Chesapeake Bay Land Subsidence and Sea Level Change An Evaluation of Past and Present Trends and Future Outlook
Ten Chesapeake Bay water level stations presently have a combined total of 647 years of water level measurements with record lengths varying between 35 years (1975-2009) at the Chesapeake Bay Bridge Tunnel, VA, and 107 years (1903-2009) at Baltimore, MD. All ten stations, with the exception of Gloucester Point, VA, are active stations in the National Water Level Observation Network of water level stations maintained by the U.S. National Oceanic and Atmospheric Administration, Center for Operational Oceanographic Products and Services. New technologies such as sea surface range measurements from earth-orbiting satellites now provide a global assessment of absolute sea level (ASL) trends relative to the center of a reference ellipsoid rather than fixed points on the earth’s surface to which relative sea level (RSL) measurements refer. New methodologies have also been applied to derive spatial averages of ASL trends over large regions with greater accuracy. Notwithstanding these advances, there is still no substitute for an accurate time series of water level measurements obtained locally, preferably one spanning several decades, when assessing RSL trends that will affect a specific community or township in the coming decades. RSL trends will determine local inundation risk whether due to vertical land movement (emergence or subsidence) or the ASL trend found as the sum of RSL trend and land movement when both are measured positive upward. In Chesapeake Bay, RSL trends are consistently positive (rising) while land movement is negative (subsiding). By choosing a common time span for the ten bay stations evaluated in this report, we are able to compare differences in RSL rise rates with approximately the same degree of confidence at each station. Uncertainty has been reduced by extracting the decadal signal present at all ten stations before using linear regression to obtain new RSL rise rates with smaller than usual confidence intervals, permitting both temporal and spatial comparisons to be made. Temporal comparisons at five bay stations over two periods, 1944-1975 and 1976-2007, suggest that, while RSL continues to rise at some of the highest rates found along the U.S. Atlantic coast, there is presently no evidence of a statistically significant increase marking an acceleration in RSL rise at any of the five bay stations. Small but steady increases in RSL rise rate with time are still a possibility as RSL trend confidence intervals remain too large for statistical inference. Spatial comparisons at ten stations for the 1976-2007 period provide new evidence on spatial variability of RSL rise rates within Chesapeake Bay. Global positioning system (GPS) data from ground stations further define the pattern of spatial variability and permit new estimates of ASL rise rates in the region, all of which are significantly less than the global ASL rise rate of 3.1 mm/yr over 1993 to 2003 reported in the IPCC Fourth Assessment Report. Present evidence suggests an ASL rise rate of about 1.8 mm/yr in Chesapeake Bay over the 1976-2007 period. Applying this rate uniformly throughout the bay, subsidence rates ranging from about -1.3 mm/yr to -4.0 mm/yr are found, leading to the general conclusion that about 53% of the RSL rise measured at bay water level stations is, on average, due to local subsidence. Outlook: Land subsidence in Chesapeake Bay is likely to continue at or near present rates. Future ASL rise in the bay region remains uncertain owing to diverse and possibly changing trends world-wide (see report cover). Their combination strongly suggests a need for future monitoring.