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.

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In this study, the Weibull distribution is tested as a possible model for fire interval data derived from dendrochronologically-dated fire scars from four sites in the American Southwest. Two- and three-parameter Weibull distributions were fit to fire interval data sets, and additional statistical descriptors based on the Weibull were derived to improve our understanding of the range of variability in presettlement fire regimes. The three-parameter models failed to provide improved fits versus the more parsimonious two-parameter models, indicating the Weibull shift parameter may be superfluous for Southwestern fire regimes. The Weibull Modal Interval (MOI) was a superior overall measure of central tendency, and appears to identify a common underlying structure in Southwestern fire regimes independent of habitat type and environmental gradients. Unusually short and long fire intervals were identified by the lower and upper exceedance intervals (LEI and UEI) and the Maximum Hazard Interval (MHI) based on the Weibull hazard function. Model statistics were nearly identical between two pairs of sites that were 260 kilometers distant that differed in topography, vegetation, and land-use history. However, differences were observed between sites only 10 kilometers apart, suggesting the influence of local factors (e.g., topography and substrate) over regional influences (e.g., climate). Although the Weibull models helped quantify the historical range of variability in presettlement fire regimes, ecological interpretations of the Weibull parameters proved difficult.

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The original version of this Article contained an error in the Data Availability section, which incorrectly read 'All data will be freely available via https://www.ams.ethz.ch/research.html.' The correct version states 'http://www.ams.ethz.ch/research/published-data.html' in place of 'https://www.ams.ethz.ch/research.html'. This has been corrected in both the PDF and HTML versions of the Article.