European summer hydroclimate has shifted markedly in recent decades, including widespread drying across many regions. However, major uncertainties remain regarding their spatiotemporal variability and underlying drivers. Here, we present the European Last Millennial Data Assimilation (EULMDA), a new reconstruction of European hydroclimate and its main drivers over the past millennium. EULMDA combines five Earth System Model simulations with more than 100 tree-ring records sensitive to moisture and temperature, and shows strong skill in reproducing instrumental variability in multiple climate fields, including large-scale circulation changes. For the warm season, we identify two dominant controls on European drought variability: circulation fluctuations linked to the Scandinavian pattern (SCAND) and long-term summer temperature changes. Together, these factors account for more than half of the spatiotemporal drought variance. The SCAND drives a pronounced north–south dipole in summer hydroclimate, explaining a larger fraction of Mediterranean drought variability than other major circulation modes, contributing to recent multidecadal drying in the Mediterranean alongside wetting in northern Europe. Meanwhile, summer warming intensifies drying across much of Europe. Taken together, these dynamic and thermodynamic processes have shaped European hydroclimate throughout the past millennium, providing critical context for interpreting recent drought trends and insight into mechanisms shaping future hydroclimate risks.

As the sub-Arctic and Arctic regions undergoes rapid changes, understanding its hydroclimate history is more critical than ever. A limited availability of moisture-sensitive proxy data severely constrains our insights, underscoring the urgent need for more hydroclimate reconstructions in the region. Here we present a May-June precipitation reconstruction based on ring width from living and dead trees of Scots pine (Pinus sylvestris L.) growing under drought-stressed conditions at 63 degrees N, near Skuleskogen National Park, on the northern part of the east coast of Sweden. The oldest deadwood sample dates back to the mid-11th century, and the Expressed Population Signal (EPS) exceeds 0.85 from 1320 CE until the present (2021 CE), making this the longest tree-ring-based hydroclimate reconstruction from high-latitude Fennoscandia. Unlike typical high-latitude forests in Fennoscandia, the trees at this site thrive under low-pressure conditions and show strong correlations up to approximately |r|=0.6 over the period 1920-2021 with drought-related variables such as precipitation, cloud cover, solar radiation, potential evapotranspiration and diurnal temperature range. The climate signal is concentrated to a short window between mid-May and early July but reflects climatic conditions over a broad region. Significant spatial correlations extend over most of Sweden as well as large parts of Norway and Finland, indicating sensitivity to large-scale climate systems. The tree-ring chronology also shows clear cyclic patterns, with a particularly strong similar to 64-year cycle. The modern era stands out for its variability, with 2018 CE emerging as one of the of the driest year in the entire record. There is potential to extend the chronology further back in time, possibly to 1000 CE or earlier, and to extract additional climate information using other tree-ring parameters such as blue intensity.

Abstract Recent decades have seen pronounced changes in European hydroclimate, including widespread summer drying, yet its spatiotemporal variability and underlying drivers remain uncertain. Here we present the European Last Millennial Data Assimilation (EULMDA), a new reconstruction of European hydroclimate and its main drivers covering the past millennium. EULMDA integrates five Earth System Model simulations with over one hundred moisture and temperature sensitive tree-ring records. It demonstrates high skill in reproducing instrumental variability across climate variables, including large-scale atmospheric circulation changes. We show that European warm-season drought variability is primarily governed by circulation changes associated with the Scandinavian pattern (SCAND) and long-term summer temperature changes, together explaining over half of the spatiotemporal drought variance. SCAND drives a pronounced north–south dipole in summer hydroclimate, explaining a larger fraction of Mediterranean drought variability than other major circulation modes, contributing to recent multidecadal drying in the Mediterranean alongside wetting in northern Europe. Meanwhile, summer warming intensifies drying across much of Europe. These results provide critical context for interpreting recent drought trends and insight into mechanisms shaping future hydroclimate risks.

Central European forests experienced high rates of air pollution in the second half of the 20th century, especially along the borders of Czechia, Germany and Poland. Consequently, tree-growth declines were detected in heavily polluted forests. However, information about how pollution has influenced growth-climate responses beyond tree-ring width (RW) in pollution-affected forests remains sparse. In this study, we investigated the impact of high-level pollution during 1960s-1980s in Central Europe on the climatic signals of various tree-ring parameters of Norway spruce, including RW, latewood Blue Intensity (LWBI), and maximum cell wall thickness (CWT), to understand how tree growth and climatic sensitivity were affected. Tree-ring cores were collected from six temperature-limited high-elevation sites within four pollution-affected regions in Czechia and northern Slovakia. RW and LWBI were measured for all samples and CWT was produced from two sites with contrasting pollution impacts. Distinct pollution-related RW growth suppression was detected in 1970s to 1980s at several sites. LWBI and CWT chronologies were highly correlated (r LWBI = 0.52-0.75; r CWT = 0.63-0.68) with growing season (April-September) temperature and did not exhibit clear signs of distortion by pollution compared to RW (r RW = 0.28-0.58). Pollution stress seemed to reduce tree growth by decreasing cell numbers and made RW less sensitive to climate. This study reveals that impacts of pollution on different tree-ring parameters varied which can further influence their climatic sensitivities. It provides valuable insight in improving the utility of pollution-affected tree-ring chronologies by choosing appropriate parameters, which can ultimately contribute to substantially improving the calibration of climate reconstructions from heavily polluted regions.

Robust, high-resolution proxy information on hydroclimate is critical for understanding the recent European drying in the context of natural variability. This is particularly true for Fennoscandia and the surrounding northern high latitude regions, where such records are scarce, and little is known about the physical drivers of these rare events. Recently, the potential of tree-ring Blue Intensity (BI) of Pinus sylvestris L. from cool droughtprone sites has emerged as a source of high-resolution hydroclimate information. Limitations to these proxies may however be linked to the analytical measurement resolution of the technique as well as potential colorrelated biases caused by the heartwood-sapwood transition and decay-associated discoloration of the tree-ring samples. These proxy uncertainties need to be addressed before the methodology can be used more systematically in the context of hydroclimate reconstructions. Here, we evaluate the reliability of BI tree-ring parameters, specifically regarding their ability to accurately track both low-frequency variations as well as hydroclimate extremes. This is accomplished by benchmarking these parameters against newly developed quantitative wood anatomical (QWA) data from Pinus sylvestris trees in southeast of Sweden. The Delta BI parameter shows a high similarity to its QWA analogue across scales ranging from interannual to centennial, suggesting that the Delta conversion efficiently overcomes the long-term trend biases seen in the raw BI data. The earlywood and latewood BI parameters record largely the same strong spring-summer precipitation signal, together explaining over 50 % of the variance in May-July precipitation, as do their QWA counterparts. However, both BI and QWA show a pronounced asymmetric response to precipitation extremes such that dry extremes are captured better than the wet extremes. This dry-biased extreme value capture needs careful consideration in future reconstruction efforts. We conclude that tree-ring BI holds significant potential for contributing to regional reconstructions, especially given the untapped opportunities to utilize the region's extensive collection of historical pine material for paleoclimate research.

Understanding long-term historical changes in cloudiness is essential for elucidating Earth’s climate dynamics and variability and its extremes. In this study, we present the first millennial-length reconstruction of the annual total cloud cover (TCC) in the western Mediterranean, covering the period from 969 to 2022 CE. Based on a comprehensive set of hydrological and atmospheric variables, our reconstruction reveals a nuanced pattern of cloudiness evolution over the past millennium. We observe an initial increase in cloudiness until 1600 CE, followed by a substantial decrease in TCC. This shift was driven by a confluence of factors, including the eruption of Mount Tambora in Indonesia in 1815, increased solar forcing, and a positive phase of the Atlantic Multidecadal Oscillation. These complex dynamics have brought modern warming cloud patterns closer to those observed during the medieval period before c. 1250, exceeding the background variability of the Little Ice Age (c. 1250 to 1849). In particular, recent decades have witnessed an unprecedented coupling of intense solar activity, high temperatures, and the lowest cloud cover on record. Our results highlight the importance of inter-oceanic-scale relationships between Atlantic forcing mechanisms and the TCC in shaping future trends in western Mediterranean cloudiness. This study provides valuable insights into the long-term dynamics of cloudiness and its implications for regional climate trends in the western Mediterranean and beyond.

The published literature of the past 20 years expresses inconsistent terminology for the Blue Intensity (BI) method that could lead to confusion in analysis and interpretation. In this technical note we propose a standard terminology based around the prevalent use of BI for the variant that is positively correlated with wood density derived from X-ray and equivalent wood anatomical techniques. We highlight significant practical advantages of this standard terminology for data analysis, scientific interpretations as well as archiving, and provide some cautionary examples that could occur if not adhering to this terminology. In future studies using BI, we recommend to explicitly clarify that the standard terminology is used with the following phrase: The BI data produced in this study is consistent with the ‘2024 BI standard terminology’.

Abstract Understanding historical soil moisture (SM) variations and their relationship with temperature in high‐latitude humid regions is essential for predicting hot droughts under widespread warming. This paper presents the first‐ever annual‐resolution summer surface SM reconstruction (1736–2006 CE) in Sweden, located in northern Europe (NE). The reconstruction utilizes the paleoclimate proxy, tree‐ring δ18O, which exhibits a strong correlation with reanalysis SM data during 1948–2007 CE (r = −0.67, p < 0.001). In contrast to an unprecedented trend suggested by previous studies, our reconstruction exhibits a wetting trend in NE since the 1960s falling within the historical spectrum of trends. Both reanalysis SM data and our reconstruction show significant negative correlations with temperature at interannual timescales. However, Coupled Model Intercomparison Project Phase 6 climate models failed to capture the significant SM‐temperature relationship during the historical (1850–1947 CE) and observation periods (1948–2006 CE). This proxy‐model comparison represents a critical step toward enhancing our understanding of SM dynamics and the reliability of SM projections in high‐latitude humid regions.

The extreme European summer heat-droughts of 2003 and 2018, compounded by the threat of continued climate change, highlight the need to better understand the frequency, severity and seasonality of such events. The widths of annual tree rings currently provide the most detailed and spatially complete proxy information of European hydroclimate of the past millennium. Ring width is easy to measure and may be well correlated with hydroclimate, but is notorious for overestimating persistence/memory in climate. Using ring width to understand frequencies of droughts may therefore be inherently misleading. In recent studies, latewood density of conifers has showed great potential to improve drought reconstructions. However, wood density is measured at a resolution unable to accurately represent latewood density in very narrow rings, which are often, but not always, the drought-related rings/years most relevant to represent correctly. Hydroclimate studies could therefore be substantially improved by using high-resolution wood anatomical parameters. Anatomical parameters are not only expected to yield higher correlations with hydroclimate, but importantly, provide unbiased reconstructions of the frequency and severity of past droughts, in contrast to ring width and density.Here we explore the potential to improve reconstructions of inter- and intra-seasonal droughts and hydroclimate by using tree-ring anatomy of Scots pine growing across cool yet drought-prone habitats in Northern Europe. This is done by measuring the anatomical cell dimensions of 15 living trees from the Isle of Gotland, south-eastern Sweden. X-ray maximum latewood density (MXD) and its blue intensity counterpart (MXBI) have previously been measured, providing an optimal opportunity for testing the accuracy and potential advantages of dendroanatomical parameters as climate proxies. Preliminary results show an exceptional variability in the latewood density notably coinciding with dry and wet years, and highlight anatomical MXD and maximum radial cell wall thickness as the two most promising wood anatomical proxy parameters for past hydroclimate.

Studies on long-term fluctuations in glacier volume and mass are crucial for understanding past climate change. In this paper, we utilized Picea schrenkiana to develop a 525-year chronology of latewood blue intensity (LWBI) in the Tianshan Mountains. Relying on temperature as the main controlling factor for tree growth and glacier mass balance (GMB) variations, the LWBI chronology was used to reconstruct the summer temperature (JJA, R-adj(2) = 47%) and the annual glacier mass balance (annual GMB, R-adj(2) = 39%) in the Tianshan Mountains over the past 400 years. The reconstruction results show that the rapid warming since 1974 has caused the Tianshan No.1 glacier (TS No.1) to experience an unprecedented melting trend within the last four centuries. It is disturbing that the glacier still remain in an ablation state for the next 80 years under both representative concentration paths (RCP) 4.5 and 8.5 scenarios, which will exacerbate the adverse environmental impacts of glacial hazards. Our study provides a continuous record for glacier research in high mountains Asian and contributes to a more detailed assessment of glacier and climate change in this region.

Understanding the long-term history of drought in the Tibetan Plateau region is important to improve understanding of drought frequency in response to future global warming. Although the Tibetan Plateau has become generally wetter recently, the southeastern part of the Tibetan Plateau has become drier since the 2000s. To place this drying trend in a historical context, we conducted tree-ring studies for Abies spectabilis and Tsuga dumosa at three sites in the southeastern Tibetan Plateau, and developed tree-ring width (TRW), earlywood width (EWW), and latewood width (LWW) chronologies to permit local drought reconstruction. Based on correlation between these tree-ring parameters and instrumental meteorological data, the EWW chronologies were identified as suitable variables for reconstructing the average April-July self-calibrating Palmer Drought Index (scPDSI). The reconstruction accounted for 45.86% of the variance in the instrumental record and allowed us to extend the drought record back to 1704 CE. Based on this new dataset, the driest interval was from 1907 to 1919 CE during which time agricultural production fell by about 70% according to written historical records, leading to severe famine. Our findings suggest that this early twentieth century drought was in phase with the previously known drought over the northern Daxing'an Mountains. However, the southeastern Tibetan Plateau drought occurred one decade earlier than the known widespread drought event in North China during the 1920s-1930s. We show that this spatial drought pattern may have been modulated by the Atlantic Multidecadal Oscillation.

Earth system models and various climate proxy sources indicate global warming is unprecedented during at least the Common Era 1 . However, tree-ring proxies often estimate temperatures during the Medieval Climate Anomaly (950–1250 ce ) that are similar to, or exceed, those recorded for the past century 2 , 3 , in contrast to simulation experiments at regional scales 4 . This not only calls into question the reliability of models and proxies but also contributes to uncertainty in future climate projections 5 . Here we show that the current climate of the Fennoscandian Peninsula is substantially warmer than that of the medieval period. This highlights the dominant role of anthropogenic forcing in climate warming even at the regional scale, thereby reconciling inconsistencies between reconstructions and model simulations. We used an annually resolved 1,170-year-long tree-ring record that relies exclusively on tracheid anatomical measurements from Pinus sylvestris trees, providing high-fidelity measurements of instrumental temperature variability during the warm season. We therefore call for the construction of more such millennia-long records to further improve our understanding and reduce uncertainties around historical and future climate change at inter-regional and eventually global scales.

The vapor pressure deficit reflects the difference between how much moisture the atmosphere could and actually does hold, a factor that fundamentally affects evapotranspiration, ecosystem functioning, and vegetation carbon uptake. Its spatial variability and long-term trends under natural versus human-influenced climate are poorly known despite being essential for predicting future effects on natural ecosystems and human societies such as crop yield, wildfires, and health. Here we combine regionally distinct reconstructions of pre-industrial summer vapor pressure deficit variability from Europe’s largest oxygen-isotope network of tree-ring cellulose with observational records and Earth system model simulations with and without human forcing included. We demonstrate that an intensification of atmospheric drying during the recent decades across different European target regions is unprecedented in a pre-industrial context and that it is attributed to human influence with more than 98% probability. The magnitude of this trend is largest in Western and Central Europe, the Alps and Pyrenees region, and the smallest in southern Fennoscandia. In view of the extreme drought and compound events of the recent years, further atmospheric drying poses an enhanced risk to vegetation, specifically in the densely populated areas of the European temperate lowlands. The atmosphere has dried across most regions of Europe in recent decades, a trend that can be attributed primarily to human impacts, according to tree ring records spanning 400 years and Earth system model simulations.

Basaltic fissure eruptions, which are characteristic of Icelandic volcanism, are extremely hazardous due to the large quantities of gases and aerosols they release into the atmosphere. The 1783 --1784 CE Laki eruption was one of the most significant high-latitude eruptions in the last millennium and had substantial environmental and climatic impacts. Contemporary observations recorded the presence of a sulfuric haze over Iceland and Europe, which caused famine from vegetation damage and resulted in a high occurrence of respiratory illnesses and related mortality. Historical records in northern Europe show that the summer of 1783 was anomalously warm, but regional tree-ring maximum latewood density (MXD) data from that year are low and lead to erroneously colder reconstructed summer temperatures. Here we measure wood anatomical characteristics of Scots pine (Pinus sylvestris) from Jämtland, Sweden in order to identify the cause of this discrepancy. We show that the presence of intra-annual density fluctuations in the majority of 1783 growth rings, a sudden reduction in lumen and cell wall area, and the measurement resolution of traditional x-ray densitometry lead to the observed reduced annual MXD value. Multiple independent lines of evidence suggest these anatomical anomalies were most likely the result of direct acidic damage to trees in Northern Europe. The common relationship between summer temperature and MXD can be disrupted by acidic haze damage to trees. Our study also demonstrates that quantitative wood anatomy offers a high resolution approach to identifying anomalous years and extreme events in the tree-ring record.

The recent warming trend, and associated shifts in growing season length, challenge the principle of uniformi-tarianism, i.e., that current relations are persistent over time, and complicates the uncritical inferences of past climate from tree-ring data. Here we conduct a comparison between tree-ring width chronologies of Pinus syl-vestris L. (Scots pine), Picea abies (L.) Karst. (Norway spruce) and Betula pubescens Ehrh. (Downy birch) and phenological observations (budburst and leaf senescence) of Fagus sylvatica L. (European beech), Quercus robur L. (European oak), Betula sp. (Birch), Norway spruce and Scots pine) in Sweden to assess to what extent the tree -ring width-temperature relationship and the timing of phenological phases are affected by increased tempera-ture. Daily meteorological observations confirm a prolongation of the thermal growing season, most consistently observed as an earlier onset of around 1-2 weeks since the beginning of the 20th century. Observations of budburst closely mimic this pattern, with budburst of the deciduous trees occurring 1-2.5 weeks earlier. In contrast to the changes seen in phenology and observational temperature data, the tree-ring width-temperature relationships remain surprisingly stable throughout the 20th century. Norway spruce, Scots pine and Downy birch all show consistently significant correlations with at least one 30 day-long window of temperature starting in late June-early July season. Norway spruce displays the largest degree of stability, with a consistent 60 day-long temperature window with significant correlation starting around Julian calendar day 150. Thus, our results suggest that the principle of uniformitarianism is not violated during the period covered by modern meteoro-logical observations. Further research is needed to determine at what thresholds the temperature sensitivity of these species may alter or deteriorate as a consequence of the ongoing climate change.

The continuous development of new proxies as well as a refinement of existing tools are key to advances in paleoclimate research and improvements in the accuracy of existing climate reconstructions. Herein, we build on recent methodological progress in dendroanatomy, the analyses of wood anatomical parameters in dated tree rings, and introduce the longest (1585–2014 CE) dendroanatomical dataset currently developed for North America. We explore the potential of dendroanatomy of high-elevation Engelmann spruce (Picea engelmannii) as a proxy of past temperatures by measuring anatomical cell dimensions of 15 living trees from the Columbia Icefield area. X-ray maximum latewood density (MXD) and its blue intensity counterpart (MXBI) have previously been measured, allowing comparison between the different parameters. Our findings highlight anatomical MXD and maximum radial cell wall thickness as the two most promising wood anatomical proxy parameters for past temperatures, each explaining 46 % and 49 %, respectively, of detrended instrumental July–August maximum temperatures over the 1901–1994 period. While both parameters display comparable climatic imprinting at higher frequencies to X-ray derived MXD, the anatomical dataset distinguishes itself from its predecessors by providing the most temporally stable warm season temperature signal. Further studies, including samples from more diverse age cohorts and the adaptation of the regional curve standardization method, are needed to disentangle the ontogenetic and climatic components of long-term signals stored in the wood anatomical traits and to more comprehensively evaluate the potential contribution of this new dataset to paleoclimate research.

Instrumental data derived from meteorological stations provide a fairly reliable record of climate variability for at least the last century for most parts of Europe. Proxy-based climate reconstructions have been extensively developed throughout the continent over recent decades to extend these records further back in time. However, to date, parts of central and eastern Europe remain underrepresented, leading to gaps in high-resolution climatic information even in recent centuries. This issue is predominantly linked to large uncertainties in existing records and limitations in data quality associated with a generally weak climatic sensitivity of available proxy records. The REPLICATE project, presented here, aims to address this deficiency by utilizing various tree-ring parameters from temperature-sensitive Norway spruce (Picea abies). The samples, collected from treeline or near-treeline environments, will be used to develop a set of temperature reconstructions across four sub-regions of the Carpathian Mountains. By doing so, we aim to contribute to filling in the spatial paleoclimatic and data quality gap in central-eastern Europe. To improve the climatic signal, we utilized a combination of tree ring width (TRW) corrected for non-climatic (disturbance) trends and blue intensity (BI) series derived from scanned images as a surrogate for maximum latewood density. We also developed a novel tree-ring parameter similar to BI based on high-resolution reflected light microscope images of the tree sample surface – termed surface intensity (SI) – which accounts for resolution and color bias limitations commonly encountered in BI datasets. Additionally, traditional thin section-based quantitative wood anatomy (QWA) parameters and their reflected light surface imaging-based counterparts (sQWA) were also included. Integrating this range of tree-ring parameters in a complementary fashion helps isolate, optimize and extract stronger climatic signals by accounting for and minimizing a range of parameter-specific limitations and biases, yielding improved calibration with a more accurate representation of low-frequency climatic trends and high-frequency extremes. From these multi-parameter tree-ring chronologies, annually resolved, robust, high-quality summer temperature reconstructions, extending to the early to mid-17th century, are under development for four Carpathian locations (i.e., northern Slovakia, western Ukraine, northern and central Romania). Initial results indicate that the reconstructions based on such a multi-parameter approach can produce paleoclimatic records with reduced uncertainty that explain between 50% and 60% of the regional temperature variability. These reconstructions will contribute to a more highly resolved temperature dataset in a part of Europe with considerable research potential, resulting in an improved spatial representation of past European temperature fluctuations. Also, by providing a reliable historical context to evaluate return periods and magnitudes of temperature extremes, they will contribute to assessing potential future socioeconomic impacts of climate change (e.g., on agriculture) and developing possible mitigation solutions.

Recently, groundwater in Sweden has attracted media attention due to supply shortages caused by dry periods and low groundwater levels. About half of Swedish drinking water stems from groundwater. The small Swedish aquifers are highly dependent on frequent and sufficient recharge. Groundwater recharge forms the link between meteorological and groundwater drought and thus the main link between climate change and sustainable water supply. This study evaluated whether the current knowledge on groundwater recharge and groundwater drought is sufficient to mitigate the impacts of climate change. A review of international literature on groundwater recharge in regions with conditions similar to Sweden was performed. National literature was compared and links to international studies evaluated. A survey among Swedish groundwater experts complemented the study. Findings are that research on groundwater recharge has been scarce in Sweden over the last decades and Swedish experts seem to not have taken much notice of international literature. It is concluded that Sweden is not well prepared to predict the impact of climate change on groundwater resources, as the most crucial process linking climate to groundwater is not well understood. The study has a strong focus on Sweden, however, the results are also relevant for countries with similar geology and climate. One main conclusion is that groundwater recharge studies often remain unique, and thus hardly transferable and comparable. Two or more independent studies are rarely applied in parallel, verification based on direct groundwater observations is not common. This raises concerns about the reliability of climate change impact predictions on groundwater.

Bulk wood density measurements are recognized for their utility in ecology, industry, and biomass estimations. In tree-ring research, microdensitometric techniques are widely used, but their ability to determine the correct central tendency has been questioned. Though rarely used, it may be possible to use bulk wood density as a tool to check the accuracy of and even correct microdensitometric measurements. Since measuring bulk wood density in parallel with X-ray densitometry is quickly and easily done, we suspect that its omission is largely due to a lack of awareness of the procedure and/or its importance. In this study, we describe a simple protocol for measuring bulk wood density tailored for tree-ring researchers and demonstrate a few possible applications. To implement real-world examples of the applications, we used a sample of existing X-ray and Blue Intensity (BI) measurements from 127 living and dead Pinus sylvestris trees from northern Sweden to produce new measurements of bulk wood density. We can confirm that the central tendency in this sample material is offset using X-ray densitometry and that the diagnosis and correction of X-ray density is easily done using bulk wood density in linear transfer functions. However, this approach was not suitable for our BI measurements due to heavy discoloration. Nevertheless, we were able to use bulk wood density to diagnose and improve the use of deltaBI (latewood BI - earlywood BI) with regard to its overall trends and multi-centennial variability in a dendroclimatological application. Moreover, we experimented with percent of latewood width, scaled with bulk wood density, as a time- and cost-effective proxy for annual ring density. Although our reconstruction only explains about half of the variation in ring density, it is most likely superior to using fixed literature values of density in allometric equations aimed at biomass estimations. With this study, we hope to raise new awareness regarding the versatility and importance of bulk wood density for dendrochronology by demonstrating its simplicity, relevance, and applicability.

Climate change poses additional risks for natural and human systems including the hydrological cycle, leading to altered temporal and spatial variation of hydroclimatic conditions. This work assessed the current understanding of the dryness and wetness conditions in Sweden over the last two millenniums based on proxy and instrumental data, as well as climate model simulations and projections until the end of the twenty-first century. The assessment represents a summary of the existing literature concerning analysis of four selected indices for dry/wet conditions relating to precipitation, potential evapotranspiration (PET), and soil moisture (SPEI, PDSI [including scPDSI], SPI, and AI). SPEI considers both precipitation and PET and can show hydroclimatic conditions at different time scales. Therefore, it was chosen to summarize the past and future changes. A focus is put on dry conditions, as drought has strong influences on groundwater which is an important freshwater resource for Sweden. The millennium historical perspective reveals that the current climate is relatively wet and the future would become even wetter as a general wetting trend started some 120 years ago. However, there have been and will be large variations of both dry and wet conditions on short time scales, especially on decadal and interannual time scales. Further, the changes since the 1950s show a regional pattern with most significant wetting in the north, a slightly overall wetting in the south but a drying in central-eastern part including the island of Gotland since 1981. This pattern is broadly consistent with climate model projections for the future.