Sea-Level Rise and Chesapeake Bay

Sea-Level Rise and Chesapeake Bay

Introduction

As the largest and most productive estuary in North America, Chesapeake Bay is a vital ecological and economic resource. The Chesapeake Bay watershed includes many low-lying areas that are threatened by rising sea level. In addition to creating the potential for erosion, inundation, and storm-surge effects, a rise in sea level can affect the salinity of and circulation in the bay (Hilton and others, 2008). Rising sea level also can affect water quality and alter major nearshore habitats that support fisheries and waterfowl and provide valuable property for the citizens near Chesapeake Bay.

Sea level changes globally and regionally over time scales ranging from millions of years to decades. These changes are a result of many complex factors (Cronin, 2011). Scientists are improving the current understanding of the causes and patterns of sea-level change, both globally and in Chesapeake Bay; determining the best available estimates of future sea-level rise; and informing decision makers about the possible effects of sea-level change so that effective mitigation measures can be developed.

U.S. Geological Survey Studies of Sea-Level Rise

This science summary is one in a series that is designed to facilitate the understanding and application of results of relevant U.S. Geological Survey (USGS) studies by Chesapeake Bay resource managers and policy makers. It provides a brief overview of the most recent published work by the USGS and collaborators on the importance of sea-level rise in Chesapeake Bay, geologic records of sea-level change, modern sea-level rise, and projected rates of future sea-level rise; an understanding of how this information can be used to develop effective management policies and practices; and a list of references for additional information. The USGS is working with the National Oceanographic and Atmospheric Administration (NOAA) and other partners to evaluate the potential effects of sea-level rise in Chesapeake Bay as part of the President’s Chesapeake Bay Executive Order (Chesapeake Bay Program, 2009).

Geologic Records of Sea-Level Change

Geologic records of sea level in and adjacent to Chesapeake Bay provide critical information on the rate and magnitude of sea-level rise during periods of warm climate. Research by the USGS and others shows that sea level rose rapidly during warm interglacial periods, inundating large regions of the Atlantic Coastal Plain during periods of high global sea level, primarily as a result of (1) changes in the volume of land ice (glaciers and ice sheets), and (2) vertical land movements. Some key findings include:

• During the last interglacial period (about 125,000 to 80,000 years ago), sea level in the Chesapeake Bay area was 5 to 6 meters (m) higher than its present level. In contrast, global sea level during glacial periods, when water was stored in large continental ice sheets, was 120 to 125 m below current levels. During these periods, Chesapeake Bay was an extension of the Susquehanna River (Colman and Mixon, 1988; Colman and others, 1990).
• As the ice sheets melted during deglaciation, global sea level rose rapidly, at times reaching rates of 50 millimeters per year (mm/yr)—more than 25 times the average rate over the last century. Toward the end of deglaciation, sea level continued to rise, forming the modern Chesapeake Bay about 9,500 to 8,000 years ago, as shown by records in sediment cores collected in 1999 (Bratton and others, 2003) and 2003 (Cronin and others, 2007) (fig. 1).
• During the period of abrupt climate change when the modern Chesapeake Bay was formed, sea level rose at roughly three times the present rate.

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Modern Sea-Level Rise

Two major factors contribute to global sea-level rise: thermal expansion from increasing ocean heat content, and the melting of glaciers and the Greenland and Antarctic ice sheets. Since about 2003, glaciers and ice sheets have contributed a greater proportion of the global sea-level rise than thermal expansion (Meier and others, 2007; Cogley, 2009; Rignot, Bamber, and others, 2008; Rignot, Box, and others, 2008). Today’s glaciers and ice sheets (fig. 2) store enough water to raise sea level by about 68 to 70 m. Calculated rates of global mean sea-level rise range from 1.7 to 3.2 mm/yr, depending on the time period examined (Cazenave and Llovel, 2010; Church and White, 2011). Detecting fluctuations in the rate of sea-level change can be difficult as a result of the short period of record of tide gages and chronological uncertainty in marsh paleo-sea-level studies (Larsen and Clark, 2006). Some key findings include:

• Chesapeake Bay tide-gage records and paleo-sea-level records from tidal marshes and the bay’s main stem, determined from sediment cores collected from 1995 to 2006 (fig. 1), show that rates of sea-level rise in Chesapeake Bay range from about 3.2 to 4.7 mm/yr depending on the location and period of record for each tide gage. These rates exceed the global average because the land is subsiding; therefore, the Chesapeake Bay area is more vulnerable than many other coastal regions to sea-level rise.
• The departure of sea-level trends in Chesapeake Bay from the global mean for the last century may not persist. Therefore, rates measured at tide gages do not necessarily reflect pre-20th century regional patterns, nor can they be expected to persist into the future.
• Estimates of local subsidence from groundwater withdrawal in parts of Virginia are 1.5 and 3.7 mm/yr for 1979–95 and 1982–95, respectively (Pope and Burbey, 2004). Subsidence is expected to increase with greater withdrawals.

Projecting Future Sea-Level Rise

Projecting future sea-level rise is one of the most challenging aspects of the study of climate change. Although uncertainty remains great, empirical and modeling methods used in recent studies have provided new estimates of the cumulative rise in sea level by 2100 (Rahmstorf, 2007, 2010; Vermeer and Rahmstorf, 2009; Pfeffer and others, 2008; Bahr and others, 2009; Rignot and others, 2011). Some key findings include:

• Evidence indicates a total rise of about 80 to 130 centimeters (cm), which is equivalent to annual rates of about 9 to 14 mm/yr, during the 21st century. Grinsted and others (2010) showed that sea-level rise could reach rates as high as 11 mm/yr by 2050. These values are estimated mean global rates and do not exclude the possibilities that slower or faster rates may persist for years to decades, that sea level may fall at times, or that regional rates for a particular coastline could be much greater than mean rates.
• Scientists believe that projected global rates will be higher than those reported in the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report and that melting ice is contributing more to sea-level rise than previously was believed. Recent research indicates that assessments of the effects of sea-level rise made as recently as in the last few years may underestimate future rates of sea-level rise. In addition, many other factors, such as large-scale ocean circulation changes and geophysical processes, can affect regional sea-level patterns (Cronin, 2012).
• The USGS has simulated the effects of sea-level rise in the Chesapeake Bay area on the Blackwater National Wildlife Refuge (NWR) (Larsen and others, 2004). Forecasts of inundation of the refuge by 2100 under two sea-level-rise scenarios, one assuming a 3-mm/yr increase and the other assuming a 6.2-mm/yr increase, are shown in figure 3. The projections show that portions of the marsh at Blackwater NWR are expected to become open water (Cahoon, 2007). In addition, the marsh-surface elevation at Blackwater NWR is decreasing in many locations, indicating that the sea-level-rise projections likely underestimate the extent of future marsh loss (Cahoon, 2007).

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Past Impacts of Sea-Level Rise on the Ecology of the Bay

The USGS has studied past conditions in Chesapeake Bay to help determine how the bay may respond to climate change in the future. Some key findings include:

• Sediment cores from Chesapeake Bay and surrounding land areas show that both land use and climate change have greatly affected the bay’s estuarine and terrestrial ecosystems (Cronin and Walker, 2006; Willard and Cronin, 2007). For example, periods of extremely wet climate led to increased river runoff, increased sediment transport, and reduced bay salinity. In contrast, “megadroughts” punctuated the last 8,000 years of the bay’s history, with frequent periods of elevated bay salinity and changes in forest composition (Cronin and others, 2005; Saenger and others, 2006; Saenger and others, 2008).
• USGS studies have demonstrated significant temperature variability in the Chesapeake Bay region during the last 8,000 years (Bratton and others, 2003; Cronin and others, 2003; Cronin and others, 2010). These climate changes are associated with changes in terrestrial vegetation in surrounding areas (Willard and others, 2005). Because climatically forced temperature and sea-level changes are commonly correlated, these results indicate that sea level also may have varied during the last 8,000 years.

Implications for Management Policies and Practices and Next Steps

• The Chesapeake Bay area is more vulnerable than many other coastal regions to sea-level rise because the land is subsiding.
• Resource managers need to identify those critical factors controlling marsh stability that can be managed in order to develop successful marsh-restoration plans. Large-scale restoration efforts coupled with an improved understanding of the effects of annual burning, nutrient enrichment, and sea-level rise are needed to reverse the trends of wetland loss at Blackwater NWR and other key areas around the bay.
• The President’s Chesapeake Bay Executive Order (Chesapeake Bay Program, 2009) places increased emphasis on understanding and addressing the combined effects of climate and land change on the Chesapeake Bay ecosystem. Future studies are planned by the USGS, NOAA, and other partners to evaluate sea level in Chesapeake Bay and the combined effects of climate and land change in the bay watershed in order to help inform development of effective adaptation strategies.

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