We find that over the Holocene, precipitation in northwestern Madagascar is actually in phase with the Northern Hemisphere Asian monsoon on multi-millennial scales, but that during some shorter centennial-scale events such as the 8.2 ka event, Anjohibe exhibits an antiphase precipitation signal to the Northern Hemisphere. The ultimate driver of precipitation changes across the Holocene does not appear to be the meridional migration of the monsoon. Instead, zonal sea surface temperature gradients in the Indian Ocean seem to play a primary role in precipitation changes in northwestern Madagascar.
... An extreme global drought and cooling event at ca. 4.2 ka has been revealed by various proxies (Verheyden et al., 2008;Railsback et al., 2018;Scroxton et al., 2023). The International Commission on Stratigraphy has formally divided the Holocene Epoch into the Northgrippian and Meghalayan before and after the 4.2 ka event because of the abrupt and broad effects of this event on agricultural societies (Walker et al., 2018). ...
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Variability of sea-surface temperature related to shifts in the mode of the El Niño–Southern Oscillation (ENSO) has been implicated as a possible forcing mechanism for the global-scale changes in tropical and subtropical precipitation known as the 4.2 ka event. We review records of coral reef development and paleoceanography from the tropical eastern Pacific (TEP) to evaluate the potential impact of the 4.2 ka event on coral reefs. Our goal is to identify the regional climatic and oceanographic drivers of a 2500-year shutdown of vertical reef accretion in the TEP after 4.2 ka. The 2500-year hiatus represents ∼40 % of the Holocene history of reefs in the TEP and appears to have been tied to increased variability of ENSO. When ENSO variability abated approximately 1.7–1.6 ka, coral populations recovered and vertical accretion of reef framework resumed apace. There is some evidence that the 4.2 ka event suppressed coral growth and reef accretion elsewhere in the Pacific Ocean as well. Although the ultimate causality behind the global 4.2 ka event remains elusive, correlations between shifts in ENSO variability and the impacts of the 4.2 ka event suggest that ENSO could have played a role in climatic changes at that time, at least in the tropical and subtropical Pacific. We outline a framework for testing hypotheses of where and under what conditions ENSO may be expected to have impacted coral reef environments around 4.2 ka. Although most studies of the 4.2 ka event have focused on terrestrial environments, we suggest that understanding the event in marine systems may prove to be the key to deciphering its ultimate cause.
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We review paleoceanographic and paleoclimatic records from the northern North Atlantic to assess the nature of climatic conditions at 4.2ka BP, which has been identified as a time of exceptional climatic anomalies in many parts of the world. The northern North Atlantic region experienced relatively warm conditions in the early Holocene (6–8ka BP) followed by a general decline in temperatures after ~5ka BP, which led to the onset of Neoglaciation. Although a few records do show a distinct anomaly around 4.2ka BP (associated with a glacial advance), this is not widespread and we interpret it as a local manifestation of the overall climatic deterioration that characterizes the late Holocene.
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The “4.2 ka event” is frequently described as a major global climate anomaly between 4.2 and 3.9 ka, which defines the beginning of the current Meghalayan age in the Holocene epoch. The “event” has been disproportionately reported from proxy records from the Northern Hemisphere, but its climatic manifestation remains much less clear in the Southern Hemisphere. Here, we present highly resolved and chronologically well-constrained speleothem oxygen and carbon isotopes records between ∼6 and 3 ka from Rodrigues Island in the southwestern subtropical Indian Ocean, located ∼600 km east of Mauritius. Our records show that the 4.2 ka event did not manifest itself as a period of major climate change at Rodrigues Island in the context of our record's length. Instead, we find evidence for a multi-centennial drought that occurred near-continuously between 3.9 and 3.5 ka and temporally coincided with climate change throughout the Southern Hemisphere.
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Climate exerted constraints on the growth and decline of past human societies but our knowledge of temporal and spatial climatic patterns is often too restricted to address causal connections. At a global scale, the inter-hemispheric thermal balance provides an emergent framework for understanding regional Holocene climate variability. As the thermal balance adjusted to gradual changes in the seasonality of insolation, the Intertropical Convergence Zone migrated southward accompanied by a weakening of the Indian summer monsoon. Superimposed on this trend, anomalies such as the Little Ice Age point to asymmetric changes in the extratropics of either hemisphere. Here we present a reconstruction of the Indian winter monsoon in the Arabian Sea for the last 6000 years based on paleobiological records in sediments from the continental margin of Pakistan at two levels of ecological complexity: sedimentary ancient DNA reflecting water column environmental states and planktonic foraminifers sensitive to winter conditions. We show that strong winter monsoons between ca. 4500 and 3000 years ago occurred during a period characterized by a series of weak interhemispheric temperature contrast intervals, which we identify as the early neoglacial anomalies (ENA). The strong winter monsoons during ENA were accompanied by changes in wind and precipitation patterns that are particularly evident across the eastern Northern Hemisphere and tropics. This coordinated climate reorganization may have helped trigger the metamorphosis of the urban Harappan civilization into a rural society through a push–pull migration from summer flood-deficient river valleys to the Himalayan piedmont plains with augmented winter rains. The decline in the winter monsoon between 3300 and 3000 years ago at the end of ENA could have played a role in the demise of the rural late Harappans during that time as the first Iron Age culture established itself on the Ghaggar-Hakra interfluve. Finally, we speculate that time-transgressive land cover changes due to aridification of the tropics may have led to a generalized instability of the global climate during ENA at the transition from the warmer Holocene thermal maximum to the cooler Neoglacial.
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The 4.2 ka BP event is defined as a phase of environmental stress characterized by severe and prolonged drought of global extent. The event is recorded from the North Atlantic through Europe to Asia and has led scientists to evoke a 300-year global mega-drought. For the Mediterranean and the Near East, this abrupt climate episode radically altered precipitation, with an estimated 30 %–50 % drop in rainfall in the eastern basin. While many studies have highlighted similar trends in the northern Mediterranean (from Spain to Turkey and the northern Levant), data from northern Africa and the central-southern Levant are more nuanced, suggesting a weaker imprint of this climate shift on the environment and/or different climate patterns. Here, we critically review environmental reconstructions for the Levant and show that, while the 4.2 ka BP event also corresponds to a drier period, a different climate pattern emerges in the central-southern Levant, with two arid phases framing a wetter period, suggesting a W-shaped event. This is particularly well expressed by records from the Dead Sea area.
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The rainfall mean state and variability of the tropical Southern Hemisphere are strongly affected by the Tropical Convergence Zones. Unlike the South Pacific and South Atlantic, the topography of Madagascar prevents, through the Mozambique Channel Trough, the direct transport of moisture from the Indian Ocean towards southern Africa that feeds the South Indian Ocean Convergence Zone (SICZ). Numerical experiments using regional climate model s reveal that a flatter than actual topography over Madagascar results in a strengthening of the SICZ through anomalously high easterly moisture fluxes transported from the Indian Ocean and the Mozambique Channel. These in turn trigger a significant increase in precipitation over southern Africa extending from Mozambique to Angola and a decrease in rainfall over Madagascar. These results have important implications for the improvement of the representation of African rainfall mean state and variability, which has been identified as a persisting issue in different generations of state-of-the-art climate models.
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Drought is a complex and multivariate phenomenon influenced by diverse physical and biological processes. Such complexity precludes simplistic explanations of cause and effect, making investigations of climate change and drought a challenging task. Here, we review important recent advances in our understanding of drought dynamics, drawing from studies of paleoclimate, the historical record, and model simulations of the past and future. Paleoclimate studies of drought variability over the last two millennia have progressed considerably through the development of new reconstructions and analyses combining reconstructions with process-based models. This work has generated new evidence for tropical Pacific forcing of megadroughts in Southwest North America, provided additional constraints for interpreting climate change projections in poorly characterized regions like East Africa, and demonstrated the exceptional magnitude of many modern era droughts. Development of high resolution proxy networks has lagged in many regions (e.g., South America, Africa), however, and quantitative comparisons between the paleoclimate record, models, and observations remain challenging. Fingerprints of anthropogenic climate change consistent with long-term warming projections have been identified for droughts in California, the Pacific Northwest, Western North America, and the Mediterranean. In other regions (e.g., Southwest North America, Australia, Africa), however, the degree to which climate change has affected recent droughts is more uncertain. While climate change-forced declines in precipitation have been detected for the Mediterranean, in most regions, the climate change signal has manifested through warmer temperatures that have increased evaporative losses and reduced snowfall and snowpack levels, amplifying deficits in soil moisture and runoff despite uncertain precipitation changes. Over the next century, projections indicate that warming will increase drought risk and severity across much of the subtropics and mid-latitudes in both hemispheres, a consequence of regional precipitation declines and widespread warming. For many regions, however, the magnitude, robustness, and even direction of climate change-forced trends in drought depends on how drought is defined, with often large differences across indicators of precipitation, soil moisture, runoff, and vegetation health. Increasing confidence in climate change projections of drought and the associated impacts will likely depend on resolving uncertainties in processes that are currently poorly constrained (e.g., land-atmosphere interactions, terrestrial vegetation) and improved consideration of the role for human policies and management in ameliorating and adapting to changes in drought risk.
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The vast Indo-Gangetic Plain in South Asia has been home to some of the world’s oldest civilizations, whose fortunes ebbed and flowed with time—plausibly driven in part by shifts in the spatiotemporal patterns of the Indian summer monsoon rainfall. We use speleothem oxygen isotope records from North India to reconstruct the monsoon’s variability on socially relevant time scales, allowing us to examine the history of civilization changes in the context of varying hydroclimatic conditions over the past 5700 years. Our data suggest that significant shifts in monsoon rainfall have occurred in concert with changes in the Northern Hemisphere temperatures and the discharges of the Himalayan rivers. The close temporal relationship between these large-scale hydroclimatic changes and the intervals marking the significant sociopolitical developments of the Indus Valley and Vedic civilizations suggests a plausible role of climate change in shaping the important chapters of the history of human civilization in the Indian subcontinent.
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A global water scarcity assessment for the 21st century was conducted under the latest socio-economic scenario for global change studies, namely Shared Socio-economic Pathways (SSPs). SSPs depict five global situations with substantially different socio-economic conditions. In the accompanying paper, a water use scenario compatible with the SSPs was developed. This scenario considers not only quantitative socio-economic factors such as population and electricity production but also qualitative ones such as the degree of technological change and overall environmental consciousness. In this paper, water availability and water scarcity were assessed using a global hydrological model called H08. H08 simulates both the natural water cycle and major human activities such as water abstraction and reservoir operation. It simulates water availability and use at daily time intervals at a spatial resolution of 0.5° × 0.5°. A series of global hydrological simulations were conducted under the SSPs, taking into account different climate policy options and the results of climate models. Water scarcity was assessed using an index termed the Cumulative Abstraction to Demand ratio, which is expressed as the accumulation of daily water abstraction from a river divided by the daily consumption-based potential water demand. This index can be used to express whether renewable water resources are available from rivers when required. The results suggested that by 2071–2100 the population living under severely water-stressed conditions for SSP1-5 will reach 2588–2793 × 106 (39–42% of total population), 3966–4298 × 106 (46–50%), 5334–5643 × 106 (52–55%), 3427–3786 × 106 (40–45%), 3164–3379 × 106 (46–49%) respectively, if climate policies are not adopted. Even in SSP1 (the scenario with least change in water use and climate) global water scarcity increases considerably, as compared to the present-day. This is mainly due to the growth in population and economic activity in developing countries, and partly due to hydrological changes induced by global warming.
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The timing and causes of paleoenvironmental changes in Madagascar have been debated, specifically in respect to human activity following the settlement in the late Holocene. Here we present δ¹⁸O, δ¹³C, layer-bounding surfaces, layer-specific width, mineralogy, and distribution of macroholes from Stalagmite MA3 from Anjohibe Cave to provide a detailed understanding of the paleoenvironmental changes in northwestern Madagascar between 370 CE and 1300 CE. The stable isotope records of Stalagmite MA3 are compared with stable isotope records of Stalagmites ANJ94-5 and MA2.
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The Paleoclimate Modelling Intercomparison Project (PMIP3) now includes the 8.2 ka event as a test of model sensitivity to North Atlantic freshwater forcing. To provide benchmarks for intercomparison, we compiled and analyzed high-resolution records spanning this event. Two previously-described anomaly patterns that emerge are cooling around the North Atlantic and drier conditions in the Northern Hemisphere tropics. Newer to this compilation are more robustly-defined wetter conditions in the Southern Hemisphere tropics and regionally-limited warming in the Southern Hemisphere. Most anomalies around the globe lasted on the order of 100 to 150 yr. More quantitative reconstructions are now available and indicate cooling of ~ 1 °C and a ~ 20% decrease in precipitation in parts of Europe as well as spatial gradients in δ18O from the high to low latitudes. Unresolved questions remain about the seasonality of the climate response to freshwater forcing and the extent to which the bipolar seesaw operated in the early Holocene.
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High-resolution isotopic analyses were performed on RL4 flowstone from Renella Cave (Alpi Apuane, central Italy), a speleothem studied previously at low resolution. The new data are discussed together with recently obtained data from Corchia and other localities in the central Mediterranean, to elucidate the possible origin and regional articulation of a climatic event centred at ca. 4.0 ka. This analyses indicates that central to southern Italy between ca 3.8 and 4.3 ka was characterized by drier conditions, whereas in Northern Italy the event seems less expressed or, as within the Alps, marked by cooler and wetter conditions. Several lines of evidence suggest that this event could be characterized by longer summer drought and possibly by little impact on precipitation during winter, even if this aspect needs to be explored in more detail. However, the event is particularly prominent in the northern sector of the African Monsoon domain, which has been robustly linked to southward shifts in the ITCZ; whereas its occurrence is uncertain on northern European latitudes. However, many proxies indicate that there aridification probably started some centuries earlier and culminated at ca. 4.0 ka. Taken as a whole, these data can be used to clarify the regional articulation of this event, but interpretations based on general circulation are still elusive.
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Annually laminated stalagmites deposited over the last 30–160 years are analysed to determine their growth rate. Three natural and artificial cave sites in England, France, and Belgium were chosen for their wide range of variability in growth rate determining variables, and multiple samples were taken from each site. The annual nature of laminae deposition within the stalagmite calcite was confirmed by comparison to the date of cave/void opening, 14C analyses, or by using dated event horizons. Measured stalagmite growth rate was determined from annual laminae thickness measurements and compared to that theoretically predicted from the chemical kinetics of the calcite precipitation reaction. A good agreement is observed between empirical observations and theoretical predictions, although two complicating factors, variations in calcite porosity, and seasonal cessation of the water supply to the samples, both affect the growth rate. Implications for the extraction of palaeoclimate information from stalagmite growth rate are discussed.
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Petrographic recognition of layer-bounding surfaces in stalagmites offers an important tool in constructing paleoclimate records. Previous petrographic efforts have examined thickness of layers (a possible proxy for annual rainfall) and alternation of layers in couplets (a possible indicator of seasonality). Layer-bounding surfaces, in contrast, delimit series of layers and represent periods of non-deposition, either because of exceptionally wet or exceptionally dry conditions. Two types of layer-bounding surfaces can be recognized according to explicitly defined petrographic criteria. Type E layer-bounding surfaces are surfaces at which layers have been truncated or eroded at the crest of a stalagmite. Keys to their recognition include irregular termination of layers otherwise present on the stalagmite’s flank, dissolutional cavities, and coatings of non-carbonate detrital materials. Type E surfaces are interpreted to represent wet periods during which drip water became so undersaturated as to dissolve pre-existing stalagmite layers, and thus they necessarily represent hiatuses in the stalagmite record. Type L layer-bounding surfaces are surfaces below which layers become thinner upward and/or layers have lesser lateral extent upward, so that the stalagmite’s layer-specific width decreases. They are thus surfaces of lessened deposition and are interpreted to represent drier conditions in which drip rate slowed so much that little deposition occurred. A Type L surface may, but does not necessarily, represent a hiatus in deposition. However, radiometric age data show that Type L surfaces commonly represent significant hiatuses. These surfaces are significant to paleoclimate research both for their implications regarding climate change (exceptionally wet or dry conditions) and in construction of chronologies in which other data, such as stable isotope ratios, are placed. With regard to climate change, recognition of these surfaces provides paleoclimatological information that can complement or even substitute for geochemical proxies. With regard to chronologies, recognition of layerbounding surfaces allows correct placement of hiatuses in chronologies and thus correct placement of geochemical data in time series. Attention to changing thickness of annual layers and thus to accumulation rate can also refine a chronology. A chronology constructed with attention to layer-bounding surfaces and to changing layer thickness is much more accurate than a chronology in which hiatuses are not recognized at such surfaces.
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The spatial pattern of Holocene climate anomalies is crucial to determining the mechanisms of change, distinguishing between unforced and forced climate variability, and understanding potential impacts on past and future human societies. The 4.2 ka event is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Yet outside the data-rich Northern Hemisphere mid-latitudes, the global pattern of climate anomalies is uncertain. In this study we investigate the spatial and temporal variability of the tropical Indian Ocean hydroclimate at the Mid- to Late-Holocene transition. We conducted Monte-Carlo principal component analysis, considering full age uncertainty, on ten high-resolution, precisely dated paleohydroclimate records from around the tropical Indian Ocean basin, all growing continuously or almost continuously between 5 and 3 kyr BP. The results indicate the dominant mode of variability in the region was a drying between 3.97 kyr BP (±0.08 kyr standard error) and 3.76 kyr BP (±0.07 kyr standard error) with dry conditions lasting for an additional 300 years in some records, and a permanent change in others. This drying in PC1, which we interpret as a proxy of summer monsoon variability, fits with a previously recognised tropic wide change in hydroclimate around 4.0 kyr BP. An abrupt event from 4.2 to 3.9 kyr BP is seen locally in individual records but lacks regional coherence. A lack of apparent 4.2 ka event in tropical Indian Ocean hydroclimate has ramifications for climate variability in the Indus valley, and for the Harappan civilization. Through a comparison of existing Indian subcontinent paleoclimate records, upstream climatic variability in the Indian Summer Monsoon and winter Westerly Disturbances source regions, and modern climatology, we present the “Double Drying hypothesis”. A winter rainfall drying between 4.2 and 3.9 kyr BP was followed by a summer rainfall drying between 3.97 kyr BP and at least 3.4 kyr BP. The Double Drying hypothesis provides more detailed climatic context for the Harappan civilization, resolves the cropping paradox, and fits the spatial-temporal pattern of urban abandonment. The consequences for the new Mid- to Late-Holocene Global Boundary Stratotype Section and Point in a stalagmite from Meghalaya are explored.
Article
The 4.2 ka Event has generally been regarded as a period of decades to at most a few centuries in which comparatively dry conditions existed in the Middle East and more broadly across the mid-latitude Northern Hemisphere. This paper presents new stable-isotopic and petrographic observations from two previously-unreported U-Th-dated stalagmites from Dante Cave in northeastern Namibia. The results are most compatible with wetter conditions during the 4.2 ka Event, and wetness during the 4.2 ka Event is the only inference supported by evidence. These new results add to observations previously reported from a third Dante Cave stalagmite suggesting a comparatively wet 4.2 ka Event in which Africa's Tropical Rain Belt migrated southward and rainfall increased along the Congo Air Boundary and/or Kalahari Discontinuity. The new results support findings from three other locations in Namibia and Botswana, from at least seven other locations in the Southern Hemisphere, and at least one in southern China, that suggest a wetter rather than drier 4.2 ka Event in those regions. The pattern emerging from these sites generally agrees with recent modeling results indicating increased moisture over broad areas (but not all) of the Southern Hemisphere. This in turn suggests a 4.2 ka Event that was not a global drought but was instead a set of latitudinally-dependent responses to global-scale southward migration of the Inter-Tropical Convergence Zone (ITCZ), and thus Africa's loosely linked Tropical Rain Belt, as a result of cooling of the Northern Hemisphere, which brought drier conditions to some areas and wetter conditions to others.
Article
The climate variability of the last deglaciation is often linked to meltwater discharges from the melting of large ice sheets. One of the best examples comes from the drainage of glacial Lake Agassiz-Ojibway (LAO) and its attendant perturbation of the Atlantic Meridional Overturning Circulation (AMOC), which has long been held responsible for a rapid cooling at ∼8.2 ka. However, recent modeling studies have argued that a large and sustained freshwater flux linked to increased surface melt and ensuing collapse of the Laurentide Ice Sheet (LIS) dam may have formed an efficient forcing for this cooling event. Yet, empirical (geological) evidence for a long-lasting meltwater flux is still equivocal while paleoceanographic data show that the freshening of the North Atlantic around the 8.2-ka cold event is characterized by multiple freshwater pulses. Part of this uncertainty arises from the lack of constraints on the structure (number) and timing of meltwater discharges involved in the drainage of LAO, which prevents a detailed assessment of the freshwater forcing mechanisms at work and their potential impact on AMOC—an important issue given the present-day increase in the melting of the cryosphere around the North Atlantic. Here, we review 597 ¹⁴C ages from marine and continental sediment archives and use 296 of these ¹⁴C ages along with LAO geomorphological and varve records to present an integrated framework constraining the timing of LAO meltwater outbursts across the final deglaciation interval. Results show that LAO drained through two distinct events: first subglacially at ∼8.22 cal ka BP and then after the breakup of the ice dam at ∼8.16 cal ka BP. These LAO meltwater discharges are coeval with two important freshwater pulses in North Atlantic sediment cores, with the largest meltwater outburst matching the onset of the 8.2 ka event in Greenland ice cores. These results suggest that, in a fast-changing ocean-climate system influenced by melting ice sheets like that of the late deglaciation, massive and short-lived freshwater injections can potentially have an impact on AMOC.
Article
Although the 8.2 ka event is a well-known globally documented abrupt climate event, its detailed expression in tropical southeastern Africa is poorly constrained. Here we present a high resolution and precisely dated record of the regional hydroclimatic variability between 8.5–7.5 ka BP from a polymorphic speleothem (ABC-1) from northwestern Madagascar. In combination with the exclusive recurrences of calcite layers (indicative of wetter conditions), the distinctly negative excursions in the ABC-1 δ¹⁸O record manifest a wet 8.2 ka event that spanned from 8.230 to 8.053 ka BP. The event is characterized by the replacement of shorter-term (interannual to interdecadal) δ¹⁸O-δ¹³C coherence with the longer-term (multidecadal to centennial) coherence, suggesting a stable and well-developed forest system during the 8.2 ka event. Our data reveal a two-stage structure of the 8.2 ka event, which is superimposed with two brief pluvial episodes in each stage. The widely documented two peaks/troughs during Stage-I suggest rapid propagation of climate signal through atmospheric processes triggered initially by the freshwater forcing, whereas the divergent behaviors of the peaks/troughs in Stage-II in different climate systems may suggest a different forcing.
Article
Robust reconstructions of paleoclimate and paleoenvironmental changes using stalagmite proxy records critically depend on detailed observations of the transfer function between the regional climate/environment, the karst aquifer hydrology, and finally the cave microclimate via monitoring, which is currently lacking in Madagascar. This paper reports the first monitoring study performed in Anjohibe Cave, in Mahajanga, NW Madagascar to understand the linkage between regional climatological changes and cave responses to such changes. In this research, we monitored (1) the drip water pH, TDS, EC, temperature, δ¹³CDIC, δ¹⁸Ow, δ²Hw, and elemental (Ca, Mg, Sr) composition, and (2) the cave atmosphere pCO2, relative humidity (RH) and temperature. Three significant findings were drawn from the results. First, the data show that air-to-air transfer is fast, and the internal parameters closely vary with the regional climatology. Second, rainfall to drip signal transfer is not immediate, and it can take few months to one season for the signals to be detected in the drip water due to the “epikarst storage effect”. Lastly, CaCO3 precipitation is likely to occur during the winter-summer transition, during which prior carbonate precipitation was detected. Since the growth of speleothems is influenced by numerous cave-specific factors, this study, although preliminary, indicates that Anjohibe Cave drip waters are capable of registering changes in its surrounding environment. Such information is ultimately archived in speleothems to reconstruct paleoclimate and paleoenvironmental changes. Results from this research will be of high significance for those working on speleothems within Madagascar, and for those working on understanding the transfer of climatic variations to cave deposits.
Article
The magnitude and frequency of extreme events is projected to increase under future climate change and understanding the major drivers of global climate variability is dependent on the development of detailed records from critical regions. Subtropical Africa is known to have undergone phases of significant change resulting from variations in broad-scale atmospheric and oceanic circulation patterns, but the nature and cause for these variations remain intensely debated. Here we present a highly resolved sedimentary record from the eastern margin of South Africa to reconstruct hydroclimate in southeast Africa over the past 7000 years. Stratigraphic and inorganic proxies preserve a highly sensitive record that document dramatic shifts in moisture balance, with phases of severe drought recorded at 4700–4200 and 3700–2600 cal yr BP. We suggest that pronounced hydroclimate variability observed in the Mkhuze record was likely triggered by changes in the activity of El Niño-Southern Oscillation (ENSO), which acted to control the transport of moisture from the Indian Ocean across southeast Africa. We show that rapid shifts in moisture availability were a characteristic feature of mid-late Holocene climate across the summer rainfall region of southern Africa, with these events broadly anti-phased with hydroclimatic shifts in East Africa and northwest Madagascar. These findings suggest that evaluating drought frequency, ecological risk and food security within the region is likely to be closely tied to ENSO and its response to future global warming.
Article
Dryland regions are generally projected to become drier under future climate change scenarios. Understanding the long-term natural variability of dryland regions via paleo-reconstructions is therefore highly desirable. The δ18O of two coeval modern speleothems from Cathedral Cave, Wellington, in semi-arid SE Australia are compared to the instrumental record to assess its efficacy as a proxy of past hydrological variability. Stalagmite δ18O was modulated by the frequency of recharge events and epikarst evaporation of storage water. Prolonged intervals between recharge events, such as droughts, resulted in higher stalagmite δ18O. Conversely, periods with more frequent recharge events and a positive water balance, resulted in lower δ18O. Disequilibrium cave processes are likely to be enhanced during dry conditions, although it is argued that these will modulate δ18Ospel in the same direction as epikarst evaporation, effectively amplifying the response of δ18Ospel. Extreme events, such as floods and droughts, were also captured in the stalagmite records, although potentially with a lag of several years. We verify that modern speleothems from semi-arid regions can be used to reconstruct hydroclimate due to variations in δ18Ospel modulated by karst processes. Such records are archives of past changes in recharge rather than precipitation amount or surface temperature, as is commonly applied to speleothem records from non-water-limited regions.
Article
Subdivision of the Holocene Series/Epoch into the Greenlandian, Northgrippian and Meghalayan Stages/Ages has recently been ratified based on stable isotope records from ice-core and speleothem archives. The base of the most recent chronostratigraphic unit, corresponding to the Northgrippian−Meghalayan boundary, coincides with the ‘4.2 ka event’ recognised as a low-latitude drought anomaly. The Global Stratotype Section and Point (GSSP) for the boundary, the oxygen isotope record from the Mawmluh Cave speleothem (India), demonstrates this event with markedly weakened Asian summer monsoon. Here we contribute to the assessments of the geological time scale and the global characteristics of this event by detailing an isotopic excursion in tree-ring carbon isotopes from high-latitude/subarctic Europe. The δ ¹³ C chronology demonstrates extremely overcast (wet) conditions, especially between 2190 and 2100 BCE, with anomalous conditions sustaining until 1990 BCE. In addition to demonstrating its exact dating and duration, the δ ¹³ C data also illustrate the two-stage nature of the event and highlight the greater magnitude of the earlier stage. This reinforces the characterisation of this ‘Meghalayan anomaly’ in the context of other proxy-sites around the world. The North Atlantic forcing, previously associated with weakened Asian summer monsoon, accords with the suggested roles of North Atlantic Oscillation and/or southwards shift of Inter-Tropical Convergence Zone in producing the hydroclimatic anomaly. In fact, it would be clarifying to separate the hydroclimatic ‘Meghalayan anomaly’ from the broadly cited and potentially longer lasting ‘4.2 ka event’, to analyse their respective forcing mechanisms. We conclude by dating the Meghalayan lower boundary to 2190 BCE.
Article
Stalagmite data from Anjohibe Cave in northwest Madagascar suggest six distinct climate periods from 9.1 to 0.94 ka. Periods I and II (9.1–4.9 ka) were wetter and punctuated by a series of prominent droughts. Periods IV-VI (4–0.94 ka) were much drier and less variable. Period III (4.9-4 ka) marks the transition between wetter and drier conditions and consists of two significant droughts: the first (4.8–4.6 ka) coincides approximately with the end of the African Humid Period and the second (4.3–4.0 ka) may be the expression of the Northern Hemisphere 4.2 ka dry event in northwest Madagascar. Strong positive correlations between δ13C and δ18O values in Periods I-IV (r = 0.63–0.91) suggest that both isotopes were influenced by natural climate changes indicating that humans may not have been present in the area. In contrast, during Periods V (r = 0.07) and VI (r = −0.12) the “decoupling” of δ13C and δ18O might signal an impact from human activities starting around 2.5 ka. Rapid changes in climate during the early and middle Holocene, with prominent droughts lasting up to 800 years, did not kill off Madagascar's megafauna, and neither did a human population, present since the early Holocene if evidence from south Madagascar is reliable. However, many extinctions occurred under the more stable climatic conditions of the late Holocene, despite an antiphase climate relationship between northern and southcentral Madagascar. This suggests that initial human colonization, or significant increase in human population, triggered the megafaunal extinctions by hunting and destruction of megafaunal habitats.
Article
Significance A speleothem geochemical record from northern Iran captures significant climate fluctuations during the mid-to-late Holocene at high resolution. Two abrupt shifts in Mg/Ca last for more than a century and are interpreted as enhanced dust activity, indicating a threshold behavior in response to aridity. Coincident gradual peaks in δ ¹⁸ O support the interpretation of regional drying. The precise chronology shows the later event, 4.26 ka to 3.97 ka, is coincident within decades of the period of abandonment of advanced urban settlements in northern Mesopotamia, strengthening the argument for association between societal and climatic change. The record demonstrates the abrupt onset of dust production in the region and ability to maintain this dry climate state for multiple centuries naturally.
Article
The 8.2 ka event is a well-known cooling event in the Northern Hemisphere, but is poorly understood in Madagascar. Here, we compare paleoclimate data and outputs from paleoclimate simulations to better understand it. Records from Madagascar suggest two distinct sub-events (8.3 ka and 8.2 ka), that seem to correlate with records from northern high latitude. This could indicate causal relationships via changes in the Atlantic Meridional Overturning Circulation (AMOC) with changes in moisture source's δ18O, and changes in the mean position of the Inter-Tropical Convergence Zone (ITCZ), as climate modelling suggests. These two sub-events are also apparent in other terrestrial records, but the climatic signals are different. The prominent 8.2 ka sub-event records a clear antiphase relationship between the northern and southern hemisphere monsoons, whereas such relationship is less evident during the first 8.3 ka sub-event. Data–model comparison have also shown a mismatch between the paleoclimate data and the model outputs, the causes of which are more or less understood and may lie in the proxies, in the model, or in both data and model. Knowing that paleoclimate proxies and climate models produce different sets of variables, further research is needed to improve the data–model comparison approach, so that both paleoclimate data and paleoclimate models will better predict the likely climate status of a region during a specified time in the past with minimal uncertainties.
Article
The Holocene is probably the most intensively studied series/epoch within the geological record, and embodies a wide array of geomorphological, climatic, biotic and archaeological evidence; yet little attention has hitherto been paid to a formal subdivision of this series/epoch. Here we report a tripartite division of the Holocene into the Greenlandian, Northgrippian and Meghalayan stages/ages and their corresponding Lower/Early, Middle, Upper/Late subseries/subepochs, each supported by a Global Boundary Stratotype Section and Point (GSSP). The GSSP for the lowermost stage, the Greenlandian, is that of the Holocene as previously defined in the NGRIP2 Greenland ice core, and dated at 11,700 yr b2k (before 2000 CE). The GSSP for the Northgrippian is in the NGRIP1 Greenland ice core, and dated at 8236 yr b2k, whereas that for the Meghalayan is located in a speleothem from Mawmluh Cave, Meghalaya, northeast India with a date of 4250 yr b2k. The proposal on which this subdivision is based was submitted by the Subcommission on Quaternary Stratigraphy, approved by the International Commission on Stratigraphy, and formally ratified by the Executive Committee of the International Union of Geological Sciences on 14 th June 2018. © 2018 International Union of Geological Sciences. All Rights Reserved.
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The evidence for collapse of human civilizations at the start of the recently defined Meghalayan Age is equivocal
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Timing and extent of ancient drought used to define the Meghalayan are uncertain.
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The climatic event between 4.2 and 3.9 ka BP known as the “4.2 ka event” is commonly considered to be a synchronous global drought that happened as one pulse. However, careful comparison of records from around the world shows that synchrony is possible only if the published chronologies of the various records are shifted to the extent allowed by the uncertainties of their age data, that several records suggest a two-pulsed event, and that some records suggest a wet rather than dry event. The radiometric ages constraining those records have uncertainties of several decades if not hundreds of years, and in some records the event is represented by only one or two analyses. This paper reports a new record from Stalagmite DP1 from northeastern Namibia in which high ²³⁰Th/²³²Th activity ratios allow small age uncertainties ranging between only 10–28 years, and the event is documented by more than 35 isotopic analyses and by petrographic observation of a surface of dissolution. The ages from Stalagmite DP1 combine with results from 11 other records from around the world to suggest an event centered at about 4.07 ka BP with bracketing ages of 4.15 to 3.93 ka BP. The isotopic and petrographic results suggest a two-pulsed wet event in northeastern Namibia, which is in the Southern Hemisphere's summer rainfall zone where more rain presumably fell with southward migration of the Inter-Tropical Convergence Zone as the result of cooling in the Northern Hemisphere. Comparison with other records from outside the region of dryness from the Mediterranean to eastern Asia suggests that multiple climatic zones similarly moved southward during the event, in some cases bringing wetter conditions that contradict the notion of global drought.
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In this study, we investigate a possible mechanism for the dichotomy in climatology of marine rainfall and sea surface temperature (SST) over a part of the Southwest Indian Ocean (SWIO) during boreal winter (January and February) with state-of-the-art satellite and reanalysis data sets. Rainfall to the north of the Mozambique Channel, bounded 10°–5°S and 40°–50°E, is found to be quite feeble despite being in the warm SST regime of up to 29–29.5 °C. The rainfall intensity is rather found to be highly associated with the atmospheric surface divergence. The vigorous rainfall is associated with the more convergence over the Intertropical Convergence Zone (ITCZ), while the weak rainfall is linked with the divergence to the north of the Mozambique Channel. The surface divergent flow to the north of the Mozambique Channel is associated with a deep southward penetration of the northerly Indian Winter Monsoon (IWM). Corresponding to the surface divergent field, a relatively high sea level pressure (SLP) compared to the SLP in the ITCZ, weak subsidence, and low-level stratiform clouds are formed to the north of the Mozambique Channel, despite the warm, tropical SST. These atmospheric conditions are most likely conductive to the inhibition of cumulus convection over the region and the unique relationship between rainfall and SST seems peculiar. Our analysis also shows that the rare occurrence of tropical cyclones over the area is attributed to a high-pressure surge and the associated positive surface vorticity (anti-cyclonic). This study suggests that the area to the north of the Mozambique Channel is dynamically interesting for climatological studies.
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Paleoclimate studies of tropical rainfall have led to a recognition of a predominant pattern of anti-phase behavior between the Northern and Southern hemispheres at both orbital and millennial timescales. Less certain is how regional tropical rainfall patterns have changed in the late Holocene, under boundary conditions and on timescales which are most relevant to the tropics' response to a warming world. Several high-resolution southern hemisphere rainfall records are at odds with meridional movement of the mean Inter-Tropical Convergence Zone location as the major driver of Holocene tropical rainfall variability, with regional precipitation patterns resembling modern day El-Niño Southern Oscillation end members. To test emerging ideas on sub-millennial tropical rainfall variability, additional records from the southern hemisphere are required.
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During the Holocene, the most notably climatic change across the African continent is the African Humid Period (AHP), however the pace and primary forcing for this pluvial condition is still ambiguous, particularly in East Africa. We present a high-resolution marine sediment record off Tanzania to provide insights into the climatic conditions of inland East Africa during the Holocene. Major element ratios (i.e., log-ratios of Fe/Ca and Ti/Ca), derived from X-Ray Fluorescence scanning, have been employed to document variations in humidity in East Africa. Our results show that the AHP is represented by two humid phases: an intense humid period from the beginning of the Holocene to 8 ka (AHP I); and a moderate humid period spanning from 8 to 5.5 ka (AHP II). On the basis of our geochemical record and regime detection, the termination of the AHP initiated at 5.5 ka and ceased around 3.5 ka. Combined with other paleoclimatic records around East Africa, we suggest that the humid conditions in this region responded to Northern Hemisphere (NH) summer insolation. The AHP I and II might have been related to an eastward shift of the Congo Air Boundary and warmer conditions in the western Indian Ocean, which resulted in additional moisture being delivered from the Atlantic and Indian Oceans during the NH summer and autumn, respectively. We further note a drought event throughout East Africa north of 10°S around 8.2 ka, which may have been related to the southward migration of the Intertropical Convergence Zone in response to the NH cooling event.
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The eastern Lesotho Highlands experience climate patterns distinct from those of surrounding lower altitude regions, representing a niche environment with a unique biodiversity, leading to well-adapted but restricted vegetation. This study explores changes in the Holocene composition of diatoms and pollen at southern Africa’s highest altitude wetland (Mafadi: 3390 m a.s.l.). The palaeoenvironmental record for Mafadi Wetland indicates fluctuations between cold, wet conditions, prevalent between ~8140 and 7580 cal. yr BP and between ~5500 and 1100 cal. yr BP, and warmer, drier periods between ~7520 and 6680 cal. yr BP and between ~6160 and 5700 cal. yr BP. Marked climatic variability is noted from ~1100 cal. yr BP with colder conditions at ~150 kyr BP. Notably, the first of these cold periods occurs soon after the Northern Hemisphere 8.2 kyr event, while a second period of notably cold conditions occurs around 1100 cal. yr BP. Variability exists between the moisture reconstructions presented in this study and those from adjacent lower altitude sites, which is hypothesised to reflect variations in the strength and extent of the Westerlies throughout the Holocene.
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Speleothems (mineral deposits that formed in caves) are currently giving us some of the most exciting insights into environments and climates during the Pleistocene ice ages and the subsequent Holocene rise of civilizations. The book applies system science to Quaternary environments in a new and rigorous way and gives holistic explanations the relations between the properties of speleothems and the climatic and cave setting in which they are found. It is designed as the ideal companion to someone embarking on speleothem research and, since the underlying science is very broad, it will also be invaluable to a wide variety of others. Students and professional scientists interested in carbonate rocks, karst hydrogeology, climatology, aqueous geochemistry, carbonate geochemistry and the calibration of climatic proxies will find up-to-date reviews of these topics here. The book will also be valuable to Quaternary scientists who, up to now, have lacked a thorough overview of these important archives. Additional resources for this book can be found at: www.wiley.com/go/fairchild/speleothem.
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Using daily rain-gauge records for Madagascar and nearby islands, this paper investigates rainfall intraseasonal variability at local and regional scales during the austral summer season (November-February), as well as the respective influences of recurrent convective regimes over the South-West Indian Ocean (SWIO) and the Madden-Julian Oscillation (MJO). Our results show a general consistency between local-scale rainfall variability in Madagascar and regional-scale features of climate variability. The influence of Tropical-Temperate Troughs in their mature phase and/or their easternmost locations is first underlined. The development of such systems over Southern Africa and the Mozambique Chanel can be considered as precursors for Malagasy wet spells, especially over the southern part of the island. Regional and local effects of the MJO are weaker on average, and only concern the northwest of the island and the north of the Mozambique Chanel. MJO and convective regimes are finally shown to explain distinct fractions of regional rainfall variability.
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Time series of Mg/Ca, Ba/Ca, and δ18O analyzed in tests of surface-dwelling planktonic foraminifer Globigerinoides ruber from two marine sediment cores recovered in the Mozambique Channel off the Zambezi River, southwestern tropical Indian Ocean, reveal climate variability over the last 17 kyr. Analysis of samples collected from the water column of the Mozambique Channel validates that Mg/Ca in G. ruber reflects calcification temperatures at 0–30 m water depth and that the surface water of the southwestern Indian Ocean is very sensitive to dissolved Ba input from adjacent rivers. Foraminiferal Ba/Ca and Mg/Ca time series are used to reconstruct hydrological and thermal changes over southeastern African and southwestern tropical oceans. The Mg/Ca-based sea surface temperature (SST) estimates indicate that the thermal evolution of the tropical southwestern Indian Ocean followed, within age model uncertainties, climate changes over Antarctica. The trend of the SST record is marked by a gradual warming from 24.7 ± 0.6 °C at 17.0 ± 0.2 kyr BP to 26.4 ± 0.3 °C at 10–11 kyr BP interrupted by two prominent coolings of ~ 1.5 ± 0.2 °C and ~ 1 °C centered at 15 ± 0.1 kyr BP and 13.4 ± 0.2 kyr BP, respectively. Declining SSTs in the early Holocene reach their minimum (25 °C) at 8.7 ± 0.2 kyr BP and give way to stable thermal conditions over the Middle and Late Holocene. The Ba/Ca record indicates that the Zambezi basin experienced relatively wet conditions during the early phase of the last deglaciation, Bølling–Allerød, and the early Holocene. These wet phases coincide with an increase of SST in the Mozambique Channel. In contrast, relatively dry conditions throughout the middle and late Holocene epoch are accompanied by relatively cold Mozambique Channel surface water. These shifts likely reflect a response to meridional shifts of the austral westerlies and subtropical front. These hypothesized shifts would have modulated the advection of cold Southern Ocean water into the Mozambique Channel. Changes in SST in the Mozambique Channel, and possible resultant changes in the zonal gradient in the tropical Indian Ocean, in turn, had a strong impact on the precipitation over southeastern Africa.