Global warming is increasing ocean temperatures at a rate that could have dire consequences for the largest estuary in the United States, according to a new study from the Batten School of Coastal and Marine Sciences and the Virginia Institute of Marine Science.

The study estimates the Chesapeake Bay area could have marine heat wave conditions for about a third of the year by the end of the century. That’s an increase from the 22 days a year the bay now sees long periods of unusually high water temperatures.

Specifically, the study focuses on estuaries — areas where freshwater meets salt water, such as the lower Chesapeake Bay. Researchers looked at estuaries across the U.S. to analyze trends in marine heat waves.

These extended heat waves can have negative effects on local wildlife, especially if prolonged. That could include mass mortality of local species, habitat loss, species migration and increased ocean stressors such as low pH and dissolved oxygen. Heat waves can critically disrupt essential processes in an ecosystem, according to the Batten School and VIMS.

Piero Mazzini, a professor at the Batten School of Coastal & Marine Sciences and coauthor of the study, said the research should “serve as a warning” for local leaders in charge of taking care of environmental assets.

“The Chesapeake Bay, for example, currently experiences marine heat wave conditions approximately 6% of the year — 22 days per year — and that is already placing stress on the ecosystem,” Mazzini said. “Our study shows that estuaries across the East Coast could experience these conditions for more than 100 days of the year by 2100.”

Estuaries serve as important nursery habitats for nearly 75% of all fish species and support more than 54 million jobs, according to the study. East Coast areas also were found to fare worse than the West Coast, where winds can churn cooler water toward the surface.

A previous study from VIMS found the bay has warmed by about 2 degrees Fahrenheit, or about .07 degrees each year, since the mid-1980s. Hot summer days already bring problems with low oxygen, and heat excites oxygen molecules so they leave the liquid more easily. This results in “dead zones,” areas where oxygen is removed faster than it can be replenished. Animals need healthy levels of oxygen in the water to survive.

In 2024, environmental groups reported that the Chesapeake Bay dead zone was at a near-average size, but despite progress, officials still warn that the bay will not meet 2025 goals for restoration.

If actions do not change, the study found that warming trends in climate are expected to persist throughout the 21st century, and some projections suggest that much of the open ocean could be in a “near-permanent” marine heat wave toward the end of the century. The rise of sea-surface temperatures is a main driver of issues for estuaries, and environmental scientists are looking to reducing greenhouse gases and increasing tree canopy as potential solutions.

Ricardo Nardi, lead author on the study, said open ocean temperatures have a long history of being studied, but estuaries are often overlooked. Longer heat wave conditions could have “disastrous ecosystem consequences,” so more research is needed to understand how conditions in the ocean and estuaries affect each other.

“These are critical ecosystems, and future conservation efforts will depend on our understanding of the factors influencing them,” Nardi said.

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