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.

] Department of Physics Purdue University West Lafayette Indiana 47907 USA [2] Department of Earth Atmospheric and Planetary Sciences Purdue University West Lafayette Indiana 47907 USA

] Oeschger Centre for Climate Change Research University of Bern 3012 Bern Switzerland [2] Climate and Environmental Physics University of Bern 3012 Bern Switzerland

] Swiss Federal Research Institute WSL 8903 Birmensdorf Switzerland [2] Oeschger Centre for Climate Change Research University of Bern 3012 Bern Switzerland [3] Global Change Research Centre AS CR 60300 Brno Czech Republic

Alfred Wegener Institut Helmholtz Zentrum für Polar und Meeresforschung 27570 Bremerhaven Germany

British Antarctic Survey Natural Environment Research Council Cambridge CB3 0ET UK

Centre for Ice and Climate Niels Bohr Institute University of Copenhagen 2100 Copenhagen Denmark

Department of Earth and Space Sciences University of Washington Seattle Washington 98195 USA

Department of Geology Quaternary Sciences Lund University 22362 Lund Sweden

Department of History The University of Nottingham Nottingham NG7 2RD UK

Department of Physics Purdue University West Lafayette Indiana 47907 USA

Desert Research Institute Nevada System of Higher Education Reno Nevada 89512 USA

School of Geography Archaeology and Palaeoecology Queen's University Belfast Belfast BT7 1NN UK

Space Sciences Laboratory University of California Berkeley California 94720 USA

The Laboratory of Tree Ring Research University of Arizona Tucson Arizona 85721 USA

Yale Climate and Energy Institute and Department of History Yale University New Haven Connecticut 06511 USA

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