| Summary: | The 2014-15 Holuhraun eruption in the central highlands of Iceland lasted from late August 2014 to late February 2015 and emitted up to 9.6 Tg of SO 2 , mainly into the lower troposphere. Its effects on aerosol-cloud interactions have been studied extensively but less attention has been given to other climate impacts. Here we focus on surface climate in the Arctic. We performed two sets of experiments to estimate the climate impacts of this eruption using the Community Earth System Model, version 2.1.3 (CESM2.1.3). In one set, only the atmospheric component was active (CAM6) and the horizontal winds were nudged towards the MERRA-2 reanalysis. For the other set we produced ten ensemble members from free-running, fully-coupled historical simulations. In our model simulations, the eruption lead to the formation of sulphate aerosols which interacted with clouds in the vicinity of the eruption. Since the eruption started in the fall and continued throughout the winter, sunlight was scarce in the domain which was mostly affected, namely the Arctic. The resulting cloud changes (mainly increased liquid water path and droplet number concentration, and decreased cloud droplet effective radius) therefore had little effects on radiative transfer in the shortwave spectrum but considerably increased downward longwave radiative flux at the surface. This lead to surface warming in the Arctic, amounting to 0.75 +/- 0.57 ˚C (95% confidence) north of 65°N for the September to November mean.
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