Marine heatwaves (MHWs) are extreme ocean warming events that can harm marine life, but their causes are not fully understood. We studied MHWs worldwide using ocean observations, satellite data, and a high-quality ocean model. Our results show that changes in the atmosphere are the main cause of these events, although ocean currents play a key role in some regions. A better understanding of MHWs will help predict them and support efforts to protect marine ecosystems and coastal communities.