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).

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We analyze the spatio-temporal patterns of temperature variability over Northern Hemisphere land areas, on centennial time-scales, for the last 12 centuries using an unprecedentedly large network of temperature-sensitive proxy records. Geographically widespread positive temperature anomalies are observed from the 9th to 11th centuries, similar in extent and magnitude to the 20th century mean. A dominance of widespread negative anomalies is observed from the 16th to 18th centuries. Though we find the amplitude and spatial extent of the 20th century warming is within the range of natural variability over the last 12 centuries, we also find that the rate of warming from the 19th to the 20th century is unprecedented. The positive Northern Hemisphere temperature change from the 19th to the 20th century is clearly the largest between any two consecutive centuries in the past 12 centuries