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.

The Arctic is warming faster than any other region on Earth. Putting this rapid warming into perspective is challenging because instrumental records are often short or incomplete in polar regions and precisely-dated temperature proxies with high temporal resolution are largely lacking. Here, we provide this long-term perspective by reconstructing past summer temperature variability at Yamal Peninsula – a hotspot of recent warming – over the past 7638 years using annually resolved tree-ring records. We demonstrate that the recent anthropogenic warming interrupted a multi-millennial cooling trend. We find the industrial-era warming to be unprecedented in rate and to have elevated the summer temperature to levels above those reconstructed for the past seven millennia (in both 30-year mean and the frequency of extreme summers). This is undoubtedly of concern for the natural and human systems that are being impacted by climatic changes that lie outside the envelope of natural climatic variations for this region.

Background: Recent warming is affecting species composition and species areal distribution of many regions. However, although most treeline studies have estimated the rates of forest expansion into tundra, still little is known about the long-term dynamic of stand productivity at the forest-tundra intersection. Here, we make use of tree-ring data from 350 larch (Larix sibirica Ledeb.) and spruce (Picea obovata Ledeb.) sampled along the singular altitudinal treeline ecotone at the Polar Urals to assess the dynamic of stand establishment and productivity, and link the results with meteorological observations to identify the main environmental drivers. Results: The analysis of stand instalment indicated that more than 90% of the living trees appeared after 1900. During this period, the stand became denser and moved 50 m upward, while in recent decades the trees of both species grew faster. The maximum afforestation occurred in the last decades of the twentieth century, and the large number of encountered saplings indicates that the forest is still expanding. The upward shift coincided with a slight increase of May–August and nearly doubling of September–April precipitation while the increase in growth matched with an early growth season warming (June + 0.27 °C per decade since 1901). This increase in radial growth combined with the stand densification led to a 6–90 times increase of biomass since 1950. Conclusion: Tree-ring based twentieth century reconstruction at the treeline ecotone shows an ongoing forest densification and expansion accompanied by an increased growth. These changes are driven by climate change mechanism, whereby the leading factors are the significant increase in May–June temperatures and precipitation during the dormant period. Exploring of phytomass accumulation mechanisms within treeline ecotone is valuable for improving our understanding of carbon dynamics and the overall climate balance in current treeline ecosystems and for predicting how these will be altered by global change.

Key message Minimum wood density exhibits strong responses to precipitation and, thus, it is a robust proxy of early season water availability. Abstract Tracheids fulfil most wood functions in conifers (mechanical support and water transport) and earlywood tracheids account for most hydraulic conductivity within the annual tree ring. Dry conditions during the early growing season, when earlywood is formed, could lead to the formation of narrow tracheid lumens and a dense earlywood. Here, we assessed if there is a negative association between minimum wood density and early growing-season (spring) precipitation. Using dendrochronology, we studied growth and density data at nine forest stands of three Pinaceae species (Larix sibirica, Pinus nigra, and Pinus sylvestris) widely distributed in three cool–dry Eurasian regions from the forest-steppe (Russia, Mongolia) and Mediterranean (Spain) biomes. We measured for each annual tree ring and the common 1950–2002 period the following variables: earlywood and latewood width, and minimum and maximum wood density. As expected, dry early growing season (spring) conditions were associated with low earlywood production but, most importantly, to high minimum density in the three conifer species. The associations between minimum density and spring precipitation were stronger (r = −0.65) than those observed with earlywood width (r = 0.57). We interpret the relationship between spring water availability and high minimum density as a drought-induced reduction in lumen diameter, hydraulic conductivity, and growth. Consequently, forecasted growing-season drier conditions would translate into increased minimum wood density and reflect a reduction in hydraulic conductivity, radial growth, and wood formation. Given the case-study-like nature of this work, more research on other cold–dry sites with additional conifer species is needed to test if minimum wood density is a robust proxy of early season water availability.

Changes in the main climatic factors (air temperature and total precipitation) and their effect on the radial increment of Scots pine ( Pinus sylvestrisL.) over the period from 1933 to 2002 were...