Hydroclimatic changes on multiple timescales since 7800 y BP in the winter precipitation-dominated Central Asia

Central Asia (CA) is one of the world's most significant arid regions, which is markedly impacted by global warming. A better understanding of the dynamical processes governing its Holocene climate variability is critical for a better understanding of possible future impacts of climate change in the region. To date, most of the existing CA paleoclimate records are from the summer precipitation-dominated eastern CA (ECA), with few records from the winter precipitation-dominated western CA (WCA). Here, we present a precisely dated (~6‰) and highly resolved (<4-y) record of hydroclimatic variations from the WCA covering the period between 7,774 and 656 y BP. Utilizing multiple proxies (δ18O, δ13C, and Sr/Ca) derived from a stalagmite from the Fergana Valley, Kyrgyzstan, we reveal a long-term drying trend in WCA, which is in contrast with the wetting trend in ECA. We propose that different responses of winter and summer westerly jets to seasonal solar insolation over the past 8,000 y may have resulted in an antiphased precipitation relationship between the WCA and ECA. Our data contain dominant quasiperiodicities of 1,400, 50 to 70, and 20 to 30 y, indicating close connections between the WCA climate and the North Atlantic. We further identified a series of droughts and pluvials on centennial-to-decadal timescales, which may have influenced regional societies and trans-Eurasian culture exchanges during historical and prehistorical times.

Modern anthropogenic drought in Central Brazil unprecedented during last 700 years

A better understanding of the relative roles of internal climate variability and external contributions, from both natural (solar, volcanic) and anthropogenic greenhouse gas forcing, is important to better project future hydrologic changes. Changes in the evaporative demand play a central role in this context, particularly in tropical areas characterized by high precipitation seasonality, such as the tropical savannah and semi-desertic biomes. Here we present a set of geochemical proxies in speleothems from a well-ventilated cave located in central-eastern Brazil which shows that the evaporative demand is no longer being met by precipitation, leading to a hydrological deficit. A marked change in the hydrologic balance in central-eastern Brazil, caused by a severe warming trend, can be identified, starting in the 1970s. Our findings show that the current aridity has no analog over the last 720 years. A detection and attribution study indicates that this trend is mostly driven by anthropogenic forcing and cannot be explained by natural factors alone. These results reinforce the premise of a severe long-term drought in the subtropics of eastern South America that will likely be further exacerbated in the future given its apparent connection to increased greenhouse gas emissions.

Dansgaard-Oeschger cycles of the penultimate and last glacial period recorded in stalagmites from Türkiye

The last glacial period is characterized by abrupt climate oscillations, also known as Dansgaard-Oeschger (D-O) cycles. However, D-O cycles remain poorly documented in climate proxy records covering the penultimate glacial period. Here we present highly resolved and precisely dated speleothem time series from Sofular Cave in northern Türkiye to provide clear evidence for D-O cycles during Marine Isotope Stage (MIS) 6 as well as MIS 2-4. D-O cycles are most clearly expressed in the Sofular carbon isotope time series, which correlate inversely with regional sea surface temperature (SST) records from the Black Sea. The pacing of D-O cycles is almost twice as long during MIS 6 compared to MIS 2-4, and could be related to a weaker Atlantic Meridional Overturning Circulation (AMOC) and a different mean climate during MIS 6 compared to MIS 2-4, leading most likely to a higher threshold for the occurrence of D-O cycles.

A 350,000-year history of groundwater recharge in the southern Great Basin, USA

Estimating groundwater recharge under various climate conditions is important for predicting future freshwater availability. This is especially true for the water-limited region of the southern Great Basin, USA. To investigate the response of groundwater recharge to different climate states, we calculate the paleo recharge to a groundwater basin in southern Nevada over the last 350,000 years. Our approach combines a groundwater model with paleo-water-table data from Devils Hole cave. The minimum water-table during peak interglacial conditions was more than 1.6 m below modern levels, representing a recharge decline of less than 17% from present-day conditions. During peak glacial conditions, the water-table elevation was at least 9.5 m above modern levels, representing a recharge increase of more than 233-244% compared to present-day conditions. The elevation of the Devils Hole water-table is 3-4 times more sensitive to groundwater recharge during dry interglacial periods, compared to wet glacial periods. This study can serve as a benchmark for understanding long-term effects of past and future climate change on groundwater resources.

Hydroclimate variability in the Caribbean during North Atlantic Heinrich cooling events (H8 and H9)

We present a speleothem record from western Cuba, spanning the period 98.7-84.9 ka BP. Our record shows two distinctive periods of high δ¹⁸O corresponding to dry and/or cold periods during 85-87.6 and 90.2-93.1 ka BP, synchronous with Heinrich events 8 and 9 (H8 and H9). Hence, we provide the first proxy evidence of the local Caribbean climate response to H8 and H9. Interestingly, H8 is more pronounced compared to H9, which may be a local response to lower temperatures in the North Atlantic resulting in a weak AMOC and reduced deep water formation, therefore a stronger south shift of the ITCZ. Our data complement existing speleothem records from western Cuba which, collectively, provide a nearly continuous paleoclimate time-series spanning the last 100 ka BP, indicating a consistent response to millennial-scale events as dry and/or cooler conditions. The comparison with regional paleoclimate records reveals an anti-phased relationship with South America, caused by the southern movements of the ITCZ during millennial-scale events which lead to dry conditions in the Caribbean and a stronger South American Monsoon System.

Coupled atmosphere-ice-ocean dynamics during Heinrich Stadial 2

Our understanding of climate dynamics during millennial-scale events is incomplete, partially due to the lack of their precise phase analyses under various boundary conditions. Here we present nine speleothem oxygen-isotope records from mid-to-low-latitude monsoon regimes with sub-centennial age precision and multi-annual resolution, spanning the Heinrich Stadial 2 (HS2) - a millennial-scale event that occurred at the Last Glacial Maximum. Our data suggests that the Greenland and Antarctic ice-core chronologies require +320- and +400-year adjustments, respectively, supported by extant volcanic evidence and radiocarbon ages. Our chronological framework shows a synchronous HS2 onset globally. Our records precisely characterize a centennial-scale abrupt "tropical atmospheric seesaw" superimposed on the conventional "bipolar seesaw" at the beginning of HS2, implying a unique response/feedback from low-latitude hydroclimate. Together with our observation of an early South American monsoon shift at the HS2 termination, we suggest a more active role of low-latitude hydroclimate dynamics underlying millennial events than previously thought.

Identification of a Quaternary rock avalanche deposit (Central Apennines, Italy): Significance for recognition of fossil catastrophic mass-wasting

Whereas deposits of extremely-rapid, 'catastrophic' mass wastings >10⁵ m³ in volume (for example, the Marocche di Dro rock avalanche in the Southern Alps and the Flims rockslide in the Western Alps) are easily recognized by their sheer mass and blocky surface, the identification of fossil catastrophic mass wastings partly removed by erosion must be based on deposit characteristics. Herein, a 'fossil' (pre-last glacial) rock avalanche, previously interpreted as either a till or debris flow, is described. The deposit, informally called 'Rubble Breccia', is located in the intramontane Campo Imperatore halfgraben that is bounded by a master fault with up to ca 900 m topographic throw. Based on documentation from field to thin section, and by comparative analysis with post-glacial rock avalanches, tills and debris flows, the Rubble Breccia is reinterpreted as a rock avalanche. The Rubble Breccia consists of an extremely-poorly sorted, disordered mixture of angular clasts from sand to block size. Many clasts show fitted subclast boundaries in crackle, jigsaw and mosaic fabrics, as diagnostic of catastrophic mass wasting deposits. Intercalated layers of angular to well-rounded clasts of coarse sand to fine pebble size, and deformed into open to recumbent folds, may represent shear belts folded during terminal avalanche propagation. The clast spectrum of the Rubble Breccia - mainly shallow-water bioclastic limestones, Saccocoma wackestones and other deep-water limestones and dolostones - is derived from the front range along the northern margin of the basin. Calcite cement found within the Rubble Breccia was dated with the U/Th disequilibrium method to 124.25 ± 2.76 ka bp, providing an ante-quam age constraint to the rock avalanche event. Because catastrophic mass wasting is a common erosional process, fossil deposits thereof should be more widespread than have been identified to date, although this may be a consequence of misidentification. The criteria outlined here provide a template to identify fossil catastrophic mass wasting deposits of any age.

Droughts and societal change: The environmental context for the emergence of Islam in late Antique Arabia

In Arabia, the first half of the sixth century CE was marked by the demise of Himyar, the dominant power in Arabia until 525 CE. Important social and political changes followed, which promoted the disintegration of the major Arabian polities. Here, we present hydroclimate records from around Southern Arabia, including a new high-resolution stalagmite record from northern Oman. These records clearly indicate unprecedented droughts during the sixth century CE, with the most severe aridity persisting between ~500 and 530 CE. We suggest that such droughts undermined the resilience of Himyar and thereby contributed to the societal changes from which Islam emerged.

Earliest parietal art: hominin hand and foot traces from the middle Pleistocene of Tibet

At Quesang on the Tibetan Plateau we report a series of hand and foot impressions that appear to have been intentionally placed on the surface of a unit of soft travertine. The travertine was deposited by water from a hot spring which is now inactive and as the travertine lithified it preserved the traces. On the basis of the sizes of the hand and foot traces, we suggest that two track-makers were involved and were likely children. We interpret this event as a deliberate artistic act that created a work of parietal art. The travertine unit on which the traces were imprinted dates to between ∼169 and 226 ka BP. This would make the site the earliest currently known example of parietal art in the world and would also provide the earliest evidence discovered to date for hominins on the High Tibetan Plateau (above 4000 m a.s.l.). This remarkable discovery adds to the body of research that identifies children as some of the earliest artists within the genus Homo.

Collapse of the Liangzhu and other Neolithic cultures in the lower Yangtze region in response to climate change

The Liangzhu culture in the Yangtze River Delta (YRD) was among the world’s most advanced Neolithic cultures. Archeological evidence suggests that the Liangzhu ancient city was abandoned, and the culture collapsed at ~4300 years ago. Here, we present speleothem records from southeastern China in conjunction with other paleoclimatic and archeological data to show that the Liangzhu culture collapsed within a short and anomalously wet period between 4345 ± 32 and 4324 ± 30 years ago, supporting the hypothesis that the city was abandoned after large-scale flooding and inundation. We further show that the demise of Neolithic cultures in the YRD occurred within an extended period of aridity that started at ~4000 ± 45 years ago. We suggest that the major hydroclimatic changes between 4300 and 3000 years ago may have resulted from an increasing frequency of the El Niño–Southern Oscillation in the context of weakened Northern Hemisphere summer insolation.

Strong links between Saharan dust fluxes, monsoon strength, and North Atlantic climate during the last 5000 years

Despite the multiple impacts of mineral aerosols on global and regional climate and the primary climatic control on atmospheric dust fluxes, dust-climate feedbacks remain poorly constrained, particularly at submillennial time scales, hampering regional and global climate models. We reconstruct Saharan dust fluxes over Western Europe for the last 5000 years, by means of speleothem strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) and karst modeling. The record reveals a long-term increase in Saharan dust flux, consistent with progressive North Africa aridification and strengthening of Northern Hemisphere latitudinal climatic gradients. On shorter, centennial to millennial scales, it shows broad variations in dust fluxes, in tune with North Atlantic ocean-atmosphere patterns and with monsoonal variability. Dust fluxes rapidly increase before (and peaks at) Late Holocene multidecadal- to century-scale cold climate events, including those around 4200, 2800, and 1500 years before present, suggesting the operation of previously unknown strong dust-climate negative feedbacks preceding these episodes.

High precise dating on the variation of the Asian summer monsoon since 37 ka BP

Comprehensive comparison of paleoclimate change based on records constrained by precise chronology and high-resolution is essential to explore the correlation and interaction within earth climate systems. Here, we propose a new stalagmite-based multidecadal resolved Asian summer monsoon (ASM) record spanning the past thirty-seven thousand years (ka BP, before AD 1950) from Furong Cave, southwestern China. This record is consistent with the published Chinese stalagmite sequences and shows that the dominant controls of the ASM dynamics include not only insolation and solar activity but also suborbital-scale hydroclimate events in the high latitudes of the northern hemisphere, such as the Heinrich events, Bølling-Allerød (BA), and Younger Dryas (YD). Benefit from the unprecedented accurate chronology, the timings of these events are precisely dated, with uncertainties of, at most, 40 years (2σ). The onset of the weak ASM during the YD began at 12.92 ka BP and lasted for 430 years. The occurrence of the 200-yr Older Dryas during the BA period was dated from 13.87 to 14.06 ka BP. The durations of the three Heinrich (H) events, H1, H2, and H3, are 14.33-18.29, 23.77-24.48, and 28.98-30.46 ka BP, respectively. Furong record shows surprisingly variable onset transitions of 980, 210, and 40 years for the corresponding weak ASM events. These discrepancies suggest different influences of the H events on ASM dynamics. During the periods of H 1-3, the obvious difference between our Furong record and NGRIP δ¹⁸O record indicated the decoupling correlation between the mid-low latitudes and high latitudes. On the other hand, synchronous climate change in high and low latitudes suggests another possibility which different to the dominant role of Northern high latitudes in triggering global climate change. Our high quality records also indicate a plausible different correlation between the high and mid-low latitudes under glacial and inter-glacial background, especially for the ASM regimes.

Megadrought and cultural exchange along the proto-silk road

Arid Central Asia (ACA), with its diverse landscapes of high mountains, oases, and deserts, hosted the central routes of the Silk Roads that linked trade centers from East Asia to the eastern Mediterranean. Ecological pockets and ecoclines in ACA are largely determined by local precipitation. However, little research has gone into the effects of hydroclimatic changes on trans-Eurasian cultural exchange. Here, we reconstruct precipitation changes in ACA, covering the mid-late Holocene with a U-Th dated, ~3 a resolution, multi-proxy time series of replicated stalagmites from the southeastern Fergana Valley, Kyrgyzstan. Our data reveal a 640-a megadrought between 5820 and 5180 a BP, which likely impacted cultural development in ACA and impeded the expansion of cultural traits along oasis routes. Instead, it may have diverted the earliest transcontinental exchange along the Eurasian steppe during the 5th millennium BP. With gradually increasing precipitation after the megadrought, settlement of peoples in the oases and river valleys may have facilitated the opening of the oasis routes, "prehistoric Silk Roads", of trans-Eurasian exchange. By the 4th millennium BP, this process may have reshaped cultures across the two continents, laying the foundation for the organized Silk Roads.

Speleothem record of mild and wet mid-Pleistocene climate in northeast Greenland

The five interglacials before the Mid-Brunhes Event (MBE) [c.430 thousand years (ka) ago] are generally considered to be globally cooler than those post-MBE. Inhomogeneities exist regionally, however, which suggest that the Arctic was warmer than present during Marine Isotope Stage (MIS) 15a. Using the first speleothem record for the High Arctic, we investigate the climatic response of northeast Greenland between c.588 and c.549 ka ago. Our results indicate an enhanced warmth of at least +3.5°C relative to the present, leading to permafrost thaw and increased precipitation. We find that δ¹⁸O of precipitation was at least 3‰ higher than today and recognize two local cooling events (c.571 and c.594 ka ago) thought to be caused by freshwater forcing. Our results are important for improving understanding of the regional climatic response leading up to the MBE and specifically provide insights into the climatic response of a warmer Arctic.

A multimillennial climatic context for the megafaunal extinctions in Madagascar and Mascarene Islands

Madagascar and the Mascarene Islands of Mauritius and Rodrigues underwent catastrophic ecological and landscape transformations, which virtually eliminated their entire endemic vertebrate megafauna during the past millennium. These ecosystem changes have been alternately attributed to either human activities, climate change, or both, but parsing their relative importance, particularly in the case of Madagascar, has proven difficult. Here, we present a multimillennial (approximately the past 8000 years) reconstruction of the southwest Indian Ocean hydroclimate variability using speleothems from the island of Rodrigues, located ∼1600 km east of Madagascar. The record shows a recurring pattern of hydroclimate variability characterized by submillennial-scale drying trends, which were punctuated by decadal-to-multidecadal megadroughts, including during the late Holocene. Our data imply that the megafauna of the Mascarenes and Madagascar were resilient, enduring repeated past episodes of severe climate stress, but collapsed when a major increase in human activity occurred in the context of a prominent drying trend.

Response to Comment on "No consistent ENSO response to volcanic forcing over the last millennium"

Robock claims that our analysis fails to acknowledge that pan-tropical surface cooling caused by large volcanic eruptions may mask El Niño warming at our central Pacific site, potentially obscuring a volcano-El Niño connection suggested in previous studies. Although observational support for a dynamical response linking volcanic cooling to El Niño remains ambiguous, Robock raises some important questions about our study that we address here.

No consistent ENSO response to volcanic forcing over the last millennium

The El Niño-Southern Oscillation (ENSO) shapes global climate patterns yet its sensitivity to external climate forcing remains uncertain. Modeling studies suggest that ENSO is sensitive to sulfate aerosol forcing associated with explosive volcanism but observational support for this effect remains ambiguous. Here, we used absolutely dated fossil corals from the central tropical Pacific to gauge ENSO's response to large volcanic eruptions of the last millennium. Superposed epoch analysis reveals a weak tendency for an El Niño-like response in the year after an eruption, but this response is not statistically significant, nor does it appear after the outsized 1257 Samalas eruption. Our results suggest that those models showing a strong ENSO response to volcanic forcing may overestimate the size of the forced response relative to natural ENSO variability.

Coupling of Indo-Pacific climate variability over the last millennium

The Indian Ocean Dipole (IOD) affects climate and rainfall across the world, and most severely in nations surrounding the Indian Ocean^1-4. The frequency and intensity of positive IOD events increased during the twentieth century⁵ and may continue to intensify in a warming world⁶. However, confidence in predictions of future IOD change is limited by known biases in IOD models⁷ and the lack of information on natural IOD variability before anthropogenic climate change. Here we use precisely dated and highly resolved coral records from the eastern equatorial Indian Ocean, where the signature of IOD variability is strong and unambiguous, to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium. Our reconstruction demonstrates that extreme positive IOD events were rare before 1960. However, the most extreme event on record (1997) is not unprecedented, because at least one event that was approximately 27 to 42 per cent larger occurred naturally during the seventeenth century. We further show that a persistent, tight coupling existed between the variability of the IOD and the El Niño/Southern Oscillation during the last millennium. Indo-Pacific coupling was characterized by weak interannual variability before approximately 1590, which probably altered teleconnection patterns, and by anomalously strong variability during the seventeenth century, which was associated with societal upheaval in tropical Asia. A tendency towards clustering of positive IOD events is evident in our reconstruction, which-together with the identification of extreme IOD variability and persistent tropical Indo-Pacific climate coupling-may have implications for improving seasonal and decadal predictions and managing the climate risks of future IOD variability.

Medieval Climate Variability in the eastern Amazon-Cerrado regions and its archeological implications

The South American Monsoon System is responsible for the majority of precipitation in the continent, especially over the Amazon and the tropical savannah, known as ‘Cerrado’. Compared to the extensively studied subtropical and temperate regions the effect of the Medieval Climate Anomaly (MCA) on the precipitation over the tropics is still poorly understood. Here, we present a multiproxy paleoprecipitation reconstruction showing a consistent change in the hydrologic regime during the MCA in the eastern Amazon and ‘Cerrado’, characterized by a substantial transition from humid to drier conditions during the Early (925-1150 C.E.) to Late-MCA (1150-1350 C.E.). We compare the timing of major changes in the monsoon precipitation with the expansion and abandonment of settlements reported in the archeological record. Our results show that important cultural successions in the pre-Columbian Central Amazon, the transition from Paredão to Guarita phase, are in agreement with major changes in the hydrologic regime. Phases of expansion and, subsequent abandonment, of large settlements from Paredão during the Early to Late-MCA are coherent with a reduction in water supply. In this context we argue that the sustained drier conditions during the latter period may have triggered territorial disputes with Guarita leading to the Paredão demise.

Changes in Circulation and Particle Scavenging in the Amerasian Basin of the Arctic Ocean over the Last Three Decades Inferred from the Water Column Distribution of Geochemical Tracers

Since the 1980-1990s, international research efforts have augmented our knowledge of the physical and chemical properties of the Arctic Ocean water masses, and recent studies have documented changes. Understanding the processes responsible for these changes is necessary to be able to forecast the local and global consequences of these property evolutions on climate. The present work investigates the distributions of geochemical tracers of particle fluxes and circulation in the Amerasian Basin and their temporal evolution over the last three decades (from stations visited between 1983 and 2015). Profiles of 230-thorium (230Th) and 231-protactinium (231Pa) concentrations and neodymium isotopes (expressed as εNd) measured in the Amerasian Basin prior to 2000 are compared to a new, post-2000s data set. The comparison shows a large scale decrease in dissolved 230Th and 231Pa concentrations, suggesting intensification of scavenging by particle flux, especially in coastal areas. Higher productivity and sediment resuspension from the shelves appear responsible for the concentration decrease along the margins. In the basin interior, increased lateral exchanges with the boundary circulation also contribute to the decrease in concentration. This study illustrates how dissolved 230Th and 231Pa, with εNd support, can provide unique insights not only into changes in particle flux but also into the evolution of ocean circulation and mixing.

Role of climate in the rise and fall of the Neo-Assyrian Empire

Northern Iraq was the political and economic center of the Neo-Assyrian Empire (c. 912 to 609 BCE)-the largest and most powerful empire of its time. After more than two centuries of regional dominance, the Neo-Assyrian state plummeted from its zenith (c. 670 BCE) to complete political collapse (c. 615 to 609 BCE). Earlier explanations for the Assyrian collapse focused on the roles of internal politico-economic conflicts, territorial overextension, and military defeat. Here, we present a high-resolution and precisely dated speleothem record of climate change from the Kuna Ba cave in northern Iraq, which suggests that the empire's rise occurred during a two-centuries-long interval of anomalously wet climate in the context of the past 4000 years, while megadroughts during the early-mid seventh century BCE, as severe as recent droughts in the region but lasting for decades, triggered a decline in Assyria's agrarian productivity and thus contributed to its eventual political and economic collapse.

Central Europe temperature constrained by speleothem fluid inclusion water isotopes over the past 14,000 years

The reasons for the early Holocene temperature discrepancy between northern hemispheric model simulations and paleoclimate reconstructions-known as the Holocene temperature conundrum-remain unclear. Using hydrogen isotopes of fluid inclusion water extracted from stalagmites from the Milandre Cave in Switzerland, we established a mid-latitude European mean annual temperature reconstruction for the past 14,000 years. Our Milandre Cave fluid inclusion temperature record (MC-FIT) resembles Greenland and Mediterranean sea surface temperature trends but differs from recent reconstructions obtained from biogenic proxies and climate models. The water isotopes are further synchronized with tropical precipitation records, stressing the Northern Hemisphere signature. Our results support the existence of a European Holocene Thermal Maximum and data-model temperature discrepancies. Moreover, data-data comparison reveals a significant latitudinal temperature gradient within Europe. Last, the MC-FIT record suggests that seasonal biases in the proxies are not the primary cause of the Holocene temperature conundrum.

Millennial and orbital scale variability of the South American Monsoon during the penultimate glacial period

The presence of large, rapid climate oscillations is the most prominent feature of the Earth’s last glacial period. These oscillations are observed throughout the Northern Hemisphere and into the Southern Hemisphere tropics. Whether similar oscillations are typical of prior glacial periods, however, has not been well established. Here, we present results of a study of the South American Summer Monsoon system that covers nearly the entire penultimate glacial period, from 195 to 135 ky BP. We use a well-dated, high-resolution (~50 y) time series of oxygen isotopes to show that the precession of the earth’s orbit is the primary control on monsoon intensity. After removing the precession signal we observe millennial oscillations that are very similar in amplitude and structure to the Dansgaard/Oeschger cycles of the last interglacial and that match well a synthetic reconstruction of millennial variability. Time series analyses shows that the most prominent of the observed cycles occur at considerably longer frequency (~3500 y) that the Dansgaard/Oeschger cycles from Marine Isotope Stages 2–4.

Moisture availability in the southwest United States over the last three glacial-interglacial cycles

The projected long-term drying of the southwest (SW) United States in response to climate warming raises a sobering alarm for this already water-limited region, yet the climatic controls on moisture availability over longer time scales remain a topic of debate. Here, we present a 350,000-year record of past water table fluctuations in Devils Hole 2 cave that are driven by variations in recharge amount to the local groundwater flow system. Because of the unprecedented length and precision of our record, we can observe variations in regional moisture availability over the last three glacial-interglacial cycles at a millennial-scale resolution. The timing of past water table rises and falls (>9 m in amplitude) closely coincides with the expansion and reduction of Northern Hemisphere ice volume, which in turn influences the position and intensity of westerly winter storms on orbital time scales. Superimposed on this long-term trend are millennial-scale highstands recorded during the last glaciation that coincide with North Atlantic Heinrich events. Earlier millennial-scale highstands provide the first evidence of multiple short-lived wet periods in the SW United States linked to coeval cooling intervals in the North Atlantic during marine isotope stages 6 and 8. The Devils Hole 2 water table record is currently the longest independently dated paleomoisture record in the SW United States and thus provides a critical testbed to examine the controls on regional moisture availability over larger time scales.

The Indian monsoon variability and civilization changes in the Indian subcontinent

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.

Hydroclimate changes across the Amazon lowlands over the past 45,000 years

Reconstructing the history of tropical hydroclimates has been difficult, particularly for the Amazon basin-one of Earth's major centres of deep atmospheric convection. For example, whether the Amazon basin was substantially drier or remained wet during glacial times has been controversial, largely because most study sites have been located on the periphery of the basin, and because interpretations can be complicated by sediment preservation, uncertainties in chronology, and topographical setting. Here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide. Our results are based on a decadally resolved, uranium/thorium-dated, oxygen isotopic record for much of the past 45,000 years, obtained using speleothems from Paraíso Cave in eastern Amazonia; we interpret the record as being broadly related to precipitation. Relative to modern levels, precipitation in the region was about 58% during the Last Glacial Maximum (around 21,000 years ago) and 142% during the mid-Holocene epoch (about 6,000 years ago). We find that, as compared with cave records from the western edge of the lowlands, the Amazon was widely drier during the last glacial period, with much less recycling of water and probably reduced plant transpiration, although the rainforest persisted throughout this time.

A high-resolution history of the South American Monsoon from Last Glacial Maximum to the Holocene

The exact extent, by which the hydrologic cycle in the Neotropics was affected by external forcing during the last deglaciation, remains poorly understood. Here we present a new paleo-rainfall reconstruction based on high-resolution speleothem δ¹⁸O records from the core region of the South American Monsoon System (SAMS), documenting the changing hydrological conditions over tropical South America (SA), in particular during abrupt millennial-scale events. This new record provides the best-resolved and most accurately constrained geochronology of any proxy from South America for this time period, spanning from the Last Glacial Maximum (LGM) to the mid-Holocene.

How well can we quantify dust deposition to the ocean?

Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2-5, suggesting that the model may overestimate dust deposition in our study area.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

How well can we quantify dust deposition to the ocean?

Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2-5, suggesting that the model may overestimate dust deposition in our study area.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Response to Comments on "Reconciliation of the Devils Hole climate record with orbital forcing"

Winograd and Coplen question the thorium-230 distribution model proposed to explain the age bias observed with increasing depth during Termination II. We have evaluated both criticisms and find that all samples display virtually identical fabrics, argue that the modern setting is not analogous to the conditions during Termination II, and reiterate the robustness of our age models. Our conclusions remain unchanged.

Strong coupling of Asian Monsoon and Antarctic climates on sub-orbital timescales

There is increasing evidence that millennial-scale climate variability played an active role on orbital-scale climate changes, but the mechanism for this remains unclear. A ²³⁰Th-dated stalagmite δ¹⁸O record between 88 and 22 thousand years (ka) ago from Yongxing Cave in central China characterizes changes in Asian monsoon (AM) strength. After removing the 65°N insolation signal from our record, the δ¹⁸O residue is strongly anti-phased with Antarctic temperature variability on sub-orbital timescales during the Marine Isotope Stage (MIS) 3. Furthermore, once the ice volume signal from Antarctic ice core records were removed and extrapolated back to the last two glacial-interglacial cycles, we observe a linear relationship for both short- and long-duration events between Asian and Antarctic climate changes. This provides the robust evidence of a link between northern and southern hemisphere climates that operates through changes in atmospheric circulation. We find that the weakest monsoon closely associated with the warmest Antarctic event always occurred during the Terminations. This finding, along with similar shifts in the opal flux record, suggests that millennial-scale events play a key role in driving the deglaciation through positive feedbacks associated with enhanced upwelling and increasing CO₂.

Indian monsoon variability on millennial-orbital timescales

The Indian summer monsoon (ISM) monsoon is critical to billions of people living in the region. Yet, significant debates remain on primary ISM drivers on millennial-orbital timescales. Here, we use speleothem oxygen isotope (δ¹⁸O) data from Bittoo cave, Northern India to reconstruct ISM variability over the past 280,000 years. We find strong coherence between North Indian and Chinese speleothem δ¹⁸O records from the East Asian monsoon domain, suggesting that both Asian monsoon subsystems exhibit a coupled response to changes in Northern Hemisphere summer insolation (NHSI) without significant temporal lags, supporting the view that the tropical-subtropical monsoon variability is driven directly by precession-induced changes in NHSI. Comparisons of the North Indian record with both Antarctic ice core and sea-surface temperature records from the southern Indian Ocean over the last glacial period do not suggest a dominant role of Southern Hemisphere climate processes in regulating the ISM variability on millennial-orbital timescales.

Onset and duration of transitions into Greenland Interstadials 15.2 and 14 in northern China constrained by an annually laminated stalagmite

The onset and duration of abrupt transitions into Dansgaard-Oeschger (DO) events can be studied in detail in Greenland ice cores given the excellent relative uncertainty of its lamina-counting chronology. For other geological archives, however, the shorter intervals are not determined accurately due to lack of clear annual lamina. Here, we present an oxygen isotope record of a stalagmite with well-developed annual lamina from Xinglong Cave, northern China, covering DO 15 and 14. Except for the absence of Greenland Interstadial (GIS) 15.1, the pattern of this record strongly resembles that of Greenland ice cores on millennial scales as well as the detailed centennial-scale cooling excursions within GIS 14. Additionally, the transitions into GIS 15.2 and 14, constrained by lamina counting, lasted 74 and 27 yr, respectively, both of which are in excellent agreement with that of the NGRIP record on the GICC05 timescales (100 ± 6 and 20 ± 1 yr, respectively). The close coupling of abrupt climatic oscillations on millennial to decadal scales between Greenland and northern China implies a rapid atmospheric teleconnection between the North Atlantic and the East Asian Summer Monsoon regions, probably via the westerlies.

Reconciliation of the Devils Hole climate record with orbital forcing

The driving force behind Quaternary glacial-interglacial cycles and much associated climate change is widely considered to be orbital forcing. However, previous versions of the iconic Devils Hole (Nevada) subaqueous calcite record exhibit shifts to interglacial values ~10,000 years before orbitally forced ice age terminations, and interglacial durations ~10,000 years longer than other estimates. Our measurements from Devils Hole 2 replicate virtually all aspects of the past 204,000 years of earlier records, except for the timing during terminations, and they lower the age of the record near Termination II by ~8000 years, removing both ~10,000-year anomalies. The shift to interglacial values now broadly coincides with the rise in boreal summer insolation, the marine termination, and the rise in atmospheric CO2, which is consistent with mechanisms ultimately tied to orbital forcing.

A Chinese cave links climate change, social impacts, and human adaptation over the last 500 years

The collapse of some pre-historical and historical cultures, including Chinese dynasties were presumably linked to widespread droughts, on the basis of synchronicities of societal crises and proxy-based climate events. Here, we present a comparison of ancient inscriptions in Dayu Cave from Qinling Mountains, central China, which described accurate times and detailed impacts of seven drought events during the period of 1520–1920 CE, with high-resolution speleothem records from the same cave. The comparable results provide unique and robust tests on relationships among speleothem δ¹⁸O changes, drought events, and societal unrest. With direct historical evidences, our results suggest that droughts and even modest events interrupting otherwise wet intervals can cause serious social crises. Modeling results of speleothem δ¹⁸O series suggest that future precipitation in central China may be below the average of the past 500 years. As Qinling Mountain is the main recharge area of two large water transfer projects and habitats of many endangered species, it is imperative to explore an adaptive strategy for the decline in precipitation and/or drought events.

North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems

The European Alps are an effective barrier for meridional moisture transport and are thus uniquely placed to record shifts in the North Atlantic storm track pattern associated with the waxing and waning of Late-Pleistocene Northern Hemisphere ice sheets. The lack of well-dated terrestrial proxy records spanning this time period, however, renders the reconstruction of past atmospheric patterns difficult. Here we present a precisely dated, continuous terrestrial record of meteoric precipitation in Europe between 30 and 14.7 ka. In contrast to present-day conditions, our speleothem data provide strong evidence for preferential advection of moisture from the South across the Alps supporting a southward shift of the storm track during the local Last Glacial Maximum (that is, 26.5–23.5 ka). Moreover, our age control indicates that this circulation pattern preceded the Northern Hemisphere precession maximum by ~3 ka, suggesting that obliquity may have played a considerable role in the Alpine ice aggradation.

Insights into Late-Pleistocene Northern Hemisphere storm track variability are hampered by a lack of well-dated proxy records. Here, the authors present a precisely dated record of meteoric precipitation between 30 and 14.7 ka, and show that obliquity may have played a vital role in Alpine glacier advance.

800,000 years of abrupt climate variability

We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.

800,000 years of abrupt climate variability

We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.

The last glacial termination

A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres.