Existing global volcanic radiative aerosol forcing estimates portray the period 700 to 1000 as volcanically quiescent, void of major volcanic eruptions. However, this disagrees with proximal Icelandic geological records and regional Greenland ice-core records of sulfate. This study uses cryptotephra analyses, high-resolution sulfur isotope analyses, and glaciochemically volcanic tracers on an array of Greenland ice cores to characterize volcanic activity and climatically important sulfuric aerosols across the period 700 to 1000. An episode of volcanic sulfur dioxide emissions (751–940), dominated by Icelandic volcanism, termed the Icelandic Active Period, is identified. This period commences with the Hrafnkatla episode (751–763), which coincided with strong winter cooling anomalies across Europe. The study reveals a significant contribution of prolonged volcanic sulfate emissions to the pre-industrial atmospheric aerosol burden, currently unaccounted for in existing forcing estimates, and highlights the need for further research to understand associated climate feedbacks.

Imogen Gabriel, Gill Plunkett, Peter M. Abbott, Melanie Behrens, Andrea Burke, Nathan Chellman, Eliza Cook, Dominik Fleitmann, Maria Hörhold, William Hutchison, Joseph R. McConnell, Bergrún A. Óladóttir, Johannes Preiser-Kapeller, Jakub T. Sliwinski, Patrick Sugden, Birthe Twarloh, Michael Sigl