Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE

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.

coastal Pacific temperate rain forest where climate conditions are more sub-Arctic boreal than the mild oceanic temperate climate typical for the biome. USA11, also Mountain hemlock from the coastal Pacific temperate rain forest in the Gulf of Alaska 1 , represents a low-elevation, distinctly non-boreal ecotype, more typical of mid-latitudes but with strong oceanic influences, producing cool summers and (for the latitude) relatively mild winters. Trees at this site were growing at an elevation of ~1500 m asl. At this elevation the growing season is less than 60 days. Despite the rather cold conditions at this elevation, the site is considered semiarid in the rain shadow of the Cascade Mountains. The climate signal of the trees is mainly hydroclimatic. USA18 comes from a very high-elevation, bristlecone pine (Pinus longaeva) site in the Sheep Mountains 4 , California, United States, in the Sierra Nevada range near the Nevada state boundary. The environment is both dry and cool, and the climate signal from the site is hard to constrain. Bristlecone pine chronologies from this region have been used to infer both, past hydroclimate as well as temperature variability.
USA07 and USA02 represent Douglas-fir (Pseudotsuga menziesii) chronologies from the El Malpais National Monument 5 , New Mexico, United States. The region is a volcanically active area on the southeast margin of the Colorado Plateau by the intersection of the Rio Grande Rift Basin.
The climate is semi-arid with steppe and half-desert environments at lower elevations, and open grasslands and forests at overall higher elevations. The tree-ring chronology, consisting of living and dead material, was originally developed to reconstruct wildfire histories and their short-and long-term relationships with climate.
South America (Chile): PAT02 comes from El Asiento in central Chile and represents material from precipitation sensitive Chilean cedar (Austrocedrus chilensis) growing on steep rocky slopes of the Andes near the species' northern distribution limit 6 . The trees were collected with the purpose to make a quantitative precipitation reconstruction. The climate in the region is on the border between cool semi-arid and Mediterranean; winters (June-August) are fairly cool and humid and summers (November-March) are rather warm and dry. Annual precipitation totals are between 300 and 400 millimetres, resulting in moisture stress during the dry summer season for the trees. PAT03 comes from Patagonian cypress (Fitzroya cupressoides) from Quildaco in the Los Lagos Region of Chile 7 . This site is typical for the Valdivian temperate rain forest region with a mild (maritime) climate receiving an annual precipitation of 4,000-4,500 mm concentrated in SH winter (June-August).
Scandinavia (Norway and Sweden): FOR01 comes from Scots pine (Pinus sylvestris L.) in Forfjorddalen 8 , a valley located on Andoya, one of the innermost islands of the mountainous archipelago Lofoten in northern Norway just above the Arctic Circle. The climate is subpolaroceanic, with surprisingly mild winters and cold summer, relative to the more continental conditions of much of the northern boreal zone. The samples come from fallen or standing dead trees. SWE01, SWE02, and SWE04 also represent relict Scots pines from the two sites in Alajärvi 9 and Gaisenjarka 10 in the Torneträsk area of the northern boreal zone in northernmost Sweden. The region is situated in the Scandes mountains close to the regional northern and upper treeline, where the overall sub-Arctic climate is slightly moderated by the maritime influences of the North Atlantic. SWE05 and SWE02 are further Scots pine samples from Håckervalen 11  with up to 500 millimetres precipitation per year, and a monthly daily mean temperature range from about -5° C in January to +14° C in July. Nearly 80% of the annual precipitation falls during the growing between May and September. JAP01 comes Japanese red cedar (Cryptomeria japonica) from the mountainous Yakushima Island in the southern Japanese archipelago 28 , Japan.
The subtropical climate is warm and humid. Although water is not expected to be a major limiting growth factor for this species and site, a mixed temperature and hydroclimatic signal is possible.  time markers in the 770s and 990s from five high-resolution ice core measurements in Greenland and Antarctica 23,24 (Fig. 1). The accumulation rate (m water equivalent year -1 ) of the individual records ranges from 0.027-0.210 (DF01 to NEEM). Please further note that the DF01 time period is based on 10 Be/ 14 C matching (*).