Marine heatwaves may hide months of cumulative ocean warming

New research suggests that traditional methods of studying marine heatwaves (MHWs) may significantly underestimate the thermal stress experienced by marine ecosystems. A study led by scientists from the Virginia Institute of Marine Science (VIMS) and William & Mary’s Batten School indicates that marine heatwaves are often embedded within much longer periods of unusually warm water that persist for weeks or months before and after the formal heatwave event.

By analyzing over 2,500 records from 20 U.S. estuaries spanning two decades, researchers found that the cumulative heat exposure during these extended periods can be as high as, or even exceed, the heat exposure during the officially classified marine heatwave itself. The study, published in Communications Earth & Environment, suggests that conventional measurements may underestimate total heat exposure by an average of more than 150%.

Researchers emphasize that viewing heatwaves as isolated events with distinct start and end points fails to provide an accurate representation of real-world conditions. This finding could fundamentally change how scientists calculate and assess the long-term impact of warming on coastal and oceanic ecosystems.