The first study of its kind shows that ecosystems experience far more cumulative heat than standard measurements reveal.
A marine heatwave may have an official beginning and end, but the warm water affecting an ecosystem often does not. Like a pot that continues absorbing heat over time, coastal waters can remain unusually warm for weeks or months surrounding the event itself.
Research led by William & Mary’s VIMS & Batten School introduces a way to measure that broader exposure. Drawing on long-term temperature records from 20 U.S. estuaries, the study treats marine heatwaves (MHWs) as parts of larger warming episodes rather than isolated spikes.
Published in Communications Earth & Environment, the work classifies the warm periods that occur before and after MHWs and combines them into a measure of cumulative heat exposure. The framework is intended to better represent the temperature conditions marine organisms actually experience and improve assessments of ecological risk.
Warm periods extend beyond heatwaves
“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 sustained warming frequently stretches across weeks or months, with the formally defined marine heatwave occurring somewhere within that longer interval. In many cases, the warm phases before and after the MHW contributed as much cumulative heat exposure as the heatwave itself, or even more.
“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.”
Standard measures miss most heat exposure
The researchers examined more than 2,580 MHWs recorded over 20 years at 20 estuaries across the United States. Their analysis found that conventional assessments underestimate total heat exposure by more than 150% on average, a difference that could substantially change how scientists evaluate threats to 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 the warm water anomalies occurring before and after a heatwave are largely independent of the MHW itself. That distinction challenges the usual practice of measuring heat stress only within the formally defined event and provides a framework for calculating full temperature exposure across coastal ecosystems.
Two patterns prolong ocean warming
The analysis separated MHWs into two categories. About two-thirds were classified as “individual” events and occurred within approximately 60 days of elevated temperatures extending beyond the heatwave window.
The remaining one-third were “compound” events. These were associated with prolonged warming lasting approximately 90 days before and after the MHW and produced more than three times the cumulative heat exposure generated by the MHW alone.
“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.”
The findings add to a series of MHW studies by faculty and researchers at VIMS & Batten School. Mazzini and Nathan Shunk, a third-year Ph.D. student, recently published research defining “vertical marine heatwaves” and creating a classification system for them in the Bay.
Last year, Nardi and Mazzini published another paper forecasting an increase in MHWs along the U.S. East Coast. That work also identified links 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 heat exposure raises ecological risks
The broader measure of warming could have practical value for coastal communities and natural resource managers. Long stretches of elevated water temperatures can intensify other environmental pressures, including low oxygen conditions and harmful algal blooms.
Together, those stressors may place additional strain on ecosystems ranging from seagrass beds to coral reefs. Accounting for the warm periods surrounding MHWs could help scientists design more realistic laboratory experiments, estimate ecosystem vulnerability more accurately, 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 marine heatwaves become more frequent and intense, measuring their full cumulative effects will be increasingly important for protecting coastal resources and helping communities prepare for future environmental change.
Reference: “Persistent warm water anomalies before and after marine heatwaves amplify heat exposure and associated risks” by Ricardo U. Nardi, Piero L. F. Mazzini, Ryan K. Walter and Jian Shen, 10 June 2026, Communications Earth & Environment.
DOI: 10.1038/s43247-026-03739-x
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