Study finds marine heatwaves hide months of extra warming in US estuaries

September 30, 2026 at 05:28 PM2 min

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Researchers at William & Mary's VIMS and Batten School found that marine heatwaves are embedded in longer warm-water periods, causing total heat exposure to be underestimated by more than 150% on average. The study, published in Communications Earth & Environment, analyzed over 2,580 marine heatwaves across 20 U.S. estuaries over two decades. The findings suggest that warming before and after a heatwave can contribute as much or more cumulative heat than the heatwave itself.

Key Facts

The study analyzed more than 2,580 marine heatwaves recorded across 20 U.S. estuaries over two decades.
Conventional assessments underestimate total heat exposure by more than 150% on average.
Warming before and after a marine heatwave can contribute as much or more cumulative heat exposure than the heatwave itself.
The research was led by Ricardo Utzig Nardi and coauthor Piero Mazzini at William & Mary's VIMS and Batten School.
The study was published in Nature's Communications Earth & Environment.

Extended Warming Periods

Researchers developed a framework to measure cumulative heat exposure not only during marine heatwaves but also during extended periods of unusually warm water before and after them. Lead author Ricardo Utzig Nardi noticed warm-water anomalies on either side of marine heatwaves and realized they are embedded within larger periods of warm water. These broader stretches of unusually warm water can continue for weeks or even months, with the marine heatwave occurring somewhere within that longer period. In many cases, the warming before and after the marine heatwave contributed as much or even more cumulative heat exposure than the heatwave itself.

Underestimated Heat Stress

The researchers examined more than 2,580 marine heatwaves recorded across 20 U.S. estuaries over two decades. Their analysis found that conventional assessments underestimate total heat exposure by more than 150% on average. Coauthor Piero Mazzini stated that marine heatwaves can no longer be viewed in isolation when assessing the impact of warming events on coastal and oceanic ecosystems. Nardi compared cumulative heat exposure to sunburn risk, explaining that biological responses depend not only on how warm the water becomes but also on how long that warming persists.