Anyone who has ever heated a pot of water on a stove knows intuitively that water temperature depends on far more than just how hot something gets. Time matters just as much. Yet, for years, marine heatwaves—often referred to by scientists as MHWs—have traditionally been studied as isolated, distinct events with clearly defined beginnings and cleanly marked endings.
Now, new research led by scientists at William & Mary’s Virginia Institute of Marine Science (VIMS) and the Batten School of Coastal & Marine Sciences suggests that this conventional approach may be overlooking a major, hidden component of the thermal stress marine ecosystems actually experience. To address this blind spot, the research team has developed a comprehensive new framework for measuring cumulative heat exposure, tracking not only the peak intensity of marine heatwaves, but also the extended periods of unusually warm water that quietly precede and follow them.
Marine Heatwaves Are Part of Longer Warming Periods
The groundbreaking study, which was recently published in the scientific journal Nature’s Communications Earth & Environment, systematically categorizes the broader warming periods that occur ahead of and in the aftermath of traditional MHWs. By utilizing decades of high-resolution, long-term environmental observations gathered from 20 distinct estuaries across the United States, the researchers successfully devised a novel methodology to measure total heat exposure. This approach views marine heatwaves as organic components of larger, continuous warming episodes rather than as abrupt, disconnected anomalies.
"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, a research specialist and M.S. student at VIMS and the Batten School, who works closely with coauthor Piero Mazzini, an assistant professor of coastal and 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."
Through their rigorous analysis, Nardi found that these broader stretches of unusually warm water can persist for weeks or even months at a time, with the formally designated marine heatwave occurring somewhere nestled within that much longer thermal timeline. In a striking number of cases, the cumulative warming that took place before and after the peak heatwave contributed as much—or in some instances, even more—to the overall thermal 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, emphasizing the paradigm shift required by the findings. "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
To arrive at these conclusions, the research team examined more than 2,580 individual marine heatwaves recorded across 20 U.S. estuaries over a span of two decades. Their meticulous analysis revealed a staggering discrepancy: conventional assessment methods underestimate total cumulative heat exposure by more than 150% on average. This massive undercount could have profound and far-reaching consequences for how scientists evaluate coastal ecosystem health, species resilience, and habitat vulnerability in a changing climate.
To illustrate the critical importance of cumulative heat exposure rather than isolated temperature spikes, Nardi offered a relatable terrestrial comparison. "Consider spending time in the sun," he explained. "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 study’s results further indicate that the warm water anomalies occurring both before and after a formal heatwave are largely independent of the MHW event itself. This finding directly challenges the traditional ways oceanographers and marine biologists interpret thermal stress, providing a powerful new analytical approach for estimating total heat exposure across vulnerable coastal environments.
Two Types of Marine Heatwaves Emerge from the Data
As part of their comprehensive evaluation, the researchers categorized the examined marine heatwaves into two broad, distinct groups based on their surrounding thermal context. Approximately two-thirds of the events were classified as "individual" heatwaves. These types of events typically occurred within approximately a 60-day window of elevated background temperatures that extended moderately beyond the core MHW period.
In contrast, the remaining one-third of the events were classified as "compound" events. These complex episodes were associated with significantly longer periods of sustained warming—averaging approximately 90 days—both before and after the peak marine heatwaves. Crucially, these compound events generated more than three times the cumulative heat exposure produced by the marine heatwaves themselves, highlighting a hidden layer of environmental stress.
"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
This latest work builds directly upon a growing body of advanced marine heatwave research spearheaded by faculty members and student researchers at VIMS and the Batten School. In recent months, Mazzini and Nathan Shunk, a third-year Ph.D. student, published another notable study that successfully defined "vertical marine heatwaves" and established a brand-new classification system for tracking them within the Chesapeake Bay.
Furthermore, last year, Nardi and Mazzini published collaborative research forecasting a concerning increase in the frequency and severity of marine heatwaves along the U.S. East Coast. That particular study successfully identified key physical connections between regional marine heatwaves and large-scale, basin-wide climate patterns, including the El Niño-Southern Oscillation 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," Mazzini noted. "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 broader implications of these findings extend far beyond academic journals. By illuminating the true scale of thermal exposure, the research can help coastal communities, resource managers, and environmental agencies better understand how prolonged warming directly affects delicate marine environments. Extended periods of elevated water temperatures are well-known to exacerbate other environmental pressures, including dangerously low dissolved oxygen concentrations and the proliferation of harmful algal blooms.
When combined, these compounding stresses can place unprecedented pressure on a diverse array of vital marine ecosystems, ranging from expansive seagrass meadows to vulnerable coastal habitats. Integrating the warming periods that occur before and after marine heatwaves into standard ecological assessments and laboratory experiments could ultimately help researchers generate much more realistic estimates of ecosystem vulnerability, leading to more informed, proactive conservation and resource 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," concluded Nardi. "I’m excited to discover what we may have missed before with this new perspective."
As marine heatwaves continue to become increasingly frequent and intense across global oceans and coastal zones, accurately measuring their full cumulative impact will only grow more vital for safeguarding valuable marine resources and helping coastal communities prepare for ongoing environmental change.