A “marine heatwave” (MHW) is often treated as a stand-alone anomaly in seawater temperature. But that framing can miss what organisms actually experience: warming that ramps up before the event and lingers after it. New research from the Virginia Institute of Marine Science (VIMS) and the Batten School reframes MHWs as embedded within longer, ecosystem-relevant heat exposures.
The team introduced a framework to quantify an ecosystem’s cumulative thermal stress by characterizing the warm-water periods that precede and follow MHWs, not just the MHW window itself. Their approach was published in Communications Earth & Environment and is based on long-term observations across 20 U.S. estuaries.
Using more than 2,580 recorded MHWs over two decades, the researchers found that conventional assessments can underestimate total heat exposure by over 150% on average. The implication is straightforward: if heat persists beyond the time range used in many studies, the biological consequences are likely underestimated as well.
Lead author Ricardo Utzig Nardi emphasized that warm-water anomalies often occur on both sides of MHWs and form part of broader warming episodes. In many cases, the pre- and post-heatwave phases contribute as much as—or more than—the embedded MHW itself.
The study shows that sustained warming phases can last for weeks to months, which matters because biological stress depends on both intensity and duration. In other words, cumulative exposure functions like an “energy dose” of heat for marine organisms, with prolonged warmth increasing risk—especially when combined with other stressors such as low oxygen or harmful algal blooms.
To test how events differ, the researchers classified MHWs into two categories. About two-thirds were “individual” events, typically surrounded by shorter periods of elevated temperatures. The remaining one-third were “compound” events, associated with longer warming episodes and more than three times the cumulative heat exposure of the MHW component alone.
This distinction also challenges laboratory designs that raise temperatures for only brief intervals. While short experiments remain valuable, the new results suggest they may not capture the prolonged thermal context that estuarine species encounter in nature.
Because coastal ecosystems experience warming in continuous sequences rather than isolated bursts, the framework can improve ecological impact assessments and help refine predictions of vulnerability for habitats ranging from seagrass beds to coral communities.
By expanding the temporal lens of heatwave research, the work offers a more realistic measure of thermal risk—an increasingly urgent need as MHW frequency and intensity rise.
Subject of Research: Marine heatwaves and cumulative thermal stress in estuaries
Article Title: Persistent warm water anomalies before and after marine heatwaves amplify heat exposure and associated risks
News Publication Date: 10-Jun-2026
Web References: https://www.nature.com/articles/s43247-026-03739-x
References: 10.1038/s43247-026-03739-x
Image Credits: Jonathan Lefcheck
Keywords: climate change, climate systems, hydrosphere, climate variability, marine ecology, marine ecosystems
