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9 Conclusions and Recommendations CONCLUSIONS 1. Relative mean sea level, on statistical average, is rising at the majority of tide gauge stations situated on continental coasts around the world. Relative mean sea level Is generally falling near geological plate boundaries and in formerly glaciated areas such as Alaska, Canada, Scandinavia, and Scotland. Relative mean sea level is not rising In limited areas of the continental United States, including portions of the Pacific Coast. 2. The contrasting signals concerning relative mean sea level behavior in different parts of the United States (and the world in general) are interpreted as due to differing rates of vertical motion of the land surfaces. Subsidence and glacial rebound are significant contributors to vertical land displacements. 3. Large, short-term (2-7 year) fluctuations worldwide are related to meteorological phenomena, notably shifts in the mean jet-strea~n path and the E} Nin>Southern Oscillation mechanisms, which lead to atmospheric pressure anomalies and temperature changes that may cause rise or fall of mean sea level by 15-30 cm over a few years. 4. Studies of a very small number of tide gauge records dating more than 100 years (the oldest being Amsterdam, started in 1682) 122

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CONCLUSIONS AND RECOMMENDATIONS 123 show that after removal of the subsidence factor where known, mean sea level has been fluctuating through a range of not more than 4~150 cm fin long-term fluctuations) for at least 300 years. 5. The geological record over the last 6,000 years or so indi- cates that there has been a general, long-term rise with short-term fluctuations probably not exceeding 200 cm during the last 1,500 years. 6. Monitoring of relative mean sea level behavior is at present inadequate for measuring the possible global result of future cli- mate warming due to rising greenhouse gases. The most serious gaps in present tide gauge coverage are in three areas: (a) high po- lar latitudes, (b) midoceanic locations, and (c) the entire Southern Herrusphere. 7. Because of localization of many extreme subsidence pro- cesses, especially those connected with anthropogenic extraction of fluids such as groundwater and hydrocarbons, tide gauges are needed at every major coastal city to gather data to assist in evaluating the long-term regional trend of relative mean sea level. 8. The risk of accelerated mean sea level rise is sufficiently established to warrant consideration ~ the planning and design of coastal facilities. Although there is substantial local variability and statistical uncertainty, average relative sea level over the past century appears to have risen about 30 cm relative to the East Coast of the United States and 11 cm along the West Coast, excluding Alaska, where glacial rebound has resulted in a lowering of relative sea level. Rates of relative sea level rise along the Gulf Coast are highly variable, ranging from a high of more than 100 cm/century in parts of the Mississippi delta plain to a low of less than 20 cm/century along Florida's west coast. 9. Accelerated sea level rise would clearly contribute toward a tendency for exacerbated beach erosion. However, in some ar- eas, anthropogenic effects, particularly in the form of poor sand management practices at channel entrances, constructed or modi- fied for navigational purposes, have resulted In augmented erosion rates that are clearly much greater than would naturally occur. Thus, for some years into the future, sea level rise may play a secondary role in these areas. 10. As noted previously, the two response options to sea level rise are stabilization and retreat. Retreat is most appropriate in areas with a low degree of development. Given that a Proper

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124 RESPONDING TO CHANGES IN SEA LEVEL choice exists for each location, selecting an incorrect response alternative could be unduly expensive. 11. There does not now appear to be reason for emergency action regarding engineering structures to mitigate the effects of anticipated increases in future eustatic sea level rise. Sea level change during the design service life should be considered along with other factors, but it does not present such essentially new problems as to require new techniques of analysis. The effects of sea level rise can be accommodated during maintenance periods or upon redesign and replacement of most existing structures and facilities. There are very limited geographic areas where current subsidence rates may require near-term action as has been the case in Japan and Terminal Island, California. 12. When not restrained by funding, availability of materials, or work force, construction of almost any conceivable protection against sea level rise can be carried out in a very short time; short, that is, relative to the rate of sea level rise. 13. Defensive or mitigative strategies are site specific and cannot be developed nationwide on the basis of a blanket general- ization or comprehensive legislation. 1lECOMMENDATIONS 1. The prognosis for sea level rise should not be a cause for alarm or complacency. Present decisions should not be based on a particular sea level rise scenario. Rather, those charged with planning or design responsibilities in the coastal zone should be aware of and sensitized to the probabilities of and quantitative un- certamties related to future sea level rise. Options should be kept open to enable the most appropriate response to future changes in the rate of sea ferret rise. Long-term planning and policy devel- opment should explicitly consider the high probability of future increased rates of sea level rise. 2. The three previously described scenarios of sea level rise used in this study (see Figure 2-2) provide a useful range of possible future sea level changes for design calculations. The general shape of these curves is concave upward with greater rates of rise in the distant future than those in the next decade or so. The confidence that these scenarios will encompass the actual levels decreases with increasing time, and significant deviations outside the range of these scenarios are possible, including an amelioration in the rate

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CONCLUSIONS AND RECOMMENDATIONS 125 of rise. Thus, the committee recommends that these projections be updated approx~nately every decade to incorporate additional data and to provide an improved basis for planning and response to the rise. 3. Practitioners can more readily incorporate the implications of sea level rise if probabilities reflecting uncertainties are attached to the projections. Thus, it is recommended that appropriate sta- tistical techniques be applied to develop a probability distribution associated with sea level rise through the year 2100 and that all updated projections include such information. 4. Feasibility studies for coastal projects (e.g., shore protection projects of the U.S. Army Corps of Engineers and storm surge studies of the Federal Emergency Management Agency) should consider the high probability of accelerated sea level rise. It may be some tune before precise estunates of future sea level rise are possible. In the meantime, the risks associated with a substantial rise should not be disregarded. Instead, feasibility studies should consider which designs are most appropriate for a range of possible future rates of rise. Strategies that would be appropriate for the entire range of uncertainty should receive preference over those that would be optional for a particular rate of rise but unsuccessful for other possible outcomes. 5. The federal government should acquire long-term reliable accurate data from a water-level measuring system for open-ocean stations at scientifically important locations throughout the world. Critical stations should include documentation of vertical ground motion and the temporal salinity and temperatures of the water column. Tide gauges should be installed at major coastal cities. 6. The unportant decision for maintaining or abandoning coastal facilities In the face of rising sea level should be well documented by scientific knowledge. Agencies that fund coastal research, such as the U.S. Navy, U.S. Army, National Science Foundation, National Oceanic and Atmospheric Administration, U.S. Geological Survey, and the Environmental Protection Agency, should increase their funding for coastal processes research. The federal research funding effort should focus on studies directed too ward understanding nature's response to relative sea level rise and developing appropriate engineering responses. A substantial por- tion of this research should be conducted at universities and other laboratories and centers throughout the coastal United States to ensure the development of requisite engineering capability in re- gions of the country where it will be most helpful.