Introduction

Sea level rise is often regarded as an issue for the future, rather than of the present day. The most alarming scenarios involve partial melting or collapse of a great polar ice sheet a hundred years or more hence due to global warming. The result of such a catastrophe would be flooding of coastal regions of the world, causing massive loss of life and property and breakdown of social order. Fortunately, plausible forecasts (Houghton et al., 1996) of sea level change for the 21st century are far less ominous. But as Chapters 2 and 8 document, the impact of even a moderate rate of sea level rise is severe, especially for island and developing nations. In fact, if global sea level rises in the 21st century at only the 20th-century rate of about 2 mm per year, the economic and social burdens will still be profound, because an increase of sea level significantly increases the impact of storms on heavily populated low-lying coastal areas. It is a matter of practical urgency to determine the amount and causes of global sea level rise so that mitigation activities can begin as soon as possible.

The global increase of sea level by as much as 20 cm over the last century, although important, is minor compared to what occurred in the distant past. Sea level rose by approximately 125 meters (over 600 times greater!) as a result of the disappearance of the great glacial ice sheets that began about 21,000 years ago. Melting was complete everywhere by 4000-5000 years ago, at which time relative sea level was within a few meters of its present value for most of the earth. Removal of the ice load also caused the elevation of certain high-latitude locations to increase by hundreds of meters and adjacent areas to subside as the earth adjusted viscoelastically. This change of elevation, referred to as glacial isostatic adjustment (GIA), is a global phenomenon that continues to this day everywhere at rates that are significant in some areas in comparison to estimates of 20th-century global sea level rise.

The last few thousand years are especially interesting as far as global sea level rise is concerned. Geological and other evidence presented in Chapter

Sea Level Rise

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2 and by Gornitz (1995a) and Varekamp et al. (1992) suggest that during this recent period, the average rate of change of global sea level has been very small, much less than the 20th-century rate. This conclusion has also been reached by Flemming (1978,1982) using a novel method. He analyzed elevations of hundreds of Mediterranean coastal archeological sites relative to modern sea level and concluded that the average rate of sea level rise during the last 2 millennia has been only 0.2 mm/yr. Apparently the 20th-century rate of about 2 mm per year is an historically recent development; one could even be tempted to attribute the increased rate to the 20th-century global warming of approximately 0.3-0.6°C (Houghton et al, 1996). But sea level began to rise at the much faster modern rate near the mid-19th century, too soon for 20th-century warming to be held responsible.

This is not to say that global warming (or cooling) has a negligible effect on sea level. A crude estimate of its importance can be calculated from the thermal expansion coefficient of sea water. This quantity varies strongly with salinity and temperature (water is a peculiar substance—consider what happens when it freezes), but for tropical and midlatitude ocean temperatures the value differs from 2.5 X 10"4/°C by less than ±50%. Using this value of the expansion coefficient gives an increase in the level of a 1000-m layer of ocean of the order of 10 cm for a temperature change of 0.5°C. So thermal expansion as a source of sea level change is not to be ignored. Detailed calculations (summarized by Warrick et al, 1996) based on the actual structure of the oceans indicate that the contribution of thermal expansion to global sea level in the 20th century is on the order of 2-7 cm.

Another potential source of increased sea level from global warming often mentioned in the media is melting of the polar glacial ice caps. But the great ice sheets of Greenland and Antarctica are unlikely to have been affected by a change of a few tenths of a degree Celsius in a century. Ice is an excellent insulator, and the thermal inertia of the polar caps is very great. However, smaller Alpine-type glaciers are likely affected. Meier (1984,1990) estimated that mountain ice sheets are retreating enough to cause global sea level to rise 4.6 ± 2.6 cm in the 20th century.

Other questions abound that relate to global sea level rise and global warming. For example, the Greenland and Antarctic ice sheets can play a role other than melting. In the case of Greenland, Zwally et al (1989) claimed that satellite altimeter data showed a thickening of ice there during 1978-1987 equivalent in volume to a fall of sea level of a few tenths of a millimeter per year. If this scenario held true over an extended time, a possible increase of global sea level from thermal expansion or other sources could be offset to some extent by increased storage of water in the form of ice. However, a later reanalysis of the satellite altimeter data with refined methods by Davis et al (1998) showed a negligible change of ice elevation for Greenland. But the issue remains. If global warming causes increased precipitation at high latitudes with concomitant storage of water in the form of ice, sea level rise due to thermal expansion of the ocean or melting of small glaciers could be offset to a certain extent. The ice mass balance (i.e., whether or not there is net accretion or loss of ice) of Greenland and Antarctica is of critical importance for accounting for changes of global sea level. However, Warrick et al. (1996) concluded that because of uncertainty of the ice mass balance of the Greenland and Antarctic ice sheets, not even the sign of their contribution to 20th-century sea level rise can be determined. But there is still concern that a response is possible.

An issue unrelated to global warming that affects the rate of global sea level rise is storage and mining of water. This question is dealt with in detail in Chapter 5. In summary, since World War II the increase in the amount of water held in large and small reservoirs (and kept out of the oceans), in combination with other effects on the hydrological cycle, has a sea level rise equivalent estimated to be 0.9 ± 0.5 mm/yr. This is enough to offset an increase in the rate of sea level rise that could have occurred in the last 50 years due to global warming.

Warrick et al. (1996) summarized the factors affecting sea level rise in the 20th century. Under the assumption that the mass balance of the Greenland and Antarctic ice caps is zero, they concluded that the remaining contributors to global sea level could add up to 0.8 mm/yr of sea level rise in the 20th century, with a substantial uncertainty. This value is much smaller than most estimates of global sea level rise based on tide gauge data. Table 3.1 shows the values published in the last decade. They range in magnitude from 1.0 to 2.4 mm/yr. All of these authors used tide gauge data from the same data base, so the differing estimates reflect selection

Table 3.1

Recent Determinations of Global Sea Level Rise from Tide Gauge Data

Table 3.1

Recent Determinations of Global Sea Level Rise from Tide Gauge Data

Author

Estimate (mm/yr)

Comments

Peltier and Tushingham (1989, 1991)

2.4 ± 0.9"

Global data

Barnett (1990)

1-2

Global data

Nakiboglu and Lambeck (1990)

1.15 ± 0.38

Global data

Trupin and Wahr (1990)

1.75 ± 0.13

Global data

Douglas (1991)

1.8 ± 0.1

Global data

Shennan and Woodworth (1992)

1.0 ± 0.15

U.K. and Europe

Mitrovica and Davis (1995)

1.1-1.6

Global data

Davis and Mitrovica (1996)

1.5 ± 0.3

U.S. east coast

Peltier (1996)

1.94 ± 0.6°

U.S. east coast

Peltier and Jiang (1997)

1.8 ± 0.6"

U.S. east coast

Douglas (1997)

1.8 ± 0.1

Global data

a Standard deviation of trends about their mean. The formal SE is a few tenths.

a Standard deviation of trends about their mean. The formal SE is a few tenths.

criteria and data analysis methods, particularly record length and the correction for glacial isostatic adjustment (GIA). Warrick et al. (1996) selected 1.8 mm/yr as the "best estimate," with the scatter of all of the estimates (1-2.4 mm/yr) taken to reflect in some measure the uncertainty of the rate. However, Douglas (1995, 1997) and Peltier and Jiang (1997) (see also Chapter 4 of this volume) conclude that there is a preponderance of evidence that the 20th-century rate of sea level rise is much closer to 2 than to 1 mm/yr. To resolve these issues of the contributors to sea level change and its observed rate of rise, at least two kinds of continuing research are needed. First, estimates of the contributions to sea level rise by thermal expansion of the ocean, melting of small glaciers, impoundment of water in large and small reservoirs, mining of groundwater, and accumulation or loss of ice on the great polar ice sheets must be improved. Second, the rate of global sea level rise for the 20th century determined from tide gauge measurements can be further refined and monitored by altimeter satellites to determine if changes in the rate are occurring. Any change in the rate of sea level rise is potentially detectable much sooner by altimetric satellites than tide gauges because of the former's near-global coverage of the oceans. Satellite measurements of sea level variation are the subject of Chapter 6. It is the purpose of the present chapter to examine the problems encountered in determining an accurate estimate of the rate of global sea level rise from tide gauge data.

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