Heat exposure from estuary heat waves is being undercounted by more than 150%

Estuary heat waves hold far more heat than the standard measurements record, and a team at the Virginia Institute of Marine Science put the shortfall at more than 150 percent on average. The count rests on 2,587 marine heat waves logged at 20 U.S. estuaries, and the missing heat sits in the weeks of unusually warm water that come before and after each officially defined event.

Ricardo Utzig Nardi, a VIMS research specialist and the lead author, and Piero Mazzini, an assistant professor at VIMS and the Batten School of Coastal and Marine Sciences, published the analysis on June 10, 2026, in Communications Earth & Environment.

Warm Water Before and After the Official Event

NOAA describes a marine heat wave as temperatures that exceed the 90th percentile of 30-year historic values for five or more days in a row. That definition draws a hard boundary around each event, and anything warm outside the boundary drops out of the record. Nardi said the team began noticing the problem in the data itself, with warm-water anomalies showing up on either side of the heat waves it was already tracking.

In the VIMS release, the pre- and post-event stretches contributed as much heat as the event itself, or more, in many cases. Mazzini’s reading is blunt: marine heat waves can no longer be examined in isolation when the impact of a warming event is being assessed.

Nardi and Mazzini’s 2,587 Events at 20 Reserves

The temperature records come from the National Estuarine Research Reserve System, which NOAA describes as 30 coastal sites with the only consistent, system-wide monitoring program among U.S. estuary programs. The paper draws on 54 stations across 20 of those reserves, with station records running from 1995 to 2023 and a climatology baseline of 2003 to 2023. Of 3,097 marine heat waves detected, 2,587 had gap-free records before and after, and those are the events analyzed.

Cumulative intensity is the measure used throughout, a sum of how far the water ran above normal for each day, expressed in degree-days Celsius, so a long mild anomaly and a short sharp one can carry the same total.

Co-authors Ryan Walter and Jian Shen complete the author list, with Shen listed at VIMS and William & Mary in Gloucester Point, Virginia, and the article is open access. The same VIMS group had already used these reserves in 2025 to show that marine heat waves are becoming more frequent in East Coast estuaries, while West Coast estuaries showed no significant warming trend, a contrast the group tied to persistent wind-driven upwelling of cold deep water.

A Ratio of Medians, Not a Per-Event Average

The central arithmetic is specific, and it comes straight from the paper’s own tables of medians. In the paper, the marine heat waves alone had a median cumulative intensity of 20.6 degree-days, and the pre- and post-event phases combined had a median of 31.6 degree-days. The abstract describes the combined phases as an approximately 153 percent higher cumulative intensity than the heat wave alone, which VIMS reports as an underestimate of “more than 150% on average.” The Discussion section states the 153.4 percent figure explicitly as relative to the median heat wave.

A second calculation in the Results gives a smaller number. Taking each event’s own ratio of pre- plus post-event intensity to the event and then the median of those ratios yields 1.4, which the paper calls 40 percent greater cumulative intensity, and the Discussion rounds the same comparison to 37 percent. The two figures answer different questions, one about the pooled middle of the distribution and one about a typical individual event, and the 150 percent claim belongs to the first.

Compound Events and the Biology of Prolonged Heat

About two-thirds of the events, 1,744 of 2,587, were individual, with elevated temperatures lasting roughly 60 days beyond the heat wave. The other 843, or 32.6 percent, were compound, meaning part of another heat wave overlapped their before or after window. Those carried warming of roughly 90 days on either side and more than three times the cumulative intensity of the heat wave alone.

The biological stakes are tied to duration. VIMS notes that prolonged warming can compound low oxygen and harmful algal blooms, and an earlier VIMS analysis of the same reserves warned that estuaries are nursery habitat for nearly 75% of fish species, with Chesapeake Bay already in marine heat wave conditions about 6 percent of the year. Mazzini adds that laboratory experiments often heat organisms for only days or weeks, which misses what long natural exposure does.

Nardi’s summary, from the VIMS release, ties the argument together: “Their biological responses depend not only on how warm the water becomes, but also on how long that warming persists.”

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

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