Study says Chesapeake Bay waters are warming

By Linda Cicoira

     Chesapeake Bay waters are warming, according to a new study made by researchers at William & Mary’s Virginia Institute of Marine Science. Long-term measurements of water temperature reveal a clear seasonal pattern, and quantifies the relative contributions to warming brought by the atmosphere, Bay tributaries, and the ocean.

     A better understanding of the seasonal timing and causes of Bay warming will improve projections of future conditions and help managers better assess water-quality goals, particularly as they relate to efforts to limit low oxygen “dead zones.”

     Findings were based on a combination of field measurements and computer modeling. The field data included more than 30 years of bay water temperatures, long-term records of river flow, weather conditions, and ocean temperatures.

     “During the last three decades the Bay has warmed three to four times faster in summer than winter,” a researcher said. “These changes have been driven primarily by atmospheric warming. But in Virginia waters, ocean warming has contributed as well.”

     Surface and bottom waters are generally warming at a similar rate throughout the Bay, except for a slightly elevated rate of bottom warming in the saltier waters near its mouth, according to the information. Overall, the team’s findings show that the Bay’s summertime bottom waters, have warmed at a rate of 0.07 degrees Fahrenheit annually since 1985, with a total warming of two degrees Fahrenheit between the late 1980s and the late 2010s.

     The team’s modeling experiments show that almost 90 percent of Bay warming is driven by atmospheric effects or warming air temperatures and the increase in heat energy emitted back to the Earth from the atmosphere due to greater concentrations of greenhouse gases.

     Sea level rise has slightly tempered summer warming, as the Bay becomes deeper it takes longer to both warm in the spring and cool in the fall, and the influence of warming due to river flow has been limited to the heads of tidal tributaries. The influence of ocean warming is greatest near the Bay mouth, where it accounts for more than half of summer warming in bottom waters.

     Given the greater loss of oxygen due to warming bottom waters, it’s highly likely that additional nutrient reductions will be necessary in the future to mitigate the impacts of temperature increases throughout the Bay,” a VIMS scientist noted.

     The study authors also stressed that future rates of Bay warming will depend not only on global atmospheric trends, but on regional changes in ocean circulation, which scientists have linked to an observed increase in temperatures within the coastal waters of the mid-Atlantic just outside the Bay mouth. These waters are currently warming at a rate of 0.4 degrees Fahrenheit per decade, around two times faster than atmospheric temperatures, and nearly four times greater than the increase in globally averaged surface ocean temperatures.

Future warming of the Bay’s bottom waters may thus largely depend on additional changes in ocean conditions, which themselves depend not only on the rate of additional heat uptake but also changes in the position of the Gulf Stream and Labrador Current.

 “We found the greatest increase in bottom temperatures in the areas of the Bay most influenced by the ocean—the saltier Virginia waters near the Bay mouth,” a lead scientist said. “This suggests these would be ideal locations for deploying long-term monitoring instruments to detect future changes in Bay temperature, which would also help us better estimate the extent of Bay hypoxia.”

.