A definition and standardised terminology for Blue Intensity from Conifers

RW acknowledges funding from NSF/NERC project (NE/W007223/1) – Understanding Trans-Hemispheric Modes of Climate Variability: A Novel Tree-Ring Data Transect spanning the Himalaya to the Southern Ocean. KS and JB were supported by the Svenska Forskningsrådet Formas (grant no. 2019-01482) and the Stiftelsen Margit Althins stipendiefond (GS2021-0023).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’.Peer reviewe

Prospects for dendroanatomy in paleoclimatology - a case study on Picea engelmannii from the Canadian Rockies

Funding: This research was supported by the Svenska Forskningsrådet Formas (grant no. 2019-01482), the National Science Foundation (grant no. 1502150), the Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (grant no. 200021_182398), and the Grantová Agentura České Republiky (grant no. 20-22351Y).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 (1585-2014CE) 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.Publisher PDFPeer reviewe

Old World megadroughts and pluvials during the Common Era

Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability

European warm-season temperature and hydroclimate since 850 CE

The long-term relationship between temperature and rainfall variables (hydroclimate) remains uncertain due to the short length of instrumental measurements and inconsistent results from climate model simulations. This lack of understanding is critical with regard to projecting future drought and flood risks. Here we assess northern Hemisphere summertime co-variability patterns between temperature and rainfall, over Europe back to 850 CE using instrumental measurements, tree-ring reconstructions, and climate model simulations. We find the temperature–hydroclimate relationship, in both the instrumental and proxt data to be more positive at lower frequencies, but less so in model simulations. In comp[arison to instrumental climate data, climate model simulations reveal a more negative co-variability between temperature and hydroclimate, across all timescales both lower and higher frequency. The reconstructions exhibit more positive co-variability. Despite observed differences in the temperature–hydroclimate co-variability patterns in instrumental, reconstructed and model simulated data, all data types share similar phase-relationships between temperature and hydroclimate, all of which indicate the common influence of external forcing of the climate system. The co-variability between temperature and soil moisture in the model simulations is overestimated, implying a possible overestimation of temperature-driven future drought risks

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

Dendroclimatology in Fennoscandia – from past accomplishments to future potential

Fennoscandia has a strong tradition in dendrochronology, and its large tracts of boreal forest make the region well suited for the development of tree-ring chronologies that extend back several thousands of years. Two of the world's longest continuous (most tree-ring chronologies are annually resolved) tree-ring width chronologies are found in northern Fennoscandia, with records from Torneträsk and Finnish Lapland covering the last ca. 7500 yr. In addition, several chronologies between coastal Norway and the interior of Finland extend back several centuries. Tree-ring data from Fennoscandia have provided important information on regional climate variability during the mid to late Holocene and have played major roles in the reconstruction of hemispheric and global temperatures. Tree-ring data from the region have also been used to reconstruct large-scale atmospheric circulation patterns, regional precipitation and drought. Such information is imperative when trying to reach better understanding of natural climate change and variability and its forcing mechanisms, and placing recent climate change within a long-term context

Reconstructed drought variability in east-central Sweden since the 1650s

In this study, we present the first regional reconstruction of summer drought for southeastern Sweden. The June-July standardized precipitation index (SPI) was reconstructed over the period 1650-2002 based on Pinus sylvestris L. tree-ring width data, where the reconstruction could account for 41.6% of the total variance in the instrumental record over 1901-2002. Our reconstruction suggests an overall wet 18th century and a dry 19th century. The most outstanding pluvial phase in the pre-instrumental period took place in the mid-1720s and lasted over 50 years, while multi-decadal periods of below average moisture conditions were reconstructed in the 1660s-1720s, 1800s-early 30s, and in the 1840s-50s. Several of these dry spells have previously been found in reconstructions from Sweden and Finland, indicating that our reconstruction reflects large-scale moisture anomalies across eastern Fennoscandia. Comparison of the SPI estimates with mid-tropospheric pressure patterns suggests that summertime drought is associated with positive pressure anomalies over British Isles and the North Sea, while an eastward movement of the Icelandic low-pressure systems over the western part of central Fennoscandia results in wetter than normal June-July conditions over the region of interest

Using Blue Intensity from drought-sensitive Pinus sylvestris in Fennoscandia to improve reconstruction of past hydroclimate variability

High-resolution hydroclimate proxy records are essential for distinguishing natural hydroclimate variability from possible anthropogenically-forced changes, since instrumental precipitation observations are too short to represent the whole spectrum of natural variability. In Northern Europe, progress in this field has been hampered by a relative lack of long and truly moisture-sensitive proxy records. In this study, we provide the first assessment of the dendroclimatic potential of Blue Intensity (BI) and partial ring-width measurements (latewood and earlywood width series) from a network of cold and drought-prone Pinus sylvestris L. sites in Sweden. Our results show that all tree-ring parameters and sites share a clear and strong sensitivity to warm-season precipitation. The ΔBI parameter, in particular, shows considerable potential for hydroclimate reconstructions, here permitting a cross-validated precipitation reconstruction capable of explaining 56% (1901–2010 period) of regional-scale warm-season high-frequency precipitation variance. Using ΔBI as an alternative to ring-width improves the predictive skill with nearly a 20 percentage points increase in explained variance, reduces signal instability over time as well as allows a broader seasonal window (May–July) to be reconstructed. Additionally, we found that earlywood BI also reflect a positive late winter through early summer temperature signal. These findings emphasize that tree-rings, and in particular wood density parameters such as from BI, are capable of providing fundamental information to advance our understanding of hydroclimate variability in regions with a cool and rather humid climate regime that traditionally has been overlooked in studies of past droughts. Increasing the spatio-temporal coverage of hydroclimate records in northern Europe, and taking full advantage of the opportunities offered by the wood densitometric properties should be considered a research priority