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

Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE

This study was funded by the WSL-internal COSMIC project (5233.00148.001.01), the ETHZ (Laboratory of Ion Beam Physics), the Swiss National Science Foundation (SNF Grant 200021L_157187/1), and as the Czech Republic Grant Agency project no. 17-22102s.Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.Publisher PDFPeer reviewe

Normalized 14C activity ratios (F14C) of an African baobab (Adansonia digitata) tree from Oman

Calender dating and annual character of the baobab's growth rings was confirmed by matching of highly resolved data of normalized 14C activity ratios (F14C) with the bomb peak. Normalized 14C activity ratio (F14C) data was determined at ETH Zürich, Dep. of Earth Sciences, Labor für Ionenstrahlphysik (LIP) and was obtained from cellulose extracted with a base-acid-base-acid-bleaching procedure after (Němec et al., 2010). Cellulose samples were graphitized using an Automated Graphitization Equipment (AGE III) coupled to an elemental analyzer (EA: Vario MICRO cube; Wacker et al., 2010). Once graphitized, the samples were immediately pressed into targets to minimize their interaction with air and their 14C content was measured with a MICADAS mass spectrometer (Wacker et al., 2010a). With intention to minimize possible carry-over effects from the implementation of previous years' non-structural carbohydrates into the current year's wood cellulose, the samples for dating were selected from the last third of each growth ring structure and comprised fibres and vessels while steering clear of terminal parenchyma bands (TPB)which usually complete each Baobab tree ring. 5 individual TPBs were analyzed for 1960-1963 (n = 4) and for 2005. In addition, intra-tree-ring F14C analysis was performed on the tree rings of 1962 (6 samples) and 1963 (8 samples)

Normalized 14C activity ratios (F14C), tree-ring width indices (RWI) and subseasonal δ13C and δ18O of tree-ring cellulose of an African baobab (Adansonia digitata) tree from Oman

Normalized 14C activity ratios (F14C), tree-ring width indices (RWI) and subseasonal δ13C and δ18O of tree-ring cellulose of an African baobab (Adansonia digitata) analyses were performed on a ca. 80cm long, SW oriented wood increment core (12mm OD) sampled in April 2015

Subseasonal δ13C and δ18O of tree-ring cellulose of an African baobab (Adansonia digitata) tree from Oman

Carbon and oxygen stable isotope data of tree-ring cellulose covers the period from 1941 to 2005CE. Tangential wood slices of approximately 1 mm were cut parallel to the fiber orientation, in radial direction from the cambial zone towards the pith resulting in resolutions from 1 up to 11 samples per year. Visible rays and other identified parenchymatous structures were removed from the wood slices to minimize possible contamination. Cellulose was extracted from wood samples after Wieloch et al. (2011). After extraction cellulose samples were homogenized by ultrasonic treatment (Laumer et al., 2009), and freeze-dried for at least 48 h. 130 180 μg of cellulose were packed in silver capsules and stored over night at 100 °C in a vacuum drying oven (ThermoFisher Heraeus VT 6060) prior to isotope ratio mass spectrometry. δ13C and δ18O were measured simultaneously by reduction of cellulose samples to carbon monoxide in a high temperature TC/EA converter (1400 °C) coupled online to an IRMS Delta V Plus (Thermo Fisher Scientific, Bremen, Germany). Values are referred to VPDB and VSMOW, respectively with a reproducibility of ≤ 0.15 for δ13C and ≤ 0.25 for δ18O

Tree-ring width indices (RWI) of an African baobab (Adansonia digitata) tree from Oman

The surface of the wood core was prepared with a razor blade perpendicular to the longitudinal stem growth and photographs were taken with a fluorescence microscope (Nikon MULTIZOOM AZ100M, V-2A: 380-420 nm; camera head: Nikon digital sight DS-Fi1c; light source: C-HGFI HG Precenteres Fiber Illumiator (130 W mercury lamp); program: NIS Elements 4.30.01© 1991-2014 Laboratory Imaging). Tree-ring width measurements were performed on a stitched panorama of overlapping core pictures by using Windendro (Regents Inst. Canada). Data was detrended using an exponential fit (R program, package dplR; Bunn et al., 2018) resulting in a dimensionless ring-width index chronology (RWI) for the time period 1911 to 2005CE