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.

AgroParisTech INRA Université de Lorraine 54000 Nancy France

AMS Laboratory University of Arizona Tucson AZ 85721 USA

Archaeological Service Kanton Thurgau 8510 Frauenfeld Switzerland

Bavarian State Office for Monument Protection 80539 München Germany

Bolin Centre for Climate Research Stockholm University SE 10691 Stockholm Sweden

Center for Climate and Resilience Research Blanco Encalada 2002 8370449 Santiago Chile

Chair of Forest Growth and Dendroecology Institute of Forest Sciences University of Freiburg Freiburg Germany

CNR IVALSA Trees and Timber Institute 38010 San Michele all'Adige TN Italy

Competence Center for Underwater Archaeology and Dendrochronology Office for Urbanism City of Zurich 8008 Zürich Switzerland

Département de biologie chimie et géographie University of Québec in Rimouski QC G5L 3A1 Canada

Department of Earth and Environmental Systems Indiana State University Terre Haute IN 47809 USA

Department of Earth Sciences University of Gothenburg 405 30 Gothenburg Sweden

Department of Environmental Science William Paterson University Wayne NJ 07470 USA

Department of Geography Johannes Gutenberg University 55099 Mainz Germany

Department of Geography Masaryk University 611 37 Brno Czech Republic

Department of Geography Swansea University Swansea SA2 8PP Wales UK

Department of Geography University of Cambridge Cambridge CB2 3EN UK

Department of Geography University of Guelph ON N1G 2W1 Canada

Department of Geography University of Tennessee Knoxville TN 37996 0925 USA

Department of Geography University of Western Ontario London ON N6A 3K7 Canada

Department of Geology and Geography West Virginia University WV 26505 6300 USA

Department of Geosciences University of Arizona Tucson AZ 85721 USA

Department of History Stockholm University SE 10691 Stockholm Sweden

Department of Humanities Siberian Federal University 660041 Krasnoyarsk Russia

Department of of Earth Sciences The College of Wooster OH 44691 USA

Department of Physical Geography Stockholm University SE 106 91 Stockholm Sweden

Department of Wood Science Mendel University in Brno 61300 Brno Czech Republic

Department Territorio e Sistemi Agro Forestali University of Padova 35020 Legnaro Italy

German Archaeological Institute 14195 Berlin Germany

Global Change Research Institute CAS 603 00 Brno Czech Republic

Harvard Forest Harvard University Petersham MA 01366 USA

Icelandic Forest Research Mógilsá 116 Reykjavik Iceland

Institute for Environmental Sciences University of Geneva 1205 Geneva Switzerland

Institute for Space Earth Environmental Research Nagoya University Nagoya 464 8601 Japan

Institute of Geography Friedrich Alexander University Erlangen Nürnberg 91058 Erlangen Germany

Institute of Geography University of Innsbruck 6020 Innsbruck Austria

Institute of Plant and Animal Ecology Ural Branch of the Russian Academy of Sciences Ekaterinburg 620144 Russia

Instituto Argentino de Nivología Glaciología y Ciencias Ambientales IANIGLA CONICET Mendoza CP 330 5500 Argentina

Isotope Climatology and Environmental Research Centre Institute of Nuclear Research H 4001 Debrecen Hungary

Key Laboratory of Desert and Desertification Northwest Institute of Eco Environment and Resources Chinese Academy of Sciences 730000 Lanzhou China

Laboratorio de Dendrocronología y Cambio Global Universidad Austral de Chile Casilla 567 Valdivia Chile

Laboratory for Ion Beam Physics ETH Zürich CH 8093 Zurich Switzerland

Laboratory of Environmental Chemistry Paul Scherrer Institute 5232 Villigen Switzerland

Laboratory of Tree Ring Research University of Arizona Tucson AZ 85721 USA

Navarino Environmental Observatory GR 24001 Messinia Greece

Palaeontology Geobiology and Earth Archives Research Centre and ARC Centre of Excellence for Australian Biodiversity and Heritage School of Biological Earth and Environmental Sciences The University of New South Wales Sydney NSW 2052 Australia

Rubenstein School of Environment and Natural Resources University of Vermont Burlington Vermont 05405 USA

School of Environment University of Auckland 1010 Auckland New Zealand

School of Geography and Geosciences University of St Andrews St Andrews KY16 9AJ Scotland UK

School of Natural and Built Environment Queen's University Belfast BT7 1NN Northern Ireland UK

School of Natural Science Hampshire College Amherst MA 01002 USA

Sukachev Institute of Forest SB RAS 660036 Krasnoyarsk Russia

Swedish Polar Research Secretariat SE 104 05 Stockholm Sweden

Swiss Federal Institute of Aquatic Science and Technology Eawag CH 8600 Dübendorf Switzerland

Swiss Federal Research Institute WSL CH 8903 Birmensdorf Switzerland

Tree Ring Laboratory Lamont Doherty Earth Observatory of Columbia University Palisades NY 10964 8000 USA

Erratum v

Nat Commun. 2018 Dec 17;9(1):5399. doi: 10.1038/s41467-018-07636-6. PubMed

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