|
Items in cart [4] |
Questions? Call 888-624-8373 |
| ||||||||||||
| Copyright © 2009. 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 24
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
OCR for page 25
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
OCR for page 26
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~.
OCR for page 27
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(t—to),
where F is a Tmear correlation coefficent and the time to represents
the lag of sea level in responding to temperature change. Robin
OCR for page 28
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.
OCR for page 29
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.
OCR for page 30
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
Representative terms from entire chapter:
level rise
