Persistent warm water anomalies before and after marine heatwaves amplify heat exposure and associated risks

Marine heatwaves are a major threat to marine ecosystems globally, yet standard marine heatwave definitions, despite providing consistent metrics, do not capture pre- and post-event warm anomalies, leading to systematic underestimation of total heat exposure experienced by marine ecosystems. Here, we analyze over 2580 marine heatwave events from 54 stations across 20 US estuaries to provide a comprehensive characterization of pre- and post-marine heatwaves thermal anomalies. We show that pre- and post-event phases are largely independent of core marine heatwave metrics and often persist longer than the marine heatwave itself. Although these phases are less intense than the marine heatwave event, when combined with the marine heatwave, they account for a substantial increase in total heat exposure—resulting in an approximately 153% higher cumulative intensity (°C·days) compared to the marine heatwave alone—that cannot be reliably inferred from standard metrics. We present a new framework to quantify pre- and post-event anomalies that can be broadly applied across coastal and open-ocean systems. This framework is critical for understanding marine heatwaves, assessing ecological impacts, guiding laboratory experiments, and informing ecosystem management.