Click for next page ( 10


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 9
Relative Mean Sea Level A significant portion of the worId's population lives within the coastal zone, with many buildings and facilities built at elevations less than 3 m (10 ft) above mean high-tide level along the shoreline. Without any secular change In the height clef mean sea level or in the height of the land, the elevations of structures are not adequate to ensure the safety of people and works in the event of major storms (including storm surges), especially when such events coincide with infrequent but predictable perigean spring tides. This hazard has grown increasingly apparent and serious along many of the worId's coastlines as local mean sea level has risen during the twentieth century. Although in some areas of the world the local sea level is falling, the predominant change is a rising sea level with rates ranging from 1 to 5 mm/yr. International attention has been drawn toward this problem by two possibly in- terrelated sets of observations: (1) relative mean sea level is rising and beach erosion is being exacerbated in many parts of the world (Bird, 1985), including many areas of the United States (Figure 1- 1~; and (2) the atmospheric level of Greenhouse gases is steadily rising as a result of the combustion of fossil fuel and deforestation. While it is tempting to correlate the two, a cause-and-effect (eu- static) response has not been proven to date, although a future eustatic response is a clear possibility (Barth and Titus, 1984~. 9

OCR for page 9
10 thy cat o i> r O O _ = a a ~ 3 9 , N lo ~ ~~, ~ _ 3 e ~ / O~ ~ ~ ~1 O u, 0 .. o z cut co A: o m as Z an Z z _ LU ~ A ~ ~ an ~ ~ Y ~ 0 at, . UJ fin c! 0 5 zip ~ US N ELI t ~ G U) ~ ~ ,., . .. _ ~ ~ O ~ ~ — U) U'J — ~ C' ~ _ Z Z ~ ~ ~ ~ 'S U) cn L' U] Z ~: CO —~ cS C:) ~ C' Y 0 m7 o) =m z ~ ~ UJ \ \ I_J ~' fo u, {D I,Le ~v ~ u: ~ ~ - ._ ~ 0 ~ u, - 0 ~ ~ ~a ._ u~ hO ~ ~: ~ ._ _ 7 C) s,4 o C) ~ . O ) ._ ~ ~ hD s" C~ V (V O O oo ~ O . - d _ ,= ._ ~ ~'~ oo ~ _ Q ~ cd ~V O "'= oq L. C~ P~ dV L' ~ O ~ O _ O'— ~ ~ ~ _ o' _ O O p,, o U] C) U: o CO C~ eS a' ~ - ~ · - u ~ ~a · - ~a b · - oo ) _1 ~ - ~

OCR for page 9
RELATWE MEAN SEA LEVEL 11 Carbon dioxide and the other trace gases that comprise "green- house gases create a greenhouse effect in the troposphere. The combined effects are still poorly understood but seem likely to es- tablish a sequence of climatic effects that could result in a general global warming (National Research Council [NRC] 1983, 1982, 1979~. The result wall be an increased rate of rise in glacioeustatic and steric-expansion eustatic sea level. Relative mean sea level change at a particular location is the difference between the eustatic (global) change and any local change in land elevation. The long-term causes of relative mean sea level rise are sixfold, but not aD of the processes are operative in every locality. 1. Eustatic Tise of world sea level. "Ecstatic means a global change of the oceanic water level. Its most ~rnportant forms at the present time are regarded as glacio-eustasy, caused by melting of land-based glacier ice, and the steric expansion of near-surface ocean water due to global ocean warming. hysterics refers to the specific volume of the medium, which expands when heated or shrinks when cooled. 2. Crustal subsidence or uplift of the land surface due to new tectonics, that is, contemporary, secular, structural downwarping of the earth's crust. Tectonic phenomena occur In five distinctive categories: subsidence of former glacio-~sostatic marginal uplift belts (e.g., the eastern United States); cooling crustal belts fol- lowing rifting (e.g., parts of the Gulf of California); subsidence in regions of long continued sediment loading (e.g., East and Gulf coasts, especially the Mississippi delta); uplift in regions of active crustal subduction (e.g., Puget Sound); and subsidence due to loading by volcanic eruptions (e.g., Hawaii, Aleutians Islands). 3. Seismic subsidence of the land surface due to sudden and irregular incidence of earthquakes. 4. Auto-subsidence due to compaction or consolidation of soft, underlying sediments, especially mud or peat. 5. Man-made subsidence due to structural loading, as well as groundwater, and of! and gas extraction. Of the four subsi- dence processes only this category,^anthropogenic subsidence, can be reversed or at least partially Instigated by recharge or other management actions. 6. variations due to climatic fluctuations are a consequence of oceanic factors including E! Nin - Southern Oscillation (ENSO)

OCR for page 9
12 RESPONDING TO CHANGES IN SEA LEVEL effects, and are related to secular changes in the size and mean latitudes of subtropical high pressure cells. Along mainland coasts (especially east coasts in the Northern Hemisphere), a decreasing current flow associated with Waring epochs causes a rise in sea level due to the Coriolis eject, whereas in rn~doceanic gyre re- gions there is no mean sea level change. This issue requires study (Cartwright et al., 1985; Barnett, 1983b). It also appears that tide gauge records contain substantial long-period fluctuations (~100 years), which indicate that the accurate extrapolation of small sea level rise values from the data is very difficult. Furthermore determining changes in rates of rise is even more difficult. ;, Of these identified causes of sea level rise, only the eustatic rise is a universal, global eject (by definition). For any one area the other causes come into play in various proportions. It should be stressed that no national survey of the local extent of the processes has ever been undertaken, but it is clear that the variations will be highly regional. Various segments of the U.S. coastline experience subsidence or uplift due to factors (2) through (5~. Superimposed on this regional subsidence is the global eustatic sea level rise. If the greenhouse e£ect/glacier melt concept is confirmed, its potential contribution to mean sea level rise will outstrip other causes of relative sea level rise along most of the U.S. coastIme by 2025. PAST CHANGES IN RELATIVE MEAN SEA REVEL Geologic Record of Sea ferret It has been establishecI that during the last Ice Age (15,000 years ago), mean sea level was perhaps as much as 10~150 m lower than it is now. Sea level rose rapidly until about 6,000 years ago, when the rate of change of global sea level became quite low compared to that earlier period of tune. An assessment developed by Shepard (1963) is presented in Figure 1-2. During the past 6,000 years there were perhaps fluctuations one or more meters over a thousand years (or more) apparent ~ some areas (Fa~rbridge, 1961~. In regions of very rapid subsidence (e.g., Mississippi delta, Rhine delta), eustatic trends tend to be obscured (Van de Plassche, 1986~. In contrast, the formerly glaciated regions of the world

OCR for page 9
RELATIVE MEAN SEA LEER o 100 ~—D ID - - Oh `~ 200 J Cal 300 x~ x xx ~2~ X. ~ \O ~ Ax · Texas shelf \ ~ Holland \ x Australia \ · Southwest Louisiana in\ O Eastern Argentina · West Louisiana shelf O Western Mexico a 0 10 2~) 13 O 25 m 50 ~ In 75 100 THOUSANDS OF YEARS BEFORE PRESENT FIGURE 1-2 Sea petrel elevations versus time as obtained from carbon 14 dates in relatively stable areas. Source: Adapted from Shepard (1963~. (notably, most of Canada, Scandinavia, and Scotland) are regions of neotectonic tilting or uplift, where rates of uplift have exceeded past rates of sea level rise. Sea [eve! Rise In the Twentieth Century For nearly a century, relative mean sea level has maintained a steady rise at many tide-record~ng stations around the world. At the same time the atmospheric greenhouse gases have shown a steady rise, and, more recently, other greenhouse gases such as chiorofluorocarbons have been increasing. To some observers the increase of greenhouse gases in the atmosphere implies a warming of the worId's climate, although the evidence to date is still being debated by the scientific community (NRC, 1983, 1982) and most experts do not expect the warming to be detected until the l990s.

OCR for page 9
14 RESPONDING TO CHANGES IN SEA LEVEL A tide gauge sunply records the current sea level at a particu- lar location. Since sea level is a basic consideration for short-range coastal construction plans, designers of long-lived coastal struc- tures shouIc3 consider the change in sea level that may occur during the structures' useful life. For this purpose, simple projection of the trend of local tide gauge records Is inadequate; the underly- ~ng causes ot sea level change must be addressed and techniques employed to forecast the effect of each cause. Tide gauge data are available for approximately the past cen- tury. However, these data provide the sea level relative to the supporting base of the gauge, and that base may be either sinking or rising. For example, Figure 1-3 presents results from long-term tide gauge records at Atlantic City, New Jersey; San Erancisco, California; and Juneau, Alaska. The approximate recorded rela- tive rates of change are Atlantic City: + 0.40 m/century, San Francisco: ~ 0.13 m/century, and Juneau: - 1.38 m/century, where a positive rate indicates a relative rise. The relative drop in sea level at Juneau reflects the rebound (rise) of the land with unloading of the land following the melting of glacial ice. A question raised early in the development of this study was whether sea level change trends based on tide gauges located inside bays and estuaries are representative of open-coast trends. This concern was posed as early as 1929 in a report sponsored by the National Research Council (Johnson, 1929~. Specifically, the issue is whether the effects of engineering works, primarily channel deepening and the construction of jetties for navigational purposes, would affect the sea level rise trend measured by bay gauges. A special study commissioned by the Marine Board to ad- dres~ this concern includes a summary of analytical relationships and empirical results relevant to this problem (Mehta and Philip, 1986~. Mehta and Philip concluded that gauges located inside and outside bays are each subject to different influences that tend to degrade the quality of the data. With locations more distant inside the mouth of the bay, gauges contain a greater amount of "noise, which Is not representative of the open-coast sea level. Analytical

OCR for page 9
RELATE MEAN SEA LEVEL - — 72 o ~ 6.4 - ct _. ID ID - 0 9.2 J ~ 8.4 7.6 14.6 hi, 13.8 - a: r Yearly Mean Sea Level Sta. No.8534720 Atlantic City, NJ 5.8 L 1850 1880 1910 n! .~ i- - r At' ~ 1 Century 0.40 m 1940 1970 YEAR Yearly Mean Sea Level Sea. No.9414290 San Franclaco, CA _~ 1 Century 1 1 1 1 1 1 1 1 1 1910 1850 1880 t YEAR Yearly Mean Sea Level Sta. No.9452210 Juneau, AK 13.0 LO 1850 1880 1910 1940 1970 -1.38 m 14 1 Century \/\|\ 1 1 1 1 1 1 _l ! 1940 1970 15 YEAR FIGURE 1-3 Tide gauge data for (a) Atlantic City, New Jersey; (b) San Francisco, California; and (c) Juneau, Alaska. Source: Hicks et al. (1983~.

OCR for page 9
16 RESPONDING TO CHANGES IN SEA LEVEL TABLE 1-1 Difference in the Secular Change of Mean Sea Level for Selected Gauge Pairs (outside minus inside) Secular Change Gauge Pair Difference (mm/yr) Outside Inside Outside- Inside Long Branch, N.J. New York, N.Y. 13.1 Atlantic City, N.J. New York, N.Y. 1.4 Duck Pier, N.C. Norfolk, Va. 2.1 Springmaid Pier, S.C. Charleston, S.C. 13.6 Daytona Beach Shores, Fla. Mayport, Fla. 6.4 Key Colony Beach, Fla. Fort Myers, Fla. 2.4 Vaca Key, Fla. Fort Myers, Fla. 2.2 Key West, Ftla. Miami, Fla. -0.3 Naples, Fla. Fort Myers, Fla. 0.7 Clearwater Beach, Flu. St. Petersburg, Fla. -0.6 Cedar Key, Fla. Pensacola, Fla. -0.4 Shell Point, Fla. Pensacola, Fla. 1.8 La Jolla, Calif. San Diego, Calif. -0.1 Santa Monica, Calif. Los Angeles, Calif. 0.8 Ricon Island, Calif. Los Angeles, Calif. 3.2 Monterey, Calif. San Francisco, Calif. -3.9 Arena Core, Calif. San E`rancisco, Calif. -13.1 Trinidad, Calif. San Francisco, Calif. 4.8 Crescent City, Calif. Astoria, Ore. -0.3 NOTE: A positive change denotes a higher outside than inside rate. SOURCE: From Hicks as reported by Mehta and Renji (1986~. considerations suggest that any short-term (several decades) did ferences will be biased toward a lower trend from this inside gauge compared to outside gauges. In addition, Hicks' (1984) results of long-term sea level trends were assessed from pairs of gauges inside bays versus open-coast gauges. The comparison included 19 gauge pairs, with each outside gauge selected as that in closest proximity to the inside gauge (Table 1-1~. The average trend difference of the 19 gauge pairs was 1.S mm/yr with the outside rate exceeding that inside. When the three absolute differences exceeding 10 rnm/yr are excluded, the average difference decreases to 1.1 mm/yr. Thus, relative to outside gauges, this study indicates that the trend rate from inside gauges has underestimated somewhat the relative mean sea level rise rate.

OCR for page 9
RELATIVE MEAN SEA LEVEL TABLE 1-2 Estimates of Eustatic Sea Level Rise per Century Based on Tide Gauge Data Author Estimated Rise (cm) Thorarinsson (1940) Gutenberg (1941) Kuenen (1950) Lisitzin (1958) F`airbridge and Krebs (1962) Hicks (1978) Emery (1980) Gornitz et al. (1982) Barnett (1983a) 11 ~ 8 12 to 14 11.2 ~ 3.6 12 15 (United States only) 30 12a 15 a Ten centimeters excluding long-term trend. SOURCE: Adapted from Barnett (1983a) and Hicks (19783. 17 Range of Sea [eve] l:sti~n~tes Available Using tide gauge results from around the world, various esti- mates have been obtained for mean rate of change (Table 1-2~. The large variance is partly the result of gross geographical imbalance of the gauge sites; most are in the Northern Hemisphere m~lat- itudes, almost Al are on continental shores, and almost none are in high latitudes. A map of gauge locations is presented in Pugh and Faull (1983~. The lack of insular tide gauges, until recently, has deprived analysts of any means of testing mean sea level change for the Coriolis effect. Gauges located toward the Huddle of an oceanic gyre that is speeded or slowed will fall or rise respectively, whereas those on continental shores show an opposing trend. The lack of a sufficient number of high-latitude gauges precludes testing for planetary spin-rate eEects (although there is astronomical proof of changes in the earth's spin rate which should be registered by sea level, especially at high latitudes). It is noted that the spin rate has decreased slightly and would result ~ relative mean sea level decreases and increases at low and high latitudes, respec- tively. This has been proposed, but not confirmed, as an indicator of antarctic melting. It is not clear whether this signal could be isolated in the presence of the other components and Noise.

OCR for page 9
18 RESPONDING TO CHANGES IN SEA LEVEL Furthermore, many gauges are located in obviously subsiding delta areas, and others ~ tectonically rising areas. The first attempt to filter out the grossly anomalous data of world tide gauge records was done by Fa~rbridge and Krebs (19623; their results indicated a mean rise for the first part of the twen- tieth century of 1.2 mm/yr. Subsequent analyses, using different filtering procedures but progressively larger data sources, showed sirn~lar results; for example, Lisitzin (1974), 1.12 mm/yr, and Gor- nitz et al. (1982), 1.2 mm/yr. Several analysts have suggested a change during the last few decades; Barnett (1983a) showed for 193~1980 an average of 2.3 mm/yr, and Emery (1980) gave a value of 3.0 mm/yr. The newest global results are by Pirazzoli (1986, 1984~. Of 1,178 records provided by the Permanent Service for Mean Sea Level, 229 stations were selected as having > Midyear records (plus a few with 30 years) of consistent trends (Figure I-4~. Only 13 percent showed the "mean ecstatic value (1.~1.5 mrn/yr); 22.5 percent showed 1.0-2.0 mm/yr; 20.5 percent showed a rise of 0.1- 1.0 mm/yr. Pirazzoli indicates that the extreme variance between the stations emphasizes the unportance of local subsidence. Fur- thermore, he stresses that there is no unequivocal demonstration of any eustatic rme at all, at least during the last 4~50 years. Gornitz (unpublished data) has prepared averages for a number Of m-Pacific atolls: over 2~30 years, Nawiliwili shows a mean rise of 0.30 mm/yr; Canton Island, 0.31 mm/yr; Eniwetok, 0.81 mun/yr; and Midway, minus 1.34 rnrn/yr. For regional studies (e.g., Australia), Aubrey and Emery (1983) were unable to iden- tify "unambiguously a eustatic signal. Munk et al. (1985) indicate that phenomena that create large amplitude fluctuations in sea level, such as the ENSO, make it very di~cult to obtain statistically reliable estimates of rates of change in mean sea level and even more difficult to detect whether the rate of change is increasing or decreasing. In surrunary, the gauge measurements, in a few cases continu- ing over 10~300 years after correction for known trends, suggest mean sea leered fluctuations that are generally consistent with the geological record of the past 6,000 years (Ters, 1986; Tooley, 1978; Fairbridge, 1961~. The nature of the related climatic changes, in- sofar as it ho been possible to document them, is also consistent.

OCR for page 9
RELATIVE MEAN SEA LEVEL 60 30 N o 30S 60 _ _ ~ +5.5?—-5.7 3 251 -2.2 -0.2 +2.6 0.0 +1.8 +~.5-~S _ 17 5 32 1 17 85 1 +0.7 +5.9 +3.0 +1.6 +3.3 -= +3.6 ~ ~~ 10 rid_ ~ 4~0~6 +5~0 +1~8 7 5 2 +2~5 +1.3 _ ~ 180W 150 120 90 60 30 0 30 60 90 120 t50 180E 19 LONGITUDE FIGURE 1-4 Geographical distribution of worldwide change of relative mean sea level (by latitude/longitude coordinates) using tide gauge records extending more than 50 years (augmented with a few of 30 years). Large figures and mathematic signs indicate average relative rise or fail in mm/yr; small figures the number of stations employed. Note that 97.4 percent of the data is from the Northern Hemisphere. The shaded box is the only rectangle (Central Asia) with no marine coastline. No data exist for 70 percent of the boxes, and 70 percent of the stations are located in only four boxes. It is evident that no statistically valid basis exists for assuming that eustatic rise is in progress, at least from the data presented here. Nevertheless, it is important that five boxes with very low rates of change are midoceanic stations. Four boxes with values over 5 mm/yr are located in areas of recognized crustal uplift or subsidence. To identify and quantify a global ecstatic rate requires not only more and better-selected gauging points, but also more-sophisticated geological and oceanographic analysis. Source: Pirazzoli (19843. Additional examination is required. The record of cInnatic fluctu- ation indicates quite appreciable variations (Lamb, 1984; Wright, 1983). METHODS O]? OBSERVING RELATIVE MEAN SEA LEVEL With reference to any one locality, relative mean sea level is

OCR for page 9
20 RESPONDING TO CHANGES IN SEA LEVEL measured by different techniques that fall within the expertise of different groups of specialists, who for the most part do not meet professionally. The techniques are as follows: 1. Tide gauge analysis is processed globally by a commission of the International Association of Physical Oceanography and nationally In the United States by the National Ocean Service (NOS). Monthly relative sea level maps of the tropical Pacific are issued by the University of Hawaii. 2. Satellite altimetry is administered by the National Aero- nautics and Space Administration (NASA) In the United States and studied internationally by groups within the International Union of Geology and Geophysics (lUGG). Satellite imaging can also be employed for geomorphic analysis. 3. Geodetic leveling is giobaDy reported by the Commission for Recent Coastal Movements of the International Association of Geodesy (LAG, which adheres to JUGG) and in the United States by the National Geodetic Survey. 4. Geomorphological and geological analysis is globally coor- dinated by the Commission on Shorelines and the Commission on Neotectonics of the International Union for Quaternary Re- search (INQUA). There is a U.S. National Committee for INQUA with individual representation on the commissions but no official governmental participation. There Is also the Sea Level Project of the International Geologi- cal Correlation Programme (No. 61, completed in 1984; followed by No. 200, led by P. Pirazzoli, CNRS-Intergeo Geographical In- stitute, Paris), which also has a U.S. committee with individual membership. Each of these organizations collect data in tune series that refer to specific locations. However, the time series cover very different intervals. The relevant geological studies deal with about the past 6,000 years; the others concentrate on periods ranging from two or three centuries to a month or less. Nevertheless, from Al these data, regional ant} global means are obtained by varied and sophisticated statistical analysis. While good agreement is possible for the regional means of relative mean sea level (coastal sectors of 50~1,000 km), there is so far no unanimity as to the global values for eustatic sea level rise. Efforts are now being directed at defining the eustatic components, and to delineating and understanding the local and regional effects.

OCR for page 9
RELATIVE MEAN SEA Lecture 21 Of the various observational techniques, tide gauge analy- sis furnishes the most detailed, accurate, and directly measured record of relative mean sea level, but its hour-by-hour variability is subject to complex disturbances (e.g., by atmospheric pressure or local rainfall), and the data therefore require extremely careful analysis. The results provide valuable monthly, seasonal, annual, and more-than-decadal trends. Tide gauge data provide basic input for the design of coastal structures. For short-lived structures, the annual tide ranges and the mean tide level are sufficient; however, for longer-lived coastal structures, the long-term trends become important in the design process. Satellite altimetry is potentially very valuable, but re- quires a longer base period (Wyrtki, 1985~. Recent availability of the Global Positioning System (GPS) provides absolute (not rela- tive) sea level to centimeter accuracy. Future contributions of this new technology, supplemented by traditional measurement tech- niques, should be extremely beneficial to understanding eustatic sea level trends and local neotectonics. Geodetic leveling involves the periodic releveling of the first- order vertical topographic survey stations, about once every two or three decades, and thus provides evidence of secular deformation of the earth's crust, as wed as indications of local compactional sum sidence. The method is extremely time consuming, and although very successful in Japan, Scandinavia, and Eastern Europe, it has been poorly funded in the United States and many of the available data have not been analyzed. The analyses provide decadal to century-Ion" trends. The Japanese have developed and applied ingenious methods of sensing local and short-term compaction rates. Fmally, geomorphological and geological analysis furnishes century to millennial trends. This method ~ particularly valuable in demonstrating the natural, long-term response of any area or region, a trend or pattern of behavior that can then be compared with the short-term time series provided by geodetic leveling and tide gauges, which may contain extraneous or anomalous short- term data. The short-term data should be evaluated in terrors of known long-term trends whenever possible. Biro Examples How these varied approaches can be constructively integrated

OCR for page 9
22 RESPONDING TO CHANGES IN SEA LEVEL NORTH 3 in, 1 aS (D ~ a) - O = - - L · 0 o ~ ~ in o o o, ~ o ~ I UPLIFT ~ a, o _ o 0 — ~ o Y - 1 SOUTH m a. Y z ~ 0 3 ~ 0 Z SUBSIDENCE ~ 0 100 200 300 KILOMETERS 400 500 FIGURE 1-5 Geodetic leveling profile from Perth Amboy, New Jersey through New York City to Rouses Point, New York on the Canadian border. Mean sea level rise at the Battery, New York City, averaged 2.7 mm/yr. Sub- tracting 1.2 mm/yr as the eustatic component leaves 1.5 mm/yr representing subsidence and long-term oceanographic factors. Source: U.S. Coast and Geodetic Survey data, 1902 to 1955; adapted from l?airbridge and Newman (1968). is illustrated by reviewing data from two well-documented sites: New York City and Long Beach, California. The tide gauge of New York City is located at the Battery and set on hard crystalline bedrock that ~ not disturbed by sediment compaction, frost action, or human action, such ~ grour~dwater withdrawal. For 90 years the record shows a systematic (though fluctuating) rise of mean sea level of about 2.7 mm/yr (Hicks et al., 1983~. Deducting about 1.2 mm/yr as the eustatic component, 1.5 mm/yr remains as a probable crustal subsidence factor. This has been checked by geodetic and geologic techniques. The geodetic leveling lines were followed up the Hudson River to the Canadian border and filtered to remove highly deviant data points (Figure 1-~. A secular tilting of the crust during the present century is shown, with the Canadian border area rising at about 1-2 mm/yr and the New York City area sinking at about the same rate; a null point is situated near Kingston (Fairbridge and

OCR for page 9
RELATIVE MEAN SEA LEVEL 23 Newman, 1968~. The geologic surveys show that over the last 6,000 years the southern end of the section has been sinking slowly and the Canadian end has been rising. About 9,0()0 years ago, Lake Champlain lay at sea level. This is a clear-cut case, demonstrated by independent data sets, that the New York City area is subsiding at about 1-2 mm/yr. The lowering of land affects the bedrock of the whole region and is not affected by human activity. Id California, despite frequent earthquakes, the tide gauge records are remarkably stable and coherent from station to station. San Francisco has the longest series (since 1855), which shows that if interannual variations are removed, mean sea level shows broad fluctuation but has generally risen at 1.3 mm/yr over 125 years (Hicks et al., 1983~. No distinguishable change is evident for the last several decades. ~ contrast to San Francisco and New York, Long Beach Har- bor, California commenced a sucIden, substantial, relative sea level rise trend in the 1950s, submerging appreciable parts of Terminal Island, which is in the harbor. This subsidence is anomalous when compared with long-term trends and was diagnosed as a short- term local phenomenon that is related to the withdrawal of oil, natural gas, and water during exploitation of the Wilmington Oil- field. Artificial recharge of the porous strata h" slowed the rate of continued subsidence, but the cost of dike building and other land preservation measures exceeded $100 million. The New York City and Long Beach examples demonstrate that areas must be considered in three contexts: local, regional, and global.