While the global extent and time-transgressive evolution of the 774 and 993 events are similar, the amplitude of the latter event is about half that of the former.
A subset of 374 rings in the 770s CE interval originates from 27 records on the Northern Hemisphere (NH) and seven records on the Southern Hemisphere (SH).
Another 110 rings that did not reach back into the 8th century CE represent eight NH and two SH records in the 990s CE.
We therefore employ a global carbon box model to generate mechanistic understanding of the intra-annual course of the radiocarbon cycle (Supplementary Fig. With 95% confidence, the model times the event, presumed to have been ephemeral, within the boreal growing season (June to August) of 774 CE (Supplementary Fig. Importantly, this seasonal timing is consistent with the observed ~10% relative difference in radiocarbon amplitude between the NH and SH.
C content of 110 tree rings from 990–1000 CE at eight and two sites in the NH and SH (blue and red lines), respectively.
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.
Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale.