The Arctic Ocean is projected to warm twice more than the global mean in a warming 21st century, following increased solar heat input due to sea ice decrease. Here we find that this increase in solar heat input is larger in a higher-resolution climate model compared to a low-resolution one. This is due to the impacts of Arctic marine heatwaves (MHWs), known as episodes of extreme ocean warming. The explicit consideration of MHWs, which are stronger and more realistic in higher-resolution models, increases melting of sea ice and thus solar heat input, thereby reinforcing the long-term Arctic Ocean warming. A positive feedback is identified between stronger MHWs and larger Arctic Ocean warming. We emphasize that Arctic Ocean warming is underestimated by the current generation of climate models, which generally have a too low spatial resolution to resolve Arctic MHWs. We conclude that future eddy- and storm-resolving models will provide a new perspective on the Earth system's response to past and future climate and environmental extremes.