Poseidon satellite altimeter, followed by later high-precision satellite altimeter missions. As a result, it is now possible to detect acceleration in sea-level rise over the past few decades.2 The current estimated rate (3 mm/year) is already at the upper limit of the range of global sea-level rise projections that were presented 20 years ago in the first Intergovernmental Panel on Climate Change (IPCC) assessment. Although this rate is small relative to the magnitude of tidal excursions at most localities, the probability of sea level rising at this rate or faster (while adding to tidal excursions and storm surges) over the next century requires serious assessment of the implications for coastal facilities.

Sea level has changed in the past with major climate cycles, dramatically so, relative to projections for future decades. During the last glacial maximum about 18,000 years ago, the accumulation of ice on land resulted in the surface ocean being approximately 120 meters below current levels.3 With the melting of most of this ice over a few thousand years, sea level rose to near its current level. As the human population grew over the past few centuries, a relatively stable sea level led to the extensive development of coastal margins for habitation and economic enterprise. This legacy of successful coastal development reflects the history of sea level within tidal variation and the degree to which related vulnerability issues are occasionally amplified by coastal storms. A 1900 hurricane in Galveston, Texas, with 8,000 fatalities, is a case in point. A rise of 0.5 meter over several decades will be of enormous consequence for coastal cities and naval installations that experience only small natural tidal variation. For example, such an increase in sea level will have significant consequences for St. Petersburg, Florida (typical tidal excursion up to 0.7 meter), but virtually none for Anchorage, Alaska (typical tidal excursion 10 meters).

The 2007 IPCC Fourth Assessment Report (AR4) projected changes in global average sea level under various scenarios considering glacier and ice sheet mass loss, but recognized that there was much uncertainty in the results. This lack of precision is because observational records of sea-level rise are short and therefore subject to uncertainty; in addition, the understanding of how glaciers and ice sheets will respond to increased temperature changes is very poor.4

Work published since the 2007 IPCC report suggests that loss of ice from small ice bodies (e.g., mountain glaciers and small ice caps) may have been

2

Martin Sommerkorn and Susan Joy Hassol (eds.). 2009. Arctic Climate Feedbacks: Global Implications, World Wildlife Fund, International Arctic Programme, Oslo, p. 13.

3

The Last Glacial Maximum refers to the period in Earth’s history when the glaciers were at their thickest and the sea levels at their lowest. See Peter U. Clarke, Arthur S. Dyke, Jeremy D. Shakun, Anders E. Carlson, Jorie Clark, Barbara Wohlfarth, Jerry X. Mitrovica, Steven W. Hostetler, and A. Marshall McCabe, 2009, “The Last Glacial Maximum,” Science, August 7, Vol. 325, No. 5941, pp. 710-714.

4

IPCC. 2007. Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, United Kingdom and New York, Chapter 10.



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