UNIVERSITY PARK, Pa. — The warming climate is expected to affect coastal regions worldwide as glaciers and ice sheets melt, raising sea level globally. For the first time, an international team has found evidence of how sea-level rise already is affecting high and low tides in both the Chesapeake and Delaware bays, two large estuaries of the eastern United States.
The team combined a computer model with 100 years of observations to tease out the fact that global sea-level rise is increasing the tidal range, or the distance between the high and low tides, in many areas throughout each bay.
Tides, or the rising and falling of the ocean's surface, occur on regular intervals and result from numerous factors, the biggest of which is the gravitational pull from the sun and moon. For centuries, people have documented and predicted the daily high and low tides because they can impact ocean navigation. Tides can also affect ocean life, flooding risk, fishing, the weather, and some energy sources such as hydroelectric power.
In 2015, Andrew Ross, meteorology doctoral student, Penn State, noticed an odd pattern emerging while testing a numerical computer model for tidal research. Adding one meter of sea-level rise to the model resulted in a distinct pattern of changes to the high and low tides throughout the Chesapeake Bay.
"We weren't sure why it was there, but it was unique enough that we thought it should show up in observations, too, if it was actually real," said Ross, now a postdoctoral research associate, Princeton University. "So we started looking at the observations, doing more comparisons."
Ross began working with a team that included his adviser, Raymond Najjar, professor of oceanography, Penn State, to pinpoint the precise effects of sea-level rise by subtracting other forces that affect changes to the tidal range. Some of these forces are predictable, including the 18.61 years it takes for the moon's orbit around the Earth to oscillate. Other forces are less predictable, like the effect from dredging a bay to create a wider, deeper space for container ships. The team analyzed tidal gauge records from 15 locations in the Delaware and Chesapeake bays, the oldest of which dates back to 1901. They also studied nearby cities outside of each bay to control for larger changes affecting the ocean more broadly.