Heat waves—like the one that blistered the Pacific Northwest last June—also occur underwater. A new study in Frontiers in Marine Science paints a worrisome picture of recent and projected trends in marine heat waves within the nation’s largest estuary, with dire implications for the marine life and coastal economy of the Chesapeake Bay and other similarly impacted shallow-water ecosystems.

The study’s authors, Drs. Piero Mazzini and Cassia Pianca of William & Mary’s Virginia Institute of Marine Science, note they saw “significant upward trends in the frequency and yearly cumulative intensity of marine heat waves within the Chesapeake Bay.”

The pair based their analysis on long-term measurements of water temperature from 6 sites along the Bay’s 200-mile length, with record length varying between 26 and 35 years. Like other researchers, they defined a marine heat wave as any period of 5 or more consecutive days with water temperatures warmer than 90% of those measured on the same date and in the same spot as in years past. They analyzed the record of Bay heat waves in terms of frequency, intensity, duration, and cumulative temperature stress.

Based on those criteria, Mazzini and Pianca determined that the Chesapeake Bay experienced an average of two 11-day marine heat waves per year between 1986 and 2020, with an average intensity of 5.4 °F (3 °C) and a maximum peak of 14.4 °F (8 °C) above the climatic norm. This translates to an average yearly cumulative intensity of 130 °F days (72 °C days), a measure of heat stress for marine systems similar to the “cooling degree days” used to determine the energy required to keep indoor spaces comfortable for people.

The researcher’s most troubling finding was that the maximum frequency of marine heat waves occurred during the last 10 years, reaching 6-8 events per year compared to only 4-5 events per year prior to 2010. That equals a gain of 1.4 annual heat waves each decade, with a corresponding increase in annual cumulative intensity. The researchers also found that years without marine heat waves were fairly common in Bay waters prior to 2010, but have occurred baywide only once since, in 2014.

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