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High‐resolution records from past interglacial climates help constrain future responses to global warming, yet are rare. Here, we produce seasonally resolved climate records from subarctic‐Canada using micron‐scale measurements of oxygen isotopes (δ18O) in speleothems with apparent annual growth bands from three interglacial periods—Marine Isotope Stages (MIS) 11, 9, and 5e. We find 3‰ lower δ18O values during MIS 11 than MIS 5e, despite MIS 11 likely being warmer. We explore controls on high‐latitude speleothem δ18O and suggest low MIS 11 δ18O values reflect greater contribution of cold‐season precipitation to dripwater from longer annual ground thaw durations. Other potential influences include changes in precipitation source and/or increased fraction of cold‐season precipitation from diminished sea ice in MIS 11. Our study highlights the potential for high‐latitude speleothems to yield detailed isotopic records of Northern Hemisphere interglacial climates beyond the reach of Greenland ice cores and offers a framework for interpreting them. Plain Language Summary: Few climate records pre‐dating the last ice age exist from high‐latitude North America, which inhibits our understanding of how regions with permafrost responded to past warming and how they might change in the future. Here, we help fill this data gap by using six speleothems (cave mineral deposits) from a cave in the Northwest Territories, Canada to produce climate records that span thousands of years during former warm periods of Earth's history. We find that speleothems that grew during an exceptionally warm super‐interglacial period 400,000 years ago have 3‰ lower oxygen isotope (δ18O) values compared to those that grew during a likely cooler interglacial 125,000 years ago. We explore potential explanations for the difference in δ18O across interglacials, and suggest that lower δ18O values during warmer periods reflect greater infiltration of cool‐season precipitation with longer annual ground thaw durations. This study highlights the importance of high‐latitude speleothems to provide detailed climate records beyond the range available from Greenland ice cores. Key Points: Long high‐latitude terrestrial climate records are rare in the Northern HemisphereHigh‐latitude speleothems can provide ultra‐high‐resolution climate records beyond the reach of Greenland ice coresMean oxygen isotopes of Arctic and subarctic speleothems likely are controlled by annual ground thaw durations [ABSTRACT FROM AUTHOR]

A ten-year record of oxygen and hydrogen isotopic composition of precipitation is here presented: from 2008 to 2017, 1483 daily precipitation samples were collected all-year round on a raised platform at Concordia Station, East Antarctica. Weather data were retrieved from the Italian Antarctic Meteo-Climatological Observatory AWS, while ERA5 was used to estimate total precipitation. The δ-temperature relationships were moderately high for daily data (r2=0.63 and 0.64 for δ18O and δ2H, respectively) and stronger using monthly data (r2=0.82 for both δ18O and δ2H), with a slope of about 0.5 ‰/°C for δ18O/TAWS (3.5 ‰/°C for δ18O/TAWS), which remains consistent also using annual averages. The isotopic composition of precipitation is the input signal of the snow/ice system and this dataset will be useful to improve the interpretation of paleoclimate records and promote a better understanding of the post-depositional processes affecting the isotopic signal in ice cores. Isotope-enabled GCM ECHAM6-wiso output for the isotopic composition of precipitation was also compared to experimental data, showing moderately good relationships for δ18O and δ2H, but not for d-excess, nonetheless marking a substantial improvement from the previous release of the model. [ABSTRACT FROM AUTHOR]

Climate field reconstructions (CFRs) combine modern observational data with paleoclimatic proxies to estimate climate variables over spatiotemporal grids during time periods when widespread observations of climatic conditions do not exist. The Common Era (CE) has been a period over which many seasonally‐ and annually‐resolved CFRs have been produced on regional to global scales. CFRs over the CE were first produced in the 1970s using dendroclimatic records and linear regression‐based approaches. Since that time, many new CFRs have been produced using a wide range of proxy data sets and reconstruction techniques. We assess the early history of research on CFRs for the CE, which provides context for our review of advances in CFR research over the last two decades. We review efforts to derive gridded hydroclimatic CFRs over continental regions using networks of tree‐ring proxies. We subsequently explore work to produce hemispheric‐ and global‐scale CFRs of surface temperature using multi‐proxy data sets, before specifically reviewing recently‐developed data assimilation techniques and how they have been used to produce simultaneous reconstructions of multiple climatic fields globally. We then review efforts to develop standardized and digitized databases of proxy networks for use in CFR research, before concluding with some thoughts on important next steps for CFR development. Plain Language Summary: Few observational records of meteorological variables like surface temperature and precipitation extend beyond 100–150 years. The length of observational records is therefore insufficient for learning how climate varies over decades to centuries, or for estimating many climate extremes. In contrast, annually‐resolved climate proxies, such as tree rings, ice cores, and corals, when used in concert with observational records, can provide information on how climate conditions have changed over decades to millennia. These proxies are also abundant enough over the last two millennia to create reconstructions in both space and time, or maps of climate conditions at seasonal or annual intervals. These kinds of reconstructions are called climate field reconstructions (CFRs) and we review their scientific history back to the 1970s when they were first attempted. We survey the underlying methods that are used to derive these reconstructions and how they have been applied, with a specific focus on the last two decades when the number of CFRs has greatly increased. We conclude with a survey of the data networks that are being cultivated for CFRs and some reflections on how the science can move forward. Key Points: Reconstructions of climate fields are valuable paleoclimatic products that have been developed since the 1970sImportant similarities and differences exist across the approaches used to derive hydroclimate and temperature climate field reconstructions (CFRs) targeting the Common Era (CE)Controlled and systematic comparisons of CFRs should be a priority for the CE paleoclimate community [ABSTRACT FROM AUTHOR]

Large uncertainties exist in climate model projections of the Asian summer monsoon (ASM). The El Niño‐Southern Oscillation (ENSO) is an important modulator of the ASM, but the ENSO‐ASM teleconnection is not stationary. Furthermore, teleconnections between ENSO and the East Asian versus South Asian subcomponents of the ASM exhibit distinct characteristics. Therefore, understanding the variability of the ENSO‐ASM teleconnection is critical for anticipating future variations in ASM intensity. To this end, we here use paleoclimate records to extend temporal coverage beyond the instrumental era by millennia. Recently, data assimilation techniques have been applied for the last millennium, which facilitates physically consistent, globally gridded climate reconstructions informed by paleoclimate observations. We use these novel data assimilation products to investigate variations in the ENSO‐ASM relationship over the last 1,000 years. We find that correlations between ENSO and ASM indices are mostly negative in the last millennium, suggesting that strong ASM years are often associated with La Niña events. During periods of weak correlations between ENSO and the East Asian summer monsoon, we observe an El Niño‐like sea surface temperature (SST) pattern in the Pacific. Additionally, SST patterns associated with periods of weak correlations between ENSO and South Asian summer monsoon rainfall are not consistent among data assimilation products. This underscores the importance of developing more precipitation‐sensitive paleoclimate proxies in the Indian subcontinental realm over the last millennium. Our study serves as a baseline for future appraisals of paleoclimate assimilation products and an example of informing our understanding of decadal‐scale ENSO‐ASM teleconnection variability using paleoclimate data sets. Plain Language Summary: A large fraction of floods and droughts in Asia are associated with extreme Asian summer monsoon (ASM) events, but unfortunately, the prediction of the ASM is still highly uncertain. Scientists often use El Niño—warm sea surface temperature (SST) events in the eastern Pacific Ocean that occur before the ASM—as a basis to predict the ASM, but it does not work perfectly all the time. In order to understand why El Niño fails to predict the ASM sometimes, we looked at climate data spanning the last 1,000 years. The data was based on paleoclimate reconstructions using archives including tree rings, corals, speleothems, and ice cores. Scientists recently synthesized all these data together with climate model simulations. We find that more intense summer monsoon rainfall in East and South Asia likely corresponds to colder SSTs in the Eastern Pacific Ocean. Also, when El Niño fails to predict the monsoon in East Asia, the background SST in the eastern Pacific is warmer than normal. Our results could help better predict ASM rainfall in the future and mitigate impacts of fire and flooding caused by extreme monsoon events. Key Points: The El Niño‐Southern Oscillation (ENSO) teleconnections to the Asian summer monsoon have evident decadal‐to‐centennial variability in the last 1,000 yearsDuring most of the last 1,000 years, El Niño tends to be associated with weak Asian summer monsoon rainfallWeak ENSO‐East Asian summer monsoon teleconnection is associated with sea surface temperature anomalies resembling El Niño events [ABSTRACT FROM AUTHOR]

Tipping points (TPs) in Earth's climate system have been the subject of increasing interest and concern in recent years, given the risk that anthropogenic forcing could cause abrupt, potentially irreversible, climate transitions. Paleoclimate records are essential for identifying past TPs and for gaining a thorough understanding of the underlying nonlinearities and bifurcation mechanisms. However, the quality, resolution, and reliability of these records can vary, making it important to carefully select the ones that provide the most accurate representation of past climates. Moreover, as paleoclimate time series vary in their origin, time spans, and periodicities, an objective, automated methodology is crucial for identifying and comparing TPs. To address these challenges, we introduce the open-source PaleoJump database, which contains a collection of carefully selected, high-resolution records originating in ice cores, marine sediments, speleothems, terrestrial records, and lake sediments. These records describe climate variability on centennial, millennial and longer time scales and cover all the continents and ocean basins. We provide an overview of their spatial distribution and discuss the gaps in coverage. Our statistical methodology includes an augmented Kolmogorov–Smirnov test and Recurrence Quantification Analysis; it is applied here, for illustration purposes, to selected records in which abrupt transitions are automatically detected and the presence of potential tipping elements is investigated. These transitions are shown in the PaleoJump database along with other essential information about the records, including location, temporal scale and resolution, as well as temporal plots. This open-source database represents, therefore, a valuable resource for researchers investigating TPs in past climates. [ABSTRACT FROM AUTHOR]

Paleoclimate proxy records have non-negligible uncertainties that arise from both the proxy measurement and the dating processes. Knowledge of the dating uncertainties is important for a rigorous propagation to further analyses, for example, for identification and dating of stadial–interstadial transitions in Greenland ice core records during glacial intervals, for comparing the variability in different proxy archives, and for model-data comparisons in general. In this study we develop a statistical framework to quantify and propagate dating uncertainties in layer counted proxy archives using the example of the Greenland Ice Core Chronology 2005 (GICC05). We express the number of layers per depth interval as the sum of a structured component that represents both underlying physical processes and biases in layer counting, described by a regression model, and a noise component that represents the fluctuations of the underlying physical processes, as well as unbiased counting errors. The joint dating uncertainties for all depths can then be described by a multivariate Gaussian process from which the chronology (such as the GICC05) can be sampled. We show how the effect of a potential counting bias can be incorporated in our framework. Furthermore we present refined estimates of the occurrence times of Dansgaard–Oeschger events evidenced in Greenland ice cores together with a complete uncertainty quantification of these timings. [ABSTRACT FROM AUTHOR]

In blue-ice areas (BIAs), deep ice is directly exposed at the surface, allowing for the cost-effective collection of large-sized old-ice samples. However, chronostratigraphic studies on blue-ice areas are challenging owing to fold and fault structures. Here, we report on a surface transect of ice with an undisturbed horizontal stratigraphy from the Larsen BIA, northern Victoria Land, East Antarctica. Ice layers defined by dust bands and ground-penetrating radar (GPR) surveys indicate a monotonic increase in age along the ice flow direction on the downstream side, while the upstream ice exhibits a potential repetition of ages on scales of tens of meters, which result from a complicated fold structure. Stable water isotopes (δ18 O ice and δ2 H ice) and components of the occluded air (i.e., CO 2 , N 2 O, CH 4 , δ15 N–N 2 , δ18 O atm (=δ18 O-O 2), δ O2/N2 , δ Ar/N2 ​​​​​​​, 81 Kr, and 85 Kr) are analyzed for surface ice and shallow ice core samples. Correlating δ18 O ice , δ18 O atm , and CH 4 records from the Larsen BIA with ice from previously drilled ice cores indicates that the gas age at various shallow vertical coring sites ranges between 9.2–23.4 kyr BP, while the ice age sampled from the surface ranges from 5.6 to 24.7 kyr BP. Absolute radiometric 81 Kr dating for the two vertical cores confirms ages within acceptable levels of analytical uncertainty. A tentative climate reconstruction suggests a large deglacial warming of 15 ± 5 ∘ C (1 σ) and an increase in snow accumulation by a factor of 1.7–4.6 (from 24.3 to 10.6 kyr BP). Our study demonstrates that BIAs in northern Victoria Land may help to obtain high-quality records for paleoclimate and atmospheric greenhouse gas compositions through the last deglaciation, although in general climatic interpretation is complicated by the need for upstream flow corrections, evidence for strong surface sublimation during the last glacial period, and potential errors in the estimated gas age–ice age difference. [ABSTRACT FROM AUTHOR]

The analysis of the stable oxygen isotopes 18O and 16O has revolutionized paleoclimate research since the middle of the last century. Particularly, δ18O of ice cores from Greenland and Antarctica is used as a paleotemperature proxy, and δ18O of deep-sea sediments is used as a proxy for global ice volume. Important terrestrial archives to which δ18O as a paleoclimate proxy is successfully applied are speleothems, lake sediments, or tree rings. By contrast, δ18 O applications to loess–paleosol sequences (LPSs) are scarce. Here we present a first continuous δ18O record (n=50) for the LPS Crvenka in Serbia, southeastern Europe, spanning the last glacial–interglacial cycle (since 145 ka). From a methodological point of view, we took advantage of a recently proposed paleoclimate/paleohydrological proxy based on bulk δ18O analyses of plant-derived lipids. The Crvenka δ18 O bulklipid values range between -10.2 ‰ and +23.0 ‰ and are systematically more positive in the interglacial and interstadial (paleo-)soils corresponding to marine oxygen-isotope stage (MIS) 1, 3, and 5, compared to the loess layers (MIS 2, 4, and 6). Our Crvenka δ18O bulklipid record provides no evidence for the occurrence of interstadials and stadials comparable to the Dansgaard–Oeschger events known from the Greenland δ18O icecore records. Concerning the interpretation of our Crvenka δ18 O bulklipid record, plant-derived lipids such as fatty acids and alcohols are certainly strongly influenced by climatic factors such as temperature (via δ18O precipitation) and relative air humidity (via 18O enrichment of leaf water due to evapotranspiration). However, pool effects in the form of non-water-correlated lipids such as sterols or the input of root-derived lipids need to be considered, too. Similarly, the input of soil-microbial lipids and oxygen exchange reactions represent uncertainties challenging quantitative paleoclimate/paleohydrological reconstructions based on δ18O bulklipid analyses from LPSs. Kurzfassung: Die Analyse der stabilen Sauerstoffisotope 18O und 16O hat die Paläoklimaforschung seit Mitte des letzten Jahrhunderts revolutioniert. Insbesondere wird δ18O von Eisbohrkernen aus Grönland und der Antarktis als Paläo-Temperaturproxy sowie δ18O von Tiefseesedimenten als Proxy für das globale Eisvolumen verwendet. Wenngleich sich in terrestrischen Archiven, wie Speläothemen, Seesedimenten oder Baumringen, paläoklimatische Rekonstruktionen unter der Anwendung von δ18O als Proxy bewährt haben, wurden solche Analysen bislang in Löß-Paläobodensequenzen (LPS) selten durchgeführt. In dieser Studie präsentieren wir einen ersten kontinuierlichen δ18O Datensatz (n=50) für die LPS Crvenka in Serbien, der den letzten Glazial-Interglazial-Zyklus (∼ 145 ka) abdeckt. Die δ18O -Werte basieren auf der Analyse von pflanzlichen Lipiden, deren Anwendung als paläoklimatischer/hydrologischer Proxy vor Kurzem vorgeschlagen wurde. Die δ18O bulk-lipid -Werte von Crvenka liegen zwischen -10.2 ‰ und +23.0 ‰ und sind in den interglazialen und interstadialen (Paläo-)Böden, die den marinen Sauerstoff-Isotopenstufen (MIS) 1, 3 und 5 entsprechen, systematisch positiver als in den Lößlagen (MIS 2, 4 und 6). Sie liefern keine Hinweise für das Auftreten von Interstadialen und Stadialen, die mit den aus den grönländischen δ18O ice-core bekannten Dansgaard-Oeschger-Ereignissen vergleichbar wären. In Bezug auf die Interpretation der δ18O bulk-lipid -Werte gilt es zu berücksichtigen, dass die Isotopie pflanzlicher Lipide, wie z.B. von Fettsäuren und Alkoholen, stark durch die Klimafaktoren Temperatur (über δ18O -Niederschlag) und relativer Luftfeuchtigkeit (über die 18O -Anreicherung des Blattwassers aufgrund von Evapotranspiration) beeinflusst werden. Weiter zu beachtende Faktoren stellen Einträge von Sterolen sowie von generell wurzel-bürtigen Lipiden dar (Pool-Effekte). In ähnlicher Weise bergen der Einfluss von bodenmikrobiellen Lipiden und Sauerstoffaustauschreaktionen Unsicherheiten, die quantitative paläoklimatische/hydrologische Rekonstruktionen auf der Grundlage von δ18O bulk-lipid -Analysen aus LPS erschweren können. [ABSTRACT FROM AUTHOR]

Volcanic fallout in polar ice sheets provides important opportunities to date and correlate ice-core records as well as to investigate the environmental impacts of eruptions. Only the geochemical characterization of volcanic ash (tephra) embedded in the ice strata can confirm the source of the eruption, however, and is a requisite if historical eruption ages are to be used as valid chronological checks on annual ice layer counting. Here we report the investigation of ash particles in a Greenland ice core that are associated with a volcanic sulfuric acid layer previously attributed to the 79 CE eruption of Vesuvius. Major and trace element composition of the particles indicates that the tephra does not derive from Vesuvius but most likely originates from an unidentified eruption in the Aleutian arc. Using ash dispersal modeling, we find that only an eruption large enough to include stratospheric injection is likely to account for the sizable (24–85 µm) ash particles observed in the Greenland ice at this time. Despite its likely explosivity, this event does not appear to have triggered significant climate perturbations, unlike some other large extratropical eruptions. In light of a recent re-evaluation of the Greenland ice-core chronologies, our findings further challenge the previous assignation of this volcanic event to 79 CE. We highlight the need for the revised Common Era ice-core chronology to be formally accepted by the wider ice-core and climate modeling communities in order to ensure robust age linkages to precisely dated historical and paleoclimate proxy records. [ABSTRACT FROM AUTHOR]

Bifurcations and tipping points (TPs) are an important part of the Earth system's behavior. These critical points represent thresholds at which small changes in the system's parameters or in the forcing abruptly switch it from one state or type of behavior to another. Current concern with TPs is largely due to the potential of slow anthropogenic forcing to bring about abrupt, and possibly irreversible, change to the physical climate system and impacted ecosystems. Paleoclimate proxy records have been shown to contain abrupt transitions, or "jumps," which may represent former instances of such dramatic climate change events. These transitions can provide valuable information for identifying critical TPs in current and future climate evolution. Here, we present a robust methodology for detecting abrupt transitions in proxy records that is applied to ice core and speleothem records of the last climate cycle. This methodology is based on the nonparametric Kolmogorov–Smirnov (KS) test for the equality, or not, of the probability distributions associated with two samples drawn from a time series, before and after any potential jump. To improve the detection of abrupt transitions in proxy records, the KS test is augmented by several other criteria and it is compared with recurrence analysis. The augmented KS test results show substantial skill when compared with more subjective criteria for jump detection. This test can also usefully complement recurrence analysis and improve upon certain aspects of its results. [ABSTRACT FROM AUTHOR]

More than two decades ago, my coauthors, Raymond Bradley and Malcolm Hughes, and I published the now iconic "hockey stick" curve. It was a simple graph, derived from large-scale networks of diverse climate proxy ("multiproxy") data such as tree rings, ice cores, corals, and lake sediments, that captured the unprecedented nature of the warming taking place today. It became a focal point in the debate over human-caused climate change and what to do about it. Yet, the apparent simplicity of the hockey stick curve betrays the dynamicism and complexity of the climate history of past centuries and how it can inform our understanding of humancaused climate change and its impacts. In this article, I discuss the lessons we can learn from studying paleoclimate records and climate model simulations of the "Common Era," the period of the past two millennia during which the "signal" of human-caused warming has risen dramatically from the background of natural variability. [ABSTRACT FROM AUTHOR]

To understand the long-term climate and glaciological evolution of the ice sheet in the region bordering the Weddell Sea, the British Antarctic Survey has undertaken a series of successful ice core projects drilling to bedrock on Berkner Island, James Ross Island and the Fletcher Promontory. A new project, WACSWAIN, seeks to increase this knowledge by further drilling to bedrock on two further ice rises in this region. In a single-season project, an ice core was recovered to bedrock at 651 m on Skytrain Ice Rise using an ice core drill in a fluid-filled borehole. In a second season, a rapid access drill was used to recover ice chips to 323 m on Sherman Island in a dry borehole, though failing to reach the bedrock which was at an estimated depth of 428 m. [ABSTRACT FROM AUTHOR]

Recent studies have shown that stratigraphically disturbed meteoric ice bedded at Vostok Station between 3318 and 3539 m dates back to 1.2 Ma BP and possibly beyond. As part of the VOICE (Vostok Oldest Ice Challenge) initiative, a new deviation from parent hole 5G-1 was made at depths of 3270–3291 m in the 2018/19 austral season with the aim of obtaining a replicate core of the old ice. Sidetracking was initiated using the standard KEMS-132 electromechanical drill routinely employed for deep ice coring at Vostok, without significant changes to its initial design. Here we describe the method and operating procedures for replicate coring at a targeted depth in an existing slant hole, involving the use of a cable-suspended electromechanical drill. The design of the milling cutter head used for sidetracking is presented. The performance characteristics and the experience of drilling branch-hole 5G-5 at Vostok are described and discussed. [ABSTRACT FROM AUTHOR]

Abstract–Climate field reconstructions (CFRs) attempt to estimate spatiotemporal fields of climate variables in the past using climate proxies such as tree rings, ice cores, and corals. Data assimilation (DA) methods are a recent and promising new means of deriving CFRs that optimally fuse climate proxies with climate model output. Despite the growing application of DA-based CFRs, little is understood about how much the assimilated proxies change the statistical properties of the climate model data. To address this question, we propose a robust and computationally efficient method, based on functional data depth, to evaluate differences in the distributions of two spatiotemporal processes. We apply our test to study global and regional proxy influence in DA-based CFRs by comparing the background and analysis states, which are treated as two samples of spatiotemporal fields. We find that the analysis states are significantly altered from the climate-model-based background states due to the assimilation of proxies. Moreover, the difference between the analysis and background states increases with the number of proxies, even in regions far beyond proxy collection sites. Our approach allows us to characterize the added value of proxies, indicating where and when the analysis states are distinct from the background states. for this article are available online. [ABSTRACT FROM AUTHOR]

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations. [ABSTRACT FROM AUTHOR]

Reconstructing past climates remains a difficult task because pre‐instrumental observational networks are composed of geographically sparse and noisy paleoclimate proxy records that require statistical techniques to inform complete climate fields. Traditionally, instrumental or climate model statistical relationships are used to spread information from proxy measurements to other locations and to other climate variables. Here ensembles drawn from single climate models and from combinations of multiple climate models are used to reconstruct temperature variability over the last millennium in idealized experiments. We find that reconstructions derived from multi‐model ensembles produce lower error than reconstructions from single‐model ensembles when reconstructing independent model and instrumental data. Specifically, we find the largest decreases in error over regions far from proxy locations that are often associated with large uncertainties in model physics, such as mid‐ and high‐latitude ocean and sea‐ice regions. Furthermore, we find that multi‐model ensemble reconstructions outperform single‐model reconstructions that use covariance localization. We propose that multi‐model ensembles could be used to improve paleoclimate reconstructions in time periods beyond the last millennium and for climate variables other than air temperature, such as drought metrics or sea ice variables. Plain Language Summary: Understanding past climate variability is important for contextualizing climate change as well as for testing the ability of climate models to simulate the climate system before global warming. However, reconstructing past climate variability remains a complex task because pre‐instrumental paleoclimate proxy records, such as tree rings, corals, ice cores, and sediment cores, are geographically sparse and are not perfect recorders of climate information. Exactly how to extrapolate information from paleoclimate proxies to other locations and climate variables remains an outstanding issue. Traditionally, information about how one location varies with another location or variable (covariance) is derived from one climate model or from one instrumental data source. Here we find that reconstructions using covariance estimated from combinations of multiple climate models produce less error than reconstructions that use just one climate model. Key Points: Ensembles drawn from single climate models and combinations of multiple models are used to reconstruct surface air temperature variabilityReconstructions from multi‐model ensembles produce lower error than reconstructions from single‐model ensemblesReconstructions from multi‐model ensembles show the largest decreases in error in regions with few observations such as high‐latitude oceans [ABSTRACT FROM AUTHOR]

The Winkie Drill is an agile, commercially available rock coring system. The U.S. Ice Drilling Program has modified a Winkie Drill for subglacial rock and ice/rock interface coring, as well as drilling and coring access holes through ice. The original gasoline engine was replaced with an electric motor though the two-speed gear reducer and Unipress hand feed system were maintained. Using standard aluminum AW34 drill rod (for 33.5mm diameter core), the system has a depth capability of 120 m. The drill uses forward fluid circulation in a closed loop system. The drilling fluid is Isopar K, selected for favorable properties in polar environment. When firn or snow is present at the drill site, casing with an inflatable packer can be deployed to contain the drill fluid. The Winkie Drill will operate from sea level to high altitudes and operation results in minimal environmental impact. The drill can be easily and quickly assembled and disassembled in the field by two people. All components can be transported by Twin Otter or helicopter to the field site. [ABSTRACT FROM AUTHOR]

Over the course of the 2014/15 and 2015/16 austral summer seasons, the South Pole Ice Core project recovered a 1751m deep ice core at the South Pole. This core provided a high-resolution record of paleoclimate conditions in East Antarctica during the Holocene and late Pleistocene. The drilling and core processing were completed using the new US Intermediate Depth Drill system, which was designed and built by the US Ice Drilling Program at the University of Wisconsin-Madison. In this paper, we present and discuss the setup, operation, and performance of the drill system. [ABSTRACT FROM AUTHOR]

Ice core measurements of the concentration and stable isotopic composition of atmospheric nitrous oxide (N2O) 74,000–59,000 years ago constrain marine and terrestrial emissions. The data include two major Dansgaard‐Oeschger (D‐O) events and the N2O decrease during global cooling at the Marine Isotope Stage (MIS) 5a‐4 transition. The N2O increase associated with D‐O 19 (~73–71.5 ka) was driven by equal contributions from marine and terrestrial emissions. The N2O decrease during the transition into MIS 4 (~71.5–67.5 ka) was caused by gradual reductions of similar magnitude in both marine and terrestrial sources. A 50 ppb increase in N2O concentration at the end of MIS 4 was caused by gradual increases in marine and terrestrial emissions between ~64 and 61 ka, followed by an abrupt increase in marine emissions at the onset of D‐O 16/17 (59.5 ka). This suggests that the importance of marine versus terrestrial emissions in controlling millennial‐scale N2O fluctuations varied in time. Plain Language Summary: Nitrous oxide is a powerful greenhouse gas that is produced naturally in soils and oceans. An important unresolved question is the extent to which anthropogenic warming will stimulate additional emissions from these sources, further adding to the warming. Past variations in the abundance of nitrous oxide have been observed using ice core reconstructions, but the reasons for the variations are not well understood. Nitrous oxide produced in soils is isotopically distinct from nitrous oxide produced in oceans. New measurements of the isotopes of atmospheric nitrous oxide provide constraints on how marine and terrestrial sources must have changed, driving fluctuations in nitrous oxide concentration during two intervals of rapid warming and a prolonged period of global cooling. The reconstructed changes in nitrous oxide sources provide insights into relationships between marine and terrestrial ecosystems and climate. Key Points: Stable isotopes of nitrous oxide constrain marine versus terrestrial production 74,000–59,000 years agoMarine and terrestrial sources varied similarly across Dansgaard‐Oeschger 19 and during the Marine Isotope Stage 5‐4 transitionMarine emissions dominated across Dansgaard‐Oeschger 16/17; thus, abrupt N2O increases were not all identical during the last glacial period [ABSTRACT FROM AUTHOR]

Paleoclimate reconstruction relies on estimates of spatiotemporal relationships among climate quantities to interpolate between proxy data. This work quantifies how structural uncertainties in those relationships translate to uncertainties in reconstructions of past climate. We develop and apply a data assimilation uncertainty quantification approach to paleoclimate networks and observational uncertainties representative of data for the last millennium. We find that structural uncertainties arising from uncertain spatial covariance relationships typically contribute 10% of the total uncertainty in reconstructed temperature variability at small (∼200 km), continental, and hemispheric length scales, with larger errors (50% or larger) in regions where long‐range climate covariances are least certain. These structural uncertainties contribute far more to errors in uncertainty quantification, sometimes by a factor of 5 or higher. Accounting for and reducing uncertainties in climate model dynamics and resulting covariance relationships will improve paleoclimate reconstruction accuracy. Plain Language Summary: Reconstructing past climate conditions is important to characterize climate variability and extremes, with implications for understanding future variability and change. An important source of information is data from proxy sources like tree rings, ice cores, ocean sediment cores, and other sources, but those data are not available everywhere on the globe. Extracting climate information from these data requires information about how climate properties covary in space and time, and that information is inevitably flawed because we lack a complete picture of how the climate behaves. This work seeks to address how much those flaws matter, how they propagate to reconstructions of past climate, and how we can reduce them. We use different physically plausible representations of climate physics from three different coupled climate models in a series of reconstruction experiments and suggest that uncertain physics typically accounts for 10% of reconstruction errors in the particular case of reconstructing surface air temperature over the Last Millennium, with regionally higher values (50% or larger). We can reduce these errors by improving climate models and by improving how we represent uncertain physics in climate reconstructions. Key Points: Perfect model experiments show how uncertain climate covariance relationships lead to uncertainties in paleoclimate reconstructionsThese uncertainties typically account for 10% of total error in reconstructed Last Millennium air temperature, with large regional variabilityImproving climate model physics and accounting for its uncertainty will improve reconstruction accuracy [ABSTRACT FROM AUTHOR]

Late Pleistocene and Holocene ground ice are common throughout the Arctic. Some forms of relict ground ice preserve local meteoric water, and their stable oxygen‐ and hydrogen‐isotope ratios can be used to reconstruct past air temperatures. In this paper, we review the formation and sampling of two forms of relict ground ice—wedge ice and pore ice—and recent (2010–2019) advances in paleoclimatological studies of ground‐ice stable isotope records in the Arctic. Recent advances are attributed to better chronological constraints and refined understandings of the systematics and seasonality of relict wedge ice and pore ice. A rich network of ice‐wedge records has emerged, primarily from the Siberian Arctic, whereas pore‐ice records are less common. The ice‐wedge network depicts a robust pattern of late Pleistocene cooling, and remarkably similar temperature depressions during Marine Isotope Stages 3 and 2. Very high‐resolution wedge‐ and pore‐ice stable isotope chronologies have been established recently and used to reconstruct winter and summer climate histories and to assess seasonal dependencies in insolation‐forced climate. Reports of ancient (>125 ka BP) ground ice demonstrate its long‐term persistence, and its potential to expand our knowledge of Quaternary climate dynamics in the terrestrial Arctic. [ABSTRACT FROM AUTHOR]

Stable oxygen isotopes (δ18O) in the Bona‐Churchill (B‐C) ice core from southeast Alaska provide a valuable, high‐resolution history of climate variability and sea ice cover in the western Arctic over the last 800 years. Multiple ice cores have been collected from the Wrangell‐St. Elias Mountain Range; however, their δ18O records exhibit little consistency as each core offers a unique view on local, regional, and/or global climate variability. To explore the primary mechanisms influencing the isotopic signature at the B‐C site, we utilize isotope‐enabled model data, reanalysis data, and observations, which all indicate a strong connection between isotopes at the B‐C site and western Arctic climate, likely established by the location of the storm track in this region. Enriched B‐C δ18O reflects increased southerly flow and warmer waters in the Bering Sea, which modulates the heat flux through the Bering Strait and into the Arctic, thereby affecting sea ice cover in the western Arctic. The B‐C δ18O paleorecord shares some remarkable similarities (r = −0.80, p < .001) with the duration of western arctic sea ice cover reconstructed from a Chukchi Sea sediment core. Interestingly, during the Little Ice Age, enriched δ18O and reduced western Arctic sea ice are observed and may be indicative of prolonged periods of the warm Arctic/cold continents pattern and a northwestward shift of the North Pacific storm track. Plain Language Summary: The Bona‐Churchill (B‐C) ice core from southeast Alaska provides a valuable, high‐resolution history of climate variability and sea ice cover in the western Arctic over the last 800 years. In the polar regions, ice core–derived stable oxygen isotopes typically serve as a temperature proxy. However, isotopic records from ice cores collected from the mountainous regions of northwestern North America exhibit little relation to local temperatures, and few similarities exist among the ice core records. Modeled and observed data in this study suggest a link between isotopic variability in the Bona‐Churchill (B‐C) ice core and climate variability in the Bering Sea, possibly through the position of the North Pacific storm track. The high‐resolution B‐C isotope record provides a rare millennial history of climate variability and sea ice cover in the western Arctic. A prominent feature in the B‐C isotopic record includes enrichment during the Little Ice Age, suggesting warmer conditions in the western Arctic, which likely resulted from a prolonged presence of the warm Arctic/cold continents pattern and an accompanying shift in storm trajectories. Key Points: Stable isotopes from the Bona‐Churchill ice core provide a millennial scale history of western Arctic climate variability and sea ice cover.Storm trajectories influence the isotopic composition and establish the connection between the Bona‐Churchill site and the Bering Sea.Isotopic enrichment during the Little Ice Age suggests a warm Arctic/cold continents pattern and a shift in the prevailing storm tracks. [ABSTRACT FROM AUTHOR]

Peat-core records have a long history of being used for paleoclimate reconstructions in northern high-latitude regions but are less common in the southern tropical zone. In this study, we present a synthesis of published peat-core reconstructions and basal ages from high-elevation (>3000 m a.s.l.) sites in the northern and central Andes of South America. We complement this database by providing a new multi-proxy peat-core-based paleoecological reconstruction from the Alta Murmurani peatland, Cordillera Vilcanota, Peru. This record, with a peat inception age of 8980 cal. yr BP, was analyzed for plant macrofossils and stable isotopes (carbon, oxygen). This review (1) assesses the timing of peat initiation and development across the tropical high-elevation region of South America (8°N to 27°S); (2) assesses the reliability of peat-core paleoclimate reconstructions by comparing the peat-core inferences across sites and against other independent archives (e.g., lake sediments, ice cores, speleothems); and (3) determines which hydroclimatic conditions are favorable for peat formation and expansion across the study area. Our results show that peat initiation occurred throughout the Holocene, progressing from the northern Andes to central Andes, with highest initiation frequencies clustered at ca. 10,700 cal. yr BP and 8300 cal. yr BP, respectively. Our synthesis reveals that peat cores can be used to assess regional paleoclimate trends in the central Andes using a multi-proxy approach combining micro- and macrofossils such as pollen, stable isotopes, as well as organic and inorganic elemental analyses. We also identify three optimal peat-forming time periods, all with optimally wet conditions occurring at transitional hydroclimate phases: (1) following deglaciation, with warming temperatures and increased moisture related to glacial runoff (ca. 11,000 cal. yr BP and ∼8000 cal. yr BP), (2) under moderate moisture regimes related to intensifying ENSO and warmer conditions (∼3500 cal. yr BP), and (3) under conditions related to increased precipitation at the beginning of a cold period (Little Ice Age). We show that either regional climate (e.g., changes in precipitation) or local conditions (e.g., changes in glacial runoff) can promote peat formation across the study region. • Peat initiation occurred in the Andes throughout the Holocene with peaks at 10,700 cal. yr BP and 8,300 cal. yr BP. • Peat-based paleoclimate reconstructions are consistent across the central Andes since the mid Holocene. • Holocene peat-forming time periods were characterized by optimal moisture conditions and land availability. [ABSTRACT FROM AUTHOR]

• We propose a method to estimate joint periodicities in multiple non- uniformly sampled data sets with uncertain sampling times. • The method allows for the estimation of common periodicity in long- term historic climate data, such as solar activity measurements from ice-core data. • We examine bounds on the expected performance accuracy using the corresponding Cramér-Rao lower bound (CRB) as well as the misspecified CRB, taking the sampling uncertainties into account. • We examine how realistic ice-core data may be simulated and illustrate the method's performance using both such data and measured ice-core data. In this work, we introduce a novel approach for determining a joint sparse spectrum from several non-uniformly sampled data sets, where each data set is assumed to have its own, possibly disjoint, and only partially known, sampling times. The potential of the proposed approach is illustrated using a spectral estimation problem in paleoclimatology. In this problem, each data point derives from a separate ice core measurement, resulting in that even though all measurements reflect the same periodicities, the sampling times and phases differ among the data sets. In addition, sampling times are only approximately known. The resulting joint estimate exploiting all available data is formulated using a sparse reconstruction framework allowing for a reliable and robust estimate of the underlying periodicities. The corresponding misspecified Cramér-Rao lower bound, accounting for the expected sampling uncertainties, is derived and the proposed method is shown to attain the resulting bound when the signal to noise ratio is sufficiently high. The performance of the proposed method is illustrated as compared to other commonly used approaches using both simulated and measured ice core data sets. [ABSTRACT FROM AUTHOR]

Investigations of chronology play a key role in the majority of archaeological research endeavors and are particularly pertinent to examinations of culture-environment relationships, especially during periods characterized by rapid and marked climatic variability and environmental reorganization. Rigorous evaluations of available data and robust methods are required if one wishes to reconstruct reliable chronologies, and this is especially the case when examining periods that are associated with a relatively few radiometric measurements. Such is the case for the Upper Paleolithic archaeological record documented in present-day France from 32,000 to 21,000 calibrated years BP. We take into account critically examined radiocarbon measurements from contextually secure archaeological contexts and employ a recently-developed method of Hierarchical Bayesian Modeling to reconstruct the chronology of archaeological cultures from the Middle Gravettian to the Badegoulian. The calculated chronological intervals for each typo-technologically defined culture phase are compared to the Greenland ice core climatic record and a terrestrial paleoenvironmental record from Bergsee Lake (Southern Germany)—itself expressed in calendar years calculated with the same calibration curve employed in our age model—thereby permitting each archaeological culture to be correlated accurately with documented paleoclimatic variability. • Robust chronologies are needed to correlate past cultures with climate variability. • A recently-developed method of Hierarchical Bayesian Modeling is employed. • Temporal intervals for archaeological cultures are determined with this method. • Arguments are presented as to why this new Bayesian method is preferable. [ABSTRACT FROM AUTHOR]

Ravine slopes at the recently discovered Nesseltalgraben site in southeastern Germany provide a unique last glacial sediment record for the Northern Calcareous Alps. The 21 m-long profile is dominated by fine-grained lacustrine-palustrine sediments overlain by several metres of glacifluvial gravels and lodgement tills of the Last Glacial Maximum and underlain by a diamicton. The age model includes 29 radiocarbon analyses and one paleomagnetic anomaly (Laschamp event) together providing a modelled age range from c. 59 to 29.6 ka cal BP, i.e. Marine Isotope Stage (MIS) 3. Apart from a description of the lithofacies, X-ray-fluorescence (XRF) scanning and elemental analyses provide high-resolution records of the geochemical composition. Multivariate analyses of XRF data separate Ca from other major elements. Carbonate contents, represented by Ca and total inorganic carbon, reach maxima in repeatedly occurring calcareous silty to sandy layers and are related to glacigenic origin. These indicate repeated inner-alpine local glaciations during stadials of MIS 3. Scanning electron microscopy and XRF data confirm the detrital origin of these layers. In contrast, organic matter and elements more resistant to chemical weathering (Si, Ti, Zr) accumulated during interstadials and concurrently elevated Rb/Sr ratios indicate intensified weathering. The high-frequency proxy variations determined for Nesseltalgraben reflect interstadial-stadial climate variability comparable with oxygen-isotope records from Greenland ice cores and Alpine speleothems. Thus, Nesseltalgraben is among the very few independently dated sediment records from continental Europe covering the entire MIS 3 and reflecting the full Dansgaard-Oeschger climate variability. • A 21 m sediment profile from the northern Alps provides a complete MIS 3 record. • Repeated inner Alpine glaciations are indicated by calcareous silt layers. • Geochemical variations reflect interstadials and stadials. • The Nesseltalgraben record resembles Dansgaard-Oeschger climate variability. [ABSTRACT FROM AUTHOR]

Millennial-scale Dansgaard Oeschger (DO) variability at northern high latitudes has influenced climatic and environmental conditions in the Mediterranean during the last glacial period. There is evidence that the hemispheric transmission of the DO variability occurred at the end of DO event 25; however, the exact timing and the trigger that activated the environmental response in the Mediterranean remains incompletely understood. Here, we provide evidence that the clear millennial-scale teleconnection between Greenland and the Mediterranean started at ∼111.4 ka BP and was initiated by a sub-millennial scale cooling in Greenland (GI-25b). High-resolution sediment proxies and the pollen record of Lago Grande di Monticchio (MON), Italy, reflect climatic instability during the last millennium of the last interglacial, which was characterised by a first and short cooling episode (MON 1) at 111.44 ± 0.69 ka BP, coinciding with the Greenland cold sub-event GI-25b in duration and timing (within dating uncertainties). MON and Greenland (NorthGRIP ice core) also agree in recording a subsequent warm rebound phase that abruptly culminated in the stadial MON 2/GS-25, marking the transition into the last glacial period. Our results show that the GI-25b triggered an early environmental response at MON to centennial-scale climate change in Greenland as a prelude to the millennial-scale teleconnection that was maintained during the glacial period. • Annual-resolution study of the transition from the Last Interglacial period to the last Glacial period. • Evaluation of the differences in proxy response to sub-millennial-scale climate variability. • Chronological discussion based on comparison of independent chronologies. [ABSTRACT FROM AUTHOR]

The potential of shrub-ring oxygen isotopes (SRO) as a paleoclimatic proxy is not well demonstrated. In this study we explored the climate signals embedded in an SRO chronology from Saga, on the northern slope of the central Himalayas by correlation analysis with climate variables and comparison with tree-ring and/or ice-core δ18O records. The Saga SRO had significant and positive correlations with May–July temperatures in both local (r = 0.43) and moisture source areas (r = 0.49) during the period 1955–2015 (CE), while no significant correlations were found with hydroclimatic variables. It is also in good agreement with two tree-ring δ18O series from the southern slope of the central Himalayas, the source of moisture to the site. The temperature signals are further verified by an ice core δ18O record from the Tanggula Mountains on the central Tibetan Plateau. However, it shows little similarity with the nearest Himalayan ice core δ18O record from Dasuopu, demonstrating temperature signals of different seasons. This study suggests that SRO could be used as an acceptable climate proxy, like tree-ring and ice-core δ18O, to broaden the spatial extent of the paleoclimatic records network. However, we should be cautious when combining different proxies with climate signals of different seasons in paleoclimate studies. • A high-elevation shrub-ring δ18O (SRO) chronology was developed. • The SRO captures summer temperature of both local and moisture source areas. • The SRO is comparable with δ18O of tree rings and ice cores. • SRO is a promising high-resolution proxy for paleoclimatology. [ABSTRACT FROM AUTHOR]

Abstract The majority of atmospheric compounds measured in ice cores are inorganic, while analysis of their organic counterparts is a less well developed field. In recent years, understanding of formation, transport pathways and preservation of these compounds in ice and snow has improved, showing great potential for their use as biomarkers in ice cores. This study presents an optimised analytical technique for quantification of terrestrial and marine biosphere emissions of secondary organic aerosol (SOA) components and fatty acids in ice using HPLC-MS analysis. Concentrations of organic compounds in snow and ice are extremely low (typically ppb or ppt levels) and thus pre-concentration is required prior to analysis. Stir bar sorptive extraction (SBSE) showed potential for fatty acid compounds, but failed to recover SOA compounds. Solid phase extraction (SPE) recovered compounds across both organic groups but methods improving some recoveries came at the expense of others, and background contamination of fatty acids was high. Rotary evaporation was by far the best performing method across both SOA and fatty acid compounds, with average recoveries of 80%. The optimised preconcentration – HPLC-MS method achieved repeatability of 9% averaged for all compounds. In environmental samples, both concentrations and seasonal trends were observed to be reproducible when analysed in two different laboratories using the same method. Graphical abstract fx1 Highlights • New and recently developed terrestrial and marine biomarkers are present in polar and alpine ice cores. • Optimised HPLC-MS analysis technique including preconcentration gives detection and quantification of novel compounds. • Concentrations of compounds in ice samples are reproducible between different laboratories using this same method. [ABSTRACT FROM AUTHOR]

Abstract According to the findings of a complex radiocarbon dated sedimentological, geochemical, anthracological, phytolite, malacological study implemented on an open 1036 cm wind-blown sand, loess, sandy loess and fossil soils profile of western part of brickyard at Katymár village in Hungary. Increased boreal mixed-leaved forest steppe and temperate grassland cover characterized the interstadials in the SE Great Hungarian Plain during the terminal phase of MIS3 stage and MIS2 stage. These interstadials correlate well with the Dansgaard-Oeschger (DO) interstadials 8, 7, 6, 5, 4, 3, 2 and the post LGM warm interval (DO1) seen in the Greenland ice core oxygen isotope records. Intervening cold phases, on the other hand, their start correlating with Heinrich event 1, 2, 3, 4 and the LGM. These data overall confirm that millennial scale climate variability during MIS3 and MIS2 stages had profound effect on the terrestrial ecosystems in the continental interior of SE Europe leading to periodic mixed-leaved boreal forest steppe within mosaic structure expansions in the interstadias and cold steppe, taiga and tundra like spots mosaic and mixed vegetation expansion in the stadial phases. 1400–1500 year long drier interstadial and humid, extended vegetation covered stadial periods interchanged at Katymár sequence, therefore the vegetation and fauna development was different from the Atlantic and Central European schemes and inferred an independent local and regional South Carpathian Basin trend. [ABSTRACT FROM AUTHOR]

Abstract Oxygen isotope ratios (δ 18 O) are the most commonly used parameters recorded in paleoclimate archives since they link different natural archives via the water cycle. Tree-ring δ 18 O (δ 18 O TR) has been widely used for hydroclimate reconstructions in the Himalaya. However, few of them record temperature signals, which are dominant in Himalaya ice-core δ 18 O. We hypothesize that the "precipitation amount effect" due to the South Asian Summer Monsoon (SASM) may overprint temperature signals in δ 18 O TR series. The purpose of this study is to investigate whether temperature signals could be found in the δ 18 O TR in locations where the influence of SASM is weak. We developed a 105-yr δ 18 O TR chronology from the northern slope of the western Himalaya which greatly blocks the SASM. Our δ 18 O TR clearly shows stronger correlations with temperature (dominant winter and weak summer) than summer precipitation signals. It also agrees well with summer soil moisture δ 18 O simulated by the global isotope model LMDZ4 (r = 0.72, 1979–2010). In LMDZ4, low winter temperature was found to increase winter snowfall and subsequent snow melt, and thus to increase the contribution of winter snowfall to soil moisture in summer at the expense of summer precipitation. Since winter snowfall is more depleted than summer precipitation, this leads to lower summer soil moisture δ 18 O. The temperature signals found in our δ 18 O TR series are consistent with those found in the Dasuopu ice-core δ 18 O. This implies that δ 18 O TR series from the southwest Tibetan Plateau (TP), with a weak monsoon, hold great potential to capture temperature signals. Climate interpretations of δ 18 O proxies in the Himalaya largely depend on the influence of seasonal water from the dominant atmosphere circulation systems of the westerlies or monsoon. The δ 18 O proxies from the monsoon-affected region have a higher potential for the reconstruction of boreal summer hydroclimate, whereas δ 18 O proxies from westerly-affected sites have a higher potential for temperature reconstructions. Highlights • A tree-ring δ 18 O record was developed from the northern slope of the Himalayas. • The δ 18 O TR recorded dominant winter and weak summer temperature signals. • No summer monsoon precipitation signals were found. • Winter temperature signals were mainly transferred into δ 18 O TR through snowmelt. [ABSTRACT FROM AUTHOR]

Abstract Frost rings can provide direct evidence of anomalously cold summertime conditions on hemispheric scales. Here we report frost rings in subfossil pinewood from Finnish Lapland dated dendrochronologically to AD 536 and 1627 BC. These exact calendar dates have been vividly discussed in the literature in the respective contexts of a cold climate anomaly/dust veil/crop failure/famine/Justinianic Plague and the Minoan eruption of Thera (Santorini). Coinciding anomalies in growth and carbon isotope ratios (δ13C) indicate cold and overcast conditions in these years in Finnish Lapland, typical to conditions induced by volcanic dust veils. This evidence reinforces the view of multiple climatic drivers strongly effecting contemporary societies around AD 536 and provides important new evidence regarding the 1627 BC event. We report a frost ring for this year, and also consider evidence from Lapland for other Thera candidate eruptions in the 16th century BC. The marker years 1560 BC and 1546 BC in bristlecone pine tree rings in North America are positioned within negative departures in our δ13C data. Most likely, the frosts occurred somewhere in July and were driven by dust veils from Northern Hemisphere. Although not indicative of volcanic provenance, frost rings and other dendroclimatic proxies dated to exact calendar years offer important time markers for chronological frameworks which can synchronize archaeological, historical and environmental data to further our understanding of environmental changes at scales that matter to human societies. Highlights • We demonstrate frost-damaged rings in AD 536 and 1627 BC in subfossil pinewood. • We produce a tree-ring δ13C chronology over 17th and 16th centuries BC. • We use the damaged rings and the growth and isotope proxies to infer past climates. • The AD 536 and 1627 BC dates may be linked with historical explosive eruption. • Comparisons with historical accounts show overlapping evidence of human calamities. [ABSTRACT FROM AUTHOR]

Abstract Newly acquired U Th dates on gypsum sands contained within Pleistocene lake sediments allow us to extend the record of climatic change in the Estancia Basin of central New Mexico back to ∼69 ka from a previous limit of ∼29 ka. Using a hydrologic balance index (HBI) that reflects variations in sediment mineralogy and bedding characteristics, we find strong similarities to speleothem oxygen isotopic records for the southwestern United States as well as to isotopic variations in Greenland ice cores. Intervals of wetter climate in the Estancia Basin generally appear to correlate to colder conditions in the North Atlantic and conversely. The existence of a pronounced dry period from 18.1 to 17 ka matches in timing the Extrapolar Climate Reversal of Asmerom et al. (2017) at Fort Stanton cave in southern New Mexico and confirms a different origin for this stadial-like event. Furthermore, the new U Th dates support the existing Estancia Basin interpretation of a dry Bølling/Allerød followed by a wet Younger Dryas, a result consistent with speleothem and wet meadow evidence from New Mexico and Arizona (Asmerom et al., 2010, 2017; Polyak et al., 2004; Hall et al., 2012; Wagner et al., 2010) but in conflict with an interpretation of speleothem evidence from central Texas (Feng et al., 2014). Highlights • U Th dates extend the record of climatic change in the Estancia Basin of New Mexico to ∼69 ka. • The dates were acquired from gypsum sands produced during lowstands of pluvial Lake Estancia. • A new U Th date provides support for a wet Younger Dryas interval in central New Mexico. • A dry period from 18.1 to 17 ka matches in timing the Extrapolar Climate Reversal at Stanton Cave. • An Estancia Basin hydrologic balance index shows similarities to the Greenland ice sheet record. [ABSTRACT FROM AUTHOR]

Abstract Paleoclimatic information recorded in polar and mountain glaciers is fundamental to study climate and environmental changes. New advanced techniques can be developed to improve the accuracy of ice core measurements, and to store the retrieved information. In this context, we developed a novel system based on a hyperspectral imaging sensor to analyse optical properties of ice cores in a non-destructive way. The system is composed of a high-precision linear stage which embeds a hyperspectral imaging sensor and a halogen light source. It was designed to collect high resolution hyperspectral images of ice cores in the 380–1000 nm spectral range with a dedicated software. The system has been tested on an ice core extracted from the Adamello glacier (Eastern Alps, Italy). Different narrow-band and broad-band spectral descriptors were computed in order to analyse the optical properties of ice and impurities enclosed in ice layers. This system can be coupled with traditional techniques applied to ice cores, while predictive algorithms can be developed to estimate key variables in ice cores at a very high spatial and spectral resolution. We believe that hyperspectral imaging offers great potential for improving the analysis of ice cores in the next years and for guaranteeing the preservation of ice core records over time. Highlights • A new hyperspectral imaging system, named HyIce, was developed to scan ice cores. • HyIce allows to collect high-resolution reflectance images of ice cores. • Different spectral descriptors were calculated from reflectance spectra. • The absorption at 800 nm in reflectance was used to estimate ice optical diameter. • Variability in spectral descriptors reflects the presence of impurities in ice. [ABSTRACT FROM AUTHOR]

In Antarctica, ice-core temperature has traditionally been regarded as a leading variable to carbon dioxide, CO 2 during the last 400,000 years before present (B.P.). This finding is in contrast to most reports on global mean surface temperature and atmospheric CO 2 for the last 150 years. However, previous techniques for establishing leading or lagging (LL) relations between paired global warming variables have required that the time series show constant frequency (stationarity). Herein, we show that on orbital and multi-millennial time scales, the Vostok Antarctic ice core displays 9 periods of 8.7 kyr ± 5 kyr during which CO 2 becomes a leading variable to temperature. Six of the 9 periods were associated with short-term pauses occurring during 4 major glaciation-deglaciation periods. We find that CO 2 also leads temperature during short pauses in the major cyclic pattern of the Greenland time series. In the latter series, there are also two contrasting cycle developments. In the first contrasting cycle developments, lasting from 103.5 to 79 ka, there is an in-phase relation between CO 2 and temperature, with a slope of 0.75. In the second contrasting cycle developments, lasting from 61.5 to 43.5 ka, there is an out-of-phase relation with a slope of −0.67. In addition, the latter shows a see-saw pattern between Arctic and Antarctic temperatures. [ABSTRACT FROM AUTHOR]

From 10Be surface-exposure dating we make the case that classic late-glacial moraines in southwestern Norway, including the iconic Esmark moraine, and in the Southern Hemisphere middle latitudes, including the iconic Birch Hill moraine of the Southern Alps of New Zealand, were constructed coevally within dating uncertainties. This finding of simultaneous glacier resurgence in the two polar hemispheres carries the implication that the millennial-scale climate reversal of the last deglaciation was global and required a change in the energy budget of the earth's climate system. • Construction of Esmark and Lysefjord moraines took place by 12,700 yrs ago. • Glacial recession to Esmark position during Heinrich Stadial 1. • Recession from the Esmark moraine during Younger Dryas time. • Coeval glacier advance in Scandinavia and in Southern Hemisphere during the ACR. [ABSTRACT FROM AUTHOR]

Using high-resolution ice core and tree ring proxies for sea ice extent (SIE), we reconstructed a robust time series of autumn SIE over the Barents-Kara (B-K) sector of the Arctic from AD1289-1993. After intercomparing the results and statistical parameters using the ordinary least squares regression (OLSR), the principle component regression (PCR) and the partial least squares regression (PLSR) methods, SIE time series were synthesized into a more robust series using the weighted average method, which used the explained variances as weights. The results showed that from the end of the 13th century to the end of 18th century, the autumn B-K SIE was large, with large variations and a slightly expanding trend overall. This reflected significant multidecadal oscillations under the Little Ice Age (LIA) background. The B-K SIE began to decrease at the end of the 18th century, and a shrinking trend became significant during the second half of the 19th century, which lasted into the 1930s-1940s. The 1930s-1940s was a period with a relatively low SIE in the B-K Sea, and the SIE had a short period of expansion from the 1940s-1970s. However, the B-K SIE has continuously and significantly shrank since the 1970s. The reduction in the B-K SIE since the end of the 18th century has been unprecedented in both duration and speed over the last 700 years. The B-K SIE has retreated significantly since the 1970s, with a speed 6.18 times greater than the former mean retreating speed. The industrial revolution may be a dominant factor in this result. The Arctic SIE in recent years may be the lowest it has been over the last millennium. [ABSTRACT FROM AUTHOR]

A key objective in palaeo-climatology is the retrieval of a continuous Antarctic ice-core record dating back 1.5 Ma. The identification of a suitable Antarctic site requires sufficient knowledge of the subglacial landscape beneath the Antarctic Ice Sheet. Here, we present new ice thickness information from the Dome Fuji region, East Antarctica, based on airborne radar surveys conducted during the 2014/15 and 2016/17 southern summers. Compared to previous maps of the region, the new dataset shows a more complex landscape with networks of valleys and mountain plateaus. We use the new dataset as input in a thermokinematic model that incorporates uncertainties in geothermal heat flux values in order to improve the predictions of potential ice-core sites. Our results show that especially the region immediately south of Dome Fuji station persists as a good candidate site for obtaining an old ice core. An initial assessment of basal conditions revealed the existence of what appears to be subglacial lakes. Further radar data analysis shows overall high continuity of layer stratigraphy in the region. This indicates that extending the age-depth information from the Dome Fuji ice core to a new ice-core drill site is a viable option. [ABSTRACT FROM AUTHOR]

Records of recent climate from ice cores drilled in 2015 on the Guliya ice cap in the western Kunlun Mountains of the Tibetan Plateau, which with the Himalaya comprises the Third Pole (TP), demonstrate that this region has become warmer and moister since at least the middle of the 19th century. Decadal-scale linkages are suggested between ice core temperature and snowfall proxies, North Atlantic oceanic and atmospheric processes, Arctic temperatures, and Indian summer monsoon intensity. Correlations between annual-scale oxygen isotopic ratios (δ 18 O) and tropical western Pacific and Indian Ocean sea surface temperatures are also demonstrated. Comparisons of climate records during the last millennium from ice cores acquired throughout the TP illustrate centennial-scale differences between monsoon and westerlies dominated regions. Among these records, Guliya shows the highest rate of warming since the end of the Little Ice Age, but δ 18 O data over the last millennium from TP ice cores support findings that elevation-dependent warming is most pronounced in the Himalaya. This, along with the decreasing precipitation rates in the Himalaya region, is having detrimental effects on the cryosphere. Although satellite monitoring of glaciers on the TP indicates changes in surface area, only a few have been directly monitored for mass balance and ablation from the surface. This type of ground-based study is essential to obtain a better understanding of the rate of ice shrinkage on the TP. [ABSTRACT FROM AUTHOR]

Polar precipitation archived in ice caps contains information on past temperature conditions. Such information can be retrieved by measuring the water isotopic signals of δ 18 O and δ D in ice cores. These signals have been attenuated during densification due to molecular diffusion in the firn column, where the magnitude of the diffusion is isotopologue specific and temperature dependent. By utilizing the differential diffusion signal, dual isotope measurements of δ 18 O and δ D enable multiple temperature reconstruction techniques. This study assesses how well six different methods can be used to reconstruct past surface temperatures from the diffusion-based temperature proxies. Two of the methods are based on the single diffusion lengths of δ 18 O and δ D , three of the methods employ the differential diffusion signal, while the last uses the ratio between the single diffusion lengths. All techniques are tested on synthetic data in order to evaluate their accuracy and precision. We perform a benchmark test to thirteen high resolution Holocene data sets from Greenland and Antarctica, which represent a broad range of mean annual surface temperatures and accumulation rates. Based on the benchmark test, we comment on the accuracy and precision of the methods. Both the benchmark test and the synthetic data test demonstrate that the most precise reconstructions are obtained when using the single isotope diffusion lengths, with precisions of approximately 1.0 ° C . In the benchmark test, the single isotope diffusion lengths are also found to reconstruct consistent temperatures with a root-mean-square-deviation of 0.7 ° C . The techniques employing the differential diffusion signals are more uncertain, where the most precise method has a precision of 1.9 ° C . The diffusion length ratio method is the least precise with a precision of 13.7 ° C . The absolute temperature estimates from this method are also shown to be highly sensitive to the choice of fractionation factor parameterization. [ABSTRACT FROM AUTHOR]

Streamflow reconstructions based on paleoclimate proxies provide much longer records than the short instrumental period records on which water resource management plans are currently based. In Australia there is a lack of in-situ high resolution paleoclimate proxy records, but remote proxies with teleconnections to Australian climate have utility in producing streamflow reconstructions. Here we investigate, via a case study for a catchment in eastern Australia, the novel use of an Antarctic ice-core based rainfall reconstruction within a Budyko-framework to reconstruct ∼1000 years of annual streamflow. The resulting streamflow reconstruction captures interannual to decadal variability in the instrumental streamflow, validating both the use of the ice core rainfall proxy record and the Budyko-framework method. In the preinstrumental era the streamflow reconstruction shows longer wet and dry epochs and periods of streamflow variability that are higher than observed in the instrumental era. Importantly, for both the instrumental record and preinstrumental reconstructions, the wet (dry) epochs in the rainfall record are shorter (longer) in the streamflow record and this non-linearity must be considered when inferring hydroclimatic risk or historical water availability directly from rainfall proxy records alone. These insights provide a better understanding of present infrastructure vulnerability in the context of past climate variability for eastern Australia. The streamflow reconstruction presented here also provides a better understanding of the range of hydroclimatic variability possible, and therefore represents a more realistic baseline on which to quantify the potential impacts of anthropogenic climate change on water security. [ABSTRACT FROM AUTHOR]

We report a previously-unexplored natural temperature cycle recorded in ice cores from Antarctica--the Antarctic Centennial Oscillation (ACO)--that has oscillated for at least the last 226 millennia. Here we document the properties of the ACO and provide an initial assessment of its role in global climate. We analyzed open-source databases of stable isotopes of oxygen and hydrogen as proxies for paleo-temperatures. We find that centennial-scale spectral peaks from temperature-proxy records at Vostok over the last 10,000 years occur at the same frequencies (±2.4%) in three other paleoclimate records from drill sites distributed widely across the East Antarctic Plateau (EAP), and >98% of individual ACOs evaluated at Vostok match 1:1 with homologous cycles at the other three EAP drill sites and conversely. Identified ACOs summate with millennial periodicity to form the Antarctic Isotope Maxima (AIMs) known to precede Dansgaard-Oeschger (D-O) oscillations recorded in Greenland ice cores. Homologous ACOs recorded at the four EAP drill sites during the last glacial maximum appeared first at lower elevations nearest the ocean and centuries later on the high EAP, with latencies that exceed dating uncertainty >30-fold. ACO homologs at different drill sites became synchronous, however, during the warmer Holocene. Comparative spectral analysis suggests that the millennial-scale AIM cycle declined in period from 1500 to 800 years over the last 70 millennia. Similarly, over the last 226 millennia ACO repetition period (mean 352 years) declined by half while amplitude (mean 0.67 °C) approximately doubled. The period and amplitude of ACOs oscillate in phase with glacial cycles and related surface insolation associated with planetary orbital forces. We conclude that the ACO: encompasses at least the EAP; is the proximate source of D-O oscillations in the Northern Hemisphere; therefore affects global temperature; propagates with increased velocity as temperature increases; doubled in intensity over geologic time; is modulated by global temperature variations associated with planetary orbital cycles; and is the probable paleoclimate precursor of the contemporary Antarctic Oscillation (AAO). Properties of the ACO/AAO are capable of explaining the current global warming signal. [ABSTRACT FROM AUTHOR]

Marine isotope stage 3 (MIS 3) was characterised by marked oscillations of extreme cold episodes with very short warm events during the stadial, and several regional differences have been recorded in the ice cores and marine deposits. The aim of this study is to reconstruct this period by evaluating both terrestrial and regional responses. Cova del Coll Verdaguer, a site located on the Iberian Peninsula, preserves a sedimentary deposit dated to between 34 and 56 ka BP and provides an opportunity for evaluating the impact of climate changes on the regional landmass during a period that coincided with the last Neanderthal population on the Iberian Peninsula. Several dating methods, including U-series, electron spin resonance, amino acid racemization and radiocarbon (14C), were applied to the site and the ages obtained show good agreement. The biotic evidence obtained is substantial, comprising floristic data from palynology and charcoal analysis, and faunal data from large and small mammals, birds and gastropods. Environmental reconstruction points to an initially open meadow landscape at the base of the sequence (∼56 ka) that progressively changes to a woodland environment dominated by conifers (∼34 ka). The presence of few thermophilous taxa, in contrast with lower latitudes of the Iberian Peninsula, is also detected. The environmental conditions of mid-altitude, Mediterranean, limestone mountains for the last Neanderthal populations appear to have been dominated by a forested landscape comprising boreal or mixed coniferous forest, characterised by a low usable biomass with poor comestible plant resources and dispersed herbivore populations. [ABSTRACT FROM AUTHOR]

Ice core samples from the polar ice shelves contain valuable paleo-climatic records and information for understanding the unique polar under-ice ecosystem. This paper describes the finite element analysis-aided design, development, and qualification of a 63-mm-inner diameter, 250-mm-long variable power underwater remotely operated vehicle-mountable inductive ice corer (IIC) for collecting ice core samples beneath the polar ice shelves. It is determined that, with the IIC operating with an input power of 1,000 W at 30 kHz, it is possible to have an ice penetration rate of 14 mm/min and obtain an ice core of 51 mm in diameter. The experimental results are found to comply with the numerical model with an accuracy of 95%. [ABSTRACT FROM AUTHOR]

The collapse of the Eastern Türk Empire (ETE, ca. 584-630 CE) in 630 CE marked the rise of Tang China as the paramount power on the Silk Road. It was followed by the Tang defeat of the Western Türk Empire in 659 and opened a phase of Chinese expansion into central Asia. Climate-induced environmental changes as well as economic and political consequences are mentioned in medieval Chinese records as major factors in the ETE collapse. The role of cooler temperatures has also been discussed in current scholarship. Here, we re-evaluate this question by assessing the available historical sources in the light of a global network of 16 tree-ring chronologies for this period, which reveal distinct summer cooling in the ETE heartland between 626 and 632 CE. Reconstructed peak cooling of up to − 3.4 °C in 627 and 628 CE (relative to the 1961-90 mean climatology) coincided with heavy snowfall and severe frost events in the territory of the ETE. A strong sulfate spike in Greenland ice cores that has been dated circa 626 CE is implicated in the abrupt surface cooling. We argue that the climatic perturbation and associated reduction in vegetation growth and livestock mortality are relevant in understanding the causes of the fall of the ETE but these indirect drivers must be evaluated within a comprehensive analysis of political relations within both the Türk and the Tang leadership. Our study underscores and contextualizes the vulnerability of past nomadic societies to small and episodic climate fluctuations, particularly when coupled with concurrent socioeconomic, political, and demographic changes. [ABSTRACT FROM AUTHOR]