The Chesapeake Bay summer water temperatures are increasing by nearly a half-degree Fahrenheit per decade and rising nearly twice as fast as global surface ocean water temperatures, according to a recent study.

But the increase, which could have profound impacts on Bay resources and water quality, is not uniform. Temperatures are rising faster in the summer and in higher salinity areas of the Lower Bay, according to a study by researchers at the Virginia Institute of Marine Science.

Choptank Riverkeeper Matt Pluta lowers a temperature probe into the headwaters of the Tred Avon River in Maryland in mid-October. The water temperature was nearly 72 degrees. Dave Harp

The study combined computer modeling with decades of measurements collected by others to help better understand the magnitude and patterns of changes in different parts of the estuary over time.

“Everyone knows the Bay is warming, but the estimates are a little bit variable,” said Kyle Hinson, a doctoral student at VIMS, who was the lead author of the recent paper published in the Journal of the American Water Resources Association.

Indeed, some studies in the past have shown greater warming in the water near urban areas, where stormwater running off pavement enters the water, or near the mouths of major Bay tributaries.

But the new work shows that such impacts tend to be local. When all of those measurements, along with routine Bay monitoring data, were fit together in the computer model, it showed that the average annual surface water temperature in the Bay had warmed by 1.3 degrees since the late 1980s.

Significantly, though, it also showed a pronounced seasonal impact. Summer surface temperatures have increased by about 2 degrees, while winter water temperatures have risen by only about 0.6 degrees.

“The difference between the winter and the summer is really pretty dramatic,” said Marjy Friedrichs, a VIMS researcher who works on models to assess climate impacts on the Bay and a co-author of the paper. “The seasonal cycle really surprised me.”

And, while the Bay overall is warming, the upward trend was higher in the lower Chesapeake, where average summertime water temperatures rose slightly faster than the Baywide average.

Hinson’s work shows that the biggest driver behind the changes is warming air temperatures, which have trended up during the same time period. Baywide, atmospheric temperatures account for around 90% of the water temperature increases, according to the study.

But other factors are at play as well, not all of which are fully understood. In the lower Chesapeake, warm water coming from the ocean shelf is also a significant factor in warming the water during the summer. Those coastal waters outside the Bay’s mouth are warming faster than atmospheric temperatures or ocean temperatures overall. If that trend continues, it could pose a threat to Lower Bay, the researchers said.

It makes sense that the Bay is warming faster than the ocean, Hinson noted. “This has a lot to do with the fact that the Bay is a really shallow system, and so the impacts of the warming atmosphere and this ocean effect are [seen] more quickly,” he said. But the reasons for the offshore warming are not clear and could relate to regional climate impacts, including alterations to currents along the coast.

What is clear is that the overall Bay trends are not likely to change soon.

A report released this summer by the United Nations’ Intergovernmental Panel on Climate Change warned that time is running out for nations to act decisively to curb greenhouse gas emissions and take other actions to limit warming. Even with strong and sustained actions, the report warned that it could take 20–30 years to see global temperatures stabilize.

Climate change is expected to have far-reaching impacts across the watershed. In October, the Chesapeake Executive Council, the top policy-making body for Bay restoration, signed a directive calling for collective action to address the threats of climate change in all parts of the Bay Program’s work. The council includes state governors, the U.S. Environmental Protection Agency administrator, the District of Columbia mayor, and the chair of the Chesapeake Bay Commission, which represents state agencies and legislatures.

The warming water trends, particularly the rapid summertime increase, could have significant consequences for some Bay resources and water quality. For instance, eelgrass, a critical species of underwater grass found in high-salinity areas of the Lower Bay, is particularly sensitive to warm temperatures and is already in decline.

Striped bass, one of the Bay’s most prized species, are also sensitive to heat stress during the summer when they die at higher rates after being handled.

Warming water temperatures could reduce the effectiveness of the region’s nutrient reduction efforts because warmer water holds less oxygen than cool water. Nutrients fuel algal growth in the Chesapeake, and when there are more algae than can be consumed by predators, the excess die and are decomposed in a process that draws oxygen — critical for aquatic life — from the water. Warmer water can make the problem worse and offset the impact of regional efforts to reduce nutrient pollution.

Related work at VIMS shows that warming temperatures have already reduced the effectiveness of some of those efforts.

Computer modeling by Luke Frankel, a graduate student working with Friedrichs, shows that if Bay water temperatures had stayed the same since the 1980s, the impact of cleanup efforts on dissolved oxygen conditions would have been about one-third greater than what actually occurred.

“That temperature effect has offset some of the improvements from nutrient reductions,” Frankel said. The good news, he said, is that — so far — nutrient reduction efforts have been great enough to improve water quality in the Bay despite the headwinds being created by warming water temperatures.

The Bay “would have been a lot worse if these reductions didn’t happen,” Frankel said.

This article was originally published on on Tuesday, November 16, 2021.

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