By Sarah Vogelsong, BayJournal.com
Tide gauge data from 5 cities reveal rate is faster in Lower Bay
As sea-level rise increasingly becomes part of public discourse and the public agenda, the Virginia Institute of Marine Sciences is ramping up efforts to provide reliable data for policy makers seeking to combat the changing circumstances.
“There’s a lot of resiliency planning going on looking at sea-level-rise projections, and we feel it’s important to know when we’re doing this planning how the data match up with the projections,” said Molly Mitchell, a marine scientist with VIMS’ Center for Coastal Resource Management.
This winter, VIMS released the second iteration of its annual Sea-Level Report Cards, a set of data from 32 coastal stations around the United States that also projects sea-level trends forward to 2050.
For the first time, VIMS has included a Chesapeake Bay-specific report card drawing on tide gauge data from five sites in the region: Norfolk and Yorktown in Virginia and Annapolis, Baltimore and Solomons Island in Maryland.
“We have a higher rate of sea-level rise in the Bay than along most of the Atlantic Coast, so there’s a lot of interest in how the rate of rise may vary around the Bay and for a lot of localities,” Mitchell said.
What the new data primarily show, Mitchell said, is stronger evidence that sea-level rise is accelerating, particularly in the Bay region and along the Gulf Coast. According to the new projections, of the 32 locations monitored in the broader study, Norfolk will have the highest rate of sea-level rise on the East Coast, with the water’s height expected to increase 5.2 millimeters per year, a slight uptick compared with last year’s projection of 5.14 mm per year.
In the Bay region, Yorktown trails Norfolk for the second highest predicted rise rate, at 4.92 mm per year. Solomons Island is next at 4.73 mm per year. Baltimore’s rise rate is expected to be 3.51 mm per year and Annapolis’ 3.84 mm per year.
These rates continue to be outstripped at three Gulf Coast locations: Grand Isle, LA; Galveston, TX; and Rockport, TX, where rise rates are 7.75 mm, 6.24 mm and 6.77 mm per year, respectively.
Depending on whether these rates remain relatively steady or accelerate over time, those projections ultimately mean that, in 2050, Norfolk’s sea level could be between 0.3 and 0.49 meters above its 1992 level, while Baltimore’s could be 0.2 to 0.38 meters higher than that baseline.
The Chesapeake has long been known to be especially vulnerable to sea-level rise because of land subsidence in the region, which Mitchell said is “a little bit higher at the southern part of the Bay than the northern part of the Bay.”
Such subsidence is the result of both geologic and human activity.
The larger geological processes involved in the sinking of the region’s land as ice sheets from the last glacial maximum continue to retreat are unavoidable. Nevertheless, these changes, as a 2015 study published by the Geological Society of America points out, risk “exacerbating the effects of global sea-level rise and impacting the region’s large population centers and valuable coastal natural resources.”
Other major contributors to subsidence include large withdrawals of groundwater and development on marshes. Paper mills in the Franklin and West Point areas of Virginia have had a particularly significant impact on the state’s groundwater reserves. A 2016 report by Virginia’s Joint Legislative Audit and Review Commission found that together, these mills “used nearly half of all permitted groundwater” withdrawals in the commonwealth. Large-scale chicken farming on the Eastern Shore has also prompted concerns about overstressing local aquifers.
“Understanding the magnitude and impact of [withdrawal-related subsidence] is critical for adaptation and management efforts, since it can be relatively easily controlled,” concluded a paper accompanying the 2018 Sea-Level Report Cards by VIMS scientists John D. Boon, Molly Mitchell and Jon Derek Loftis and communications director David Malmquist.
The report cards are intended to provide policymakers and local communities with the data needed to plan for the rising water.
“Success here may mean that a coastal community receiving these reports will be able to use the information to its advantage when revising or updating its flood defense plans,” the 2018 VIMS paper noted.
Mitchell described the VIMS projections as an “outlier” among forecasts because of its planned yearly frequency. The two other major sea-level rise forecasts, put forward by the U.S. Army Corps of Engineers and the National Oceanic and Atmospheric Administration, are issued less frequently.
VIMS projections differ from those of the Corps and NOAA in other ways as well. Most notably, they are based on the observed water level record, which Mitchell said would be the closest to “the experience of a person standing on the land” and monitoring sea level in person.
In contrast, the Corps and NOAA forecasts are based on computer models involving global sea-level trends that incorporate global, regional and local factors. They also project forward much further than the VIMS estimates, providing forecasts to 2100 rather than VIMS’ 2050 cutoff.
VIMS has chosen that year as its horizon, a Center for Coastal Resources Management newsletter explains, “because of the likelihood that patterns controlling sea level rise (and therefore, sea level rise trends) will change in the future.”
“There are pros and cons for both of those [approaches], and that’s why having both of them is very informative,” Mitchell said.
Among other uses, the new projections have been incorporated into the AdaptVA portal developed by the VIMS Center for Coastal Resources Management, Virginia Coastal Policy Center, College of William & Mary Public Policy Program, and Wetlands Watch.
For Mitchell, the next step in applying the data collected by VIMS is to increase awareness of how communities affected by rising sea level will be impacted by events like hurricanes and high tides, because sea-level rise calculations focus on the average level of the water without taking into account the high and low tides that occur throughout the day.
“We’ve been talking about mean sea level for so long, [and] for a while that was just trying to get people to understand that sea level is changing,” she said. “But now we’ve been ignoring the tide and the storms, so we need to bring those back in the conversation.”