Volcanic eruptions contribute to climate variability, but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings. Here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual-to-decadal temperature variability in the Northern Hemisphere during the past 2,500 years. Our results are based on new records of atmospheric aerosol loading developed from high-resolution, multi-parameter measurements from an array of Greenland and Antarctic ice cores as well as distinctive age markers to constrain chronologies. Overall, cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes. Our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth-century pandemics, famines, and socioeconomic disruptions in Eurasia and Mesoamerica while allowing multi-millennium quantification of climate response to volcanic forcing.

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).