Anyone who has ever heated a pot of water on a stove knows intuitively that temperature depends on far more than just how hot something ultimately gets. Time matters just as much. The longer a pot sits on a burner, the more heat energy it absorbs, altering its state entirely. Yet, despite this fundamental thermodynamic principle, marine heatwaves—often referred to by scientists as MHWs—have traditionally been studied by the scientific community as isolated, distinct events with clearly defined, sharply drawn beginnings and endings.
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 massive, crucial component of the actual thermal heat marine ecosystems experience over time. To bridge this critical gap in our understanding, the researchers have developed an innovative analytical framework for measuring cumulative thermal exposure not only during the peak of marine heatwaves themselves, but also across the extended periods of unusually warm water that systematically occur both before and after them.
Marine Heatwaves Are Part of Longer Warming Periods
The groundbreaking study, which was recently published in Nature’s Communications Earth & Environment, categorizes and analyzes the warming periods that systematically occur ahead of and following traditional MHW windows. By leveraging extensive, decades-long observational datasets gathered from 20 distinct U.S. estuaries, the research team successfully engineered a novel methodology to measure total heat exposure. This approach reframes marine heatwaves as organic, embedded components of much broader, sustained warming episodes rather than treating them as sudden, isolated meteorological 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. Nardi conducted the study working closely with co-author Piero Mazzini, an assistant professor at VIMS and 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."
Through rigorous data analysis, Nardi discovered that these broader stretches of unusually warm water can persist continuously for weeks or even months at a time, with the technically defined marine heatwave simply occurring somewhere within that much longer duration. In a significant number of cases, the gradual warming that preceded and followed the official MHW contributed as much—or in some instances, even more—cumulative heat exposure to the environment than the heatwave event itself.
"As you can see, these are remarkably long periods of thermal exposure," noted Mazzini. "We can no longer look at marine heatwaves in isolation when assessing the impact of warming events on coastal and oceanic ecosystems. 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 researchers conducted an exhaustive examination of more than 2,580 individual marine heatwaves recorded across 20 U.S. estuaries over a span of two decades. Their deep-dive analysis revealed a startling discrepancy: conventional assessment methods underestimate total cumulative heat exposure by more than 150% on average. This massive analytical oversight could have profound consequences for how scientists understand, predict, and manage coastal ecosystem health in an era of rapid global climate change.
To illustrate the critical importance of cumulative heat exposure over mere peak intensity, Nardi offered a familiar 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."
Furthermore, the results of the study indicate that the warm-water anomalies occurring before and after a heatwave are largely independent of the MHW itself. This pivotal finding directly challenges the traditional way scientists interpret environmental thermal stress and establishes a fresh, more accurate approach for estimating true total heat exposure across vulnerable coastal and estuarine ecosystems.
Two Types of Marine Heatwaves Emerge
As part of their comprehensive evaluation, the researchers successfully separated the thousands of analyzed MHWs into two distinct, broad categories, shedding light on the varying structural dynamics of coastal warming. Approximately two-thirds of the studied events were classified as "individual" heatwaves. These particular occurrences typically took place within an approximate 60-day window of elevated temperatures that extended only moderately beyond the official MHW timeframe itself.
In contrast, the remaining one-third of the events were categorized as "compound" heatwaves. These complex events were closely associated with dramatically longer periods of ambient warming—stretching across approximately 90 days—both before and after the actual MHW peak. Crucially, these compound events generated more than three times the cumulative heat exposure produced by the marine heatwave event by itself.
This structural insight has immediate implications for experimental marine biology. "Many laboratory experiments simulate marine heatwaves by increasing temperatures for a few days or weeks in total," Mazzini explained. "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 newly published work builds seamlessly upon a rapidly expanding foundation of marine heatwave research spearheaded by faculty members and student researchers at VIMS and the Batten School. Demonstrating the breadth of this institutional focus, Mazzini and Nathan Shunk, a third-year Ph.D. student, recently published a complementary study that formally defines "vertical marine heatwaves" and establishes an innovative classification system for tracking them within the Chesapeake Bay.
Moreover, just last year, Nardi and Mazzini collaborated on published research that successfully forecasted a concerning, long-term increase in the frequency and severity of MHWs along the United States East Coast. That earlier predictive work successfully mapped out vital connections between marine heatwaves and large-scale global 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 reflected. "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
Ultimately, the implications of this research extend far beyond academic journals, offering vital tools that can help coastal communities and natural resource managers better comprehend how prolonged seasonal warming directly affects delicate marine environments. Extended stretches of elevated water temperatures do not act in a vacuum; instead, they act as powerful multipliers that intensify other compounding environmental pressures, such as severe low-oxygen dead zones and toxic, harmful algal blooms.
Combined, these interrelated stresses place unprecedented, compounding pressure on a diverse array of marine habitats, ranging from vital underwater seagrass meadows to complex coral reef networks. By formally incorporating the critical warming periods that occur both before and after MHWs into standard ecological assessments and controlled laboratory experiments, researchers can generate far more realistic, granular estimates of overall ecosystem vulnerability. In turn, this data can provide conservationists and policy decision-makers with the precise information needed to protect coastal resources effectively.
"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," Nardi concluded. "I’m excited to discover what we may have missed before with this new perspective."
As marine heatwaves continue to grow demonstrably more frequent and intense across the globe, accurately measuring and accounting for their full cumulative impact will only become more vital for safeguarding invaluable coastal resources and ensuring that human communities can effectively prepare for an era of ongoing environmental change.