U.S. East Coast Sea Level

Steric Processes

At present, steric processes—primarily an increase in the volume of seawater as it warms—contribute relatively uniformly to sea-level rise at tide stations along the U.S. East Coast.

Glacial Isostatic Adjustment (GIA)

This process is causing most of our East Coast tide stations to subside (and thus relative sea level to rise). Exceptions are Eastport, Maine; Portland, Maine; and Boston, Massachusetts, which fall near the “zero line” between fore-bulge collapse and isostatic rebound. Jacksonville, Florida is uplifting slightly (causing relative sea level to fall), but not necessarily due to GIA (Sella et al. 2007). The maximum subsidence due to GIA (1.5 millimeters per year) is found around New Jersey (Karegar et al. 2017).

Water Storage/Withdrawal

Two anomalies in East Coast tide-gauge records suggest an influence of water storage and withdrawal practices (Karegar et al. 2017). A “southern anomaly”—with the highest subsidence rates along the entire East Coast—affects Norfolk, Virginia, Wilmington, North Carolina and Charleston, South Carolina. This is attributed to excessive groundwater withdrawal between the late 1970s and early 2000s, followed by a recovery. A “northern anomaly” impacts Eastport and Portland, Maine, where geologic loading from water storage above a dam contributes to local subsidence.

Ice Sheet Melting (Greenland and Antarctica)

Ice sheets contribute to sea-level rise as they melt and calve, and as their consequent loss of mass decreases their gravitational pull on adjacent waters. NASA’s Gravity Recovery and Climate Experiment (GRACE) shows significant acceleration in the loss of ice-sheet mass in both Greenland and Antarctica (Velicogna et al. 2014). GRACE-based modeling shows that the Greenland ice sheet’s contribution to sea-level rise via melting decreases southward along the U.S. East Coast, from 0.15 millimeters per year in Maine to 0.07 mm/year in Florida. The contribution from Antarctic ice-sheet melting is distributed evenly along East Coast stations, affecting acceleration but not station-to-station variability in sea-level-rise trends (Davis and Vinogradova 2017).

At New England tide stations, the gravitational effects of Greenland’s loss of ice-sheet mass slightly counteract (-0.03 mm/year) the 0.15 mm/year of sea-level rise contributed by its meltwaters. Moving south, this contribution reverses. By Florida, the effects of gravitational loading contribute a small amount (0.1mm/year) to sea-level rise (Mitrovica et al. 2001).

Contributions to sea-level acceleration from melting of the Greenland and Antarctic ice sheets exceed those from ocean dynamics (including steric processes) and air-pressure effects at all stations south of Sandy Hook, New Jersey, and become relatively more important south of Norfolk, Virginia (Davis and Vinogradova 2017). In addition, the specific origin of glacial melt may affect the geographic distribution of its contribution to sea-level rise. For example, contributions to sea-level rise at the New York gauge can be connected to melting of the North-East Greenland Ice Stream, and the sub-continent’s Petermann and Humboldt glaciers (Larour et al. 2017).

Ocean Dynamics

Ezer et al. (2013) suggest that a recent slow-down in the Gulf Stream has contributed to higher sea levels along the U.S. East Coast, with the magnitude greatest at Cape Hatteras (between Norfolk, VA and Wilmington, NC) and decreasing northward. Kopp (2013) links acceleration in this area to the Atlantic Multidecadal Oscillation, North Atlantic Oscillation, and Gulf Stream North Wall indices. The long-term prognosis for changes in these dynamic processes are yet unresolved, hampering projections.

Other Factors

Piecuch and Ponte (2015) suggest that a string of low-pressure systems may be significantly impacting year-to-year variations in sea level along the U.S. East Coast by generating extreme upsurges in sea level. They suggest this may account for ~50% of sea level rise in New England during 2008 and 2010 and 10-30% of decade-scale sea-level accelerations in the mid-Atlantic “hot spot” (Boon 2012, Ezer and Corlett 2012; Sallenger et al. 2012). However, Davis and Vinogradova (2017) found this effect to be nil in the south and small (0.05 mm/yr) in the north; we thus believe it impacts short-term variability more than long-term sea-level rise.