VIMS study finds marine heatwave tallies miss most cumulative ocean heat

A study led by William & Mary's Virginia Institute of Marine Science and Batten School argues that ecosystems experience far more cumulative heat than standard marine heatwave (MHW) tallies capture, SciTechDaily reported. Drawing on long-term temperature records from 20 U.S. estuaries, the team treats MHWs as parts of larger warming episodes that include warm periods before and after the formal event. Lead author Ricardo Utzig Nardi and coauthor Piero Mazzini said sustained warming often stretches across weeks or months, with the defined heatwave embedded inside that longer interval. In many cases, pre- and post-event warm phases contributed as much cumulative heat as the heatwave itself, or more. Analyzing more than 2,580 MHWs over 20 years at 20 estuaries, the researchers found conventional assessments underestimate total heat exposure by more than 150% on average. About two-thirds of events were classed as individual and occurred within about 60 days of elevated temperatures outside the heatwave window; compound patterns were linked to roughly 90 days of surrounding warmth and more than three times the cumulative heat of the MHW alone. The paper appears in Communications Earth & Environment (DOI: 10.1038/s43247-026-03739-x).