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 changeespecially the processes by which it occurs, the rates, and the record of past changeis 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
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
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.
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.
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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.
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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.
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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.
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