Understanding Sea Level Rise in the Mid-Atlantic

by Ben Horton, Ph.D., Associate Professor, Department of Earth and Environmental Science and Director of the Sea Level Research Laboratory, University of Pennsylvania and Ken Miller, Ph. D., Professor and Chair of the Department of Earth and Planetary Sciences, Rutgers University
 
     Global sea-level rise is one of the more certain impacts of human induced global warming, although future projections of magnitude show marked variations. Given the large and growing concentration of population and economic activity in the coastal zone of New Jersey, as well as the importance of its coastal ecosystems, the potential impacts of sea-level rise have elicited widespread concern for the last two decades. To understand sea-level change, it becomes critical to assess the sum of global, regional and local trends related to changing ocean and land levels. Indeed, coastal managers are concerned about the interplay among global sea level rise, regional and local subsidence, and variations in sediment supply, as these determine the impacts at the coast and forms the basis of management response plans. With this in mind, the University of Pennsylvania and Rutgers University have begun to document past and present sea level changes in New Jersey to reduce 21st century uncertainties of the magnitude of sea-level rise.
 
Relative sea level
     The interplay of global sea-level changes and subsidence or uplift of the crust has been termed relative sea level (RSL). Glacial and interglacial cycles are characterised by temperature changes of approximately 10°C and sea-level fluctuations of 120 meters. As temperature falls, water is removed from the oceans and stored on the land in the form of ice sheets and glaciers, resulting in a eustatic lowering of sea level. Sea level is further lowered through the steric effect of reduced volume of ocean waters caused by lower temperatures. Upon deglaciation, these processes are reversed and sea level rises. Loading and unloading of the Earth's crust as a consequence of ice sheet growth and decay can introduce large vertical movements in land level, known as isostatic movement. 20,000 years ago at the last glacial maximum a massive ice sheet (known as the Laurentide) covered much of North America in thicknesses of up to 4 kilometers. Areas beneath this thick ice cover were depressed while the ice sheet was in place, but subsequently experienced uplift as the ice sheet melted. Conversely, at the margins of the ice sheet and beyond, land was uplifted during the glaciations as a forebulge, which subsequently collapsed during deglaciation. Due to the nature of Earth's slow response to loading, this compensatory adjustment of land level continues to be an important factor today, several thousand years after the Laurentide ice sheet vanished.
 

Past sea-level changes
     This isostatic movement is the primary cause of the spatial variation of past RSL changes along the U.S. Atlantic coast. For example, in Maine, the retreat of the Laurentide ice sheet exposed large areas of isostatically depressed land, which were rapidly submerged by the sea. The maximum inland extent of the marine invasion was 70 to 129 meters above present sea level at 14,000 years ago. After the ice retreat, the land rebounded, resulting in a RSL fall to a lowstand of -60 to -65 meters at 11,000 years ago. Subsequent slowing of the rate of rebound and overtaking by the rise of eustatic contributions resulted in SL rise at a generally decreasing rate to present. In contrast, observations from the U.S. Atlantic coast close to the southern limit of the ice, and beyond, such as New Jersey, record a general trend of rising RSL throughout the last 10,000 years as isostatic subsidence from the collapse of the forebulge complemented the eustatic increase in ocean volume.
 
     Drilling of the New Jersey coast has provided detailed estimates of sea-level for the past 9,000 years from preserved remains of former salt-marsh surfaces which contain microscopic organisms called foraminifera. These organisms have varying tolerances and preferences for submergence by ocean water, which drives a natural pattern of zonation whereby assemblages from low, middle and high salt-marsh environments are easily distinguished. When the foraminifera preserved in coastal sediments are combined with estimates of the sediment's age, it is possible to produce a reconstruction of former RSL. The age of sediment layers can be evaluated using many methods including carbon isotopes contained within fossilized salt-marsh plant roots and leaves or by using pollen to identify historical records of European settlement.
 

     Our work in New Jersey shows that RSL roses from 30 meters below present (-30 meters) at 8,500 years ago to -5 meters at 4,000 years ago at a rate of roughly 4 millimeters per year. This high rate of rise was due primarily to eustatic contributions from the melting of the Laurentide and other ice sheets coupled with isostatic subsidence. From 4,000 years ago to AD 1900, the melting of the ice these sheets ceased and the eustatic input to RSL was near zero, thus rate of rise decreased to 1.3 millimeters annually. This rate is almost entirely from land subsidence from the collapse of the forebulge. Through the application of similar techniques to other areas of the U.S. Atlantic coast from Maine to Georgia, we found significant spatial variations in the land movement for the past 4,000 years. The mid Atlantic coastlines of New Jersey, Delaware and Maryland have been subsiding twice as much as areas to the north and south during the past 4,000 years. We must remember that these coastal subsidence values serve as background rates on which future sea-level rise must be superimposed; the ongoing subsidence enhances sea-level rise.
 
Present-day sea level trends
     All tide-gauge locations along the U.S. Atlantic coast show an acceleration in the rate of sea-level rise between the past 4,000 years of geological data and the 20th century tide gauges. The acceleration is the result of an increase in the ocean volume, due to melting of continental glaciers and ice sheets, and expansion of ocean water as it warms. Tide-gauge data for the U.S. Mid Atlantic region show a regional rate of approximately 3.1 millimeters per year of RSL rise in the 20th century versus the 1.8 millimeters per year global average. The 1.3 millimeter per year difference in this region is due to the previously mentioned ongoing coastal subsidence.. The rates of RSL rise are higher locally (around 4 millimeters per year) at Atlantic City and Sandy Hook, New Jersey due to sediment compaction; compaction at Atlantic City is caused by groundwater withdrawal, a similar mechanism that causes the high rates of subsidence observed in Venice, Italy.

What about the future?
     21st century RSL rise will cause a number of direct and indirect impacts for New Jersey. Predictions of future global sea-level rise is uncertain, but can be bracketed as a eustatic increase of ~0.75-1.90 m by 2100. The 2007 IPCC Fourth Assessment Report provided a best estimate of 0.4 meters, with a range of 0.2-0.6 meters by 2100. However, this report only accounted for the steric effects of warming and mountain glacier retreat. Because of the timing of the report, it assumed contributions from Greenland and Antarctic ice sheet melting was unknown. Not only have subsequence studies shown that Greenland is contributing over one quarter of the most recent rise inferred from satellite data from the past 15 years (~3 millimeters per year of sea-level rise from 1993 to present), it is clear the modern rise is tracking a trajectory that will result in at least 0.8 meters of rise by 2100. The upper limit of sea-level rise is not well constrained, but glaciological and other constraints very likely limit the total rise to significantly less than 2 meters by 2100. We have revealed that the magnitude of impacts will vary along the U.S. Atlantic Coast and will be particularly pronounced in the mid-Atlantic region (including New Jersey) due to high rates of land subsidence (0.1-0.2 meters by 2100), which will exacerbate sea-level rise caused by ocean volume changes. The actual impacts from place to place along the New Jersey shore will depend on a variety of factors, including other aspects of climate change and local scale factors such as coastal morphology, human modifications and the general starvation of beaches in this region.
 
     It is clear that no matter what emissions scenario of greenhouse gases is realized, that the shoreline in the Garden State is in great peril. By understanding the interplay of global sea-level rise, subsidence, and sediment changes afforded by the geological record, future scenarios for the impact of climate change on the New Jersey shore can be evaluated.