Ocean heatwaves are hiding months of extra warming

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.

Two types of marine heatwaves emerge

The researchers also separated MHWs into two broad categories. About two thirds were classified as "individual" events. These occurred within approximately 60 days of elevated temperatures extending beyond the MHW itself.

The remaining one third were classified as "compound" events. These were associated with much longer periods of warming (approximately 90 days) before and after MHWs and generated more than three times the cumulative heat exposure produced by the MHW itself.

"As you can see, these are remarkably long periods of thermal exposure," explained Mazzini. "Many laboratory experiments simulate marine heatwaves by increasing temperatures for a few days or weeks in total. Our findings demonstrate that while these experiments hold value, they often fail to capture the prolonged thermal exposure organisms experience in nature. Our research provides a new framework for designing experiments that reflect natural marine heatwave conditions and quantify cumulative heat exposure, allowing scientists to better evaluate the biological impacts of marine heatwaves across ecosystems."

Rethinking how marine heatwaves are studied

The work adds to a growing body of MHW research from faculty and researchers at VIMS & Batten School. Mazzini and Nathan Shunk, a third-year Ph.D. student, recently published a study defining "vertical marine heatwaves" and developing a classification system for them in the Bay.

Last year, Nardi and Mazzini also published research forecasting an increase in MHWs along the U.S. East Coast. That work identified connections between marine heatwaves and large scale climate patterns, including El Niño and the Pacific Decadal Oscillation.

"This study is an example of outstanding science born from a simple question about temperature's relationship to water quality conditions in estuaries," said Mazzini. "It was made possible by NOAA's National Estuarine Research Reserve System and its long-term, high-frequency temperature observations. Comprehensive monitoring programs like these allow us to ask bigger questions and uncover patterns that would otherwise remain hidden."

Longer warming could intensify ecosystem stress

The findings could also help coastal communities and resource managers better understand how prolonged warming affects marine environments. Long periods of elevated water temperatures can intensify other environmental pressures, including low oxygen conditions and harmful algal blooms.

Together, these stresses can place additional pressure on ecosystems ranging from seagrass beds to coral reefs. Including warming periods before and after MHWs in ecological assessments and laboratory experiments could help researchers create more realistic estimates of ecosystem vulnerability and provide better information for conservation and management decisions.

"This research has the potential to shift our understanding of the role warming waters play in ecosystem health by widening our focus beyond the heatwave window, so that we consider the full impact of temperature across time," explained Nardi. "I'm excited to discover what we may have missed before with this new perspective."

As MHWs become more frequent and intense, measuring their full cumulative impact may become increasingly important for protecting coastal resources and helping communities prepare for future environmental change.