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 140
Page 140
16
Sea Level
Average sea level over the oceans has not been constant
throughout earth's history, and it is changing slightly today.
Global sea level was about 100 m (a little more than 300 feet)
lower at the peak of the last ice age, about 18,000 years ago.
During the geologic past, there have been repeated variations from
present sea level of more than this amount during times of intense
glaciation and during periods in which the earth was free of ice.
During the whole period of human civilization, however, the average
sea level has been roughly as it is today. Current understanding of
sea level change—especially the processes by which it occurs,
the rates, and the record of past change—is described in
detail in Sea-Level Change (National Research Council,
1990).
Tide gauges measure sea level variations in relation to a fixed
point on land and thus record "relative sea level" (RSL). RSL at
any particular place varies over time and space. The direct causes
of these variations include vertical motions of the land to which
the tide gauge or other measuring device is attached, and changes
in the volume of sea water in which the gauge is immersed.
Differences in atmospheric pressure, water runoff from land, winds,
ocean currents, and the density of sea water all cause spatial and
temporal variations in sea level in comparison to the "geoid" (the
surface of constant gravitational potential corresponding, on
average, to the global mean sea surface). An atmospheric pressure
differential of 1 millibar is equivalent to a sea level difference
of 1 cm (0.4 inch). Variation in the runoff of large rivers can
result in local sea level changes of as much as 1 m (about 3 feet).
In exceptional circumstances, in the North Sea, along the Chinese
coast, and in the Bay of Bengal, sea level may rise by 5 m (about
15 feet) or more in a "storm surge." These changes are generally no
more than a few days in duration. Both irregular and seasonal
changes in
OCR for page 141
Page 141
temperature or salinity of the upper ocean layers cause
expansion or contraction of the water volume. These relatively
short-term changes in sea level may persist for a few days, several
months, or even several years, and their magnitude may be as much
as 5 to 15 cm (2 to 6 inches).
Climate-Related Sea Level Change
Climate-related contributions to sea level change can be
associated either with variations in the actual mass of water in
the ocean basins or with thermal expansion (due to changing density
and thus variations of temperature and salinity).
The mass of water at or near the earth's surface is practically
constant for periods of 10,000 years or less. What matters for sea
level is the partitioning of this mass of water among the major
hydrologic reservoirs. The four major reservoirs are the oceans
(1,370 million km3), ice (30
million km3), surface waters (8 to
19 million km3), and atmospheric
moisture (0.01 million km3)
(National Research Council, 1990). The melting of the northern
continental ice sheets between 15,000 and 7,000 years before the
present probably accounted for most of the rise of the sea to
current levels.
Some have suggested that greenhouse warming could lead to
disintegration of the West Antarctic Ice Sheet, most of which is
grounded below sea level. If climate becomes warmer, and warmer
ocean water intrudes under the ice sheet, the release of ice from
the sheet would accelerate. Estimates suggest that several hundred
years would be required to achieve this amount of warming (Bryan et
al., 1988; Meier, 1990). The current estimated effect on sea level
of the West Antarctic Ice Sheet is -0.6 ± 0.6 mm (-0.02
± 0.02 inches) per year, or a net decrease. Glaciers other
than the West Antarctic and Greenland ice sheets have been
estimated to have contributed about 0.46 ± 0.26 mm (0.017
± 0.01 inches) per year to sea level rise since 1900 (Meier,
1990).
Differences in water temperature, or in a combination of
temperature and salinity, account very well for seasonal and
interannual variations in sea level (National Research Council,
1990). This thermal expansion is not large enough, however, to
account for the changes over tens of thousands of years. Warming
the entire ocean from 0°C (32°F) to the current global
average temperature of about 15°C (59°F) would involve
thermal expansion of only about 10 m (about 30 feet).
Evidence of Sea Level Rise over the
Last 100 Years
Several studies of various periods during the last 100 years are
in general agreement that mean sea level is rising (see the
following reviews: Aubrey, 1985; Barnett, 1985; Robin, 1986).
Estimates range from about 0.5 to 3.0
OCR for page 142
Page 142
mm (0.019 to 0.1 inches) per year, with most lying in the range
of 1.0 to 1.5 mm (0.039 to 0.058 inches) per year. More recent
studies show similar or slightly higher estimates, ranging from
1.15 (Barnett, 1988) to 2.4 ± 0.9 mm (0.045 to 0.095
± 0.036 inches) (Peltier and Tushingham, 1989).
There are several possible sources of error common to all these
studies. First, they all use the same global mean sea level data
set. Although this is based on about 1,300 stations worldwide, only
about 420 have a time series of greater than 20 years. In practice,
the variability is such that 15 to 20 years of data are needed to
compute accurate trends, which significantly reduces the size of
the data set. Second, there is a historical bias in the data set in
favor of northern Europe, North America, and Japan. This
geographical bias can be reduced, but not eliminated, by treating
regional subsets of the data set as independent information.
Finally, the most important source of error results from the
difficulties in removing vertical land movements from the data set.
Although efforts have been made to address each of these sources of
potential bias, it cannot be said unequivocally that these factors
have not systematically biased all the studies in the same
direction.
Projecting Future Sea Level Rise
Various estimates of future sea level rise have been made
(Intergovernmental Panel on Climate Change, 1990). In general, most
of these studies foresee a sea level rise of between 10 and 30 cm
(4 and 12 inches) over the next four decades. This is significantly
faster than the estimated rise over the last 100 years. The IPCC
(Intergovernmental Panel on Climate Change, 1990) estimates a sea
level rise of between 8 and 29 cm (3 and 12 inches) by 2030, with a
"best estimate" of 18 cm (7 inches) for its "business-as-usual"
scenario (no reduction in emissions of greenhouse gases). By the
year 2070, IPCC projects a rise of between 21 and 71 cm (8 and 28
inches), with a best estimate of 44 cm (17.6 inches).
These estimates, however, are subject to considerable
uncertainty. In order to estimate oceanic thermal expansion,
changes in the interior temperature, salinity, and density of the
oceans have to be considered. Observational data are scant.
Alternatively, estimates can be based on numerical models of ocean
circulation. Ideally, detailed three-dimensional models would
describe the various oceanic mixing processes and simulate transfer
and expansion effects throughout the oceans. However, such models
are in the early stages of development, and applications are few in
number. Instead, simple box upwelling-diffusion models are used.
This type of model typically represents land and oceans by a few
"boxes." The complicated processes of oceanic mixing are simplified
in one or more parameters. Inclusion of expansion coefficients in
the model (varying with depth and possibly with latitude) allows
sea level changes to be estimated as well.
OCR for page 143
OCR for page 144
Representative terms from entire chapter:
level rise
Page 143
Two types of box upwelling-diffusion models yield somewhat
different results (National Research Council, 1990). In a ''pure
diffusion" (PD) model, heat is carried downward by eddy diffusion.
In an "upwelling diffusion" (UD) model an upwelling rate balances
some of the transfer into the deep oceans. The PD model transports
heat relatively rapidly into the oceans, which slows the
atmospheric temperature rise but increases the rate of sea level
rise. The UD model reduces heat penetration into the ocean,
allowing the climate to warm more rapidly but reducing the sea
level response. Different choices of parameters in these models
probably are more important than the differences in the models
themselves. PD and UD models have been run assuming that the deep
water of the world ocean remains relatively unchanged (ocean
circulation is "surprise free") except that the deep water becomes
somewhat warmer because of vertical and lateral mixing. The
projected rise in sea level from thermal expansion estimated by the
PD model ranges from 20 to 110 cm (8 to 44 inches) and by the UD
model from 10 to 50 cm (4 to 20 inches). Both estimates are for the
year 2100 and a radiative equivalent of doubling the preindustrial
atmospheric concentration of CO2.
Because of the uncertainties about key phenomena described in
this chapter, this panel uses a range of sea level rise from 0 to
60 cm (about 24 inches) associated with an equivalent doubling of
preindustrial levels of CO2. There
would be a lag of from a few years to several decades before the
level would be reached, with a greater delay the higher the rise.
This expected sea level rise is based on a combination of factors,
including the possibility that the net change associated with ice
at the high latitudes may be a lowering of sea level combined with
the thermal expansion of the oceans. This range is slightly lower
than those found elsewhere. For example, the IPCC
(Intergovernmental Panel on Climate Change, 1990) anticipates a
doubling of preindustrial levels of CO2 by about 2030, and estimates a sea level
rise of between 8 and 29 cm (3 and 12 inches) by 2030 and of
between 21 and 71 cm (8 and 8 inches) by 2070.
References
Aubrey, D. G. 1985. Recent sea levels from tide gauges: Problems
and prognosis. In Glaciers, Ice Sheets and Sea Level: Effect of a
CO2-induced climatic change.
DOE/ER/60235-1. Washington, D.C.: U.S. Department of Energy, Carbon
Dioxide Research Division.
Barnett, T. P. 1985. Long-term climatic change in observed
physical properties of the oceans. Pp. 91–107 in Detecting
the Climatic Effects of Increasing Carbon Dioxide, M. C. MacCracken
and F. M. Luther, eds. DOE/ER-0235. Washington, D.C.: U.S.
Department of Energy.
Barnett, T. P. 1988. Global sea level change. In Climate
Variations over the Past Century and the Greenhouse Effect. A
report based on the First Climate Trends
Page 144
Workshop, September 7–9, 1988. Rockville, Md.: National
Oceanic and Atmospheric Administration.
Bryan, K. S., S. Manabe, and M. J. Spelman. 1988.
Interhemispheric asymmetry in the transient response of a coupled
ocean-atmosphere model to a CO2
forcing. Journal of Physical Oceanography 18(6):851ú867.
Intergovernmental Panel on Climate Change. 1990. Climate Change:
The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins, and
J. J. Ephraums, eds. New York: Cambridge University Press.
Meier, M. F. 1990. Role of land ice in present and future sea
level change. In Sea-Level Change. Washington, D.C.: National
Academy Press.
National Research Council. 1990. Sea-Level Change. Washington,
D.C.: National Academy Press.
Peltier, W. R., and A. M. Tushingham. 1989. Global sea level
rise and the greenhouse effect: Might they be connected? Science
244:806–810.
Robin, G. de Q. 1986. Changing the sea level. In the Greenhouse
Effect, Climate Change and Ecosystems, B. Bolin, B. Döös,
J. Jäger, and R. A. Warrick, eds. Chichester, United Kingdom:
John Wiley and Sons.
