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Abstract The 10th‐century Eldgjá eruption in Iceland is the largest basaltic flood lava eruption of the Common Era. However, the extent of its impacts is unclear due to limited historical records. Combining volcanology and ice‐core analyses from both recent and earlier studies, we present a gas emissions scenario for the Eldgjá eruption spanning 937 to 940 CE, co‐injecting volcanic sulfur and halogens. The combined tropospheric and stratospheric sulfur emissions are 3–8 times higher than those adopted for Eldgjá in existing paleoclimate simulations. Earth system modeling of this scenario under pre‐industrial conditions reveals a compound event with maximum northern extratropics surface cooling of ∼2°C in summer‐autumn of 939 and 940 CE, prolonged Arctic sea ice growth, and large‐scale precipitation changes, concurrent with stratospheric ozone depletion and elevated pollution. These results imply that the combined climatic and environmental effects of the Eldgjá eruption may have significantly impacted human populations at the time.

Abstract High-latitude explosive volcanic eruptions can cause substantial hemispheric cooling. Here, we use a whole-atmosphere chemistry-climate model to simulate Northern Hemisphere (NH) high-latitude volcanic eruptions of magnitude similar to the 1991 Mt. Pinatubo eruption. Our simulations reveal that the initial stability of the polar vortex strongly influences sulphur dioxide lifetime and aerosol growth by controlling the dispersion of injected gases after such eruptions in winter. Consequently, atmospheric variability introduces a spread in the cumulative aerosol radiative forcing of more than 20%. We test the aerosol evolution’s sensitivity to co-injection of sulphur and halogens, injection season, and altitude, and show how aerosol processes impact radiative forcing. Several of these sensitivities are of similar magnitude to the variability stemming from initial conditions, highlighting the significant influence of atmospheric variability. We compare the modelled volcanic sulphate deposition over the Greenland ice sheet with the relationship assumed in reconstructions of past NH eruptions. Our analysis yields an estimate of the Greenland transfer function for NH extratropical eruptions that, when applied to ice core data, produces volcanic stratospheric sulphur injections from NH extratropical eruptions 23% smaller than in currently used volcanic forcing reconstructions. Furthermore, the transfer function’s uncertainty, which propagates into the estimate of sulphur release, needs to be at least doubled to account for atmospheric variability and unknown eruption parameters. Our results offer insights into the processes shaping the climatic impacts of NH high-latitude eruptions and highlight the need for more accurate representation of these events in volcanic forcing reconstructions.