Warm summers during the Younger Dryas cold reversal

Patterns and drivers of Holocene moisture variability in mid-latitude eastern North America

Proxy data for eastern North American hydroclimate indicate strong and persistent multi-millennial droughts during the Holocene, but climate model simulations often fail to reproduce the proxy-inferred droughts. Diagnosing the data-model mismatch can offer valuable insights about the drivers of hydrological variability and different regional sensitivities to hydroclimate forcing. Here we present a proxy-modeling synthesis for Holocene climates in the eastern North American mid-latitudes, including machine-learning-based water balance reconstructions and high-resolution climate simulations. These data-model results resolve prior-generation inconsistencies, show consistent spatiotemporal patterns of Holocene hydroclimate change, and enable assessment of the driving mechanisms. This agreement suggests that the secular summer insolation trend, combined with the Laurentide Ice Sheet deglaciation and its effect on atmospheric circulation, together explain the extent and duration of drier-than-present climates. In addition, our high-resolution proxy data and transient simulations reveal clear multi-centennial climate variability. In our simulations, temperature-driven increases in evapotranspiration exceed regional precipitation gains, drying much of the region during the mid Holocene. This suggests that the mid-Holocene multi-millennial drought was driven by similar processes compared to the drying trajectory projected for mid-latitude North America over this century, which is also primarily driven by warming.

Large scale and regional demographic responses to climatic changes in Europe during the Final Palaeolithic

The European Final Palaeolithic witnessed marked changes in almost all societal domains. Despite a rich body of evidence, our knowledge of human palaeodemographic processes and regional population dynamics still needs to be improved. In this study, we present regionally differentiated population estimates for the Greenland Interstadial 1d-a (GI-1d-a; 14-12.7 ka cal BP) and the Greenland Stadial 1 (GS-1; 12.7-11.6 ka cal BP) for Southern, Western, Northern and Central Europe. The data were obtained by applying the Cologne Protocol, a geostatistical approach for estimating prehistoric population size and density, to a newly compiled dataset of Final Palaeolithic sites. On a large spatio-temporal scale and compared to preceding Upper Palaeolithic phases, areas north of the Alps become the dominant demographic growth area for the first time since the dispersal of anatomically modern humans into Europe. At smaller scales, we observe divergent regional trends, with a conspicuous lack of archaeological evidence appearing in previously occupied areas of central France and Germany. Our study also shows that during the Final Palaeolithic, the climatic cooling of GS-1 coincides with a pronounced population decline in most parts of the study area. An apparent increase in population density occurs only in north-eastern Central Europe and north-eastern Italy. Our estimates suggest that the total population was reduced by half. Similar results, with a relationship between decreasing temperatures and decreasing populations, have already been observed for the late phase of the Gravettian, when populations were reduced to only one third of those estimated for the early phase. Yet, in contrast to the collapse of local populations during the late Gravettian, the increase in population densities in Central Europe during GS-1 indicates population movements eastwards, possibly in response to deteriorating climatic conditions, particularly in western regions during the Younger Dryas.

Late Glacial summer paleohydrology across Central Europe

It is generally accepted that a weakening of the North Atlantic thermohaline circulation caused the Younger Dryas cooling. Although the role of seasonality was emphasized previously, this aspect is rarely considered yet, and it remains elusive how this impacted hydroclimate during winters and summers across Central Europe. Here, we coupled biomarker-based δ18O and δ2H from Bergsee in southern Germany to reconstruct deuterium excess as a proxy for evaporation history from the Bølling-Allerød to the Preboreal. We compared this dataset with other biomarker isotope records in Central Europe. They are all lacking a strong isotopic depletion during the Younger Dryas, which is best explained by the summer sensitivity of the biomarker proxies: As Younger Dryas summers were relatively warm, there is an absence of the strong winter cooling signals recorded in annual water isotope records like Greenland or Lake Steißlingen. Lake evaporation at Bergsee together with other paleohydrological reconstructions draw a coherent picture of the Late Glacial hydroclimate, with strong evidence for warm and dry Younger Dryas summers. Rather than a southward shift of the Westerlies during winter, we suggest that a recently proposed feedback mechanism between North Atlantic sea ice extend, strong winter cooling and summer atmospheric blocking serves as a suitable explanation for summer dryness. Additional confidence to the robustness of these biomarker records is provided by the overall agreement of paleohydrological fluctuations during the Preboreal.

Human response to the Younger Dryas along the southern North Sea basin, Northwest Europe

Currently in NW Europe little is known about the human response to the extensive cold reversal at the end of the Pleistocene, the Younger Dryas (ca. 12,850 till ca. 11,650 cal BP), mainly due to the poor chronological resolution of the archaeological sites belonging to the Ahrensburgian Culture. Here we present a series of 33 radiocarbon dates performed on the seminal cave site of Remouchamps, situated in the Belgian Meuse basin. Combined with a revision of the available radiocarbon evidence along the southern North Sea basin (Belgium, southern Netherlands, western Germany), it is suggested that the first half of the Younger Dryas, characterized as extremely cold and wet, faced a significant population reduction. Repopulation started around the middle of the Younger Dryas, from ca. 12,200 cal BP onward, probably in response to a slight climatic improvement leading to somewhat warmer summers. This might be considered a prelude to the subsequent population boost of the Early Holocene (Mesolithic).

Legacies of millennial-scale climate oscillations in contemporary biodiversity in eastern North America

The Atlantic meridional overturning circulation (AMOC) has caused significant climate changes over the past 90 000 years. Prior work has hypothesized that these millennial-scale climate variations effected past and contemporary biodiversity, but the effects are understudied. Moreover, few biogeographic models have accounted for uncertainties in palaeoclimatic simulations of millennial-scale variability. We examine whether refuges from millennial-scale climate oscillations have left detectable legacies in the patterns of contemporary species richness in eastern North America. We analyse 13 palaeoclimate estimates from climate simulations and proxy-based reconstructions as predictors for the contemporary richness of amphibians, passerine birds, mammals, reptiles and trees. Results suggest that past climate changes owing to AMOC variations have left weak but detectable imprints on the contemporary richness of mammals and trees. High temperature stability, precipitation increase, and an apparent climate fulcrum in the southeastern United States across millennial-scale climate oscillations aligns with high biodiversity in the region. These findings support the hypothesis that the southeastern United States may have acted as a biodiversity refuge. However, for some taxa, the strength and direction of palaeoclimate-richness relationships varies among different palaeoclimate estimates, pointing to the importance of palaeoclimatic ensembles and the need for caution when basing biogeographic interpretations on individual palaeoclimate simulations. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Summer paleohydrology during the Late Glacial and Early Holocene based on δ2H and δ18O from Bichlersee, Bavaria

Isotope-based records provide valuable information on past climate changes. However, it is not always trivial to disentangle past changes in the isotopic composition of precipitation from possible changes in evaporative enrichment, and seasonality may need to be considered. Here, we analyzed δ2H on n-alkanes and δ18O on hemicellulose sugars in sediments from Bichlersee, Bavaria, covering the Late Glacial and Early Holocene. Our δ2Hn-C31 record documents past changes in the isotopic composition of summer precipitation and roughly shows the isotope pattern known from Greenland. Both records show lower values during the Younger Dryas, but at Bichlersee the signal is less pronounced, corroborating earlier suggestions that the Younger Dryas was mainly a winter phenomenon and less extreme during summer. δ18Ofucose records the isotopic composition of the lake water during summer and is sensitive to evaporative enrichment. Coupling δ2Hn-C31 and δ18Ofucose allows calculating lake water deuterium-excess and thus disentangling changes in the isotopic composition of precipitation and evaporative enrichment. Our deuterium-excess record reveals that the warm Bølling-Allerød and Early Holocene were characterized by more evaporative enrichment compared to the colder Younger Dryas. Site-specific hydrological conditions, seasonality, and coupling δ2H and δ18O are thus important when interpreting isotope records.

Chronology and dynamics of fluvial style changes in the Younger Dryas and Early Holocene in Central Europe (lower San River, SE Poland)

Reconstruction of fluvial style changes in the San River in the Subcarpathian Basins is based on geomorphological and sedimentological analyses. The time control of alluvial fills and temporal changes in the river channel are derived from radiocarbon and optically stimulated luminescence dating combined with independent pollen-based biochronostratigraphy. The results showed that the alluvial plain of the braided (BR) or braided-meandering (BR-M?) river was abandoned before 12,800 cal BP. Large meanders (LM) were cut off in the older part of the Younger Dryas (YD; ca. 12,600 cal BP), and in the younger part of this period (ca. 12,450 cal BP). The small meanders (SM) developed at the end of the YD and were abandoned at the onset of the Preboreal (PB; ca.11,550 cal BP). The erosion phase at the YD-PB transition, reported from many valleys in Central Europe, was not confirmed in the study area. The full cycle of San River channel transformation (BR (BR-M?) → LM → SM); was estimated to be approximately 1200 years. According to the palynological data, open pine forests with birch that survived from the end of the Allerød dominated the landscape of the river valley during the YD cooling and did not undergo major changes during the warming in the early PB. Therefore, we assume that the influence of vegetation changes in the San River channel pattern transformation was nonsignificant. The location of the studied palaeochannels in the floodbasin filled with silty clayey deposits may have influenced the formation of relatively narrow and deep channels, than that of much the wider and shallower meanders from the YD, situated several kilometres downstream of the surveyed sites.

Rapid northern hemisphere ice sheet melting during the penultimate deglaciation

The rate and consequences of future high latitude ice sheet retreat remain a major concern given ongoing anthropogenic warming. Here, new precisely dated stalagmite data from NW Iberia provide the first direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation. These records reveal the penultimate deglaciation initiated with rapid century-scale meltwater pulses which subsequently trigger abrupt coolings of air temperature in NW Iberia consistent with freshwater-induced AMOC slowdowns. The first of these AMOC slowdowns, 600-year duration, was shorter than Heinrich 1 of the last deglaciation. Although similar insolation forcing initiated the last two deglaciations, the more rapid and sustained rate of freshening in the eastern North Atlantic penultimate deglaciation likely reflects a larger volume of ice stored in the marine-based Eurasian Ice sheet during the penultimate glacial in contrast to the land-based ice sheet on North America as during the last glacial.

Stalagmites from NW Iberia record the rapid demise of large ice sheets during the penultimate deglaciation, and reveal decadal-scale feedbacks between warming and ice melting.

Glacier fluctuations in the northern Patagonian Andes (44°S) imply wind-modulated interhemispheric in-phase climate shifts during Termination 1

The Last Glacial Termination (T1) featured major changes in global circulation systems that led to a shift from glacial to interglacial climate. While polar ice cores attest to an antiphased thermal pattern at millennial timescales, recent well-dated moraine records from both hemispheres suggest in-phase fluctuations in glaciers through T1, which is inconsistent with the bipolar see-saw paradigm. Here, we present a glacier chronology based on 30 new ¹⁰Be surface exposure ages from well-preserved moraines in the Lago Palena/General Vintter basin in northern Patagonia (~ 44°S). We find that the main glacier lobe underwent profound retreat after 19.7 ± 0.7 ka. This recessional trend led to the individualization of the Cerro Riñón glacier by ~ 16.3 ka, which underwent minor readvances at 15.9 ± 0.5 ka during Heinrich Stadial 1, during the Antarctic Cold Reversal with successive maxima at 13.5 ± 0.4, 13.1 ± 0.4, and 13.1 ± 0.5 ka, and a minor culmination at 12.5 ± 0.4 ka during Younger Dryas time. We conclude that fluctuations of Patagonian glaciers during T1 were controlled primarily by climate anomalies brought by shifts in the Southern Westerly Winds (SWW) locus. We posit that the global covariation of mountain glaciers during T1 was linked to variations in atmospheric CO₂ (atmCO₂) promoted by the interplay of the SWW-Southern Ocean system at millennial timescales.

Enhancing computational fluid dynamics with machine learning

Machine learning is rapidly becoming a core technology for scientific computing, with numerous opportunities to advance the field of computational fluid dynamics. Here we highlight some of the areas of highest potential impact, including to accelerate direct numerical simulations, to improve turbulence closure modeling and to develop enhanced reduced-order models. We also discuss emerging areas of machine learning that are promising for computational fluid dynamics, as well as some potential limitations that should be taken into account.

Synchronous or Not? The Timing of the Younger Dryas and Greenland Stadial-1 Reviewed Using Tephrochronology

The exact spatial and temporal behaviour of rapid climate shifts during the Last Glacial–Interglacial Transition are still not entirely understood. In order to investigate these events, it is necessary to have detailed palaeoenvironmental reconstructions at geographically spread study sites combined with reliable correlations between them. Tephrochronology, i.e., using volcanic ash deposits in geological archives as a dating and correlation tool, offers opportunities to examine the timing of events across wider regional scales. This study aims to review the posited asynchrony of the Younger Dryas stadial in comparison with Greenland Stadial-1 by correlating new proxy data from southernmost Sweden to previous palaeoclimate reconstructions in Europe based on the presence of the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra. μ-XRF core-scanning data were projected using a recently published age–depth model based on these tephras and several radiocarbon dates, and compared to previous findings, including by adapting previous chronologies to the recently proposed earlier date of the Laacher See Tephra (13,006 ± 9 cal. a BP). Although the results to some extent support the idea of a more synchronous Younger Dryas event than previously assumed, this issue requires further high-resolution proxy studies to overcome limitations of temporal precision.

Disruption of the Atlantic Meridional Circulation during Deglacial Climates Inferred from Planktonic Foraminiferal Shell Weights

Changes in the density structure of the upper oceanic water masses are an important forcing of changes in the Atlantic Meridional Overturning Circulation (AMOC), which is believed to widely affect Earth’s climate. However, very little is known about past changes in the density structure of the Atlantic Ocean, despite being extensively studied. The physical controls on planktonic foraminifera calcification are explored here, to obtain a first-order approximation of the horizontal density gradient in the eastern Atlantic during the last 200,000 years. Published records of Globigerina bulloides shells from the North and Tropical eastern Atlantic were complemented by the analysis of a South Atlantic core. The masses of the same species shells from three different dissolution assessed sediment cores along the eastern Atlantic Ocean were converted to seawater density values using a calibration equation. Foraminifera, as planktonic organisms, are subject to the physical properties of the seawater and thus their shells are sensitive to buoyancy forcing through surface temperature and salinity perturbations. By using planktonic foraminifera shell weight as an upper ocean density proxy, two intervals of convergence of the shell masses are identified during cold intervals of the last two deglaciations that may be interpreted as weak ocean density gradients, indicating nearly or completely eliminated meridional circulation, while interhemispheric Atlantic density differences appear to alleviate with the onset of the last interglacial. The results confirm the significance of variations in the density of Atlantic surface waters for meridional circulation changes.

On the Experimental, Numerical and Data-Driven Methods to Study Urban Flows

Understanding the flow in urban environments is an increasingly relevant problem due to its significant impact on air quality and thermal effects in cities worldwide. In this review we provide an overview of efforts based on experiments and simulations to gain insight into this complex physical phenomenon. We highlight the relevance of coherent structures in urban flows, which are responsible for the pollutant-dispersion and thermal fields in the city. We also suggest a more widespread use of data-driven methods to characterize flow structures as a way to further understand the dynamics of urban flows, with the aim of tackling the important sustainability challenges associated with them. Artificial intelligence and urban flows should be combined into a new research line, where classical data-driven tools and machine-learning algorithms can shed light on the physical mechanisms associated with urban pollution.

Synergistic impacts of global warming and thermohaline circulation collapse on amphibians

Impacts on ecosystems and biodiversity are a prominent area of research in climate change. However, little is known about the effects of abrupt climate change and climate catastrophes on them. The probability of occurrence of such events is largely unknown but the associated risks could be large enough to influence global climate policy. Amphibians are indicators of ecosystems’ health and particularly sensitive to novel climate conditions. Using state-of-the-art climate model simulations, we present a global assessment of the effects of unabated global warming and a collapse of the Atlantic meridional overturning circulation (AMOC) on the distribution of 2509 amphibian species across six biogeographical realms and extinction risk categories. Global warming impacts are severe and strongly enhanced by additional and substantial AMOC weakening, showing tipping point behavior for many amphibian species. Further declines in climatically suitable areas are projected across multiple clades, and biogeographical regions. Species loss in regional assemblages is extensive across regions, with Neotropical, Nearctic and Palearctic regions being most affected. Results underline the need to expand existing knowledge about the consequences of climate catastrophes on human and natural systems to properly assess the risks of unabated warming and the benefits of active mitigation strategies.

Julián Velasco et al. use climate model simulations to show how the collapse of the Atlantic meridional overturning circulation and unabated global warming under the RCP 8.5 scenario affect the global distribution of 2509 amphibian species. These results show severe and synergistic impacts of global warming, with particularly strong effects shown in the Neotropical, Nearctic and Palearctic regions.

Atmospheric circulation over Europe during the Younger Dryas

The Younger Dryas (YD) was a period of rapid climate cooling that occurred at the end of the last glaciation. Here, we present the first palaeoglacier-derived reconstruction of YD precipitation across Europe, determined from 122 reconstructed glaciers and proxy atmospheric temperatures. Positive precipitation anomalies (YD versus modern) are found along much of the western seaboard of Europe and across the Mediterranean. Negative precipitation anomalies occur over the Fennoscandian ice sheet, the North European Plain, and as far south as the Alps. This is consistent with a more southerly and zonal storm track, which is linked to a concomitant southern location of the Polar Frontal Jet Stream, generating cold air outbreaks and enhanced cyclogenesis, especially over the eastern Mediterranean. This atmospheric configuration resembles the modern Scandinavian (SCAND) circulation over Europe (a blocking high pressure over Scandinavia pushing storm tracks south and east), and by analogy, a seasonally varying palaeoprecipitation pattern is interpreted.

Comparison of Climate Model Simulations of the Younger Dryas Cold Event

Results of five climate model simulation studies on the Younger Dryas cold event (YD) are compared with a focus on temperature and precipitation. Relative to the Bølling-Allerød interstadial (BA), the simulations show consistent annual cooling in Europe, Greenland, Alaska, North Africa and over the North Atlantic Ocean and Nordic Seas with maximum reduction of temperatures being simulated over the oceans, ranging from −25 °C to −6 °C. Warmer conditions were simulated in the interior of North America. In two experiments, the mid-to-high latitudes of the Southern Hemisphere were also warmer, associated with a strong bi-polar seesaw mechanism in response to a collapse of the Atlantic meridional overturning circulation (AMOC). The modelled YD-BA temperature response was in general agreement with proxy-based evidence. The simulations reveal reduced YD-BA precipitation (up to 150 mm yr−1) over all regions with major cooling, and over the northern equatorial region. South of the equator, modelled precipitation seemed to increase due to a southward shift of the InterTropical Convergence Zone (ITCZ). The largest uncertainty in the YD is the high-latitude response, where the models show diverging results. This disagreement is partly related to uncertainties in the freshwater forcing. Most model studies assume an AMOC shutdown, but this is incompatible with proxy evidence.

Machine-learning based reconstructions of primary and secondary climate variables from North American and European fossil pollen data

We test several quantitative algorithms as palaeoclimate reconstruction tools for North American and European fossil pollen data, using both classical methods and newer machine-learning approaches based on regression tree ensembles and artificial neural networks. We focus on the reconstruction of secondary climate variables (here, January temperature and annual water balance), as their comparatively small ecological influence compared to the primary variable (July temperature) presents special challenges to palaeo-reconstructions. We test the pollen-climate models using a novel and comprehensive cross-validation approach, running a series of h-block cross-validations using h values of 100-1500 km. Our study illustrates major benefits of this variable h-block cross-validation scheme, as the effect of spatial autocorrelation is minimized, while the cross-validations with increasing h values can reveal instabilities in the calibration model and approximate challenges faced in palaeo-reconstructions with poor modern analogues. We achieve well-performing calibration models for both primary and secondary climate variables, with boosted regression trees providing the overall most robust performance, while the palaeoclimate reconstructions from fossil datasets show major independent features for the primary and secondary variables. Our results suggest that with careful variable selection and consideration of ecological processes, robust reconstruction of both primary and secondary climate variables is possible.

Deep-water circulation changes lead North Atlantic climate during deglaciation

Constraining the response time of the climate system to changes in North Atlantic Deep Water (NADW) formation is fundamental to improving climate and Atlantic Meridional Overturning Circulation predictability. Here we report a new synchronization of terrestrial, marine, and ice-core records, which allows the first quantitative determination of the response time of North Atlantic climate to changes in high-latitude NADW formation rate during the last deglaciation. Using a continuous record of deep water ventilation from the Nordic Seas, we identify a ∼400-year lead of changes in high-latitude NADW formation ahead of abrupt climate changes recorded in Greenland ice cores at the onset and end of the Younger Dryas stadial, which likely occurred in response to gradual changes in temperature- and wind-driven freshwater transport. We suggest that variations in Nordic Seas deep-water circulation are precursors to abrupt climate changes and that future model studies should address this phasing.

The response time of North Atlantic climate to changes in high-latitude deep-water formation during the last deglaciation is still unclear. Here the authors show that gradual changes in Nordic Seas deep-water circulation systematically lead ahead of abrupt regional climate shifts by ~400 years.