It’s well known that climate change will be having a major impact on coastal regions around the world as glaciers and ice sheets melt, causing sea level to rise globally. But now this isn’t just a concern for the future, as for the first time an international research team has discovered evidence that sea-level rise is already affecting high and low tides in both the Chesapeake and Delaware bays.
Through combining a computer model with 100 years of observations, the researchers found that global sea-level rise is increasing the tidal range — which is the distance between high and low tides — in many areas throughout each bay.
In 2015, Andrew Ross, a meteorology doctoral student at Penn State, noticed a bizarre pattern occurring while testing a numerical computer model for tidal research. A distinct pattern of changes to the high and low tides of Chesapeake Bay would result upon adding one meter of sea-level rise. “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,” says Ross, now a postdoctoral research associate, Princeton University. “So we started looking at the observations, doing more comparisons.”
Together with a team of researchers, Ross began working to pinpoint the precise effects of sea-level rise through subtracting other forces that affect changes to the tidal range. These forces include predictable ones such as the orbit of the moon around Earth, as well as less predictable ones like the effect of dredging a bay. They also analyzed tidal gauge records from 15 locations in the Delaware and Chesapeake bays, the oldest of which dated back to 1901.
The researchers paired information from isolated data of the influence of sea-level rise on tides to a computer model that could adjust the overall sea level, simulating the effects of either the past or future. They found that both the tide gauge data and the model generally agreed on the main effects of sea-level rise on tides. Analysis showed that one meter of sea-level rise increased the tidal range by up to 20 percent in some areas. Based on the physical characteristics of the bay, sea-level rise varied. “In the Delaware Bay, as you go upstream toward Philadelphia, the shore lines are converging in a sort of funnel shape, and so we see that amplifies sea-level rise’s effects on the tides,” said Ross. “That amplification gets magnified the farther you go upstream.”
For the Chesapeake Bay, the shape resulted in a different pattern of effects – in some cases slightly decreasing the tidal range. The shape of the bay is longer and less of a funnel, compared to the Delaware Bay. When a wave enters the Chesapeake Bay from the ocean, it hits the end of the bay and reflects back towards the ocean, either adding to or subtracting from the size of the waves it hits. Waves travel faster in deeper water, so a higher sea level leads to faster wave speed. This changes where incoming and outgoing waves meet, which can have a cascading effect on tide patterns throughout the bay.
“When people think about flooding associated with sea-level rise, they often think everything will just go up, including the tides, but the reality is that in some cases, the tidal range may stay the same or decrease,” explains Raymond Najjar, a professor of oceanography at Penn State. “It all depends on the geometry of the bay and the speed of waves, which we know are affected by rising sea levels.”