Climate variations influenced the agricultural productivity, health risk, and conflict level of preindustrial societies. Discrimination between environmental and anthropogenic impacts on past civilizations, however, remains difficult because of the paucity of high-resolution paleoclimatic evidence. We present tree ring-based reconstructions of central European summer precipitation and temperature variability over the past 2500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration. Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~250 to 600 C.E. coincided with the demise of the western Roman Empire and the turmoil of the Migration Period. Such historical data may provide a basis for counteracting the recent political and fiscal reluctance to mitigate projected climate change.

An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability.

The supposed role of climate change on societal reorganizations in Europe 1,2 and Asia 3,4 during the first half Common Era (CE) is difficult to prove without adequate annually resolved and absolutely dated climate proxy archives 5,6. Interpretation of concurrences between cooling in the 6 th century and pandemic 7,8 , rising and falling civilizations 1-6 , human migrations and political turmoil 8-13 lacks understanding of scalar and causal mechanisms. Here we use tree-ring chronologies from the Russian Altai and Austrian Alps to reconstruct summer temperatures over the past two millennia. In both regions, conditions during Roman and recent times were warmer than throughout the medieval period. Unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 CE 14 , was likely sustained by ocean and sea-ice feedbacks 15,16 , superimposed on a solar minimum 17. This newly defined Late Antique Little Ice Age (LALIA, 536 to ~660 CE) exceeded the LIA in severity. Covering much of the Northern Hemisphere, it should be considered as an additional environmental factor contributing to the establishment of the Justinian plague 7,8 , transformation of the eastern Roman and collapse of the Sasanian Empire 1,2,5 , movements out of the Asian steppe and Arabian Peninsula 8,11,12 , spread of Slavic-speaking people 9,10 , and upheavals in China 13. Annually resolved and absolutely dated insight into late Holocene climate variability is crucial in order to distinguish anthropogenic from natural forced variation 18 , and to evaluate the performance of climate model simulations 19. Spatially well-distributed palaeoclimatic archives are also essential for answering questions surrounding possible relationships between climate variability and human history 5,6. However, around the world today, there are only 13 temperature sensitive tree-ring chronologies that span the entire CE (Table S1).

Aim To evaluate the climate sensitivity of model-based forest productivity estimates using a continental-scale tree-ring network

Growing scientific evidence from modern climate science is loaded with implications for the environmental history of the Roman Empire and its successor societies. The written and archaeological evidence, although richer than commonly realized, is unevenly distributed over time and space. A first synthesis of what the written records and multiple natural archives (multi-proxy data) indicate about climate change and variability across western Eurasia from c. 100 b.c. to 800 a.d. confirms that the Roman Empire rose during a period of stable and favorable climatic conditions, which deteriorated during the Empire's third-century crisis. A second, briefer period of favorable conditions coincided with the Empire's recovery in the fourth century; regional differences in climate conditions parallel the diverging fates of the eastern and western Empires in subsequent centuries. Climate conditions beyond the Empire's boundaries also played an important role by affecting food production in the Nile valley, and by encouraging two major migrations and invasions of pastoral peoples from Central Asia.

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.

No abstract

Summary1. The effects of climate change on Arctic ecosystems can range between various spatiotemporal scales and may include shifts in population distribution, community composition, plant phenology, primary productivity and species biodiversity. The growth rates and age structure of tundra vegetation as well as its response to temperature variation, however, remain poorly understood because high-resolution data are limited in space and time. 2. Anatomical and morphological stem characteristics were recorded to assess the growth behaviour and age structure of 871 dwarf shrubs from 10 species at 30 sites in coastal East Greenland at~70°N. Recruitment pulses were linked with changes in mean annual and summer temperature back to the 19th century, and a literature review was conducted to place our findings in a pan-Arctic context. 3. Low cambial activity translates into estimated average/maximum plant ages of 59/204 years, suggesting relatively small turnover rates and stable community composition. Decade-long changes in the recruitment intensity were found to lag temperature variability by 2 and 6 years during warmer and colder periods, respectively (r = 0.85 1961-2000 and 1881-1920 ). 4. Synthesis. Our results reveal a strong temperature dependency of Arctic dwarf shrub reproduction, a high vulnerability of circumpolar tundra ecosystems to climatic changes, and the ability of evaluating historical vegetation dynamics well beyond the northern treeline. The combined wood anatomical and plant ecological approach, considering insights from micro-sections to community assemblages, indicates that model predictions of rapid tundra expansion (i.e. shrub growth) following intense warming might underestimate plant longevity and persistence but overestimate the sensitivity and reaction time of Arctic vegetation.