Climate Reconstructions Research Articles

Recent summer warming over the western Mediterranean region is unprecedented since medieval times

Contextualising anthropogenic warming and investigating linkages between past climate variability and human history require high-resolution temperature reconstructions that extend before the period of instrumental measurements. Here, we present maximum latewood density (MXD) measurements of 534 living and relict Pinus uncinata trees from undisturbed upper treeline ecotones in the Spanish central Pyrenees. Spanning the period 1119–2020 CE continuously, our new MXD composite chronology correlates significantly with gridded May–September mean temperatures over the western Mediterranean region (r = 0.76; p ≤ 0.001; 1950–2020 CE). Based on an integrative ensemble approach, our reconstruction reveals unprecedented summer warming since 2003 CE. The coldest and warmest reconstructed temperature anomalies are −3.4 (±1.4) °C in 1258 and 2.6 (±2.2) °C in 2017 (relative to 1961–90). Abrupt summer cooling of −1.5 (±1.0) °C was found after 20 large volcanic eruptions since medieval times. Comparison of our summer temperature reconstruction with newly compiled historical evidence from the Iberian Peninsula suggests a lack of military conflict during or following exceptionally hot or cold summers, as well as a general tendency towards less warfare and more stable wheat prices during warmer periods. Our study demonstrates the importance of updating and refining annually resolved and absolutely dated climate reconstructions to place recent trends and extremes of anthropogenic warming in a long-term context of natural temperature variability, and to better understand how past climate and environmental changes affected ecological and societal systems.

Ископаемые кости миоцен-голоценовых верблюдов Евразии (Россия): минералого-геохимические свойства и экологические реконструкции

Using a wide complex of mineralogical-geochemical and isotope-geochemical methods we studied, a unique series of samples of fossil camels’ bone remains taken from Eurasian localities from the northwestern Black Sea region to western Transbaikalia and Mongolia, dated in the chronological range from the Late Miocene (6 Ma) until the 14th century AD. The obtained results indicated the mineralogical and geochemical properties of fossil bones as very effective way of ecological and climatic reconstructions and interpretation of the evolutionary history of mammals.

Initialization shock in the ocean circulation reduces skill in decadal predictions of the North Atlantic subpolar gyre

Due to large northward heat transport, the Atlantic meridional overturning circulation (AMOC) strongly affects the climate of various regions. Its internal variability has been shown to be predictable decades ahead within climate models, providing the hope that synchronizing ocean circulation with observations can improve decadal predictions, notably of the North Atlantic subpolar gyre (SPG). Climate predictions require a starting point which is a reconstruction of the past climate. This is usually performed with data assimilation methods that blend available observations and climate model states together. There is no unique method to derive the initial conditions. Moreover, this can be performed using full-field observations or their anomalies superimposed on the model's climatology to avoid strong drifts in predictions. How critical ocean circulation drifts are for prediction skill has not been assessed yet. We analyze this possible connection using the dataset of 12 decadal prediction systems from the World Meteorological Organization Lead Centre for Annual-to-Decadal Climate Prediction. We find a variety of initial AMOC errors within the predictions related to a dynamically imbalanced ocean states leading to strongly displaced or multiple maxima in the overturning structures. This likely results in a blend of what is known as model drift and initial shock. We identify that the AMOC initialization influences the quality of the SPG predictions. When predictions show a large initial error in their AMOC, they usually have low skill for predicting internal variability of the SPG for a time horizon of 6-10 years. Full-field initialized predictions with low AMOC drift show better SPG skill than those with a large AMOC drift. Nevertheless, while the anomaly-initialized predictions do not experience large drifts, they show low SPG skill when skill also present in historical runs is removed using a residual correlation metric. Thus, reducing initial shock and model biases for the ocean circulation in prediction systems might help to improve their prediction for the SPG beyond 5 years. Climate predictions could also benefit from quality-check procedure for assimilation/initialization because currently the research groups only reveal the problems in initialization once the set of predictions has been completed, which is an expensive effort.

Identifying Detrital and Diagenetic Minerals in Paleosols of the Illinois Basin

Phyllosilicates are hypothesized to be primarily of pedogenic origin in shallowly buried paleosols (≤3 km depth), regardless of the age of the paleosol. To test this hypothesis, this work evaluates the possible presence of detrital and diagenetic phyllosilicates in middle and upper Pennsylvanian paleosols, collected from three drill cores along a north–south transect in the Illinois Basin. The abundances of 2M1 muscovite, quartz, and K-feldspar are greater in a morphologically immature Protosol from the southernmost core; 1Md illite and interstratified illite-smectite with R1 and R0 stacking orders are more abundant in the more mature Vertisols of the central and northern cores. K-Ar age values of multiple clay-size fractions from each paleosol averaged ~260 Ma in the northern core, 270 Ma in the central core, and 295 Ma in the southern core. While considering the complex tectonic and thermal history of the Illinois Basin, detrital minerals are more abundant in immature paleosols that experienced relatively greater maximum burial depths and thus greater sediment supply whereas illitization in more mature paleosols was probably initiated primarily during protracted burial diagenesis. As the present study found evidence for diagenetic and detrital minerals in clay-size fractions of shallowly buried, deep-time paleosols, caution is advised when using paleosol minerals for ancient climate and environment reconstructions.

Late Neogene terrestrial climate reconstruction of the central Namib Desert derived by the combination of U–Pb silcrete and terrestrial cosmogenic nuclide exposure dating

Abstract. The chronology of the Cenozoic “Namib Group” of the Namib Desert is rather poorly understood and lacks direct radiometric dating. Thus, the paleoclimate and landscape evolution of the central Namib Desert remains imprecise, complicating the detailed search for global and/or local forcing factors for the aridification of the Namib. The widespread occurrence of calcretes and silcretes in the Namib Desert allows us to apply the novel application of the U–Pb laser ablation dating technique on silcretes and calcretes to date important phases of landscape stability and to retrieve critical paleoclimatic and environmental information on desertification and its paleoclimatic variability. Microscale silcrete formation (maximum of 8 mm) due to pressure solution by expanding calcrete cementation provides the opportunity to date multiple phases (multiple generations of silcrete as growing layers or shells) of silcrete formation. Groundwater silcrete and calcrete formation occurred at our study site during the Pliocene, a period of relatively stable climate and landscape conditions under semi-arid to arid conditions. Terrestrial cosmogenic nuclide (TCN) exposure ages from flat canyon rim surfaces indicate the cessation of groundwater calcrete formation due to incision during the Late Pliocene–Early Pleistocene and mark a large-scale landscape rejuvenation due to climate shifts towards more arid conditions in the Pleistocene, which can be connected to global climate patterns. This study demonstrates the feasibility of applying U–Pb laser ablation to groundwater silcrete and calcretes, discusses several important issues associated with this technique, and opens up the possibility of dating numerous sedimentary sequences containing silcretes and calcretes in arid environments. In particular, the use of silcretes (as described above) reduces potential effects of detrital components and bulk signal measurements by using massive calcretes. Our study redefines and improves the generally accepted Late Cenozoic chronostratigraphy of the Namib Desert (Miller, 2008).

Effects of Climate Change and Fire on the Middle and Late Holocene Forest History in Yenisei Siberia

This study presents the long-term forest history in the forest–tundra ecotone of the Low Yenisei River basin. The new high-resolution pollen and macroscopic charcoal data were inferred from the 8.6 m long peat archive covering the last 6300 years. Climate reconstructions are based on the application of the best modern analogue technique using pollen data. Our findings suggest an alternation of phases of middle-taiga forests of Larix sibirica, Abies sibirica, Picea obovata, and Pinus sibirica (intervals of climate warming: 6320–6050, 5790–5370, 4480–4220, and 3600–2700 cal yr BP, respectively) and open larch woodlands with the participation of Betula, Picea, and Pinus sibirica, typical for northern taiga (intervals of climate cooling and increasing humidification: 5370–4480, 4220–3600 cal yr BP, respectively). The vegetation pattern of the region became similar to the modern one around 2700 cal yr BP. Climate warming caused a northward shift of vegetation-zone boundaries in Yenisei Siberia and an expansion of the range of Abies sibirica by about 200 km to the north compared to the present day. The increased frequency of fires and biomass burning during warm periods may promote the melting of the local permafrost, thereby enhancing the tree growth and regeneration.

The climate in Poland (central Europe) in the first half of the last millennium, revisited

Abstract. The article presents updated knowledge on climate change in Poland (central Europe) in the first half of the last millennium (1001–1500). This knowledge is required to delimit the existence, duration, and scale of the Medieval Warm Period (MWP, also called the Medieval Climate Anomaly, MCA, here). To this end, it employs all available quantitative climate reconstructions created for Poland in the last 2 decades and four new reconstructions using three dendrochronological series and an extensive database of historical source data on weather conditions. The growth of conifers in lowland and upland Poland depends on the temperature in the cold season, especially in February and March. All available reconstructions based on dendrochronology data represent this time of the year. Summer temperatures were reconstructed using biological proxies and documentary evidence. The latter, however, is limited to the 15th century only. Winter temperature was used as the proxy for annual temperature proxies instead of the more usual use of summer temperature. The MWP occurred in Poland probably from the late 12th century to the first halves of the 14th or 15th centuries. All the analysed quantitative reconstructions suggest that the MWP in Poland was comparable to or warmer than the mean temperature in the period 1951–2000. The coldest conditions in the entire study period were noted in the first half of the 11th century (both winter and summer) and the second half of the 15th century (only winter). The greatest climate continentality occurred in the 15th century. Good agreement was found between the reconstructions of Poland's climate and many reconstructions available for Europe.

Dendroclimatology: Basic Concepts, Measurement Techniques and Research Possibilities in the Republic of Srpska

Dendrochronology is a new scientific discipline whose methods are used in paleogeographic research in order to gain insight into the state, as well as changes in the environment. Dendrochronology has given rise to dendroclimatology, which is used to study climate trends in the recent period, on the basis of which it is possible to determine the changes that have taken place in the climate complex in the past. Thanks to the ability of trees to form rings in accordance with climatic influences, it is possible to determine whether there is a correlation between meteorological data and data obtained from dendroclimatological analyzes. Dendroclimatology has become an essential tool for analyzing perennial climate fluctuations and changes. The chapter presents the results of previous research and perspectives of climate reconstruction using dendroclimatological methods in the Republic of Srpska. Although it is still undeveloped in our country, this science is very important for the study of climate in the past, as well as other cause-andeffect changes related to climate oscillations. The area of the Republic of Srpska is covered by a significant percentage of forests, in which old trees are frequent. This area offers a number of advantages, but also limitations for dendroclimatological research. In the area of the Republic of Srpska, these studies were conducted sporadically until several years ago. More intensive researches in recent years create the basis for a better understanding of recent climate and paleoclimate. Having in mind the above, it can be concluded that the Republic of Srpska has significant potential for dendroclimatological research.

Anion elements incorporation into corals skeletons: Experimental approach for biomineralization and paleo-proxies.

The elemental composition of coral skeletons provides important information for palaeoceanographic reconstructions and coral biomineralization. Partition of anions and their stable isotopes in coral skeleton enables the reconstruction of past seawater carbonate chemistry, paleo-CO2, and past climates. Here, we investigated the partition of B, S, As, Br, I, and Mo into the skeletons of two corals, Acropora cervicornis and Pocillopora damicornis, as a function of calcium and carbonate concentrations.* Anion-to-calcium ratio in the corals (An/CaCoral) were correlated with the equivalent ratios in the culturing seawater (An/CO32-SW). Negative intercepts of these relationships suggest a higher CO32- concentration in the coral extracellular calcifying fluid (ECF) relative to seawater, from which the skeleton precipitates. The enrichment factor of CO32- at the ECF was 2.5 for A. cervicornis and 1.9 for P. damicornis, consistent with their relative calcification rates. The CO32-ECF concentrations thus calculated are similar to those proposed by previous studies based on B/Ca coupled with δ11B, as well as by direct measurements using microsensors and fluorescent dyes. Rayleigh fractionation modeling demonstrates a uniform Ca utilization at various CaSW concentrations, providing further evidence that coral calcification occurs directly from a semiclosed seawater reservoir as reported previously. The partition coefficients reported in this study for B, S, As, Br, I, and Mo open up wide possibilities for past ocean chemistry reconstructions based on Br having long residence time (~160 Ma) in the ocean. Other elements like S, Mo, B, as well as pCO2 may also be calculated based on these elements in fossil coral.

Reconstructing 15 000 years of southern France temperatures from coupled pollen and molecular (branched glycerol dialkyl glycerol tetraether) markers (Canroute, Massif Central)

Abstract. Climatic changes in southern Europe during the Holocene are characterized by a strong spatial and temporal heterogeneity whose patterns are still poorly understood, notably the presence or not of a Holocene thermal maximum (HTM; 10 000–6000 cal BP). The climatic patterns also differ according to the proxies used (e.g. pollen, chironomid) and the latitude of the record. Here, a multi-proxy approach combining pollen and lipid biomarkers (branched glycerol dialkyl glycerol tetraethers, brGDGTs) is applied to the Canroute sedimentological sequence (Massif Central, France) to reconstruct the climatic variation over the last 15 000 years in southern Europe. This area is poorly documented in terms of vegetation and climate change. To provide reliable climate reconstructions, we have (1) performed a multi-method approach applied to pollen (modern analogue technique, MAT; weighted averaging partial least squares regression, WA-PLS; boosted regression trees, BRT; and random forest, RF) and molecular biomarkers brGDGTs (five calibrations) and (2) investigated the role of modern databases and calibrations in climate reconstructions. Three different databases were tested for pollen data: one global database based on a Eurasian pollen database and two regional databases corresponding to Mediterranean–Temperate Europe and Temperate Europe–Scandinavian databases respectively. Five global calibrations were tested for lipid biomarkers including four for soil and one for peat. Results show that the use of different modern databases highlights the importance of considering environmental and ecological constraints when using transfer functions on pollen sequences. Pollen- and brGDGT-inferred climate trends are consistent, notably for the Late Glacial and the Early and Late Holocene. However, the reconstructions notably differ concerning the presence of a Holocene thermal maximum with the MAT pollen-based method, but no difference is apparent with the BRT pollen method nor brGDGT. The temperature reconstructions estimated from the two independent pollen and lipid proxies are then compared to regional climate signals (chironomids, pollen, molecular biomarkers) to better understand global regional climatic patterns in southern Europe. Altogether, our results from the Canroute sequence and those already available in southern Europe reveal that for the Late Glacial and Early Holocene, the regional climate trends are consistent between sites and proxies, supporting the reliability of their reconstructions despite some discrepancies. During the Holocene, the temperature signal of Canroute does not indicate the clear presence of a pronounced HTM, but rather stable temperatures.

Comparing stem growth of strip-bark and whole-bark growth morphologies in a subarctic conifer (Pinus banksiana), Yellowknife, Northwest Territories

Generally, organisms living near the periphery of their range experience increased environmental stress compared to individuals of the same species growing at the core of their geographic distribution. The greater environmental stress at the ecological limits of a species distribution normally leads to greater sensitivity to growth-limiting factors. For some tree species, this can lead to the development of unique growth morphologies such as strip-bark or partial cambial dieback. Previous studies have concluded that the growth differences induced by CO2 fertilization effects in high elevation strip-bark trees makes them unsuitable for climate reconstructions. However, comparisons of strip-bark and whole-bark growth morphologies at high latitude and low elevation sites are limited. This study compares the growth of strip-bark and whole-bark jack pine (Pinus banksiana Lamb.) trees near Yellowknife, Canada. Annual ring widths from trees of each morphological type were measured, and the results indicate the growth of the two populations are statistically different from one another prior to the late 19th century. After the late 19th century, growth of the two populations remains statistically different, but the average mean growth of strip-bark trees increases 44% after 1891 while average mean growth of whole-bark trees decreases nearly 11%. Climate sensitivity of the two populations was tested using monthly and seasonal climate indices from 1943 to 2015 and results indicate the two populations share similar responses to climate. The results of this research suggest these strip-bark and whole-bark trees share similar climate-growth relations at inter-annual timescales, however, their growth trends diverge at decadal to centennial timescales. The timing of cambial dieback was dated to a short time interval in the early 19th century which coincided with a period of cool summer temperatures across the northern hemisphere. Increased growth in strip-bark trees after ∼1891 could be related to CO2 fertilization effects, although further studies into intrinsic water use efficiency would be helpful in assessing this possibility more accurately.

Were climatic forcings the main driver for mid-holocene changes in settlement dynamics on the Varamin Plain (Central Iranian Plateau)?

Settlement crises in ancient cultures of Western Asia are commonly thought to be caused by climatic events such as severe droughts. However, the insufficient climate proxy situation in this region challenges the inference of clear relationships between climate and settlement dynamics. We investigate the Holocene climatic changes on the Varamin Plain in the context of the climatic history of Western Central Asia by using a transient comprehensive Earth System Model simulation (8 ka BP to pre-industrial), a high-resolution regional snapshot simulation and a synthesis of pollen-based climate reconstructions. In line with the reconstructions, the models reveal only slightly varying mean climatic conditions on the Varamin Plain but indicate substantial changes in seasonality during the Holocene. Increased precipitation during spring, combined with lower temperature and potentially stronger snow accumulation on the upstream Alborz mountains may have led to an increased water supply on the alluvial fan during the vegetation period and thus to more favourable conditions for agricultural production during the Mid-Holocene compared to modern times. According to the model, dry periods on the Central Iranian Plateau are related to particularly weak Westerly winds, fostering the subsidence in the mid-troposphere and hampering precipitation over the region. The model reveals that dry periods have spatially heterogenous manifestations, thus explaining why they do not appear in all proxy records in the wider study region. In fact, the climatic signal may depend on local environmental conditions. The interaction of the topography with the atmospheric circulation leads to additional spatial heterogeneity. Although our results provide several indications for a connection between climate and settlement dynamics, the small overall changes in moisture call into question whether climate is the main driver for settlement discontinuities on the Central Iranian Plateau. To shed further light on this issue, more high-resolution long-term proxy records are needed.

Quantitative pollen-based paleoclimate reconstructions for the past 18.5 ka in southwestern Yunnan Province, China

Quantitative reconstruction of past climate plays an important role in understanding global and regional climate changes and validating climate models. Although some important progress has been made in quantitative paleoclimate reconstructions based on terrestrial records in the Indian summer monsoon region, high-resolution quantitative studies spanning the last ∼20 ka are still relatively sparse with differing results. This study presents high-resolution quantitative variations in mean temperature of the coldest month (MTCM, the first controlling factor of the regional vegetation), mean annual temperature (MAT, the second controlling factor), and mean annual precipitation (MAP) from Lake Tengchongqinghai in southwestern China, based on an updated modern pollen dataset and the fossil pollen record spanning the last 18.5 ka. The results show that temperature and precipitation increased gradually from 18.5 ka, and peaked from 7.2 to 4.5 ka when MAT was on average 1.0 °C higher than the modern observational value (14.5 °C), corresponding to the mid-Holocene thermal maximum (HTM), and then decreased gradually. The total reconstructed ranges are between −2.2 and 9.2 °C for MTCM, 7.7 and 17.2 °C for MAT, and 840 and 1300 mm for MAP. On top of this overall climate trend, seven abrupt cold and dry events were detected during the periods of 16.2–14.8 ka, 12.8–11.5 ka, ∼11.1 ka, 9.1–8.4 ka, ∼7.7 ka, 4.3–3.7 ka, and 0.68–0.009 ka (1270–1950 CE). The results of this quantitative reconstruction were validated by both statistical and ecological evaluations. We conclude that the trend of climatic change since 18.5 ka in this study area was primarily driven by June, July, August, and September solar insolation and changes in radiative forcing and greenhouse gas concentrations. The abrupt changes may be caused by changes in the Atlantic meridional overturning circulation, solar activity, the position of the Intertropical Convergence Zone, and volcanic activity.

Palynological response deduced through spatially distinct surface samples to reconstruct palaeoecology and palaeoclimate of the Barak Valley, Assam (Indo-Burma region), northeast India

Establishing modern pollen analogues under various vegetation types in a location is necessary without which it is difficult to accurately interpret fossil pollen assemblages in any region in terms of the past environment and palaeoclimate. We have analysed the modern pollen dataset of 30 surface soil and sediment samples from the forested, cropland, wetland and river sites of the Karimganj District in south Assam, to comprehend the connection between the recent pollen assemblage and the vegetation patterns in the Barak Valley. The overall pollen data reflect the tropical moist deciduous and semi-evergreen forest comprising Syzygium, Schleichera, Terminalia, Lagerstroemia, Duabanga, Sapotaceae, Ilex, Mesua and Schima under a warm and humid climate in response to rainfall variations in the region. The presence of Euphorbiaceae and Convolvulaceae inferred high monsoonal activity in and around the region. A robust relationship between local herbaceous vegetation and pollen was observed. The steady occurrence of cereal pollen (average 18.67%) along with other cultural pollen taxa like Brassicaceae, Coriandrum and Solanaceae illustrate intense agricultural activity around the valley areas. Multivariate principal component analysis (PCA) and box plot were applied to the quantified data obtained from pollen frequency analyses of the surface samples which clearly revealed a significant variation and similarity in vegetation types. Thus, an attempt has been commenced to precisely observe the behavioural pattern of modern pollen deposition in a varied depositional setting, which could, in turn, help in assessing the expanse of agricultural practices and the depth of deterioration of pristine forests and surrounding areas. This modern training dataset could also help in the precise reconstruction of past climate and vegetation shifts in the Barak Valley region. A correlation of regional palynodata with other regions of the Indian subcontinent and Indo-Burma will also be assessed for better interpretation.

Improving signal strength of tree rings for paleoclimate reconstruction by micro-hyperspectral imaging

ABSTRACT In dendroclimatology, tree ring chronology is ordinarily established to reveal the fluctuation law of climate change on the interannual, interdecadal, and centennial scales. However, since traditional dendrochronology can only use one variable (tree ring width) to reflect environmentally related information, this causes the richer information recorded in the tree rings to be discarded. In this study, we examined the potential of hyperspectral chronological indices (shortened as “hyperspectral index/indices”) with samples collected in Shennongjia woodland in central China. The correlation analysis of the tree ring series on different samples indicated that hyperspectral indices outperform the traditional width index in chronology statistics including Signal-to-noise Ratio (SNR) and Expressed Population Signal (EPS). The reliability test shows that hyperspectral chronologies have more periods reaching the threshold of EPS or Subsample Signal Strength (SSS) > 0.85, which means that hyperspectral chronologies provide more reliable periods for accurate climate reconstruction. Based on this, chronologies built by the three dendroclimatic indices were used to reconstruct the average temperature changes in Shennongjia over the last 103 years. The reconstruction results indicate that in our study area, the traditional width index model failed the split-sample calibration test and exhibited a low reconstruction accuracy, while the hyperspectral index model has a higher explained variance of 46.4% (p < 0.01), compared to the width index (21.4%) and the grayscale index (38.3%). Our research results show that hyperspectral indices have greater potential for climate reconstruction in regions with lower susceptibility to climate stress. This is attributed to their ability to effectively extract subtle climate signals from the spectral variations on the surface of tree rings. Such ring spectral changes may be caused by complex and currently unknown responses of the trees to the climate.

Winter hydroclimate contrasts between southern and northern Europe at ∼ 5.2 ka

Winter climate variations in midlatitude regions have far-reaching effects on both natural and human systems worldwide. However, most climate reconstructions focus on either annual or summer climate conditions. Here we present a stalagmite stable isotope record from Urşilor Cave (Romania) where the δ18O values are controlled by winter temperature. Because of its East-Central Europe (ECE) location, the cave site is well suited to capture variations in precipitation and temperature in response to changes in the North Atlantic region. Present day precipitation and temperature in this region are controlled by two teleconnection patterns, the Arctic Oscillation (AO) and the East Atlantic (EA). By analyzing composite maps for four different scenarios of AO and EA phase, we suggest that between 5.6 and 5.2 ka the positive phases of AO and EA became increasingly dominant. This shift led to generally warmer winters in northern Europe, but drier conditions in southern Europe and the Levant. Considering the compound effect of AO and EA on the hydroclimate of ECE, our study highlights the importance of examining the combined impact of teleconnection patterns on climate proxies influenced by various forcing mechanisms.

Tropical Pacific Modulation of the Asian Summer Monsoon Over the Last Millennium in Paleoclimate Data Assimilation Reconstructions

AbstractLarge 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.

A transient coupled general circulation model (CGCM) simulation of the past 3 million years

Abstract. Driven primarily by variations in the earth's axis wobble, tilt, and orbit eccentricity, our planet experienced massive glacial/interglacial reorganizations of climate and atmospheric CO2 concentrations during the Pleistocene (2.58 million years ago (Ma)–11.7 thousand years ago (ka)). Even after decades of research, the underlying climate response mechanisms to these astronomical forcings have not been fully understood. To further quantify the sensitivity of the earth system to orbital-scale forcings, we conducted an unprecedented quasi-continuous coupled general climate model simulation with the Community Earth System Model version 1.2 (CESM1.2, ∼3.75∘ horizontal resolution), which covers the climatic history of the past 3 million years (3 Myr). In addition to the astronomical insolation changes, CESM1.2 is forced by estimates of CO2 and ice-sheet topography which were obtained from a simulation previously conducted with the CLIMBER-2 earth system model of intermediate complexity. Our 3 Ma simulation consists of 42 transient interglacial/glacial simulation chunks, which were partly run in parallel to save computing time. The chunks were subsequently merged, accounting for spin-up and overlap effects to yield a quasi-continuous trajectory. The computer model data were compared against a plethora of paleo-proxy data and large-scale climate reconstructions. For the period from the Mid-Pleistocene Transition (MPT, ∼1 Ma) to the late Pleistocene we find good agreement between simulated and reconstructed temperatures in terms of phase and amplitude (−5.7 ∘C temperature difference between Last Glacial Maximum and Holocene). For the earlier part (3–1 Ma), differences in orbital-scale variability occur between model simulation and the reconstructions, indicating potential biases in the applied CO2 forcing. Our model-proxy data comparison also extends to the westerlies, which show unexpectedly large variance on precessional timescales, and hydroclimate variables in major monsoon regions. Eccentricity-modulated precessional variability is also responsible for the simulated changes in the amplitude and flavors of the El Niño–Southern Oscillation. We further identify two major modes of planetary energy transport, which played a crucial role in Pleistocene climate variability: the first obliquity and CO2-driven mode is linked to changes in the Equator-to-pole temperature gradient; the second mode regulates the interhemispheric heat imbalance in unison with the eccentricity-modulated precession cycle. During the MPT, a pronounced qualitative shift occurs in the second mode of planetary energy transport: the post-MPT eccentricity-paced variability synchronizes with the CO2 forced signal. This synchronized feature is coherent with changes in global atmospheric and ocean circulations, which might contribute to an intensification of glacial cycle feedbacks and amplitudes. Comparison of this paleo-simulation with greenhouse warming simulations reveals that for an RCP8.5 greenhouse gas emission scenario, the projected global mean surface temperature changes over the next 7 decades would be comparable to the late Pleistocene glacial-interglacial range; but the anthropogenic warming rate will exceed any previous ones by a factor of ∼100.

Stratigraphic noise and its potential drivers across the plateau of Dronning Maud Land, East Antarctica

Abstract. Stable water isotopologues of snow, firn and ice cores provide valuable information on past climate variations. Yet single profiles are generally not suitable for robust climate reconstructions. Stratigraphic noise, introduced by the irregular deposition, wind-driven erosion and redistribution of snow, impacts the utility of high-resolution isotope records, especially in low-accumulation areas. However, it is currently unknown how stratigraphic noise differs across the East Antarctic Plateau and how it is affected by local environmental conditions. Here, we assess the amount and structure of stratigraphic noise at seven sites along a 120 km transect on the plateau of Dronning Maud Land, East Antarctica. Replicated oxygen isotope records of 1 m length were used to estimate signal-to-noise ratios as a measure of stratigraphic noise at sites characterised by different accumulation rates (43–64 mm w.e. a−1), snow surface roughnesses and slope inclinations. While we found a high level of stratigraphic noise at all sites, there was also considerable variation between sites. At sastrugi-dominated sites, greater stratigraphic noise coincided with stronger surface roughnesses, steeper slopes and lower accumulation rates, probably related to increased wind speeds. These results provide a first step to modelling stratigraphic noise and might guide site selection and sampling strategies for future expeditions to improve high-resolution climate reconstructions from low-accumulation regions.

Tree Core Analysis with X-ray Computed Tomography.

An X-ray computed tomography (CT) toolchain is presented to obtain tree-ring width (TRW), maximum latewood density (MXD), other density parameters, and quantitative wood anatomy (QWA) data without the need for labor-intensive surface treatment or any physical sample preparation. The focus here is on increment cores and scanning procedures at resolutions ranging from 60 µm down to 4 µm. Three scales are defined at which wood should be looked at: (i) inter-ring scale, (ii) ring scale, i.e., tree-ring analysis and densitometry scale, as well as (iii) anatomical scale, the latter approaching the conventional thin-section quality. Custom-designed sample holders for each of these scales enable high-throughput scanning of multiple increment cores. A series of software routines were specifically developed to efficiently treat three-dimensional X-ray CT images of the tree cores for TRW and densitometry. This work briefly explains the basic principles of CT, which are needed for a proper understanding of the protocol. The protocol is presented for some known species that are commonly used in dendrochronology. The combination of rough density estimates, TRW and MXD data, as well as quantitative anatomy data, allows us to broaden and deepen current analyses for climate reconstructions or tree response, as well as further develop the field of dendroecology/climatology and archeology.