Arctic hydroclimate variability during the last 2000 years : current understanding and research challenges

Reanalysis data show an increasing trend in Arctic precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydro-climate changes are expected to continue and possibly accelerate in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through the atmospheric and ocean circulations. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand Arctic hydroclimate and its variability prior to the instrumental record, climate proxy records are needed. The purpose of this review is to summarise the current understanding of Arctic hydroclimate during the past 2000 years. First, the paper reviews the main natural archives and proxies used to infer past hydroclimate variations in this remote region and outlines the difficulty of disentangling the moisture from the temperature signal in these records. Second, a comparison of two sets of hydroclimate records covering the Common Era from two data-rich regions, North America and Fennoscandia, reveals inter- and intra-regional differences. Third, building on earlier work, this paper shows the potential for providing a high-resolution hydroclimate reconstruction for the Arctic and a comparison with last-millennium simulations from fully coupled climate models. In general, hydroclimate proxies and simulations indicate that the Medieval Climate Anomaly tends to have been wetter than the Little Ice Age (LIA), but there are large regional differences. However, the regional coverage of the proxy data is inadequate, with distinct data gaps in most of Eurasia and parts of North America, making robust assessments for the whole Arctic impossible at present. To fully assess pan-Arctic hydroclimate variability for the last 2 millennia, additional proxy records are required.Peer reviewe

The utility of bulk wood density for tree-ring research

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

The influence of decision-making in tree ring-based climate reconstructions.

Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794-2016 CE at 0.79 (p < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability

Arctic hydroclimate variability during the last 2000 years: current understanding and research challenges

Reanalysis data show an increasing trend in Arctic precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydroclimate changes are expected to continue and possibly accelerate in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through the atmospheric and ocean circulations. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand Arctic hydroclimate and its variability prior to the instrumental record, climate proxy records are needed. The purpose of this review is to summarise the current understanding of Arctic hydroclimate during the past 2000 years. First, the paper reviews the main natural archives and proxies used to infer past hydroclimate variations in this remote region and outlines the difficulty of disentangling the moisture from the temperature signal in these records. Second, a comparison of two sets of hydroclimate records covering the Common Era from two data-rich regions, North America and Fennoscandia, reveals inter- and intra-regional differences. Third, building on earlier work, this paper shows the potential for providing a high-resolution hydroclimate reconstruction for the Arctic and a comparison with last-millennium simulations from fully coupled climate models. In general, hydroclimate proxies and simulations indicate that the Medieval Climate Anomaly tends to have been wetter than the Little Ice Age (LIA), but there are large regional differences. However, the regional coverage of the proxy data is inadequate, with distinct data gaps in most of Eurasia and parts of North America, making robust assessments for the whole Arctic impossible at present. To fully assess pan-Arctic hydroclimate variability for the last 2 millennia, additional proxy records are required.</p

Hydroclimate variability in Scandinavia over the last millennium - insights from a climate model-proxy data comparison

The integration of climate proxy information with general circulation model (GCM) results offers considerable potential for deriving greater understanding of the mechanisms underlying climate variability, as well as unique opportunities for out-of-sample evaluations of model performance. In this study, we combine insights from a new tree-ring hydroclimate reconstruction from Scandinavia with projections from a suite of forced transient simulations of the last millennium and historical intervals from the CMIP5 and PMIP3 archives. Model simulations and proxy reconstruction data are found to broadly agree on the modes of atmospheric variability that produce droughts–pluvials in the region. Despite these dynamical similarities, large differences between simulated and reconstructed hydroclimate time series remain. We find that the GCM-simulated multi-decadal and/or longer hydroclimate variability is systematically smaller than the proxy-based estimates, whereas the dominance of GCMsimulated high-frequency components of variability is not reflected in the proxy record. Furthermore, the paleoclimate evidence indicates in-phase coherencies between regional hydroclimate and temperature on decadal timescales, i.e., sustained wet periods have often been concurrent with warm periods and vice versa. The CMIP5–PMIP3 archive suggests, however, out-of-phase coherencies between the two variables in the last millennium. The lack of adequate understanding of mechanisms linking temperature and moisture supply on longer timescales has serious implications for attribution and prediction of regional hydroclimate changes. Our findings stress the need for further paleoclimate data–model intercomparison efforts to expand our understanding of the dynamics of hydroclimate variability and change, to enhance our ability to evaluate climate models, and to provide a more comprehensive view of future drought and pluvial risks

Late Holocene spatiotemporal hydroclimatic variability over Fennoscandia inferred from tree-rings

There is a broad scientific consensus that the global climate is changing, and that human activity is a significant factor contributing to the change. The response of the hydrological cycle to the warming is far reaching, including increases in the intensification and frequency of extreme hydroclimatological events. The underlying physical mechanisms driving this changes are poorly understood, and the observational record, which rarely predates the 20th century, is too short to resolve the full range of natural moisture variability or make predictions of longer-term hydroclimatic patterns. Tree-rings provide precisely dated and annually resolved paleoclimatic archives, which can be used to infer climate in the pre-instrumental era. Focused on the Fennoscandian region, the core efforts of this dissertation work are (1) to examine the potential of Fennoscandian tree-ring data as proxies of past moisture variability, (2) to increase the network of moisture sensitive tree-ring chronologies in the region, and finally (3) to combine the newly sampled data with already existing dendrochronological material to develop a first spatiotemporal reconstruction of Fennoscandian hydroclimatic variability spanning over the past millennium. A unique network of twenty-seven moisture sensitive chronologies was provided for southern and central Scandinavia. A subset of the network, combined with existing tree-ring data, was used to produce the first regional hydroclimatic reconstruction, as expressed by the Standardized Precipitation Index (SPI), for southeastern Sweden, spanning the last 350 years. The reconstruction revealed decadal scale alterations in wet and dry regimes, and proved xeric-site tree-ring data from the region to contain valuable hydroclimatic information. Moreover, a pilot study using Scots pine tree-ring carbon (δ13C) and oxygen (δ18O) measurements from the central Scandinavian mountains assessed the potential of each record as a proxy of local moisture conditions. Results showed that both isotope ratios recorded the moisture signal strongly enough to be used as a proxy of past hydroclimatic conditions. Based on these results, the potential of using multi-parameter tree-ring data (including ring-width, maximum latewood density, stable isotopes) from Fennoscandia to make spatiotemporal reconstructions of past moisture variability was first tested, and then applied to produce an “atlas” of past hydroclimatic conditions, defined by the Standardized Precipitation-Evapotranspiration Index (SPEI), spanning back to 1000 CE. The resulting reconstruction gave a unique opportunity to examine the frequency, severity, persistence, and spatial characteristics of Fennoscandian climate variability in the context of the last 1000 years. The reconstruction highlighted the 17th century as an epoch of frequent severe and widespread hydroclimatic anomalies, and the 15th-16th centuries as surprisingly free from any spatially extensive droughts/pluvials. No explicit shifts towards more frequent and intense extremes over the region were observed in the reconstructed data over the most recent century. Moreover, the analysis suggests that the spatial hydroclimatic patterns over Fennoscandia may be divided into two major modes, remarkably stable over the past seven centuries, and that the controls on these patterns may come from the summer North Atlantic Oscillation

The climatic drivers of normalized difference vegetation index and tree‐ring‐based estimates of forest productivity are spatially coherent but temporally decoupled in Northern Hemispheric forests

Aim Radial growth and foliage dynamics of trees both play a significant role in the terrestrial carbon cycle. Yet, crucial knowledge gaps exist in how these two growth components are linked. Our goal is to help bridge these gaps by providing a Northern Hemispheric survey of the connections between, and drivers of, inter‐annual wood and canopy–landscape dynamics and phenology. Location Northern (>30° N) forest ecosystems. Methods We compared a multispecies network of ca. 700 annually resolved radial tree‐growth records with the global inventory modelling and mapping studies‐normalized difference vegetation index (GIMMS‐NDVI) estimates of foliage greenness between 1982 and 2012. Tree‐ring data were assimilated into the simple process‐based Vaganov–Shashkin Lite model to derive xylem phenology on a monthly basis and were contrasted against NDVI estimates of canopy phenology. We additionally determined the response of all these vegetation measures to temperature and precipitation. Results We found broad‐scale agreement in the phenology and growing season climate response between radial tree growth and seasonally integrated canopy–landscape dynamics. On a monthly basis, however, a temporal asynchrony in the climate signals at mid‐ and high latitudes was observed, where the strongest climate response of the NDVI record occurred around leaf flush, whereas an early‐ to mid‐growing season signal dominated the tree‐ring growth. Main conclusions Our comprehensive study helps to elucidate the unique contributions of foliar and radial growth to terrestrial carbon cycling and the time‐scales at which they operate. Although we observed that both measures have similar overall climate constraints, these two growth components are sensitive to distinct seasonal windows. Our study suggests that joint assessment of both leaf and stem growth is required to address productivity of forests and demonstrates that these seasonal sensitivities must be considered before combining and interpreting these two metrics

Ensemble standardization constraints on the influence of the tree growth trends

Tree growth trends can affect the interpretation of the response of tree-ring proxies (especially tree-ring width) to climate in the low-frequency band, which in turn may limit quantitative understanding of centennial-scale climate variability. As such, it is difficult to determine if long-term trends in tree-ring measurements are caused by age-dependent growth effects or climate. Here, a trend similarity ranking method is proposed to define the range of tree growth effects on tree-ring width chronologies. This method quantifies the inner and outer boundaries of the tree growth effect following two extreme standardization methods: curve fitting standardization and regional curve standardization. The trend similarity ranking method classifies and detrends tree-ring measurements according to the ranking similarity between the regional growth curve and their long-term trends through curve fitting. This standardization process mainly affects the secular trend in tree-ring chronologies, and has no effect on their inter-annual to multi-decadal variations. Applications of this technique to the Yamal and Torneträsk treering width datasets and the maximum latewood density dataset from northern Scandinavia reveals that multi-centennial and millennial-scale temperature variations in the three regions provide substantial positive contributions to the linear warming trends in the instrumental period, and that the summer warming rate during the 20th century is not unprecedented over the past two millennia in any of the three regions

Blue intensity and density from northern Fennoscandian tree rings, exploring the potential to improve summer temperature reconstructions with earlywood information

Here we explore two new tree-ring parameters, derived from measurements of wood density and blue intensity (BI). The new proxies show an increase in the interannual summer temperature signal compared to established proxies, and present the potential to improve long-term performance. At high latitudes, where tree growth is mainly limited by low temperatures, radiodensitometric measurements of wood density, specifically maximum latewood density (MXD), provides a temperature proxy that is superior to that of tree-ring widths. The high cost of developing MXD has led to experimentation with a less expensive method using optical flatbed scanners to produce a new proxy, herein referred to as maximum latewood blue absorption intensity (abbreviated MXBI). MXBI is shown to be very similar to MXD on annual timescales but less accurate on centennial timescales. This is due to the fact that extractives, such as resin, stain the wood differentially from tree to tree and from heartwood to sapwood. To overcome this problem, and to address similar potential problems in radiodensitometric measurements, the new parameters Δblue intensity (ΔBI) and Δdensity are designed by subtracting the ambient BI/density in the earlywood, as a background value, from the latewood measurements. As a case-study, based on Scots pine trees from Northern Sweden, we show that Δdensity can be used as a quality control of MXD values and that the reconstructive performance of warm-season mean temperatures is more focused towards the summer months (JJA – June, July, August), with an increase by roughly 20% when also utilising the interannual information from the earlywood. However, even though the new parameter ΔBI experiences an improvement as well, there are still puzzling dissimilarities between Δdensity and ΔBI on multicentennial timescales. As a consequence, temperature reconstructions based on ΔBI will presently only be able to resolve information on decadal-to-centennial timescales. The possibility of trying to calibrate BI into a measure of lignin content or density, similarly to how radiographic measurements are calibrated into density, could be a solution. If this works, only then can ΔBI be used as a reliable proxy in multicentennial-scale climate reconstructions