Effusive, long-lasting volcanic eruptions impact climate through emission of gases and subsequent production of aerosols. Although previous studies have shown that the sulphate aerosol produced by these eruptions cools Earth's climate, this Earth system modelling study shows that high-latitude effusive eruptions can cause Arctic warming during the fall and wintertime. This warming effect is caused by the enhancement of downward longwave radiation from very optically thin clouds. The results have implications for our understanding of future Arctic climate change as well as any efforts to deliberately modify the climate through solar radiation management.