Major volcanic eruptions inject large quantities of sulfur-rich gases into the atmosphere, which can drive seasonal to multiyear cooling on hemispheric to global scales. Such volcanically forced cooling can cause profound societal disruption. A prominent example is the 1108–1110 CE volcanic episode, marked by one of the largest sulfate deposition events recorded in polar ice cores over the past 2500 years and linked to some of the most severe Northern Hemisphere (NH) summer cooling and widespread societal disruption of the past 1500 years. Our understanding of this episode has relied largely on ice-core sulfate concentration records, and key questions remain about whether it represents a single eruption or multiple eruptions and about the plume injection heights, source regions, and climatic impacts of the volcanism involved.</p> <p>Here, we reassess this volcanic episode by integrating new high-resolution sulfur isotope records and cryptotephra geochemistry from Greenland ice cores. Our isotopic data provide direct evidence that the 1108–1110 CE episode, which is currently considered a single tropical event in volcanic forcing databases, in fact comprises a cluster of eruptions. These include an NH extratropical eruption beginning in mid-1108 CE, a subsequent tropical eruption, and a minor NH extratropical eruption in early 1111 CE, with the first two injecting sulfur into the stratosphere. Cryptotephra analysis identifies basaltic glass shards geochemically consistent with Icelandic sources; however, this evidence does not provide definitive provenance constraints for the two stratospheric sulfur-injecting eruptions.</p> <p>Based on these new constraints, we revise the volcanic stratospheric sulfur injection for this episode to 9.0 ± 1.4 Tg S, approximately 53 % lower than the previous estimate of 19.2 ± 4.7 Tg S, indicating that the stratospheric sulfur load associated with this episode has been overestimated. The timing of sulfate deposition further indicates that the initial NH extratropical eruption was the primary driver of the severe 1109 CE summer cooling, largely through sulfate transported via the troposphere and lowermost stratosphere. We therefore suggest that such aerosols, which are typically excluded from total forcing estimates, may play an underappreciated role in the climate response through aerosol-cloud interactions. These results provide a more nuanced representation of the volcanic forcing over this time period that can be used in state-of-the-art climate model simulations.