Standard counts miss more than 150% of the heat in marine heatwaves, a study of 20 US estuaries finds

Conventional assessments of marine heatwaves underestimate total heat exposure by more than 150% on average, according to a study of 20 U.S. estuaries by researchers at the Virginia Institute of Marine Science. The team analyzed more than 2,580 marine heatwaves recorded over two decades and found that most are embedded in longer stretches of warm water that standard methods leave out of the tally.

Help yourself and the planet: The Cool Down makes it easy to help yourself while helping the planet, with smart, simple changes to save more and waste less in your everyday life.

Get The Cool Down →

We may earn a commission if you sign up.

Marine heatwaves are normally studied as discrete events with a clear start and end. Organisms in a bay do not experience them that way.

Heatwave windows and the warmth around them

A common definition treats a marine heatwave as a period when temperatures exceed the 90th percentile of 30-year historic values for five or more days in a row, a framework NOAA’s marine heatwave indicator page describes and attributes to a 2016 paper by Hobday and colleagues. NOAA’s page says such events can change an ecosystem’s biology and chemistry and can cut access to areas used for fishing, recreation and cultural practices. Under that approach, only the heatwave window gets measured, and the framework treats events separated by three or more days as distinct, so warm water that lingers just below the threshold, or returns after a short cool spell, is not counted as part of the event. The study, published in Communications Earth & Environment in 2026, asks what happens to the water temperature just before and just after that window.

Lead author Ricardo Nardi said studies usually focus only on the marine heatwave window. The research team, which also includes Piero Mazzini, Ryan Walter and Jian Shen, measured cumulative heat exposure during the heatwave and during the warm periods on either side, then sorted the episodes into types, as described in the ScienceDaily summary of the paper.

Individual and compound events

The classification produced two groups. About two-thirds of the heatwaves were individual events, with elevated temperatures extending roughly 60 days beyond the heatwave itself. The remaining third were compound events, in which warm periods lasted about 90 days before and after the heatwave and produced more than three times the cumulative heat exposure of the heatwave alone.

Warm-water periods before and after a heatwave were largely independent of the heatwave itself, according to the Virginia Institute of Marine Science’s account of the work. In many cases they contributed as much cumulative heat as the heatwave phase, or more. That independence is what makes the underestimate so large: the adjacent warmth is not a tail of the event but a separate pulse of heat that standard counting never sees.

Read against the heatwave-only measure, an underestimate of more than 150% implies that total exposure was more than two and a half times what the standard count captured. The study reports the figure as an average across its estuaries, so individual sites and events will fall above and below it. The two-thirds and one-third split also means that even the individual events, the more common type, carried roughly two months of extra warmth beyond the heatwave itself, so the undercount is not confined to rare extremes.

Temperature records and the biology of warm water

The analysis rests on long-term, high-frequency temperature monitoring at sites in NOAA’s National Estuarine Research Reserve System, whose system-wide monitoring program data are public. Two decades of readings taken at short intervals are what allow a heatwave to be placed inside the weeks of warmth around it; a record of occasional spot measurements could not show that.

Mazzini, an assistant professor at the Virginia Institute of Marine Science and William & Mary’s Batten School of Coastal & Marine Sciences, said researchers can no longer look at marine heatwaves in isolation when assessing the impact of warming events. His concern extends to the laboratory: many experiments simulate marine heatwaves by raising temperatures for a few days or weeks in total, which he argues often fails to capture the prolonged thermal exposure organisms experience in nature. The contrast is stark against the study’s own measurements: a lab animal held warm for a few days and then returned to normal has faced a far shorter stress than a wild one sitting in warm water for the roughly 60 to 90 days of extra warmth the team found around heatwaves.

Nardi made the biological point directly. Responses, he said, depend not only on how warm the water becomes but also on how long that warming persists. Extended warm water can intensify stressors such as low oxygen and harmful algal blooms, adding strain to ecosystems from seagrass beds to coral reefs, and the authors suggest that counting the adjacent warming periods could produce more realistic predictions of vulnerability for conservation and management.

The study is a single analysis of 20 estuaries, and its 150% figure describes how much a particular counting convention understates exposure at those sites, not a measured change in ocean temperature over time. The paper does not claim that heatwaves are getting longer; it claims that the way they are counted hides much of the heat. Because the data come from reserve sites in estuaries, the results describe those settings and should not be assumed to carry over unchanged to the open ocean.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

Help yourself and the planet: Get The Cool Down →

We may earn a commission if you sign up.