Millennial-length tree-ring records: A basis for climate reconstruction and assessment of climate extremes and trends at local to global scales

Over the last two decades, tree-rings have become one of the most important paleoclimatic archives in terms of studying high-resolution climate over the Common Era (CE) in the mid-to-high latitudes. Such analyses are crucial, not only to put current temperature and hydroclimatic changes in a longterm context, but also to assess the full range of natural and anthropogenic climate forcing. However, estimates of the first millennium remain poorly constrained due to a paucity of millennial-length treering records, causing a spatial under-representation of some regions in larger-scale climate reconstructions and increasing uncertainty in the interpretation of climatic trends and extremes. Thus, a major activity towards the assessment of long-term climate dynamics at local to regional scales is the development of new millennial-length proxy records. This thesis addresses the aforementioned topic by (i) developing two millennial-length climate reconstructions based on tree-ring width (TRW) and maximum latewood density (MXD) measurements from Pinus heldreichii trees growing in the highelevation environment at the tree line of Mount Smolikas in the Pindus Mountains in northern Greece and by (ii) assessing millennial-length temperature trends in tree-ring records in a global proxy database compiled by the PAGES 2k consortium to identify potential drivers that limit long-term temperature trends in tree-ring data. Calibration studies showed that P. heldreichii TRW is controlled by June-July precipitation and April temperatures, with differences in the climate signal of four sites arising from varying meltwater supply and exposure effects. MXD formation is controlled by summer temperatures. Climate signal strength of both parameters is particularly strong in the high-frequency spectrum, mirroring inter-annual climate variability patterns and thus allowing for a detection of climate extremes. Based on the standardized precipitation index and TRW, regional summer drought variability is reconstructed over the period 730-2015 CE. MXD of the trees is translated in an August- September temperature reconstruction covering the period 738-2014 CE. Regional high-resolution, annually resolved climate information is extended for the first time back into the first millennium, thus both reconstructions fill a temporal and spatial gap in a larger-scale hydroclimate and temperature reconstruction network in Europe. The records help to advance our understanding of climate variability over Europe, associated atmospheric processes, and external forcings. The PAGES 2k multiproxy database offers a new and unique opportunity to study the lack of long-term cooling trends in tree-ring data which can be expected in Northern Hemisphere summers, particularly in the high latitudes, due to orbitally driven changes in solar irradiance. Tests of different influencing factors reveal that preserving millennial-scale cooling trends related to orbital forcing is not feasible in most tree-ring datasets. This result is of great importance as it touches the current debate by showing that tree-ring data do not lack this trend as a consequence of inappropriate tree-ring standardization. Evaluation of the database provides a basis not only to improve future climate reconstructions, but also to enhance our understanding of long-term climate variations as well as to reduce associated uncertainties

Downscaling und multivariate Bias-Adjustierung ; Im Rahmen des BMVI-Expertennetzwerkes entwickelte Verfahren zum Postprocessing von Klimamodelldaten

Zusammenfassung: Klimaprojektionsdaten liegen originär auf einem Gitter vor, das für die Ableitung von Anpassungsmaßnahmen an den Klimawandel vor Ort zu grob ist. Zudem sind Klimaprojektionsdaten eventuell mit systematischen Ungenauigkeiten (Bias) behaftet, die insbesondere bei der Analyse schwellenwertbezogener Indizes die Ergebnisse verfälschen können. In diesem Bericht wird ein Verfahren zur multivariaten Bias-Adjustierung vorgestellt und bewertet. Multivariat bedeutet in diesem Zusammenhang, dass korrelierte Variablen gemeinsam adjustiert werden, was die Korrelation zwischen solchen Variablen erhält und insbesondere in der Klimafolgenforschung von enormer Wichtigkeit ist. Es schließt sich die Vorstellung eines statistischen Verfahrens zur Generierung von höheren räumlichen Auflösungen der Modelldaten (Downscaling) an. Hierbei wird die originäre Modellauflösung von ~ 11 km x 11 km unter Zuhilfenahme einer Hauptkomponentenanalyse (Principal Component Analysis), der Ermittlung statistischer Beziehungen zwischen den originären Modelldaten und der ermittelten Hauptkomponenten und der Anwendung dieser Beziehungen auf hochauflösende Daten, zur Generierung eines Datensatz mit einer Zielauflösung von 5 km x 5 km genutzt. Die Methode wurde im Rahmen des BMVI-Expertennetzwerkes „Wissen – Können – Handeln“ entwickelt und auf ein Ensemble von Klimaprojektionsdaten angewendet. Die Ergebnisse liefern wesentliche Beiträge in diesem Ressortforschungsprogramm, im Rahmen der Klimawirkungs- und Risikoanalyse 2021 und im Kontext der Deutschen Anpassungsstrategie an den Klimawandel sowie für Beratungsleistungen des Deutschen Wetterdienstes.

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

Pre‐instrumental summer precipitation variability in northwestern Greece from a high‐elevation Pinus heldreichii network

The spatiotemporal variability of precipitation is of vital importance to Mediterranean ecology and economy, but pre‐instrumental changes are not well understood. Here, we present a millennial‐length June–July precipitation reconstruction derived from a network of 22 Pinus heldreichii high‐elevation sites in the Pindus Mountains of northwestern Greece. Tree‐ring width chronologies from these sites cohere exceptionally well over the past several hundred years (r1467–2015 = 0.64) revealing coherence at inter‐annual to centennial timescales across the network. The network mean calibrates significantly against instrumental June–July precipitation over the past 40 years (r1976–2015 = 0.71), even though no high‐elevation observational record is available representing the moist conditions at the treeline above 1,900 m a.s.l. For the final reconstruction, the instrumental target data are adjusted to provide realistic estimates of high‐elevation summer rainfall back to 729 CE. The reconstruction contains substantially more low‐frequency variability than other high‐resolution hydroclimate records from the eastern Mediterranean including extended dry periods from 1,350 to 1,379 CE (39 ± 4.5 mm) and 913 to 942 (40 ± 8.4 mm), and moist periods from 862 to 891 (86 ± 11 mm) and 1,522 to 1,551 (80 ± 3.5 mm), relative to the long‐term mean of 61 mm. The most recent 30‐year period from 1986 to 2015 is characterized by above average June–July precipitation (73 ± 2 mm). Low‐frequency changes in summer precipitation are likely related to variations in the position and persistence of storm tracks steering local depressions and causing extensive rainfall (or lack thereof) in high‐elevation environments of the Pindus Mountains.Associated with a strengthening of circum‐global sub‐tropical high‐pressure belts, climate models unequivocally predict a decrease of Mediterranean precipitation, accompanied by an increase of extreme events in the upcoming decades. Long‐term desiccation will amplify evaporative demand challenging plant metabolism and foster an even greater need to irrigate Mediterranean crops. We place these recent hydroclimate dynamics into a long‐term context and explore the feasibility of reconstructing low‐frequency precipitation variability by employing a large network of high‐elevation Pinus heldreichii sites from northwestern Greece.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

Recent hot and dry summers in Germany in comparison to climate projections

This study investigates whether the recent extremely hot and dry summers (2003, 2015, 2018 and 2019) in Germany will be normal summer conditions under future climate change scenarios. Abnormally persistent high-pressure systems during these recent German summers maintained clear skies and dry conditions on the ground, resulting in record-breaking heat and drought conditions. Here, climate indices are calculated from the German Meteorological Service's (DWD) climate model reference-ensembles for periods 2031–2060 and 2071–2100 under greenhouse gas emission scenarios RCP 2.6 (11 ensemble members) and RCP 8.5 (21 ensemble members) and are compared with climate indices calculated from three DWD observational datasets (station data and gridded datasets: HYRAS and DWD Climate Monitoring Grids). The climate projections show increasing summer heat conditions for Germany throughout this century and a slight shift to dryer conditions, especially for the RCP 8.5 scenario. These recent extreme summers are often in the 15 % hottest summers for RCP 2.6 in 2031–2060, 2071–2100 and the RCP 8.5 2031–2060 scenarios, but would be considered normal or even cool summers under the RCP 8.5 scenario in 2071–2100. Due to the combination of extreme heat and strong precipitation deficits, the climatic water balance and derived meteorological drought indicator values of these recent summers are often within the top 15 % of projected extreme summer drought indices and are sometimes only matched by the most anomalous hot and dry summers in the DWD reference-ensembles, even for the high-emission RCP 8.5 scenario in 2071–2100

A 1200+year reconstruction of temperature extremes for the northeastern Mediterranean region

Proxy evidence is necessary to place current temperature and hydroclimatic changes in a long-term context and to assess the full range of natural and anthropogenic climate forcings. Here, we present the first millennium-length reconstruction of late summer (August-September) temperature variability for the Mediterranean region. We compiled 132 maximum latewood density (MXD) tree-ring series of living and relict Pinus heldreichii trees from a network of four high-elevation sites in the Pindus Mountains of Greece. Forty series reach back into the first millennium and the oldest sample dates to 575 CE. At annual to decadal scales, the record correlates significantly with August-September temperatures over the Balkan Peninsula and northeastern Mediterranean (r1950-2014 = 0.71, p < 0.001). We produce two reconstructions emphasizing interannual and decadal scale variance over the past millennium. Analysis of temperature extremes reveals the coldest summers occurred in 1035, 1117, 1217, 1884 and 1959 and the coldest decades were 1061-1070 and 1811-1820. The warmest summers occurred in 1240 and 1474, and the warmest decades were 1141-1150 and 1481-1490. Comparison of this new reconstruction with MXD-based summer temperature reconstructions across Europe reveals synchronized occurrences of extreme cool summers in the northeastern Mediterranean, and an antiphase-relationship with warm summer temperatures over the British Isles and Scandinavia. This temperature dipole is related to anomalies in the latitudinal position of the North Atlantic Jet. Despite the representation of common atmospheric forcing patterns, the occurrence of warm extremes is limited to few events, suggesting potential weaknesses of MXD to record warm temperature anomalies. In addition, we acknowledge problems in the observational data to capture local temperature variability due to small scale topographic differences in this high-elevation landscape. At a broader geographical scale, the occurrence of common cold summer extremes is restricted to years with volcanically induced changes in radiative forcing.Deutsche Forschungsgemeinschaft [ES 161/9-1Inst 247/665-1 FUGG]; National Science Foundation [AGS-1349942]12 month embargo; published online: 29 November 2018This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Resumos em andamento - Fisiopatologia

Resumos em andamento - Fisiopatologi