In many cases, the researchers found, warming before and after the heatwave contributed as much — or even more — cumulative heat than the heatwave itself.

What did two decades of estuary data reveal?

Drawing on long-term observations from NOAA's National Estuarine Research Reserve System, the team examined 20 U.S. estuaries and found that warm-water anomalies surrounding heatwaves are largely independent of the heatwave itself. That finding challenges the usual way scientists interpret thermal stress in coastal waters.

The analysis also split marine heatwaves into two broad categories:

• Individual events — about two-thirds of the total. These occurred within roughly 60 days of elevated temperatures extending beyond the heatwave itself.
• Compound events — the remaining one-third. These were associated with much longer warming periods, approximately 90 days before and after the heatwave, and generated more than three times the cumulative heat exposure produced by the heatwave alone.

"As you can see, these are remarkably long periods of thermal exposure," Mazzini said.

How could this change laboratory experiments?

The findings carry a caution for experimental science. Many laboratory studies simulate marine heatwaves by raising water temperatures for only a few days or weeks. Mazzini said such experiments hold value but often fail to capture the prolonged thermal exposure organisms actually face in nature.

The new framework, he explained, offers a way to design experiments that reflect real conditions and quantify cumulative heat exposure, allowing scientists to better evaluate biological impacts across ecosystems.

The stakes are ecological. Prolonged heat exposure, especially when combined with other stressors such as low-oxygen conditions and harmful algal blooms, can create conditions some species may not survive. These pressures affect ecosystems ranging from seagrass beds to coral reefs.

What are the study's limits and next steps?

The analysis draws on estuarine data from U.S. waters, so the exact percentages may not translate directly to open-ocean or tropical systems. Still, the researchers argue the underlying pattern — heatwaves nested inside longer warming episodes — likely applies broadly, and the method gives coastal managers a fuller picture of ecosystem vulnerability.

"We can no longer look at marine heatwaves in isolation when assessing the impact of warming events on coastal and oceanic ecosystems," Mazzini said.

The study builds on earlier work from the same group. Mazzini and Ph.D. student Nathan Shunk recently published research defining "vertical marine heatwaves" and a classification system for them in the Chesapeake Bay region. Last year, Nardi and Mazzini published a forecast of increasing marine heatwaves along the U.S. East Coast, linking them to large-scale climate patterns including El Niño and the Pacific Decadal Oscillation — a long-term cycle of Pacific Ocean temperature variability.

For Nardi, the new perspective opens fresh questions. "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," he said. "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 impact may prove increasingly important for protecting coastal resources and helping communities prepare for future environmental change.