Anyone who has ever heated a pot of water knows that temperature depends on more than how hot something gets. Time matters too. Yet marine heatwaves (MHWs) have traditionally been studied as distinct events with clearly defined beginnings and endings.
New research led by William & Mary's VIMS & Batten School suggests that approach may overlook a major part of the heat marine ecosystems actually experience. The researchers developed a framework for measuring cumulative exposure not only during MHWs, but also during the extended periods of unusually warm water that can occur before and after them.
Marine heatwaves are part of longer warming periods
The study, published in Nature's Communications Earth & Environment, categorizes warming periods that occur ahead of and following MHWs. Using long term observations from 20 U.S. estuaries, the researchers developed a way to measure total heat exposure by viewing marine heatwaves as part of broader warming episodes rather than as isolated events.
"We started to notice these warm-water anomalies on either side of marine heatwaves and realized that they're actually embedded within larger periods of warm water," explained lead author Ricardo Utzig Nardi M.S. '25, a research specialist working with coauthor Piero Mazzini, assistant professor at VIMS & the Batten School of Coastal & Marine Sciences. "It sounds obvious, but studies usually focus only on the marine heatwave window and that's not an accurate representation of real-world conditions."
Nardi found that these broader stretches of unusually warm water can continue for weeks or even months, with the marine heatwave occurring somewhere within that longer period. In many cases, the warming before and after the MHW contributed as much or even more cumulative heat exposure than the heatwave itself.
"We can no longer look at marine heatwaves in isolation when assessing the impact of warming events on coastal and oceanic ecosystems," said Mazzini. "We must acknowledge the larger framework in which they exist and this research provides a way to do just that."
Decades of data reveal underestimated heat stress
The researchers examined more than 2,580 MHWs recorded across 20 U.S. estuaries over two decades. Their analysis found that conventional assessments underestimate total heat exposure by more than 150% on average, a difference that could have important consequences for understanding ecosystem health.
"Consider spending time in the sun," said Nardi, explaining the importance of cumulative heat exposure. "Your risk of sunburn depends on both sunlight intensity and how long you're exposed to it. A few minutes may cause little harm, but hours of exposure can take a toll. It's similar for marine organisms and warm water. Their biological responses depend not only on how warm the water becomes, but also on how long that warming persists. Prolonged, cumulative heat exposure, especially when combined with other stressors, can create conditions that some species may not be able to survive."
The results indicate that warm water anomalies occurring before and after a heatwave are largely independent of the MHW itself. That finding challenges the usual way scientists interpret thermal stress and provides a new approach for estimating total heat exposure across coastal ecosystems.
The researchers also separated MHWs into two broad categories. About two thirds were classified as "individual" events. These occurred within approximately 60 days of el…