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2 Assessment of Changes in Relative Mean Sea Level ESTIMATES OF FUTtJ8E MEAN SEA LEVEL RISE During the past 4 years, several groups have attempted to estimate the rise in eustatic mean sea level that could result from projected global warming. Studies have focused on four factors: the thermal expansion of ocean water; the melting of mount awn glaciers; the melting of Greenland glaciers; and the possibility that antarctic glaciers may slide into the oceans. To estimate the significance of these processes also requires an estimate of future global warming, which in turn depends on future concentrations of "greenhouse gases and the sensitivity of the cInnate to changes in these concentrations. I,ong-term carbon clioxide monitoring stations are situated to minimize localized effects of industrialization. Records dating back to 1958 from the top of Mauna Boa, Hawaii are presented in Figure 2-1. They indicate that in addition to substantial sea- sonal variations, there is an upward trend in the concentration of greenhouse gases, with an increase from 315 to 340 ppm occurring from 1958 to 1981. A similar increase was detected at an antarctic observation station. ~ addition to these documented increases, an increase of 50 ppm since preindustrial times (around 1850) has been est~rnated using tree ring data. 24
ASSESSMENT OF CHANGES IN RELATIVE MEAN SEA LEVEL 25 340 g At O 330 He C' 0 320 C' Cal o · Estimated value Based on estimated dam 310 1958 1960 1965 1970 1975 1980 YEAR FIGURE 2-1 Mean monthly CO2 concentration at Mauna Loa, Hawaii. Source: NRC (1983~. ~ Carbon Diozzd;e and Climate: A Scientific Assessment (NRC, 1979), it was concluded that a doubling of CO2 would raise the earth's average surface temperature 1.5~.5°C, with the warming at the poles two to three times as great as the average warming. The pane] concluded: ewe have tried but have been unable to find any overlooked physical effect that could reduce the currently estimated global warming due to a doubling of CO2 to negligible proportions. Nor~haus and Yohe (NRC, 1983) estimated the likely rate of increase for CO2 considering uncertainties regarding future energy use patterns, economic growth, and the ability of the oceans to absorb carbon dioxide. The pane! estimated that there is a 98 percent probability that CO2 concentrations will be at least 450 ppm (1.7 times the preindustrial level) by the year 2050, and a 55 percent chance that the concentration will be 550 ppm. They estimated the probability of doubling CO2 concentrations by 2100 as 75 percent. Ramanathan et al. (1985) estimate that the com- bined impacts of methane, chIorofluorocarbons, nitrous oxide, and
26 RESPONDING TO CHANGES IN SEA LEVEL several other trace gases will be approximately comparable to the warming resulting from greenhouse gases buildup. On the basis of a global warming of ARC, Revelle (1983) assessed the likely rise In sea level assuming that no antarc- tic deglaciation takes place. He estimated that Greenland and mountain glaciers could each contribute 12 cm to sea level in the next century, and that thermal expansion could contribute 30 cm. Based on current trends, Revelle concluded that other factors could contribute an additional 16 cm, for a total rise of 70 cm, with an est~rnated uncertainty of ~25 percent. Hoffman et al. (1983) developed a Variety of sea level rise scenarios based on high and low assumptions for all the major uncertainties. Although they used a fairly sophisticated mode! for projecting global warmung and thermal expansion, they cautioned that the absence of glacial process models kept them from making accurate projections of snow and ice contributions to sea level. They estimated that sea level was likely to rise between 26 and 39 cm by the year 2025 and between 91 and 137 cm by 2075. The report Glaciers, Ice Sheets, and Sea Level (NRC, 1985b) provided the first detailed Took at the possible glacial contribution to sea level rise. Meier (1984) estimated that a global warming of 1.5~.5°C could lead to a 1(~30 cm alpine contribution to sea level rise. BindschadIer (1985) estimated a similar contribution from Greenland. There was less consensus regarding the antarctic contribution. Thomas (1985) estimated that the antarctic con- tribution resulting from a 4°C warring would most likely be 28 cm, but could be as high as 2.2 m. The pane} concluded that the antarctic contribution was likely to be a few tenths of a meter, and possibly as great as 1 m or as little as a 10 cm drop. The pane] did not estimate the likely contribution of thermal expansion (see discussion below). Hoffman et al. (1986) revised their earlier projections in light of the glacial process models provided by the Polar Research Board (NRC, 1985b) and new information on future concentrations prm vided by the Board on Atmospheric Sciences and Climate (NRC, 1983) and Ramanathan et al. (1985~. Although the revised as- sumptions had a minor impact on the estimates of thermal ex- pansion, they substantially lowered the estimates of snow and ice contributions until after 2050. Hoffman et al. estimated the rise by 2025 to be between 10 anal 21 cm, and by 2075 to be between 36 and 191 cm (Tables 2-1 and 2-2~.
ASSESSMENT OF CHANGES IN RL13LATIYE MEAN SEA LEVEL 27 TABLE 2-1 Contributions to Future Sea Level Rise in the Year 2100 (in centimeters) Thermal Alpine Study Expansion Glaciers Greenland Antarctica Total Hoffman et al. (1986) 28-83 12-37 6-27 12-220 57-368 Thomas (1985) -- -- -- 0-220 -- Hoffman et al. (1983) 28-115 b b b 56-345 NRC (1983) a ~~ 10-30 10-30 -10- +100 -- Revelle (1983) 30 12 12 c 70 . bContributions in the year 2085. Hoffman et al. assumed that the glacial contribution would be one too two times the contribution of thermal expansion. Revelle attributes 16 cm to other factors. TABLE 2-2 aTemporal Estimates of Future Sea Level Rise (in centimeters) Study Year 2000 2025 205C) 2075 2085 Hoffman et al. (1986) Low 3.5 10 20 36 44 High 5.5 21 55 191 258 Hoffman et al. (1983) Low 4.8 13 23 38 -- Mid-range low 8.8 26 53 91 -- Mid-range high 13.2 39 79 137 -- High 17.1 55 177 212 Re~relle (1983)a -- -- -- -- 70 aOther studies only provided an estimate for a specific year. Based on an exarn~nation of the various component processes, Robin (1986) has proposed and applied a linear correlation be- tween rise ~ mean global sea level, ~ S1;, and mean global temper- ature, AK, that is ASL(t) = FAK(tto), where F is a Tmear correlation coefficent and the time to represents the lag of sea level in responding to temperature change. Robin
28 RESPONDING TO CHANGES IN SEA LEVEL notes that although nonlinear correlation Is more appropriate, the data are not sufficiently well conditioned to quantify adequately the coefficients modifying the nonlinear terms. Additionally, over temperature changes of interest, the sea level response should be nearly linear. Based principally on an analysis of the Gornitz et al. (1982) results, the following range of correlation coefficients was developed 16 cm/°C ~ F < 30 cm/°C. Applying these values to an estimated temperature change of 3.5 ~ 2°C due to a doubling of greenhouse gases over the next century, the associated range in global mean sea level change is from 24 to 154 cm. All of the studies have focused on changes in the worId's average sea level. However, as discussed previously, there is reason to believe that the relative rise will be somewhat greater along most of the U.S. coast. Tm Atiantic City, for example, relative sea level has risen 40 cm in the last century (Hicks et al., 1983), while the global rise has been estimated at 1~15 cm (Barrett, 1983a; Gornitz et al., 1982~. Assuming that the processes responsible for local and regional subsidence do not change, over the next century the rise in sea level at Atlantic City and some of the U.S. Atlantic and Gulf coasts will be I~30 cm greater than the global average. SCENARIOS USED IN THIS REPORT Because the rate of filture sea level rise is uncertain, there must be uncertainties in any assessment of the unplications. For its analyses to reflect these uncertainties, the committee examined three possible scenarios of eustatic sea level rise to the year 2100: rises of 0.5 m, 1.0 m, and 1.5 m. The total relative sea level change above present levels at time t, T(t), is the sum of the local, A, and eustatic, E, components and is expressed as T(t) = L(t) ~ E(t). One possible equation for the eustatic contribution, the form of which appears generally consistent with anticipated future sea leveb and which will be adopted for this study, is E(t) = 0.0012t ~ bt2, in which E(t) is the additional eustatic component in meters above present levels and t is the time in years from present.
ASSESSMENT OF CHANGES TV RELATIVE MEAN SEA LEVEL 29 4.0 3.0 o a Ct 20 In - ~: J J Oh ~ 1.0 In 0.0 · Hoffman (1983) high * Glacier volume estimate of Polar Board augmented with thermal expansion estimates by NRC (t983) r _ Past Century 1 ~ ,1 I estimated I 0.12 m rise I j ~ 1 2000 · Hoffman (lS83) mid-high NRC (1985b) high · Robin (t986) high 111/ ~ Hoffman (1983) mid-low /~/ NRC(1983) / / ~ - · Floffman (1983) low / / 1/ / / / ~ NRC (198Sb) low. //~ · Robin (1986) low YEAR 2100 FIGURE 2-2 Eustatic sea level rise scenario adopted in this report compared with other estimates. This component is presented in Figure 2-2 for the three sce- narios considered, which results in eustatic components at the year 2100 of 0.5 m, 1.0 m, and 1.5 m, respectively. Also shown are estimates developed in various studies. The local component varies greatly from subsidence to uplift. The total component is T(t) = (0.0012+M/1, OOO)t+bt2 in which M (in mrn/yr; M = dl/(lt) is obtained from Table 2-3, and b for each of the three scenarios Is presented in Table 2-4.
30 RESPONDING TO CHANGES IN SEA LEVEL TABLE 2-3 Estimate of M, the Local Subsidenace (~) or Uplift (-) Rates as Determined from Tide Gauge Measurements Rate Rate Location (mm/yr) Location (mm/Yr) Portland, Maine 1.0 Grand Isle, La. 8.9 Boston, Mass. 1.0 Sabine Pass, Tex. 12.0 Providence, R.I. 0.5 Galveston, Tex. 5.1 Montauk, N.Y. 0.6 Port Isabel, Tex. 2.1 New York, N.Y. 1.S St. Georges 1.4 Atlantic City, N.J. 2.9 (Bermuda) Philadelphia, Pa. 1.4 San Diego, Calif. -0.4 Lewes, Del. 1.9 Los Ankles, Calif. -0.4 Baltimore, Md. 2.0 San Francisco, Calif. 0.1 Hampton Roads, Va. 3.1 Crescent City, Calif. -1.7 Wilmington, N.C. 0.6 Astoria, Ore. -1.3 Charleston, S.C. 2.2 Seattle, Wash. 0.8 Mayport, Fla. 1.0 Juneau, Alaska -13.8 Miami Beach, Fla. 1.1 Skagway, Alaska -19.5 Key West, Fla. 1.0 Honolulu, Hawaii 0.4 St. Petersburg, Fla. 0.8 Apra Harbor, Guam -2.8 Pensacola, Fla. 1.1 Wake Island -0.1 aThe eustatic component of sea level change has been treated as steady (1.2 mm/yr) in developing this table. In reality, this effect is global but it may not be steady. SOURCE: Based on Hicks (1983). TABLE 2-4 Value of Coefficient b for Three Scenarios Considered in This Report Scenario Eustatic Component by Year 2100 (m) b 2 (m/yr ) I. 0.S II. 1.0 III. 1.5 0.000028 0.000066 0.000105