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and 13 IMPACTS OF FUTURE SEA LEVEL RISE James M. Broadus Economists like to tell a story about the famous Gilded Age financier, J. P. Morgan. It seems a yuppie of those days found himself seated next to the old wizard and decided to play for some free advice. ''What do you think the market will do?'' he asked. Morgan looked at him sternly, glanced about, and leaned closer to whisper, "Fluctuate." Exactly the same can be said of sea level, but with even greater certainty and a much longer record of experience. Changes in sea level are recorded on epochal scales as well as observed in real time (Figure 13.1~. They are associated with regional tectonics, mesoscale oceano- graphic features, local land subsidence and erosion, and tides, waves, and ripples. We can speak of changes in average global eustatic sea level (determined by the volume of the ocean) and of changes in local relative sea level (local average height of the sea level relative to the land). In some places, local relative sea level has been rising for some time (Figure 13.2a). In other places it has been falling (Figure 13.2b). What changes have been taking place recently in average global eustatic sea level we do not really know. What changes to expect in coming decades and what their implications are for us now are the issues at hand. The presumption in this case is that global warming, driven by the buildup of greenhouse gases from human activities, will increase the volume of water in the oceans (through a combination of thermal expansion and melting ice) and lead to sea level rises throughout the world (UNEP, 1986; Titus, 1986; Robin, 1987~. Before discussing the status of current efforts to estimate the impacts of future sea level change, it is useful to consider a dozen brief observations that condition the exercise. 1. The expected physical impacts of rising sea level (Titus, 1986, 1987; Bird, 1986; Bruun, 1986; Park et al., 1986; EPA, 1988) include: o inundation of low coastal lands; o relocation or destruction of coastal wetlands; o shoreline erosion and beach loss; o exacerbated exposure to storm surge and flooding (Figure 13.3~; increased salinity of rivers, bays, and aquifers. 12S

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126 2 L`J 50 Ad LU an 111 ~ 100 At, I 150 I]J ~ (PRESENT TIME) Present Sea Level 0 5,000 10,000 t5,O00 20,000 25,000 3O,000 35,000 YEARS BEFORE PRESENT FIGURE 13.1 Recent changes in sea level. Small-scale fluctuations are , not shown. (Adapted from D. A. Ross. 1977. Introduction to Oceanog- raphy, Prentice-Hall, Englewood Cliffs, N.J.) 7200 7100 7000 6900 BALTIMORE i' Air ~ 1 1 1 1 1 6800 1 900 1920 1940 1960 1980 ILI an > 7300 LLI By: - STOCKHOLM hip\ A A I 6800 _ 6700 1910 1930 YEAR I 1 1 1 1950 1970 1990 YEAR (a) (b) FIGURE 13.2 Tide-gauge records from the early twentieth century in- dicating (a) rising local relative sea level at Baltimore on North American East Coast and (b) falling local relative sea level at Stockholm in Scandinavia, as the land rises in "rebound" from weight of receding glaciers. (Courtesy D. G. Aubrey and A. R. Solow, Woods Hole Oceano- graphic Institution.)

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127 2. Coasts are dynamic, ever-shifting places (Pilkey et al., 1982; Bird and Schwartz, 1985; NRC, 1987~. Build too close and you will pay a price (Figure 13.4~. 3. Humans have long experience with coastal natural disasters (Figure 13.5~. Sea level rise will only add to the problem, not create it. 4 Impacts depend on the magnitude and pace of sea level rise, and this is very uncertain (IAPSO, 1985~. 5. Sea level impacts depend on local relative sea level change, which varies from place to place. We still cannot distinguish local from global sea level change (Barrett, 1984; Solow, 1987), and in any case global trends are less pertinent for impact assessments and policy planning. 6. Most efforts to determine a long-term trend in global average sea level change suggest a gradual rise of 1 to 2 mm/yr over the past century (Barrett, 1983~. The tide-gauge data from which such estimates are derived are extremely patchy and variable in quality, and the data are extremely "noisy" at best. Also, the statistical techniques employed are unsettled and require further refinement (Solow, 1987; Gornitz and Solow, 1989~. Evidence of an acceleration in the rate of sea. level rise in response to an enhanced greenhouse warming (Robin, 1987) has not been detected, with the most likely change point identified so far coming in the late nineteenth century (Gornitz and Solow, 1989~. That would be far too early to be ascribed to the modern buildup of greenhouse gases. 7. The projections we do have for future sea level changes, on which estimates of future impacts must be based, are only scenarios describing hypothesized future conditions (Hoffman et al., 1986~. These scenarios usually span a reported range, including low, medium, and high cases, for example; but typically they are not associated with estimates of relative likelihood. What is needed are probabilistic forecasts, so that estimated future impacts can be weighted by their likelihood. In fact, there is currently no valid quantitative estimate of the extent and size of expected sea level increases. 8. Impacts depend on human responses (Schelling, 1983; Bird, 1986; Bruun, 1986; Gibbs, 1986; NRC, 1987; Broadus, 1989~. People are good at incremental adaptation, risk management, and technological advance. 9. Economic impacts are sensitive to property values and durable fixed capital. In market economies, property values of land provide a workable approximation of the present value of the future flow of economic services supplied exclusively by the land. Property values can thus be used in estimating the value of projected land loss. As property values increase, so does the economic cost of potential inundation. Labor employed in many economic activities taking place in exposed areas can relocate, and so inundation will only impose a cost of adjustment and some penalty for moving to ''next-best" productive employment. For physical capital in exposed areas (such as tools, cars, trailers, houses, wharves, shops, and factories), some can be moved and some will have worn out anyway before inundation. Therefore, it is really the value of long- lived or durable fixed capital (such as power plants, waste treatment facilities, nuclear waste disposal emplacements, highways, and port infrastructure) that must be reckoned into estimates of future losses.

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FICURE 13.3 Twelve-foot storm surge comes ashore at Galveston, Iexas, during Hurricane Frederick in September 1979. (Courtesy of the National Oceanic and Atmospheric Administration.) FIGURE 13.4 Cape Cod house tumbles into the sea in 1988 after a break in offshore barrier spit increased shoreline eros10n at ChaLham, Massachu- setts. (Courtesy of Cape Cod Times.)

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FIGURE 13.5 "Lamentable News out of Monmouthshire:" Old woodblock illustration of fourteenth-century coastal flooding in England. (Courtesy of University of East Anglia, Norwich, United Kingdom.) The more the useful lifetime of such installations extends into the period of expected inundation, the greater the potential impact. 10. The present economic value of future impacts depends critically on the social rate of discount. This is the factor by which future costs and benefits are reduced to make them comparable with present costs and benefits. It makes good economic sense to apply some discount rate to future values, because people do tend to value a current payment or benefit more heavily than a nominally equal payment at a future date. That is smart because to do otherwise would be to ignore the additional earnings that could be gained from the current payment (for instance, through investing it for compounded interest payments) in the time before the future payment becomes due. Selecting the appropriate discount rate to apply in practice, however, is quite difficult (Lied et al., 1982~. It involves strong judgments about the preferences of society and en- counters serious ethical complications when intergenerational effects are at stake. Higher discount rates depress the estimated cost of future sea level impacts. Lower discount rates make them appear larger. 11. Impacts (and responses) will change with changes in tastes, preferences, and relative scarcity. Not so long ago, for example, wetlands were widely considered to be little more than useless wasteland, better to be filled and built on. With growing environmental awareness, increased scientific understanding of their functions, and changing aesthetic appreciation, wetlands are assuming a much loftier status. A similar effect might well have been expected anyway as wetlands became

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130 TABLE 13.1 Nationwide Impacts of Sea Level Rise Hypothetical Sea Level Rise Response Scenario 50 cm 100 cm 200 cm If densely developed areas are protected Shore protection costs (billions of dollars) Dry land lost (mid) Wetlands lost (%) If no shores are protected Dry land lost (mi2) Wetlands lost (%) If all shores are protected 32-53 73-111 169-309 2,200-6,100 4,100-9,200 6,400-13,500 20-45 29-69 33-80 3,300-7,300 5,100-10,300 8,200-15,400 17-43 26-66 29-76 Wetlands lost (%) 38-61 50-82 66-90 SOURCE: EPA (1988). scarcer under the pressure of various assaults. Changing tastes and public preferences are also altering favored responses. Where "hardy' defensive measures such as dikes or seawalls might once have been selected, "soft" responses such as setback requirements and abandonment seem to be growing in popularity. 12. An interesting trade-off has been identified between response measures aimed at preservation of wetlands by allowing them to migrate with rising sea level and defensive measures aimed at protection of developed dry land behind them (EPA, 1988; Titus, 1988; Titus et al., 1984~. EPA estimates suggest, for example, that up to 61 percent of the nation's wetlands might be lost to a 50-cm sea level rise if all coastal dry land were defended, while as little as 20 percent would be sacrificed if only densely developed areas were protected (Table 13.1~. Most attempts to assess the economic impact of future sea level rise employ a kind of "coloring book" approach. First, a scenario is selected that describes a hypothetical rise, often within a specified time period. The area subject to inundation from such a rise is then identified with topographic information and ''colored in" on a map (Figure 13.6~. The impact analyst then applies cost-benefit techniques, often based on property values, to estimate the potential loss in economic terms. The exercise is usually repeated for a range of scenarios in an attempt to bound the actual value.

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131 M FD/FE~RA /JEA /V 5 FA ~W a__. Alexandria ~ Of .1~ Density per Square Kilometer 1 /, , ',, ~ ,~ ,,,, . O - 399 i,',..... I CalRO o our I said I'\ he- ~ X, Suez Canal ~ 1( k//omelers 50 : 1 ~ , .:: m//es 25 ., ~ < , .! FIGURE 13.6 Inundation scenarios, superimposing 1-m, 3-m, and 4-m re- lative sea level rises on population density for the Nile Delta, Egypt (Milkman et al., 1989~. The scenarios are usually selected from a range of projections that have been developed on the basis of crude models of the relationship between atmospheric temperature and sea level change (Figure 13.7~. Often the scenarios are adapted from these ranges to account for local conditions such as land subsidence (Barth and Titus, 1984~. Recent estimates by Raper et al. (1988), which center on a "best-guess" range of a 12- to 18-cm increase in eustatic sea level by 2030, are more moderate than some earlier projections but still fall roughly within the low to medium range proposed by Hoffman et al. (1986~. A relatively rapid de- parture from the apparent background trend is seen for all the scenarios in Figure 13.7. Recall that such an acceleration has yet to be detected in the statistical record, and Raper et al. (1988) are careful to include a zero increase within their broader range of possible changes. A selection of economic impact estimates are reported in Table 13.2. These focus on a variety of locales, time frames, and sea-level-rise

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132 cm 120- 100- 80: sol 40 201 Raperetal. 1985 2030 Hoffman (1986) FIGURE 13.7 Typical range of sea-level-rise scenarios from Hoffman et al. (1986) (in dark gray stippled region) compared to shorter-term projections (black zone) by Raper et al. (1988) and contrasted with apparent trend over past century (light grey line) of about 0.15 cm/yr. Notice degree of acceleration above trend that is required if projections and scenarios are to be realized. scenarios. They also employ somewhat different estimation techniques using different assumptions about economic growth, human responses, and the social rate of discount. The original sources also reported their cost estimates in differing constant dollar terms (corrected for in- flation), but in Table 13.2 all are expressed in constant 1987 dollars for easy comparison. Perhaps the most careful and useful impact assessment to date is that of Gibbs (1984) for a range of scenarios for Charleston, South Carolina, and Galveston, Texas. Low and medium scenarios for the year 2075 are shown in Table 13.2. These estimates show the great sensitivity to rate- of-increase assumptions , jumping from a present value (using a 3 percent discount rate) cost estimate of $760 million for the low case in Galve- ston (92.4 cm) to $1.3 billion for the medium case (164.5 cm). (Note too, however, that both the ,'low'' and Medium' cases are in the upper portion of the range of scenarios given by Hoffman et al. (1986~.) The Gibbs estimates in Table 13.2 also illustrate the remarkable influence of the choice of discount rate. For example, moving from a 3 percent discount rate to a TO percent discount rate reduces the higher Charleston cost estimate of $2.6 billion to a mere $68 million.

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133 TABLE 13.2 Selected Cost Estimates Suggesting Economic Impact of Sea Level Rise at Various Sites (millions of constant 1987 dollars) Hypothet- Social ical Sea Rate of Level Source Discount Site Date Rise <%' (cm) Gibbs (1984) 3 Charleston, S.C. 2075 87.6 1,712 159.2 2,616 Gibbs (1984) 10 Charleston, S.C. 2075 87.6 27 159.2 68 Gibbs (1984) 3 Galveston, Tex. 2075 92.4 760 164.5 1,322 Gibbs (1984) 10 Galveston, Tex. 2075 92.4 14 164.5 97 Schneider O United States 2130 760.0 555,555 and Chen (1980) Broadus (19 89 ~ ga Nile Delta 2050 100.0 Broadus (1989) ga Bangladesh 2050 100.0 417 Yohe (1988) 0 Long Beach 2050 100.0 345 Island, N.J. Yohe (1988) 0 Long Beach 2100 200.0 1,942 Island, N.J. Wilcoxen (1986) 3 San Francisco 2100 177.4 74 asocial rate of discount equals economic growth rate, g. Similarly for Galveston, the $760 million present-value lower cost estimate using a 3 percent discount rate falls to only $14 million if a 10 percent discount rate is used instead. Schneider and Chen (1980) attempted an estimate for national losses to future sea level rise, using a scenario of a 760-cm (25-ft) rise by the year 2130. Based on 1971 property values and implicitly assuming a zero rate of discount, they calculated total property losses of over

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134 $0.5 trillion ($200 billion in 1971 dollars) and speculated that ancillary damages might double the total. Their estimate is reported here only as an instructive historical curiosity. No student of the subject would now take the 25-ft projection seriously. National impact estimates have also been attempted for Egypt and Bangladesh (Broadus et al., 1986; Broadus, 1989; Milliman et al., 1989~. Again, areas subject to future inundation were delineated for various scenarios. Then demographic information, land use patterns, and national income accounts were used to estimate the current scale of economic activity taking pi ace within the potentially affected areas. Thus, 7 percent of habitable land and 5 percent of current population were estimated to occupy the area subject to inundation by a 1-m rise in Bangladesh, with 12 percent of habitable land and 14 percent of population in the potentially affected area of Egypt (Broadus, 1989~. Although property value data were not available (and would be of dubious use in any case because of market distortions), a crude attempt was made to extend these estimates to present-value cost estimates. Using strong but reasonable parametric assumptions (discount rate equals economic growth rate, most of the productive land inundated is agricul- tural, agricultural rents are a modest proportion of total agricultural output, and absent mitigating responses), it was estimated that a "worst- case" relative sea level rise of 1 m by 2050 could impose a cumulative loss in present-value terms of roughly $0.5 billion in both Bangladesh and Egypt (Broadus, 1989~. In work currently under way, Yohe (1988) has examined the case of Long Beach Island, New Jersey. Using a sample of actual property values in strips extending across the island and applying a range of scenarios for rates of sea level rise, he has estimated the losses that might be incurred by ''not holding back the sea." For example, a 1-m sea level rise by 2050 is estimated to threaten some $345 million in property values, while a 2-m rise by 2100 could wipe out some $2 billion in property values, essentially the entire current property value of the island. Yohe (1988) implicitly assumes a zero rate of discount. In an interesting and somewhat different economic impact assessment, Wilcoxen (1986) estimated the additional cost that sea level rise could impose on the lifetime cost of a major sewer transport installation near San Francisco. Sea level rise had not been factored into the original engineering cost estimates for the project. Considering various sea level scenarios and weighting them by a subjective likelihood of re- alization, Wilcoxen projected $74 million as the present value (at a 3 percent rate of discount) of cumulative expenditures on beach nourish- ment that might be required to protect the system from erosion and wave attack over its 100-year planned life. Wilcoxen's (1986) effort to form an aggregate scenario by weighting each of his various other scenarios with a ''rough probability of occur- rence" is noteworthy (although how the rough probabilities were arrived at was not reported). It results in a true estimate of expected cost, rather than the usual "certainty equivalent" assumption implied in the estimates derived from all the other scenarios reported in Table 13.2. It also calls attention again to the need for probabilistic forecasts of future sea level changes rather than the more questionable scenarios

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FIGURE 13.8 ''Going to School.' Boston flood. Harper's Weekly, February 27, 1886. currently in use. This is an important area for further progress in our ability to understand the present implications of future sea level change. Progress is being made. Major survey efforts to extend and improve estimates of potential impact have been mounted by the U.S. Environmental Protection Agency and by the United Nations Environment Programme, among others. Increasingly, impact assessments are being linked to considera- tion of prospective response strategies. In the physical sciences, rapid progress is being made in statistical analysis, modeling techniques, and observational capability. Headway has been made in the effort to adjust trends in long-term sea level change for background effects from global tectonism (Peltier and Tushingham, 1989; Emery and Aubrey, 1985), and satellite-based radio altimetry will permit direct and precise observa- tion of changes in eustatic sea level. Fundamental knowledge of physi- cal coastal and oceanographic processes is growing, and recognition of the presence and importance of these processes to human activities is becoming more widespread. The immensity of our uncertainty about future sea level change should not be understated, and a sustained commitment of effort and resources is required to maintain our progress in reducing that uncertainty. Like the children in a flooded Boston of a century past, we are still going to school (Figure 13.8~. But we are fast learners. On balance, it appears,

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136 we are getting better at understanding and addressing the problem of sea level rise faster than it is getting worse. ACKNOWLEDGMENTS The author is indebted for instruction and guidance on this subject to John Milliman, Andy Solow, Dave Aubrey, K. O. Emery, Eric Bird, Jim Titus, and Gary Yohe, although all errors and misjudgments are his own. Financial support from The Pew Charitable Trusts and from the U.S. Environmental Protection Agency is gratefully acknowledged. W.H.O.I. Contribution No. 7181. REFERENCES Barnett, T.P. 1984. The estimation of "global" sea level change: A problem of uniqueness. J. Geophys. Res. 89(C5~:7980-7988. Barnett, T.P. 1983. Recent changes in sea level and their possible causes. Climate Change 5:15-38. Barth, M.C., and J.G. Titus (eds.~. 1984. Greenhouse Effect and Sea Level Rise. New York: Van Nostrand Reinhold Company. Bird, E.C.F. 1986. Potential effects of sea level rise on the coasts of Australia, Africa and Asia. In Titus, J.G., (ed.), 1986. Bird, E.C.F., and M.L. Schwartz (eds.~. 1985. The World's Coastline. Stroudeburg: Van Nostrand Reinhold. Broadus, J.M. 1989. Possible impacts of and adjustments to sea level rise: The cases of Bangladesh and Egypt. In T. Wigley and R. Warrick (eds.), The Effects of Climate Change on Sea Level, Severe Tropical Storms and their Associated Impacts, Climatic Research Unit, University of East Anglia, Norwich, U.K., in press. Broadus, J.M., J.D. Milliman, S.F. Edwards, D.G. Aubrey, and F. Gable. 1986. Rising sea level and damming of rivers: Possible effects in Egypt and Bangladesh. In Titus, J.G. (ad.), 1986. Bruun, P. 1986. Worldwide impact of sea level rise on shorelines. In Titus, J.G. (ed.), 1986. Emery, K.O., and D.G. Aubrey. 1985. Glacial rebound and relative sea levels in Europe from tide-gauge records. Tectonophysics 120:239- 255. Environmental Protection Agency (EPA). 1988. Sea level rise. In Draft Report to Congress on the Potential Effects of Global Climate Change on the United States. Gibbs, M.J. 1986. Planning for sea level rise under uncertainty: A case study of Charleston, South Carolina. In Titus, J.G. (ed.), 1986. Gibbs, M.J. 1984. Economic analysis of sea level rise: Methods and results. In Barth, M.C., and J.G. Titus (eds.), 1984. Gornitz, V., and A. Solow. 1989. Observations of long-term tide-gauge records for indications of accelerated sea-level rise. Proceedings of the Department of Energy Workshop on Greenhouse Gas Induced

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137 Climatic Change, Amherst, Mass., May 1989. Washington, D.C.: Department of Energy, forthcoming. Hoffman, J.S., J. Wells, and J.G. Titus. 1986. Future global warming and sea level rise. In C. Sigbjarnarson (ed.), Iceland Coastal and River Symposium. Reykjavik, Iceland: National Energy Authority. IAPSO Advisory Committee on Tides and Mean Sea Levele 1985. Changes in relative mean sea level. EOS, Transaction, Am. Geophys. U. 66(45):754-756. Lind, Robert C., K.J. Arrow, G.R. Corey, P. Dasgupta, A.K. Sen, T. Stauffer, J.E. Stiglitz, J.A. Stockfisch, and R. Wilson. 1982. Discounting for Time and Risk in Energy Policy. Washington, D.C.: Resources for the Future, Inc. Milliman, J.D., J.M. Broadus, and F. Gable. 1989. Environmental and economic impact of rising sea level and subsiding deltas: the Nile and Bengal examples. Ambio, in press. National Academy of Sciences (NAS). 1983. Changing Climate. Washington, D.C.: National Academy Press. National Research Council (NRC). 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, D.C.: National Academy Press. Park, R.A., T.V. Armentano, and C.L. Cloonan. 1986. Predicting the effects of sea level rise on coastal wetlands. In Titus, J.G. (ad.), 1986. Peltier, W.R., and A.M. Tushingham. 1989. Global sea level rise and the greenhouse effect: Might they be connected? Science 244:806-810. Pilkey, O., et al. 1982. Saving the American Beach: A Position Paper by Concerned Coastal Geologists. Savannah: Skidaway Tnstitute of Oceanography. Raper, S.C.B., R.A. Warrick, and T.M.L. Wigley. 1988. Global sea level rise: Past and future. In J.D. Milliman and S. Sabhasri (eds.), Proceedings, SCOPE Working Group on Rising Sea Level and Subsiding Coastal Areas, 9-13 November 1988, Bangkok, Thailand. New York: John Wiley & Sons, in press. Robin. G. 1987. Projecting the rise in sea level caused by warming of the atmosphere. In Bolin, B., B.R. Doos, J. Jager, and R.A. Warrick (eds.), The Greenhouse Effect, Climatic Change, and Ecosystems. New York: John Wiley & Sons. Schelling, T.C. 1983. Climatic change: policy. Pp. 449-482 in NAS, 1983. Schneider, S.H., and R.S. Chen. 1980. Carbon dioxide warming and Implications for welfare and coastline flooding: Physical factors and climatic impact. Ann. Rev. Energy 5:107-140. Solow, A.R. 1987. The application of eigenanalysis to tide-gauge records of relative sea level. Continental Shelf Research 7~6~:629- 641. Titus, J.G. (ed.~. 1988. Greenhouse Effect, Sea Level Rise, and Coastal Wetlands. Washington, D.C.: Environmental Protection Agency. Titus, J.G. 1987. Causes and effects of sea level rise. In Preparing for Climate Change, Proceedings of the First North American Conference on Preparing for Climate Change: A Cooperative Approach,

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138 October 27-29, 1987 Institutes, Inc. Titus, J.G. (ed.~. 1986. Effects of Changes in Stratospheric Ozone and Global Climate, Volume 4: Sea Level Rise. U.S. Environmental Protection Agency, Washington, D.C. Titus, J.G., T.R. Henderson, and J.M. Teal. 1984. Sea level rise and wetlands loss in the United States. National Wetlands Newsletter , Washington, D.C. Rockville, Md. Government 6(5):3-6. United Nations Environment Programme (UNEP). 1986. Report of the International Conference on the Assessment of the Role of Carbon Dioxide and of Other Greenhouse Gases in Climate Variations and Associated Impacts. World Climate Programme, Villach, Austria, October 9-15, 1985. Geneva, Switzerland: World Meteorological" r Organization. Wilcoxen, P.J. 1986. Coastal erosion and sea level rise: Implications for ocean beach and San Francisco's westside transport project. Coastal Zone Management Journal 14~39:173-191. Yohe, G.W. 1988. The cost of not holding back the sea. Report for the U.S. EPA, 1988.