University of Oulu

Brehm, N., Bayliss, A., Christl, M. et al. Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings. Nat. Geosci. 14, 10–15 (2021).

Eleven-year solar cycles over the last millennium revealed by radiocarbon in tree rings

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Author: Brehm, Nicolas1; Bayliss, Alex2; Christl, Marcus1;
Organizations: 1Laboratory of Ion Beam Physics, ETHZ, Otto-Stern Weg 5 HPK, 8093 Zurich, Switzerland
2Historic England, Cannon Bridge House, 25 Dowgate Hill, London, EC4R 2YA, UK
3Department of Geology, Lund University, Sölvegatan 12, Lund, Sweden
4Climate and Environmental Physics, Physics Institute & Oeschger Centre for Climate Change Research, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
5Eawag, 8600 Dübendorf, Switzerland
6Heidelberg University, Institute of Environmental Physics, Heidelberg, Baden-Württemberg, Germany
7Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen
8School of Space Research, Kyung Hee University, Yongin, Gyeonggi-Do, 446-701, Republic of Korea
9Space Physics and Astronomy Research unit and Sodankylä Geophysical Observatory, University of Oulu, Finland
10Underwater archaeology and Dendroarchaeology, Office for Urbanism, Zurich
Format: article
Version: accepted version
Access: open
Online Access: PDF Full Text (PDF, 1.4 MB)
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Language: English
Published: Springer Nature, 2021
Publish Date: 2021-07-04


The Sun provides the principal energy input into the Earth system and solar variability represents a significant external climate forcing. Although observations of solar activity (sunspots) cover only the last about 400 years, radionuclides produced by cosmic rays and stored in tree rings or ice cores serve as proxies for solar activity extending back thousands of years. However, the presence of weather-induced noise or low temporal resolution of long, precisely dated records hampers cosmogenic nuclide-based studies of short-term solar variability such as the 11-yr Schwabe cycle. Here we present a continuous, annually resolved atmospheric ¹⁴C concentration (fractionation-corrected ratio of ¹⁴CO₂ to CO₂) record reconstructed from absolutely dated tree rings covering nearly all of the last millennium (ad 969–1933). The high-resolution and precision ¹⁴C record reveals the presence of the Schwabe cycle over the entire time range. The record confirms the ad 993 solar energetic particle event and reveals two new candidates (ad 1052 and ad 1279), indicating that strong solar events that might be harmful to modern electronic systems probably occur more frequently than previously thought. In addition to showing decadal-scale solar variability over the last millennium, the high-temporal-resolution record of atmospheric radiocarbon also provides a useful benchmark for making radiocarbon dating more accurate over this interval.

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Series: Nature geoscience
ISSN: 1752-0894
ISSN-E: 1752-0908
ISSN-L: 1752-0894
Volume: 14
Pages: 10 - 15
DOI: 10.1038/s41561-020-00674-0
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: The Laboratory of Ion Beam Physics is partially funded by its consortium partners PSI, EAWAG and EMPA. N.B. is funded by the Swiss National Science Foundation (SNSF grant number SNF 197137). I.U. acknowledges support from the Academy of Finland (project number 321882 ESPERA).
Academy of Finland Grant Number: 321882
Detailed Information: 321882 (Academy of Finland Funding decision)
Copyright information: © The Author(s), under exclusive licence to Springer Nature Limited 2020. This is a post-peer-review, pre-copyedit version of an article published in Nat. Geosci. The final authenticated version is available online at