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.

Time series of seed production and tree-ring width of Norway spruce (Picea abies (L.) Karst.) in southern Norway were analysed for their relationship to various climatic factors occurring during "key" stages, which a priori might be expected to show a strong climate response. The following factors combined in a multiple linear regression model were found to predict seed production (based on withheld data points) with considerable accuracy, at high levels of statistical significance: June–July mean temperature and August lowest temperature in the previous year, late spring frost and June–July precipitation of the last 2 years, and January–February lowest temperature in the current year. Tree ring width was negatively correlated with the seed production index of the current year and the lowest July temperature in the previous year and positively correlated with June-July precipitation in the current year. It is suggested that habitat constraints for seedling establishment should also be considered in a more general life-history cost theory to explain masting behaviour in forest trees.