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Early Warning Signs of Global Warming: Sea-Level Rise and Coastal Flooding

Worldwide measurements from tidal gauges indicate that global mean sea level has risen between 10 and 25 cm (18 cm average) during the last 100 years (Warrick et al., 1996). This rate is greater than the average of the last few thousand years estimated from geological and archaeological records; the timing of the onset of this acceleration, however, is uncertain (Gornitz, 1995; Warrick et al., 1996). There is no evidence for an acceleration of sea-level rise during this century, nor would any necessarily be expected from the observed climate change to date (Warrick et al., 1996). The IPCC Second Assessment Report estimated a sea-level rise for the next 100 years of about 49 cm, with a range of uncertainty of 20-86 cm. New emissions scenarios being prepared for an IPCC Special Report have much lower sulfur dioxide levels, which have a cooling effect via the production of sulfate aerosols. Thus, preliminary projections of sea-level rise using the new emissions data are slightly higher (Wigley, 1999). In either scenario, the projected rate of sea-level rise is at least two to four times the rate of the last century.

A major source of uncertainty about sea-level rise is the future behavior of the Greenland and Antarctic ice sheets. For example, disintegration of the West Antarctic Ice Sheet (WAIS) which lies grounded on land below sea level could eventually raise sea level by 4 to 6 meters (Bindschadler, 1998; Kerr, 1998; Rignot, 1998; Bindschadler et al., 1999). WAIS is presently fringed by floating ice shelves that slow the release of icebergs into the sea. Although increasing temperatures in this region would be insufficient to cause melting, the warmer air would likely bring increased snowfall. The resulting positive mass balance of the ice sheet could lead to a rapid advance, or "surge," accelerating the discharge of ice and raising sea level (Warrick et al., 1996).

Most scientists believe that collapse of WAIS within the next century is unlikely, but there is some concern for an irreversible destabilizing of the ice sheet even with relatively modest warming (e.g., Oppenheimer, 1998). A new study suggests that the modern grounding-line retreat is part of an ongoing recession that has been under way for a few thousand years (Conway et al., 1999). The authors conclude that the retreat is independent of anthropogenic warming or sea level rise but nevertheless could lead to complete disintegration of the WAIS within the present interglacial period.

In Greenland, the southern regions of the ice sheet are likely to be most susceptible to climate change and could contribute to sea-level rise through melting and runoff at the margins. Observations are insufficient to determine if the ice volume changed significantly during the past century. Recent aircraft surveys using laser-altimetry, however, indicate that between 1993 and 1998 the southeastern part of the ice sheet thinned overall (Krabill et al., 1999).

Sea-level rise in a given location will vary depending on factors such as vertical land movement, wind and pressure patterns, ocean circulation, and rate of warming. Coastal wetlands and lowlands, beaches and barrier islands, and ocean islands and atolls are especially vulnerable to rising seas. Depending on the rate of sea-level rise, the rate of vertical wetland build-up, and the capacity for wetlands to migrate inland, a 50-cm sea-level rise could inundate up to 50% of North American coastal wetlands (Shriner and Street, 1998). These areas are critical habitat for large numbers of coastal bird and fish species, and provide ecosystem services such as pollution filtration, sediment trapping, erosion mitigation, and flood control. Wetlands in most areas have been able to keep pace with historic sea-level rise by accreting sediment and growing vertically and by moving inland with the encroaching sea. But the accelerated rates projected for the next 100 years may be too fast for natural accretion and migration to keep up. Sediment deficits and development barriers are among the most important factors that limit the survival of this important coastal habitat.

Sea-level rise leads to increased coastal flooding through direct inundation and an increase in the base for storm surges, allowing flooding of larger areas and higher elevations. One study, for example, estimated that a rise in sea level of 30 to 90 cm would increase the size of the 100-year floodplain in the United States by 10,000 to 20,000 km2 (FEMA, 1991). Put differently, storms of a given magnitude will have a shorter return interval. High tide peaks, for example, that occur once every one hundred years on average may occur every ten years, making now rare events more common. In some areas, flooding could be further exacerbated by an increase in extreme precipitation events resulting from an intensification of the hydrological cycle (see Downpours, Heavy Snowfalls, and Flooding). Heavy precipitation associated with coastal storms causes increased runoff and river surges that intensify the effects of storm surges from the sea. Levees and seawalls currently protect many coastal areas, but these structures have been designed for current sea level and may be overtopped in the future or undermined by increased erosion.

The costs of responding to a sea-level rise of 50 cm by 2100 are estimated at between $20 and $200 billion in the United States alone (Shriner and Street, 1998). The wide range in this estimate reflects the different options, extent, and timing of response. Adaptation measures such as the construction of bulkheads, dikes, and pumping systems can protect property, but these measures are likely to result in further loss of wetlands and beaches with detrimental effects on fish and wildlife, recreation, and tourism. Elevation of structures and land surfaces, and land-use policies that allow shorelines to retreat naturally, are less disruptive response strategies but are challenging to implement in areas already highly developed. Maine and Rhode Island, for example, have regulations prohibiting structures that will prevent the inland migration of wetlands (Shriner and Street, 1998). Land elevation and beach nourishment are attractive options in many ways; yet they are not feasible in all locations, and they require extraordinary financial and political commitments into the indefinite future.

References

Bindschadler, Robert. 1998. Future of the West Antarctic Ice Sheet. Science 282, 428-429.

Bindschadler, R.A., R.B. Alley, J. Anderson, S. Shipp, H. Borns, J. Fastook, S. Jacobs, C.F. Raymond, C.A. Shuman, 1999. What is happening to the West Antarctic Ice Sheet? EOS 79 (22), 2 June 1998. http://igloo.gsfc.nasa.gov/wais/eos.html

Conway, H., B.L. Hall, G.H. Denton, A.M. Gades, E. D. Waddington, 1999. Past and future grounding-line retreat of the West Antarctic Ice Sheet. Science 286, 280-283.

FEMA, 1991. Projected impact of sea-level rise on the National Flood Insurance Program. Report to Congress. Federal Insurance Administration, Washington, DC, 172 p.

Gornitz, V., 1995. Sea-level rise: A review of recent, past and near-future trends, Earth Surface Processes and Landforms 20, 7-20, 1995.

Kerr, Richard. 1998. West Antarctica s weak underbelly giving way? Science 281, 499-500.

Krabill, W., E. Frederick, S. Manizade, C. Martin, J. Sonntag, R. Swift, R. Thomas, W. Wright, and J. Yungel, 1999. Rapid thinning of parts of the southern Greenland ice sheet, Science 283, 1522-1524.

Oppenheimer, M., 1998. Global warming and the stability of the West Antarctic ice sheet. Nature 393, 325-332.

Rignot, E.J. 1998. Fast recession of a West Antarctic glacier. Science 281, 549-551.

Shriner, D.S. and R.B. Street, 1998. North America in The Regional Impacts of Climate Change: An Assessment of Vulnerability, 253-330, (Eds RT Watson, MC Zinyowera, RH Moss), Cambridge University Press, Cambridge.

Warrick, R. A., C. L. Provost, M. F. Meier, J. Oerlemans, and P. L. Woodworth, 1996. Changes in sea level, in Climate Change 1995: The Science of Climate Change, 359-405,

(Eds JT Houghton, LG Meira Filho, BA Calander, N Harris, A Kattenburg, and K Maskell), Cambridge University Press, Cambridge.

Wigley, T.M.L., 1999. The Science of Climate Change: Global and U.S. Perspectives. Pew Center on Global Climate Change, Arlington, Virginia, 48 p.

Additional Resources

Titus, J. 1998. Greenhouse effect and sea level rise: America starts to prepare. http://www.erols.com/jtitus/index.htm

Titus, J. G. and V.K. Narayanan, 1995. The probability of sea-level rise. EPA 230-R-95-008, Environmental Protection Agency, Washington, DC.

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