Using machine learning on tree-ring data to determine the geographical provenance of historical construction timbers

Dendroclimatology offers the unique opportunity to reconstruct past climate at annual resolution and wood from historical buildings can be used to extend such information back in time up to several millennia. However, the varying and often unclear origin of timbers affects the climate sensitivity of individual tree-ring samples. Here, we compare tree-ring width and density of 143 living larch (Larix decidua Mill.) trees at seven sites along an elevational transect from 1400 to 2200 m asl and 99 historical tree-ring series to parametrize state-of-the-art classification models for the European Alps. To achieve geographical provenance of the historical series, nine different supervised machine learning algorithms are trained and tested in their capability to solve our classification problem. Based on this assessment, we consider a tree-ring density-based and a tree-ring width-based dataset for model building. For each of these datasets, a general not species-related model and a larch-specific model including the cyclic larch budmoth influence are built. From the nine tested machine learning algorithms, Extreme Gradient Boosting showed the best performance. The density-based models outperform the ring-width models with the larch-specific density model reaching the highest skill (f1 score = 0.8). The performance metrics reveal that the larch-specific density model also performs best within individual sites and particularly in sites above 2000 m asl, which show the highest temperature sensitivities. The application of the specific density model for larch allows the historical series to be assigned with high confidence to a particular elevation within the valley. The procedure can be applied to other provenance studies using multiple tree growth characteristics. The novel approach of building machine learning models based on tree-ring density features allows to omit a common period between reference and historical data for finding the provenance of relict wood and will therefore help to improve millennium-length climate reconstructions

The IPCC’s reductive Common Era temperature history

J.E. acknowledges support by the Gutenberg Research College, J.E. M.T. and U.B. by the project AdAgrif (CZ.02.01.01/00/22_008/0004635) and ERC (AdG 882727), J.E.S. by the US NSF (OISE-1743738, AGS-2101214 and AGS-2303352), K.J.A. by the US NSF (AGS-1803946 and AGS-2102993), K.A. by the ARC (FT200100102), R.D. by the US NSF (OPP-2112314, OPP-2124885, and AGS-2102759), S.G and M. Stoffel by the SNSF (Sinergia CRSII5_183571), F.C.L. by the SRC (grant no. 2018-01272), Marianne and Marcus Wallenberg Foundation (grant no. MMW 2022-0114) and Swedish Collegium for Advanced Study (Pro Futura Scientia XIII Fellow), LS by the German Research Foundation (SCHN 1645/1-1), M. Sigl by the ERC (CoG 820047), and R.W. by the NSF-NERC (NE/W007223/1).Common Era temperature variability has been a prominent component in Intergovernmental Panel on Climate Change reports over the last several decades and was twice featured in their Summary for Policymakers. A single reconstruction of mean Northern Hemisphere temperature variability was first highlighted in the 2001 Summary for Policymakers, despite other estimates that existed at the time. Subsequent reports assessed many large-scale temperature reconstructions, but the entirety of Common Era temperature history in the most recent Sixth Assessment Report of the Intergovernmental Panel on Climate Change was restricted to a single estimate of mean annual global temperatures. We argue that this focus on a single reconstruction is an insufficient summary of our understanding of temperature variability over the Common Era. We provide a complementary perspective by offering an alternative assessment of the state of our understanding in high-resolution paleoclimatology for the Common Era and call for future reports to present a more accurate and comprehensive assessment of our knowledge about this important period of human and climate history.Peer reviewe

Assessing earlywood-latewood proportion influence on tree-ring stable isotopes

Tree-ring stable isotopes are typically measured in latewood cellulose to mitigate potential carry-over effects from previous year storage pools. The isotopic composition of individual tree-ring segments is thought to include considerable intra-annual variability. This sampling strategy may be complicated by steep intra-annual isotope gradients that can rival the inter-annual variability, however. Consistent sampling of latewood material may not always be possible due to low sample availability or high prevalence of narrow rings or low amounts of latewood because of species-specific changes in ring width. Therefore, years that contain samples with higher portions of non-latewood (earlywood) material may influence the final chronology of isotopic variability. Here, we analyze the potential influence that changing earlywood and latewood components of individual tree rings can have on stable carbon and oxygen records from Quercus spp. and Pinus heldreichii chronologies. Analysis of stable isotopes in oak tree rings with varying amounts of latewood show no statistically significant differences in the range of isotopic composition, nor any major differences when considering the same calendric year. Similar results were found for the pine data, when comparing stable isotope measurements with earlywood-to-latewood ratio and maximum density. We argue that this simple approach should be applied to any long-term tree-ring stable isotope record in order to provide a better understanding of the potential biases that could arise from previously recorded intra-annual variability in the wood

Increasing volatility of reconstructed Morava River warm-season flow, Czech Republic

Study region: The Morava River basin, Czech Republic, Danube Basin, Central Europe. Study focus: Hydrological summer extremes represent a prominent natural hazard in Central Europe. River low flows constrain transport and water supply for agriculture, industry and society, and flood events are known to cause material damage and human loss. However, understanding changes in the frequency and magnitude of hydrological extremes is associated with great uncertainty due to the limited number of gauge observations. Here, we compile a tree-ring network to reconstruct the July–September baseflow variability of the Morava River from 1745 to 2018 CE. An ensemble of reconstructions was produced to assess the impact of calibration period length and trend on the long-term mean of reconstruction estimates. The final estimates represent the first baseflow reconstruction based on tree rings from the European continent. Simulated flows and historical documentation provide quantitative and qualitative validation of estimates prior to the 20th century. New hydrological insights for the region: The reconstructions indicate an increased variability of warm-season flow during the past 100 years, with the most extreme high and low flows occurring after the start of instrumental observations. When analyzing the entire reconstruction, the negative trend in baseflow displayed by gauges across the basin after 1960 is not unprecedented. We conjecture that even lower flows could likely occur in the future considering that pre-instrumental trends were not primarily driven by rising temperature (and the evaporative demand) in contrast to the recent trends

Decadally resolved lateglacial radiocarbon evidence from New Zealand kauri

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of Arizona Board of Regents on behalf of the University of Arizona for personal use, not for redistribution. The definitive version was published in Radiocarbon 58 (2016): 709-733, doi: 10.1017/RDC.2016.86.The Last Glacial-Interglacial Transition (LGIT; 15,000-11,000 cal BP) was characterized by complex spatiotemporal patterns of climate change, with numerous studies requiring accurate chronological control to decipher leads from lags in global paleoclimatic, -environmental and archaeological records. However, close scrutiny of the few available tree-ring chronologies and 14C-dated sequences composing the IntCal13 radiocarbon calibration curve, indicates significant weakness in 14C calibration across key periods of the LGIT. Here, we present a decadally-resolved atmospheric 14C record derived from New Zealand kauri spanning the Lateglacial from ~13,100 - 11,365 cal BP. Two floating kauri 14C time series, curve-matched to IntCal13, serve as a radiocarbon backbone through the Younger Dryas. The floating Northern Hemisphere (NH) 14C datasets derived from the YD-B and Central European Lateglacial Master tree-ring series are matched against the new kauri data, forming a robust NH 14C time series to ~14,200 cal BP. Our results show that IntCal13 is questionable from ~12,200 - 11,900 cal BP and the ~10,400 BP 14C plateau is approximately five decades too short. The new kauri record and re-positioned NH pine 14C series offer a refinement of the international 14C calibration curves IntCal13 and SHCal13, providing increased confidence in the correlation of global paleorecords.This work was part funded by the Foundation for Research, Science and Technology (FRST)—now Ministry for Business, Innovation & Employment (MBIE)-PROP-20224-SFK-UOA), a Royal Society of New Zealand grant, the Australian Research Council (FL100100195 and DP0664898) and the Natural Environment Research Council (NE/H009922/1, NE/I007660/1, NER/A/S/2001/01037 and NE/H007865/1)

Return of the moth: rethinking the effect of climate on insect outbreaks

Abstract: The sudden interruption of recurring larch budmoth (LBM; Zeiraphera diniana or griseana Gn.) outbreaks across the European Alps after 1982 was surprising, because populations had regularly oscillated every 8–9 years for the past 1200 years or more. Although ecophysiological evidence was limited and underlying processes remained uncertain, climate change has been indicated as a possible driver of this disruption. An unexpected, recent return of LBM population peaks in 2017 and 2018 provides insight into this insect’s climate sensitivity. Here, we combine meteorological and dendrochronological data to explore the influence of temperature variation and atmospheric circulation on cyclic LBM outbreaks since the early 1950s. Anomalous cold European winters, associated with a persistent negative phase of the North Atlantic Oscillation, coincide with four consecutive epidemics between 1953 and 1982, and any of three warming-induced mechanisms could explain the system’s failure thereafter: (1) high egg mortality, (2) asynchrony between egg hatch and foliage growth, and (3) upward shifts of outbreak epicentres. In demonstrating that LBM populations continued to oscillate every 8–9 years at sub-outbreak levels, this study emphasizes the relevance of winter temperatures on trophic interactions between insects and their host trees, as well as the importance of separating natural from anthropogenic climate forcing on population behaviour

Prominent role of volcanism in Common Era climate variability and human history

Climate reconstructions for the Common Era are compromised by the paucity of annually-resolved and absolutely-dated proxy records prior to medieval times. Where reconstructions are based on combinations of different climate archive types (of varying spatiotemporal resolution, dating uncertainty, record length and predictive skill), it is challenging to estimate past amplitude ranges, disentangle the relative roles of natural and anthropogenic forcing, or probe deeper interrelationships between climate variability and human history. Here, we compile and analyse updated versions of all the existing summer temperature sensitive tree-ring width chronologies from the Northern Hemisphere that span the entire Common Era. We apply a novel ensemble approach to reconstruct extra-tropical summer temperatures from 1 to 2010 CE, and calculate uncertainties at continental to hemispheric scales. Peak warming in the 280s, 990s and 1020s, when volcanic forcing was low, was comparable to modern conditions until 2010 CE. The lowest June–August temperature anomaly in 536 not only marks the beginning of the coldest decade, but also defines the onset of the Late Antique Little Ice Age (LALIA). While prolonged warmth during Roman and medieval times roughly coincides with the tendency towards societal prosperity across much of the North Atlantic/European sector and East Asia, major episodes of volcanically-forced summer cooling often presaged widespread famines, plague outbreaks and political upheavals. Our study reveals a larger amplitude of spatially synchronized summer temperature variation during the first millennium of the Common Era than previously recognised

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

© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)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.The National Park “Aigüestortes i Estany de Sant Maurici” provided sampling permissions, and Meteo France temperature measurements from Pic du Midi. U.B. and J.E. were supported by the Czech Science Foundation grant HYDRO8 (23-08049S), and the ERC Advanced grant MONOSTAR (AdG 882727).Peer reviewe

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