Sea-Level Change (1990)

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Overview and Recommendations EXECUTIVE SUMMARY i: This study addresses current scientific understanding of sea-level change-particularly the processes of sea-level change, their rates, and the record of past change. An important part of such an assessment is an evaluation of the adequacy of the geophysical knowledge base and the opportunities to improve upon it. Discussion of engineering and societal responses to sea-level change is not included in this study as these issues are fully discussed in a report, Responding to Changes in Sea Level: Engineering Implications (NRC, 1987). Average sea level over the oceans has never been constant throughout earth history, and it is changing slightly today. The entirety of civilization has occurred within a single high stand of the sea, and yet global sea level was more than 100 m lower than it is at present only 18,000 yr ago. And during the geologic past there have been repeated variations of more than 100 m from present sea level, both during times of intense glaciation and during times of an ice-free Earth. Relative sea level (RSL; i.e., sea level relative to a fixed point on land) at any particular place in the ocean varies over a wide range of time and space scales. The direct causes of these variations are vertical motions of the land to which the tide gauge or other measuring instrument is attached, and changes in the volume of sea water in which the tide gauge is immersed. But changes in climate, plate tectonics, ice and snow, and ocean circulation are all indirect causes of changing sea level. The relative importance of the forcing functions varies with the time scales of interest. On the basis of estimates of global warming of the atmosphere and ocean resulting from increasing concentrations of carbon dioxide and other greenhouse gases, it is possible to make an approximate forecast of global rise in sea level during the next 100 yr. Two processes will be principally involved: thermal expansion of ocean waters as they become warmer and changes in the mass of land ice in both continental ice sheets and mountain

4 OVERVIEW AND RECOMMENDATIONS glaciers. One hundred years from now it is likely that sea level will be 0.5 to 1 m higher than it is at present. There are two principal uncertainties at present about global sea level. (1) What if any is the value of the long-term trend over the next few centuries? (2) If a secular trend exists, what proportion of this trend results from changes in the specific volume of sea water (called steric changes) and what from changes in the total mass of water in the oceans? The apparent trend of sea level at any particular place as measured by a tide gauge is the sum of the trend in motion of the gauge itself as the land on which it is mounted moves vertically, the trend of change in steric sea level, and the trend of change in water mass under the tide gauge. To understand what is happening, one needs to be able to make measurements that will separate these three components of the observed sea level. In principle, a combination of inverted echo sounders (which in effect measure thermal expansion or contraction of the water column) or systematic observations of ocean tem- perature as a function of depth with conductivity-temperature-depth recorders to determine the steric component, plus one or more of three methods (very-long-baseline interferom- etry, global positioning system, and absolute gravimetry) for measuring the vertical mo- tions of the tide gauge, plus tide-gauge measurements at the sea surface should allow us to separate the three major components of changes in RSL. Within the next few years, it should be possible to measure accurately the combined effects on global sea level of steric changes and changes in the mass of sea water from laser or radar altimeters mounted on Earth-orbiting satellites. Recommendation 1. Long-term sea-level measurements of sufficient accuracy over the world's oceans could provide one of the most significant data sets for understanding global change, particularly climatic change resulting from greenhouse warming. It is for this reason that the planning committees for the World Climate Research Program and the Intergovernmental Oceanographic Commission of UNESCO have given a very high prior- ity to extending the global sea-level network in the Indian, South Atlantic, and South Pacific oceans. We strongly recommend that national oceanographic and meteoro- logical communities-lend moral and intellectual support to this sea-level program. Recommendation 2. Possible changes in the mass balance of the Antarctic and Greenland ice sheets are fundamental gaps in our understanding and are crucial to the quantification and refinement of sea-level forecasts (the probable contribution from ice wastage makes up more than half of various forecasts). A polar-orbiting satellite altimeter would be invaluable in monitoring the mass balance of these ice sheets. Recommendation 3. To refine estimates of sea-level change related to greenhouse warming, it is necessary to develop and improve coupled atmosphere-ocean-cyrosphere global circulation models in which greenhouse gas concentrations in the atmosphere are gradually increased. Recommendation 4. The Cretaceous period offers special opportunities to understand global processes and their variations, in particular, large, long-term changes in sea level. One of the major projects of the Global Sedimentary Geology Program, under the auspices of the International Union of Geological Sciences, is entitled "Cretaceous Resources, Events, and Rhythms." We urge national and international support of this and similar programs that will improve our understanding of past sea-level changes and the processes that produced them. Recommendation 5. To separate epeirogeny from eustasy and steric components, it is important to measure repeatedly the absolute heights of tide gauges. We recommend that global measurements of absolute heights of these gauges be undertaken using abso- lute gravimetry and space-based techniques.

OVERVIEW AND RECOMMENDATIONS s INTRODUCTION It is easy to think of sea level as a stable baseline against which changes on land can be measured, and it is so used by politicians, engineers, land planners, and households. But as we have become more aware of the Earth and its metabolism, we have recognized that sea level is highly unstable both in time and in space. We can regain our former confidence in its usefulness only by learning how and why it varies and compensating for these vari- ations. , 1 - Average sea level over the globe has never been constant throughout earth history: and it is changing slightly today. The entirety of civilization has occurred within a single high stand of the sea, and yet global sea level was more than 100 m lower than it is at present during the maximum of the last glacial period only 18,000 yr ago (yrBP). Relative sea level (RSL; i.e., sea level relative to a fixed point on land) at any particular place in the ocean varies over a wide range of time and space scales. Among the causes of these variations are vertical motions of the land to which the tide gauge or other measuring instrument is attached and changes in the volume of sea water in which the tide gauge is immersed. Wind-driven waves produce the shortest-period variations in the height of the sea surface. Variations with periods of 12 hours or more are caused by lunar and solar tides. Variations in atmospheric pressure cause inverse variations in sea level- an atmospheric pressure differential of 1 mbar is equivalent to a sea-level differential of 10 mm. A series of depressions in atmospheric pressure can cause a rise in sea level in a shallow ocean basin of 0.3 m or more (Hekstra, 1988~. Variations in the runoff of large rivers can result in local sea-level changes of as much as 1 m. In relatively shallow water, large variations in sea level are also caused by offshore and onshore winds that pile up water against the shore or drive it away from the shore. (This process is called wind setup.) In exceptional circumstances, in the North Sea, along the Chinese coast, and in the Bay of Bengal, sea level may rise by 5 m or more in a "storm surge" under the action of strong winds (Hekstra, 19881. Both irregular and seasonal variations in temperature or salinity of the upper ocean layers cause expansion or contraction of the water volume in different regions. These relatively short-term steric changes in sea level may persist for a few days, several months, or even several years, and the magnitude may be as much as 50 to 150 mm. Changes in sea level have many practical consequences, often disruptive but sometimes beneficial. The disruptive consequences can sometimes be avoided and the benefits enhanced if the ways and means by which sea level varies are understood. Although disruptive consequences, especially from sea-level rise, are far more common, occasional examples of benefits are being obtained as a result of changes in policy. For example, in the Wadden Sea on the northwest coast of the Netherlands, where the land has subsided as much as 260 mm during the past 20 yr, the Dutch government has decided not to follow age-old tradition by attempting to reclaim more land for agriculture. The area has been designated as a "Declared UNESCO Biosphere Reserve" in which only those economic ac- tivities are allowed that do not conflict with natural conditions or processes. In view of agricultural surpluses in the Netherlands, and indeed throughout the European community, the Wadden Sea can be used much more effectively as a nursery for young fish and shrimp and other valuable invertebrates (Hekstra, 1988) than as reclaimed land for agriculture. Chances of sea level on the order of 300 mm can have significant implications for J ' coastal communities and coastal engineering practices. engineering responses lo sea-level change are largely a function of the rate of change. Many of these issues and engineering responses are described in the report Responding to Changes in Sea Level: Engineering Implications prepared by the Marine Board of the National Research Council (NRC, 19871. A 1-m change in average sea level can translate into major shifts in shoreline positions, positions that have both economic and legal significance.

6 O VER VIEW AND RECOMMENDA TI ONS Sea-level change, seemingly so simple and straightforward, is in fact the product of many interrelated processes. Insight into these processes can be gained by intensive study of sea-level change in the context of related environmental phenomena, remembering that changes in sea level are an integrated measure of environmental change, in terms of both causes and consequences. Changes occur on all space and time scales, from local to global, and from a few seconds to geologic ages. This volume is primarily concerned with future sea-level changes over the next few centuries and past changes over a wide range of times from which greater understanding can be gained and used as an aid in the prediction of future changes and in the search for fossil fuels and other natural resources. Also covered are the mechanisms and processes involved in past changes, in order to gain greater knowledge of the Earth as a dynamic system, i.e., how the Earth works. _ ~ ~^ ~_ V _. ~ ~ ~ ~ ~ ~ · ~ ~ Climate, plate tectonics, the cryosphere, and ocean circulation all contribute to changing sea level. The relative importance of the forcing functions varies with the time scales of interest. The effects of changing sea level are also broad on the one hand with direct feedbacks to the causative forcing functions, e.g., albedo change, and on the other hand with effects on other processes such as sedimentation or coastal ecology. PROCESSES AND FEEDBACKS Many processes can cause a change in RSL at any particular location. They include the following: 1. local or regional uplift or subsidence of the land; 2. changes in atmospheric pressure, winds, or ocean currents; 3. changes in the mass of ocean water brought about by wastage or accumulation of ice sheets and mountain glaciers (glacio-eustatic) or by increased or decreased retention of liquid water within or upon the continents, and possibly also by a slow release through geologic time of juvenile water from the Earth's interior; 4. steric changes in the volume of ocean water without changes in water mass (Patullo et al., 1955) in response to temperature or salinity changes (also called thermohaline changes in Table 11; and 5. changes in the volume of the ocean basins owing to changes in the rate of plate divergence (seafloor spreading), plate convergence (subduction, overthrusting), epeiro- genic changes in the elevation of the seafloor (largely from mid-ocean volcanism), marine sedimentation, or isostatic adjustment of the Earth's crust under the sea resulting from glaciation or deglaciation on land. The latter three processes can affect global mean sea level or eustatic sea level. But all processes need to be considered, even though, depending on the time scale of interest or the magnitude of the sea-level change, some of them may be insignificant. In a following section on forecasting sea-level change due to greenhouse-induced climate warming, a projected global sea-level rise of 0.5 + 1 m by the year A.D. 2100 is ascribed to a combination of thermal expansion of ocean water and melting of glaciers and ice sheets. Time Scales of Sea-Level Change Sea-level change encompasses a broad range of time scales, with different mechanisms associated with change over different times. The oceanographer concerned with storm tides will not have much interest in the factors explaining Cretaceous sea levels; likewise, the geologist's glacio-eustatic theories have little application to seasonal events. The problem of sorting out time scales and processes afflicts studies of climate change in general. Complexities multiply in attempts to link different processes together. Table 1 summarizes mechanisms of sea-level change by time scale and magnitude.

OVERVIEW AND RECOMMENDATIONS 7 TABLE 1 Some Mechanisms of Sea-Level Change Time Scale (years) Order of Magnitude of Change (mm) Ocean Steric (thermohaline) Volume Changes Shallow (0 to S00 m) 10-~ to 102 10° to 103 Deep (500 to 4000 m) 10' to 104 10° to 104 Glacial Accretion and Wastage Mountain Glaciers 10t to 102 10' to 103 Greenland Ice Sheet 102 to 105 10~ to 104 East Antarctic Ice Sheet 103 to l 05 104 to l 05 West Antarctic Ice Sheet 102 to 104 103 to 104 Liquid Water on Land Groundwater Aquifers 102 to 105 1'02 to 104 Lakes and Reservoirs 102 to 105 10° to 102 Crustal Deformation Lithosphere Formation and Subduction 105 to 108 103 to 105 Glacial Isostatic Rebound 102 to 104 102 to 104 Continental Collision 105 to 108 104 to 105 Sea Floor and Continental Epeirogeny 105 to 108 104 to 105 Sedimentation 104 to 108 103 to 105 Heat exchange is relatively rapid within the uppermost few hundred meters of the oceans. A one-dimensional treatment implies that thermal expansion of these waters can occur on time scales of months to decades. Sea level will rise about 100 mm for every degree of temperature increase throughout the uppermost 500 m. Heat exchange with ocean deep waters is slower (Chapter 131. If the deep ocean were to warm everywhere by 10°C, as was perhaps the case during the early Tertiary and Cretaceous, sea level could rise by about 10 m. The time scales and magnitudes of melting ice can be estimated from both historical data and mass balance considerations. The present Greenland and Antarctic ice caps are remnants of the late Pleistocene ice sheets that increased sea levels about 100 m by disintegrating over a period of several thousand years encompassing the end of the Pleis- tocene (Chapters 4 and 5~. Mass balance estimates suggest modern ice residence times on the order of 102 to 1os yr. The Antarctic Ice Sheet contains enough water to raise sea level by about 60 m, and the Greenland Ice Sheet contains water equivalent to a 6-m sea-level rise. Sea Level and the Geoid The sea surface departs significantly from the geoid, owing to waves, the tidal attraction of the Sun and Moon, ocean circulation that tilts the ocean surface, atmospheric distur- bances, and steric regional variations in water temperature and salinity. If these effects can be taken into account, the resulting mean sea level accurately follows the geoid, which however is far from being an ideal spheroidal shape. Deep mantle phenomena and gravity anomalies associated with subduction can cause deviations from the spheroid of many tens of meters. Local crustal geological features, such as seamounts, fracture zones, and other abrupt topographic forms that are not in isostatic equilibrium, can cause deviations of

8 OVERVIEW AND RECOMMENDATIONS several meters. In fact, it is possible to derive a partial picture of ocean floor topography from a knowledge of the mean shape of the sea surface. If the effects of vertical tectonic movements, in addition to the local or regional variations of sea-surface topography, are removed from tide-~au~e records nresumahlv =_ _ =- ~ 7 or-- -a any remaining trend is global (eustatic sea-level change). Eustatic changes can result from processes that change the mass or volume of water in the ocean basin or those that change the volume of the ocean basin itself. These processes are discussed in later sections. Land Elevation Changes Tide gauges are anchored to the land, which itself can be moving vertically at rates comparable to sea-level change. These vertical tectonic movements of the land will result in an RSL change as measured by a tide gauge. If the land where the tide-gauge station is located is subsiding, RSL will show a rise; likewise, uplift will result in an RSL fall. The Viking city now called Old Uppsala was a major port for ships sailing Lake Malaren; however, the port and the city of Uppsala were forced to move downstream around the eleventh century because of a 10-mm/yr uplift in the Fennoscandian region while the lake remained connected with the worldwide mean sea level. Vertical tectonic motions can be very localized, such as subsidence along a coastline owing to sediment load or from the withdrawal of groundwater or hydrocarbons. Subsi- dence is occurring along the Louisiana gulf coast because of the deposition and dewatering of sediment from the Mississippi River system. On the other hand, vertical motions can be systematically related to one another through isostasy. Land areas that were ice covered during the last glacial episode (~18,000 yrBP) were depressed; this depression created a forebulge in adjacent areas (see Pettier, Chapter 4~. The North American or Laurentide ice sheets began disintegrating about 15,000 yrBP and by about 7000 yrBP had all but vanished. Along the east coast of North America, those areas that were ice covered are still being uplifted due to isostatic rebound at rates of up to about 10 mm/yr. In the peripheral area where the forebulge is collapsing, the land is subsiding at more than 1 mm/yr. The east coast of the United States is perhaps the best location illustrating this "drowning" effect, which is responsible for many of the unique features of its nearshore environment, including the extensive occurrence of salt marshes. In these areas and with current models of isostasy and the Earth, the relative vertical motions are predictable; thus, their relative contribution to sea-level changes as measured by tide gauges can be taken into account in arriving at eustatic sea-level changes. Effects of Atmospheric Pressure, Winds, and Ocean Currents Local RSL variability can result from several forcing functions including air pressure, wind stress, ocean circulation, and thermohaline changes. These processes contribute to the high-frequency noise in tide-gauge data and need to be compensated for in extracting eustatic changes. The difference between the average air pressure over the world ocean and the local barometric pressure can result in a change of sea level at annual and shorter periods. One millibar of pressure differential is equivalent to 10 mm of sea-surface change. Wind stress can have an important effect on the sea level. The coastal sea-level response to a steady longshore wind stress can be similar in magnitude to the air-pressure effect. The time to achieve a steady state is of the order of a few days. Sea-surface changes caused by wind stress are localized and are not a major contributor to low-frequency sea- level change. Ocean circulation can also result in sea-level variability from long-period waves, as Sturges (Chapter 3) shows. Sea-level signals of about 50 to 150 mm with periods of 5 to

OVERVIEW AND RECOMMENDATIONS 9 10 yr and longer are coherent between the U.S. Pacific coast and Hawaii, and on both sides of the Atlantic. These signals are out of phase, with delays of several years, and are consistent in part with baroclinic wave propagation across the ocean. Also coherent are sea-surface variabilities resulting from El Nino and related effects (Figure 1~. From these, sea level can vary on the time scale of a few years by 50 to 150 mm resulting from warming of the ocean subsurface waters by a few degrees Centigrade (TIC). Changes in the Mass of Ocean Water For all intents and purposes, the mass of water at or near the Earth's surface is constant over time scales of less than 104 yr. It is how the water is partitioned between the major hydrologic reservoirs that is of importance to sea-level change. The four major reservoirs, in order of abundance, are the oceans (1370 x 106 km3), ice (30 x 106 km3), ground and surface waters (8 x 106 to 19 x 106 km3), and atmospheric moisture (0.01 x 106 km33. The principal exchange of water over the past several million years involved ice. Sea level was over 100 m lower during the peak of the most recent glacial 18,000 yrBP. The melting of the northern continental ice sheets between 15,000 and 7000 yrBP probably accounted for most of the rise of the sea to present levels. Indeed, sea-level change within the next few thousand years probably will also be dominated by water within the global ice budget and by ice sheet dynamics. If the polar ice sheets were to disappear, sea level would be some 60 to 70 m higher than at present. Mountain glaciers make up about 1 percent of the volume of land ice. Their potential contribution to sea level is about 1 m if they were to melt totally and all the meltwater reached the sea. Data cited by Meter (Chapter 10) suggests that wastage of the world's Boo s Too ~ 30°4 ool S Ano. 120° 150° E 180° W 150° 120° 90° 1 1 ~DEC 19 75 _ to _ ,~1 ,.,6 1 1 ~I I I 1 1 1 1 1 1 _ -so977 .~~ - ,~ l - 1 1 1 1  120° 150° E 180° W 1 FIGURE 1 Maps of sea-level anomaly for December 1975 and December 1977. Contours show sea-level anomalies in millime- ters after removal of seasonal cycle. The two cases were selected for their large contrast. From Wyrtki and Nakahoro (1984).

10 OVERVIEW AND RECOMMENDATIONS mountain glaciers and small ice caps contributed about 0.46 + 0.26 mm/yr to higher sea level between 1900 and 1961 corresponding to a total sea-level rise of 28 + 16 mm, which is about a third of the estimated sea-level rise during that period. The Antarctic and GreenlandAice sheets gain material mainly through the accumulation of snow and lose material through several processes. These processes include surface melt and runoff of meltwater, calving (discharge) of icebergs, and melting of the underside of floating ice shelves. Surface melt/runoff is a minor process for the Antarctic Ice Sheet, but is important to the balance of the Greenland Ice Sheet. Iceberg calving is an important loss process for both ice sheets, and is predominant in Antarctica. Melting of the underside of ice shelves has no effect on sea level, but does control the speed of discharge of land-based ice from Antarctica and is important for predicting the behavior of that ice sheet in the next century. The increase in air temperature caused by a rise in concentration of CO2 and other greenhouse gases may cause increased snow and ice melting, and some of this meltwater may run off to the oceans, causing a rise in sea level. Increased air temperature and/or meltwater production may also cause some outlet glaciers and ice streams to flow faster, transferring land-based ice to the ocean and causing a further rise in sea level. On the other hand, a rise in CO2 concentration may, in some regions, lead to increased snow precipita- tion on glaciers and ice sheets, which will have the opposite effect on sea level. Predicting the effects of climate change on ice growth and wastage is a complex problem because several different, interacting processes must be considered. Five important factors in future changes of glaciers and ice sheets are (1) the variation of energy and mass balance components with altitude, (2) the warming of cold firn to allow meltwater runoff, (3) the dynamic response of ice masses to changes in thickness, (4) increased flow and iceberg calving of tidal glaciers due to increased meltwater, and (5) the stability of ice-sheet/ice-stream/ice-shelf systems. The time frame is restricted to the next 100 yr, approximately the time of doubling of the present level of CO2. The first and fifth points are briefly discussed below; see Chapter 10 for additional details on all five proc- esses. observable functions of altitude. radiation and precipitation of snow. For glaciers, many of the mass and energy fluxes related to melting are known or The two most sensitive of these are absorbed solar The first is critically dependent on the albedo (reflectivity) of the surface, which in turn depends on how long the surface is covered with high-albedo snow during the course of the melt season. The persistence of the snow cover depends, of course, on the amount of snowfall and the intensity of melt processes, both of which also depend on altitude. The attitudinal dependence of the mass and energy fluxes leads to a potential instability, which is one of the reasons for the sensitivity of glaciers to slight climate changes. An increase in melting causes a lowering of the ice surface, which in turn may cause a further increase in melting or decrease in snow accumulation leading , _ is, to further changes accentuating the melting. Some have suggested that a climatic change due to increased CO2 in the atmosphere could lead to disintegration of the West Antarctic Ice Sheet, most of which is grounded below sea level, causing a 6-m rise in global sea level. The discharge of ice from this ice sheet is mainly through rapidly moving ice streams, which flow into floating ice shelves. An ice sheet that rests on a flat bed situated below sea level can be inherently unstable. Floating ice shelves at its seaward edge, which are "pinned" in position by shallow bottom areas, act as buttresses that prevent the ice sheet from quickly flowing out into the ocean. The rise of 100 m in sea level during the past 15,000 yr caused a substantial collapse of a large part of the West Antarctic Ice Sheet until it anorox'imatelv stahili7.er1 a~ its nr~.ce^.n~ ~ r r ., A A ~ ,, _ A ~ JO ~ ~ ~ ~ ~ ~ A_~ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ ~ A ~ ~ · ~ . ~ ~ ~ ~ ~ _ _ . _ _ ,, _ size. A relatively small further sea-level rise could act to "unpin" the buttressing ice shelves that allow the remnant ice sheet to exist (Thomas and Bentley, 1978). If the climate in the future becomes warmer with the result that warmer ocean water intrudes under the ice shelves causing increased melting under the shelves, then the back

OVERVIEW AND RECOMMENDATIONS 11 pressure exerted on the ice streams by the shelves will be reduced and the ice streams will accelerate, draining the ice sheet itself. The critical questions then are: (1) How rapidly will the temperature of Antarctic subsurface waters rise in response to increased atmospheric concentrations of greenhouse gases? (2) How much sub-ice melting will be caused by the circulation of this warmer water under the ice shelves? (3) How will the changed conditions affect calving rates and thus the dimensions of ice shelves? (4) How rapidly will the ice streams react to changes in the back pressure? For the next one or two centuries we need to consider only the first question because warming of the ocean south of the Antarctic convergence is likely to be markedly delayed. Bryan et al. (1988) used a combined oceanic and atmospheric general circulation model to show that convective mixing from the surface down to 4000 m will slow the rate of ocean warming because the entire water column must be warmed by the same amount. If we consider the added heat energy of around 4 watts/m2 transferred from the atmosphere to the surface ocean layers, a time of several hundred years would be required to accomplish this warming. Bentley (1985) has summarized a number of other reasons why disintegration of the West Antarctic Ice Sheet should not occur within the next one or two centuries. Accepting Bentley's arguments, sea level will not rise catastrophi- cally in the near future resulting from the demise of this ice sheet. Scientific concern about possible disappearance of the West Antarctic Ice Sheet has largely been based on Mercer's (1978) hypothesis that this body of ice disappeared during the last interglacial, 125,000 yrBP, with the result that a terrace about 5 m above present sea level was created around many shorelines around the world. An alternative hypothesis is that the surface of the East Antarctic Ice Sheet was lower by some 300 to 350 m than today (Robin, 1987~. This idea is supported by the investigation by Lorius et al. (1985) of INTO from the ice core collected by Soviet engineers at Vostok in the East Antarctic Ice Sheet. At 125,000 yrBP, the oxygen isotope values were about 2 Ho higher than present. Robin points out that this apparently higher temperature could be caused by a reduction in surface elevation of about 300 m. Surface lowering of this amount for the East Antarctic Ice Sheet would correspond to a volume of ice of about 2 million km3 and a correspond- ing rise of sea level of 5 to 7 m during the last interglacial. Eustatic Elects of Changes in Liquid Water on Land In the absence of large-scale glaciation and deglaciation, a possible mechanism for relatively rapid eustatic sea-level change could be changes in the mass of liquid water sequestered on the continents, both above and below the ground surface. Such a mecha- nism is needed to explain the apparently eustatic sea-level changes in a virtually ice-free Earth during Mesozoic and early Cenozoic time described in papers by Haq et al. (1987) and by Christie-Thick et al. (Chapter 71. The topic presented first is the possible variations in groundwater. A global climate change toward less precipitation will lower the water table in ground- water aquifers, transfer water from the land to the sea, and raise sea level. Less precipita- tion on land should result from atmospheric cooling, lower wind velocity, or changes in atmospheric circulation which would alter the balance between precipitation over the ocean and over the land. Increased precipitation resulting from atmospheric warming or other causes will raise the water table and lower sea level. Removal of fresh-water-bearing porous sediments chiefly sands and calcareous de- posits by erosion should have the same effect as a decline of precipitation, while accre- tion of such sediments will create a greater potential aquifer volume and hence be roughly equivalent to a rise in the water table. Land subsidence, which may now be occurring in many coastal areas, will reduce the volume of sedimentary aquifers above sea level, and thus have much the same effect as erosion of sediments or a decline in the level of the water table due to decreased precipi

12 OVERVIEW AND RECOMMENDATIONS ration. Emergence of previously submerged aquifers should have an opposite effect, especially in carbonate terrains where karst formation can occur. It is also possible that infiltration or discharge rates could change with time, leading to larger or smaller accumulations of groundwater as the balance between infiltration and discharge approaches a new equilibrium determined by changes in hydrostatic pressure in the aquifer. For example, canyon cutting during times of low sea level should increase discharge rates while deposition of unconsolidated coarse sediments should increase infil- tration rates. According to Hay and Leslie (Chapter 9) the late Cenozoic fluctuations in sea level caused by glaciation and deglaciation resulted in an offloading of sediments from coastal plain regions and continental shelves into the continental slopes and abyssal plains of the oceans. Thus the potential groundwater reservoir that exists today may well represent a minimum for much of geologic history. In past times, the potential change in sea level resulting from fluctuations in groundwater storage could have been double that which exists today. These statements may be roughly quantified by assuming an equivalent rise or fall of the water table by 100 m, 13.3 percent of the average height of continental surfaces above sea level. Hay and Leslie (Chapter 9) estimate that porosities are close to 40 percent in the upper layers of the sands and calcareous sediments resting on the continents in geosyn- clines (intracratonic basins), coastal plains, and cratonic platforms. They calculate that these two types of sediments make up 38 to 47 percent of the total deposits in these three sedimentary environments, and that they are the only sediments that take part in significant water exchange with the environment outside the aquifers. Table 2 shows the volumes and pore space of the top 100 m of sands and calcareous sediments in cratonic platforms, geosynclines, and coastal plains, computed from these estimates by Hay and Leslie and supplemented by estimates made by Southam and Hay ( 198 1 ) and by Ronov ( 19821. The total volume of pore space in the top 100 m of the continental sediments 2.5 x 106 km3 is equivalent to a rise or fall of sea level by 7 m. Hay and Leslie assume that the rates of filling or discharge in these coarse sediments would be less than 13.5 x 103 km3/yr. Thus more than 185 yr would be required to fill or empty an aquifer 100 m thick, corresponding to a rate of sea-level change of less than 4 mm/yr. Where only a slight imbalance exists between infiltration and discharge the times required for filling or emptying an aquifer 100 m thick could be tens to hundreds of thousands of years. For example, Meier (1984) TABLE 2 Aquifers and Pore Space in Top 100 Meters of Sediments on Land Volume of sandy and Volume of Area covered calcareous pore space by sandy or sediments in in top Area covered calcareous top 100 m 100 m of Sea-level by sediments sediments of aquifers aquifersa equivalent Location (lo6 km2) (percent) (106 km3) (106 km3) (meters) Cratonic platforms 55 47 2.6 1.0 2.9 Geosyclines 59 38 2.3 0.9 2.6 Coastal plains and shelves 31 47 1.4 0.6 1.7 Total 145 6.3 2.5 7.2 aAssuming 40 percent porosity

OVERVIEW AND RECOMMENDATIONS 13 estimates that global depletion of groundwater during this century has been between 1600 and 2400 km3/yr or 20 to 30 km3/yr. At this rate, filling or emptying a 100-m-thick global aquifer would take 85,000 to 130,000 yr and the corresponding rate of rise or fall of sea level would be less than 0.1 mm/yr. Baumgartner and Reichel (1975) and Woods (1984) estimate the global volume of groundwater at 8 x 106 km3, about 22 percent of the Earth's fresh water, equivalent to 22 m of sea level. This may be compared to 70 m of sea-level equivalent for the Greenland and Antarctic ice sheets. From data given by Hay and Leslie, assuming an average porosity of 20 percent for sandy and calcareous sediments, a pore volume in sediments has been computed at 64.8 x 106 km3. But SS.S x 106 km3 of this pore space is below sea level and hence presumably saturated with water. The pore space above sea level, which could be drained or filled by the various mechanisms discussed herein, is 9.4 x 106 km3, equivalent to 27 m of sea level, very close to the estimate of groundwater volume given by Baumgartner and Reichel. However, as Hay and Leslie suggest, the porosity of the 750 m of sediments above sea level may be 30 to 40 percent, corresponding to a pore volume of IS x 106 to 19 x 106 km3. Geological evidence shows that large quantities of shallow water carbonates and non- marine sands were laid down in mid-Paleozoic, late Paleozoic to early Mesozoic, and mid- Cretaceous times. During these periods, their abundance was probably more than twice that of the present time and the potential for groundwater storage was higher. Insofar as these porous sediments occupied a greater percentage of the land area than their present counterparts, infiltration rates must also have been higher than today, compared with rates of discharge, which can occur only at the edges of sedimentary columns. The mass of liquid water above ground in lakes and rivers is only a small fraction of the mass of groundwater. Residence times in rivers vary from less than a week to about a year depending on size and length and on the slope of the river bed. The volume of water stored in lakes is about 0.22 x 106 km3 (Robin, 1987), probably about 50 times the volume in rivers but only about 1 percent of the volume of groundwa- ter above sea level. Changes in lake volume result from climate variation and change and from human activities, primarily diversion of inflows for irrigation or other purposes. The Aral Sea in the Uzbek Republic of the Soviet Union is a striking example (Micklin, 1971~. This lake without an outlet, fed by the Amur Darya and Syr Darya rivers, was formerly the world's fourth largest lake, behind the Caspian Sea, Lake Superior, and Lake Victoria. In 1960 its area was 68,000 km2, its average depth was 16 m, and its volume was 1090 km3. Beginning in 1960, there was a large increase in diversions of the river flows for irrigation caused by expansion and intensification of the irrigated areas. These increased diversions were not compensated for by conservation measures as previously, and the lake began to shrink rapidly. By the beginning of 1970, the area had decreased by 40 percent, the volume had decreased by 66 percent, and the water depth had dropped to 9 m. This change in lake volume must have been accompanied by a eustatic rise in sea level of slightly less than 2 mm. Destruction of the lake is still occurring; without drastic changes in irrigation practices, it will have largely disappeared by the early part of the twenty-first century, and there will be a further eustatic rise in sea level of about 1 mm. On a worldwide basis, Robin (1987) estimates lake volumes are diminishing by 72 km3/ yr. He bases his estimates on the observed annual decline of the Caspian Sea by 10.9 km3, assuming that this decline in the Caspian, the world's largest lake, is IS percent of the global diminution of lake volumes. The corresponding eustatic rise in sea level should be 0.2 mm/yr or 20 mm/century. To this should probably be added the sea-level rise of about 1 mm due to the future decline of the Aral Sea. Robin (1987) also points out that a long-term warming of the atmosphere by 3°C at low latitudes and 6°C at higher latitudes should result in an increase of the atmospheric content of water vapor, and a corresponding fall in sea level of about 7 mm.

14 OVERVIEW AND RECOMMENDATIONS Changes in the Volume of Water Without Changes in Mass Steric height change (i.e., temperature and/or salinity variation) contributes to global sea-level fluctuations from year to year and decade to decade. It has been shown in regional studies that steric height changes account very well for sea-level variations (on the order of 100 mm) observed on seasonal and interannual time scales. On the other hand, sea-level fluctuations over periods of tens of thousands of years are so large that steric expansion can be ruled out as a major contributor. A warming of the entire ocean from 0°C to the currently observed temperatures would involve a thermal expansion of only about 10 m. The thermal expansion coefficient increases significantly both with increasing tempera- ture and with increasing pressure. The steep increase with increasing temperature has sometimes been used as an argument that deep cold water may be ignored in steric height calculations relative to warm surface water. As Roemmich (Chapter 13) shows, this is not so, because a parcel of water at 4°C at about 2000 m depth has a thermal expansion coefficient 60 percent as large as that of a parcel at the ocean surface at 20°C. We need to discover how much variability in steric height is contained in time scales of decades or longer and to determine what vertical and horizontal scales of variability correspond to these long-term changes. These essential questions must be answered first before we can ask how long the ocean must be sampled at a single location, how many locations must be sampled, and to what depths the sampling should extend to determine the relationship of steric height to the long-term trend in sea level. Trends in steric height over three decades in the central north Pacific appear to be about 1 mm/yr, but these trends are so small in comparison with the large interannual variations of 10 to 100 mm that a much longer time series is needed to confirm the trend (Thomson and Tabata, 1987~. On time scales of tens to hundreds of years, the effects of salinity changes on global sea level are slight. The total salt content of the oceans remains approximately constant, and sea-level changes due to vertical redistribution of salt in the water column are likely to be small. Heat, unlike salt, is freely exchanged with the atmosphere, and even the combined heat content of the ocean-atmosphere system may change significantly over a few decades or centuries. Thus global expansion or contraction of ocean waters must be dominated by temperature change rather than by salinity change except under some circumstances in high latitudes and semi-isolated basins. In general, there is a fairly tight temperature/ salinity correlation; a temperature increase of 1°C or above in the thermocline can be ac- companied by a salinity increase of approximately 0.1 parts per thousand. This tempera- ture change produces an increase in specific volume whose magnitude is more than three times the corresponding decrease in specific volume caused by the salinity change. Thus the effects are of opposite sign, and although salinity is not negligible, the temperature effects dominate. Steric changes in the water column on time scales of a decade and longer are not confined to or concentrated in the upper ocean. In the subtropical North Atlantic such changes extend to depths of at least 3000 m (Chapter 13~. For observational programs the large vertical extent of the signal means that the entire depth of the ocean should be sampled. In the upper water layers, both vertical diffusion and advection are important, as are water mass transport along quasi-horizontal isopycnal (constant density) surfaces from subsurface regions in higher latitudes toward the mid-latitude ocean interior. Critical processes are air-sea interactions in the regions where the isopycnals intercept the sea surface, and such cross-pycnal mixing processes as double diffusion (diffusion of heat and salinity at different molecular diffusion rates) and intermittent internal wave breaking. In the deeper layers (below the permanent thermocline in the North Atlantic) we are con- cerned with the formation of North Atlantic deep water by convective mixing and sinking in the Greenland and Norwegian seas and by horizontal advection and mixing as this water mass moves southward. The critical problems are the mechanisms and rates by which

OVERVIEW AND RECOMMENDATIONS  FIGURE 2 Volume estimates for six tectonic processes affecting the volume of the ocean basins. An increase in volume as time progresses produces a sea-level fall (or freeboard increase). See Harrison (Chapter 8) for additional detail. 15 waters of lower-density can gradually replace higher density water in the deep ocean basins. Roemmich (Chapter 13) shows that at mid-latitudes in the North Atlantic the th- ermocline fluctuations in steric height exhibit greater variance in the time domain than the deeper changes and also more variability around the subtropical gyro. For the 27 yr of Panuliris hydrographic data taken by the Bermuda Biological Station (discussed by Roem- mich, Chapter 13), the thermocline fluctuations make a greater contribution to steric height change than the deep changes, but on a longer time scale, it is not clear whether mid-depth or deep variations would dominate. The therrnocline comes to the surface along the northern rim of the subtropical gyre, and these waters are therefore subject to atmospheric forcing relatively near the Panuliris area. The deep water is farther removed from contact with the atmosphere, which can account for the more gradual changes and larger spatial scale of the deep signal. Volume of Ocean Basins Changes in the volume of the ocean basins can have very significant long-term effects on sea level~ffects that can be as great as or greater than that caused by ice. The dominant cause of these large-scale changes is the variable volume (Figure 2) of ocean ridge material from seafloor spreading processes. If spreading increases, the volume of the ridge crest increases, displacing water and flooding the continental areas. The critical quantity is the area of seafloor produced per unit time, which can be changed either by an increase in spreading rate over a ridge crest of constant length, or by increasing the length of the ridge crest, or by a combination of the two. Harrison (Chapter 8) shows that during the past 80 million yr (m.y.) a diminution in the volume of ocean-ridge crests by 9.55 x 10~6 m3 could have produced an increase in freeboard (essentially equal to a fall in sea level) of about 180 m, with a very large uncertainty of +140 and-180 m. The continents themselves can undergo vertical motion termed epeirogeny. Evidence shows that epeirogenic movements of the continents can result in several hundreds of meters of elevation over very long time periods. Similar elevation changes occurring in ocean basins can significantly affect the volume of the ocean basin and, thus, sea level. For example, there is evidence for a thermally induced swelling or uplift in the equatorial Pacific during the Cretaceous, which was accompanied by large amounts of extrusive IC 6t IE 2 o -2  0 10 20 30 40 50 AGE, MYBP /4 RIDGE CREST / VOLUME /\ CONTINENTAL ICE PACIFIC VOLCANISM CONTINENTAL COLLISION OCEAN COOLING -------art -- O 60 70 80 A/ O~1 1~TAI AG.'

16 OVERVIEW AND RECOMMENDATIONS volcanic activity. A decrease of the volume of the ocean basins of about 2 X 10~6 ma was produced by this "ocean-floor epeirogeny" between 110 m.y. ago (Ma) and 70 Ma. Many other possibilities exist for changing sea level by tens to hundreds of meters. For instance, a change of the pattern of subduction can cause the ocean basins to become, on average, younger or older and so produce a sea-level change that is in principle similar to that caused by varying the amount of seafloor produced per unit time interval. If the area of the ocean basin changes due to continental growth or continental destruction (e.g., India colliding with Asia and decreasing the continental surface), a change in ocean volume will also occur. Harrison (Chapter 8) suggests three processes bY which the auantitv or distribution of · A, · __ ~1 ~ sediments in the deep sea could be changed: ( 1 ) the evolution of pelagic foraminifera in the Late Cretaceous, (2) the greater average age of the ocean basins today than 80 Ma, and (3) higher sedimentation rates during the past ~ m.y. than earlier times. The last two processes would tend to counteract about one-eighth of the effect of greater ridge areal volume in earlier times compared with today. The evolution of pelagic foraminifera may not have changed the rates of deposition of calcareous deposits, which depend almost entirely on the rate of weathering of calc-silicates and volcanic emission of CO2, but it should have resulted in a redistribution of these sediments from mainly shallow-water reef deposits to a more or less uniform blanket over the deep-sea floor. This would have had a significant isostatic effect, replacing marked subsidence of shallow water areas and probably a forebulge under both adjacent land areas and the offshore deep ocean, with a relatively uniform sinking of the deep-sea floor and a rise under the continents. Harrison (Chapter 8) calculates that the total increase in the average freeboard of the continents from all causes over the past 80 m.y., not counting the possible effects of redistribution of calcareous sedimentation, should have been about 260 m. But estimates based on paleogeographic sediment distribution for the past 80 m.y. indicate a much smaller value 150 m. He suggests that the discrepancies could be resolved if the continental hypsometric curves were steeper in the past than they are today. However, the uncertainties of the estimates of the various volume effects are sufficient to account for this freeboard discrepancy. RECORD OF CURRENT AND PAST CHANGE Sea level has changed dramatically over the course of earth history, with repeated variations of more than a 100 m from present sea level. These large sea-level changes occurred both during times of intense glaciation and during times of an ice-free Earth. Even during times of glacio-eustatic change, there were other broad-scale variations resulting from long-term climatic and tectonic changes. The details of sea-level variations are well documented for much of the record of the past few hundred thousand years but gradually deteriorates with older records, mainly because the time resolutions of dating and correlation methods become less precise. In addition, the numbers of assumptions needed to extract sea-level change from proxy records increase with age. Historic and Tide-Gauge Records-The Past 100 Years < - Large sections of the world's coastlines are experiencing changes in RSL that have been analyzed as being spatially and temporally coherent, but there is no unique way to average the existing sea-level data from different regions to obtain a global measure of RSL change. variations on the order of flu percent in the estimate of relative change can be induced simply by use of different averaging methods. Barnett (Chapter 1J, using both a key tide-gauge station approach and a grouping of records from different regions, has estimated that RSL has increased by 1 to 2 mm/yr over ~ r

OVERVIEW AND RECOMMENDATIONS 20 iOL -10~ 1 1 1 1 1 ~ 1 1 1 1 REGION I EUROPE/AFR I CA -20 1 1 1 1 1 1900 1920 1940 1960 2CI (l) IC LLJ I1J it REGION 3 EAST COAST NORTH AMER I CA ·-. . e ~ - ee e _ O -10 _ -MU 1 1 1 1 1 1 1 1 1 1 1 ~1 1 1 1 1900 1920 1940 1960 2 1 1 _ -10 _ 1 ^~ MU _ 10 _ O _ -10 1 -20 1 980 20 10  1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -20 1900 1920 1940 1960 1.980 1 REG I ON 5 INDO PACI FIC -10 _ -on l l l 17 I 1 1 1 REGION 2 WEST COAST NORT H AME R I CA e -mu _ L I 1980 1900 1920 1940 1960 1 980 ~I I I I I 1- T I I I I I I REGION 4 EAST COAST ASIA e e . eee 1900 1920 1940 1960 1980 I I 1 1 1 1 1 1 1 1 1 1 1 REGION 6 WEST COAST CENTRAL SOUTH AMERICA - 1 1 ~ 1 1 1 1 ~ 1 1 1 1 1 1 1 1900 ' 1920 1940 1960 1980 FIGURE 3 Regional averages of annual sea-level anomaly (see Chapter 1, for further discussion). From Barnett (1984) with permission of the American Geophysical Union. the past 80 yr or so (Figure 31. There is no strongly coherent change on a global scale (coherence is primarily regional), nor is there an obviously accelerating rate of RSL . . increase in recent years. Interpretation of sea-level records from tide-gauge measurements faces many problems. The principal problems are summarized below: 1. Station locations have been subject to position changes as, for example, during harbor development, which can result in discontinuity of the time-series record. 2. Tectonic uplift/subsidence of the land mass on which the tide gauge sits will induce

18 OVERVIEW AND RECOMMENDATIONS an RSL change. These vertical tectonic movements are further discussed in a previous section and in the discussion by Pettier (Chapter 4~. 3. A variety of physical processes that include changes in water temperature and salinity, river runoff, local and nonlocal currents, winds, waves, and atmospheric pressure all can affect RSL, sometimes in conflicting ways. Thompson (Chapter 2) found that the standard error of an estimate of longer term trend in North Atlantic sea-level records can be halved (from 0.4 to 0.2 mm/yr) on removal of short-term meteorological and oceano- graphic effects. Munk et al. (Chapter 14) discuss subsidiary measurements that can be used to take account of some of the oceanographic effects. Sturges (Chapter 3) finds that the dominant sea-level signals at periods of 5 to 10 yr and longer are coherent between the U.S. Pacific coast and Hawaii, and on both sides of the Atlantic. The amplitudes, at these periods, are ~50 to 150 mm. The signals are out of phase, with delays of several years, but are partly consistent with baroclinic wave propa- gation across the ocean. The longest records appear to be "contaminated" by this ocean- wave propagation. A wave of 10-yr period and 50-mm amplitude has a rate of rise of the order of 20 mm/yr, which is about an order of magnitude larger tin Barnett's estimate of the mean rate of sea-level rise during the past 80 yr. Consequently, records of short duration, even if they have high precision, can hide the existence of long-term variability or change. The Past 18,000 Years Eighteen thousand years before present, huge continental ice sheets covered most of Canada, parts of the northern United States, Greenland, northwestern Europe and Antarc- tica, and sea level was more than 100 m lower than it is today; this is known as the Weichsel-Wisconsin glacial maximum (see discussion by Matthews, Chapter 5~. The ice sheets stored the water that corresponded to nearly all of this lowering of global sea level. The mass of the ice sheets also deformed the shape of the Earth, and affected the volume of the ocean basins. The ice sheets in North America and Europe began to dissipate by 15,000 yrBP. The meltwater was returned to the ocean basins with a concomitant rise in sea level. The causes of ice-sheet disintegration are not clearly understood, but cyclical Earth-orbital variations in the inclination of the Earth's axis and in the seasonal Earth-Sun distance (COHMAP, 1988) together with ice-sheet dynamics and changes in ocean circulation probably formed a complex set of interrelated forcing functions that initiated breakup and continued wast- age of the ice sheets between 15,000 and 7000 yrBP. The evidence for past sea levels is partly geological the existence of coral reef terraces containing the littoral zone coral, A. palmata; submerged or buried strand-line complexes; peat deposits just above subaerial exposure surfaces; and articulated valves of shallow water or tidal zone pelecypods. Evidence of this kind includes a series of paired pelecypod valves off the Texas gulf coast at-57 m with a carbon-14 date of 12,900 + 400 yrBP and a brackish water peat from New England at almost the same depth and age. Similar data are found at younger ages, but the only high-quality data between about 12,500 and 18,000 yrBP are the deep-sea oxygen-isotope records in planktonic and benthic foraminifera of pelagic sediments. The oxygen-isotope ratios reflect both the quantity of light oxygen sequestered in the ice sheets and the temperature of the waters in which the foraminifera lived. The relative importance of these two effects is uncertain. Matthews shows that this uncertainty gives a range of 120 to 108 m for the maximum sea-level lowering in the tropical North Atlantic near Barbados. Chappell and Shackleton (1986) estimate a value of-130 m for the Huon Peninsula in New Guinea, and they cite a maximum sea-level lowering estimate of 150 m on the broad shelf off northern Australia. They state that such

OVERVIEW AND RECOMMENDATIONS 19 variations around the globe "are to be expected as a result of adjustment to glacial/ interglacial change of ice and ocean volumes." During most of the time between 15,000 and 7000 yrBP, the rate of dissipation of the ice caps was very rapid, and the average rate of rise in sea level was 12.5 mm/yr. But there were two brief periods of renewed cold in the European, though not the American, record at about 11,000 to 12,000 yrBP. These are called the Older and Younger Dryas. Pollen records from European bogs show that the forests that had begun to repopulate northern Europe after the ice departed were destroyed and replaced by Arctic shrubs. These colder intervals lasted for several hundred years. They may have resulted from a reduction in the temperature of the near-surface water lying to the west of Europe (Broecker, 1987~. By 7000 to 8000 yrBP the North American and European ice sheets had disappeared. In high northern latitudes, RSL was 10 to 30 m higher than it is today because of the isostatic depression of the land under the previous weight of ice. In England and southern New England and in lower latitudes, the isostatic "forebulge" had produced an RSL 10 to 20 m lower than today's sea level. Bloom and Yonekura (Chapter 6) examined 13 well-documented]Iolocene sea-level records from published literature that covers the past 8000 radiocarbon years and a range of latitudes from the Arctic to Panama. In the north, RSL declined exponentially at initial rates of 210 mm/yr and a final rate during the past 1000 yr of the order of 1 or 2 mm/yr. South of the "hinge line" in central New England, RSL rose at an exponentially declining rate. Interestingly enough, the amount of emergence or submergence for each 1000-yr interval at any particular place is proportional to the total amount of emergence or submer- gence at that place over the past 8000 yr. This would imply that there has been no eustatic change in sea level during the later Holocene, only isostatic adjustment-upward RSL rise in the south and downward RSL fall in the north, which are strictly proportional to each other. If so, in the absence of climatic change, future sea-level change could be controlled by residual isostatic response to the loading and unloading completed 7000 to 8000 yrBP. This suggestion is consistent with Peltier's model of isostatic adjustment in the mantle (see Chapter 4) and with a recent discussion by Stewart (1989~. It is given further credibility by Barnett's (1984) observation that sea level is falling on the east coast of Asia, where presumably different isostatic adjustments are taking place. The Past 250,000 Years The record of sea-level change for this time period can be interpreted from two principal proxy sources: (l) oxygen isotope records in sediment cores from the deep sea and (2) the study of ancient shorelines (coral reefs) that have been tectonically uplifted. Although each type of record presents several interpretive problems and requires assumptions in deriving sea level, a recent correlation of an oxygen isotope record and a flight of raised coral ter- races in the southwest Pacific has provided improved estimates of sea level (Table 3 and Figure 4) (Chappell and Shackleton, 1986~. Coral terraces from the Huon Peninsula (Papua New Guinea) are described by Bloom and Yonekura in Chapter 6. These uplifted terraces are like a continuous tape recorder; each coral reef developed when the rising sea level overtook the rising land, and the reef crests represent approximately the peaks of each sea-level rise. RSL can be estimated from the current heights of the terraces. It has been assumed that sea level at ~125,000 yrBP was +6 m above the present sea level- a number derived from several different terrace se- quences around the world. On the basis of this assumption, the uplift rates for different portions of the Huon Peninsula can be derived. Further from the assumption that these uplift rates have remained essentially constant for the past 125,000 yr, sea level can be calculated (Figure 4) for the other Huon Peninsula terraces as described in Chapter 6. Oxygen isotope ratios, 6~80 [6 = (~80/~60 · 18O/160SMOW) _ 1, where SMOW is standard

20 OVERVIEW AND RECOMMENDATIONS TABLE 3 Sea Level and Calculated Rates of Average Change for the Past 135,000 Years Agea Sea (1000 Levela Rate Stage yrBP) (m, Whoa Change (mm/yr) Modern 0 0 3.42 6 ob 3 47 I/II II II/IIIb IIIb 17 -130 5.09 29 -46 4.48 35 -65 4.67 40 -41 4.47 IIIb/a 42 -52 4.66 IIIa 44 -45 4.51 IVb IVb/a IVa IVa/Vb Vb Vb/a Va 53 -30 4.30 56 - 4 4.58 59 -28 4.20 64 -61 4.66 72 -36 4.21 74 - 4 4.33 81 -19 3.99 ValVIb 88 -44 4.37 VIb 96 -26 4.09 VIa 106 -19 4.03 VIa/VIIb 112 -62 4.38 VIIb 118 0 3.49 VIIb/a VIIa VII/VIII 122-8 124+6 3.37 135-130 5.20 +118 +10.7 -84 -7.0 +20 +3.3 -24 -4.8 +11 +55 -7 -3.5 (-15) (-2.5) +14 +4.7 -16 -5~3 +33 +6.6  -25 -3.1 +8 +4.0 -25 -3.6 +25 +3.6 -18 -2.2 (+7) (+0.7) +43 +7.2 -62 -10.3 +8 +2.0 -14 -7.0 +136 +12.4 aData from Chappell and Shackleton (1986). bSea level at 6000 yrBP has been within a meter or so with fluctuations arising from isostatic and tectonic adjustments (Bloom and Yonekura, Chapter 61.

OVERVIEW AND RECOMMENDATIONS  FIGURE 4 (A) 6~80 record for the past 240,000 yr from east equatorial Pacific core V19-30; and (B) sea-level curve for the Huon Peninsula recalculated to co~Te- late with the 6'8O record from core V19- 30. Modified after Chappell and Shackle- ton (1986). 21 mean ocean water], of foraminifera vary with the temperature, salinity, and 680 of the water from which their carbonate tests are deposited. Ocean water 6~80 varies with the quantity of isotonically light ice stored on the continents so that the record from foraminif- era is a blend of global ice volume and local water temperature and salinity components. For the late Pleistocene, temperature and salinity components of the signal are demonstra- bly small compared to the ice volume signal. A 6~80 record from the east equatorial Pacific (core V19-30; Shackleton et al., 1983) is shown in Figure 4. Peaks represent the influx of isotonically light water derived from continental ice and are correlated with high stands of sea level indicated by the Huon Peninsula terraces. If these sea-level change magnitudes and timing are representative of global sea level, then rates of eustatic sea-level change during the period from 25O,000 yrBP to about 6000 yrBP varied between -10 mm/yr and +12 mm/yr, with typical rates being on the order of +3 to +7 mm/yr. These rates exceed those that have prevailed over the past few thousand years but may be approached by those projected for the next millennium or so (see section on Forecasting Changes in Sea Level Related to Greenhouse Gases). At the peak of glaciation 18,000 and 135,000 yrBP, sea level near New Guinea was lower than it is today by about 130 m (Table 34. But between these extremes, during glacial episodes, sea level oscillated between 19 and 65 m below the interglacial levels over time intervals of 2000 to 12,000 yr. These oscillations resulted from variations in the area and thickness of both the continental ice sheets in northern Europe and northern North America and the Antarctic Ice Sheet. As shown by Labeyrie et al. (1986), at least some intervals of rising sea level may have been initiated by surges of ice streams behind ice shelves in Antarctica, which greatly increased the rates of iceberg calving. The resulting rise of a few meters in global sea level may have then destabilized the marine portions of the Northern Hemisphere ice sheets, leading to the much larger rise in sea level shown in the oxygen isotope and Huon Peninsula terrace records. The Past 250 Million Years The sea-level record becomes much more cloudy as we look back in time. Much of the past 250 m.y. was dominated by an ice-free Earth or at least a world lacking extensive continental ice caps (see Frakes and Francis, 1988~. Oxygen isotope ratios of tropical planktonic foraminifera show that continental ice volumes during only the past 40 to 45 m.y. were consistently as large as or larger than that of today (Prentice and Matthews, 1988~. Between 50 and 65 Ma the world was intermit ~ 4.0 o 5.0 o 1 _ -50 a) a, J `~5 -1 00 cn -150 ~ AA it. ~ I 1 1 1 1 l l ~,h//~\J  1~ l l ~ , ha" ? 1 1 1 1 1 1 1 1 1 1 ,W, /^ \ 40 80 120 160 200 240 Age (kyr)

22 OVERVIEW AND RECOMMENDATIONS tently ice free; and continental ice was apparently absent throughout the Mesozoic (65 to 250 Ma). Consequently, sea level was considerably higher than in later times, and vast continental areas on the order of 50 million km2 (one third of present land area) were flooded, e.g., the Cretaceous Interior Seaway in North America. Sea-level changes were dominated by three major forcing factors: climatic change, tectonic processes, and sedimentation. Climate was much warmer than it is at present, with ocean-bottom temperatures at some 10 to 15°C. Thermal contraction of seawater (steric effect) could account for a sea-level fall of more than 10 m between the climatically warm and equable Cretaceous (144 to 66 Ma) and the onset of an ice-dominated system in the Cenozoic. Tectonic processes such as seafloor spreading, subduction of oceanic crust, broad-scale ocean-floor epeirogeny, continental collisions, and other activity affected the volume of the ocean basins. Epeirogenic uplift or subsidence of the continents also affected RSL. The volume of the ocean basins was also changed by changes in sedimen- tation. The evolution of the calcareous algae called Coccolithophoridae at the end of the Jurassic and of pelagic foraminifera in the Middle Cretaceous resulted in deposition, perhaps for the first time, of a thick blanket of calcareous sediments over the deep seafloor and in a possible rise in sea level. Direct evidence for sea level at different times during the past 250 m.y. comes from paleogeographic maps, from which the area of the continents covered by marine sediments can be estimated. Sea level was highest, about 180 + 40 m above the present level, at 100 Ma, having risen from 80 m about 180 Ma. There was also a pronounced regression of sea level (commonly associated with a fall) during the Paleocene at 60 Ma. For the past 80 m.y., there is evidence concerning changes in the volume of the ocean basins, particularly in the volume of ancient mid-ocean ridges, based on estimates of spreading rates between times of magnetic reversals and estimates of ridge lengths. From the data analyzed by Kominz (1984), ridge-crest volume changes since 80 Ma would have resulted in a fall of sea level of 180 + ~ 150 m. If ice volume changes, the effects of Pacific volcanism, and changes in sedimentation are added, the calculated sea level fall since 80 Ma is about 230 m, some 40 percent greater than that computed from modern paleogeogra- phic maps. Although the errors in both sets of estimates are more than sufficient to account for the discrepancy, it is possible, as Harrison has suggested (Chapter 8), that the continen- tal hypsographic curves were steeper in the past than they are today, so that a greater change of freeboard was necessary to produce a given increase in the areas of flooding. Little or no oceanic crust older than about 80 m.y. still exists in the present oceans; hence there is little direct evidence concerning the volume of the ocean basins. We must rely on paleogeographic reconstructions of the continents and analysis of their sediment covers to infer ancient sea levels. Within the past dozen or so years, seismic stratigraphic methods have been developed and applied to the analysis of scores of events of coastal onlap/offlap within stratigraphic successions. P. R. Vail and his colleagues, e.g., Haq et al. (1987), have ascribed most of the boundaries between offlapping and onlapping stratal units to sea-level change (Figure 5) and have implied that the vast majority reflect eustatic or global sea-level variation. Although this approach has proven valuable in exploration for hydrocarbon resources, many investigators question whether these boundaries are necessarily of eustatic origin. Some boundaries may be caused by variations in the local rate of subsidence or by changes in the rate of sediment supply. Vail and his co-workers have estimated magnitudes of sea- level change that are on the order of 10 to 100 m and are superimposed on the long-term trends from climatic and tectonic processes discussed above, but questions also remain about whether such large variations are indicated by the data in hand, even if the signal is a eustatic one. It is particularly difficult to explain sea-level changes in excess of 100 m in intervals of 1 m.y. or less during the Cretaceous, a time when the Earth was largely ice free. These and related issues are discussed by Christie-Thick et al. in Chapter 7. Unconformity-bounded sequences, which form largely in response to changes in de

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24 O VER VIEW AND RECOMMENDA TI ONS positional base level, can be identified in outcrops and boreholes as well as in seismic profiles. Unlike transgressions and regressions of the shoreline or changes in paleobathymetry, which are used in classical stratigraphy to gauge sea-level fluctuations, the formation of regional unconformities is relatively insensitive to the rate of sediment supply, and this constitutes the main advantage of the sequence stratigraphic approach. According to Christie-Thick et al. (Chapter 7) and Christie-Thick (1989) most sequence boundaries record times at which the rate of sea-level fall increased (or reached a maxi- mum) or the rate of tectonic subsidence decreased. Those of eustatic origin should be formed in all basins connected to the open ocean and should be nearly correlative, what- ever the local tectonic history. Unconformities of tectonic origin may also be present, but these are not expected to extend beyond a region more than a few hundred or a few thousand kilometers across. The identification of a global sea-level signal therefore depends on demonstrating that particular unconformities are present in widely separated basins, and are synchronous (ideally to within 0.5 m.y.~. More difficult than establishing the timing of sea-level change is the problem of estimating amplitudes and rates of change. Amplitudes of sea-level oscillations may be estimated through a combination of sequence stratigraphy and geophysical modeling of subsidence history, but practical difficulties and sensitivity of results to model assumptions may limit estimates to no better than a factor of 2 or 3 larger or smaller than true amplitudes (Christie-Brick et al., Chapter 71. The principal uncertainties are in age control, paleobathymetry, compaction history, the effects of sediment loading on the lithosphere, and the tectonic subsidence that must be subtracted from corrected strati~ra- phic data to obtain the sea-level signal. A, Harrison (Chapter 8) has suggested that 10 transgressive-regressive cycles of sedimen- tation with average periods of 7 to 10 m.y. during the Cretaceous resulted from alternating increases and decreases in the rates of seafloor spreading. These would have caused eustatic rises and falls of sea level of about 15 m. The eustatic sea-level changes were greatly amplified in the Western Interior Seaway by subsidence of the basin resulting from subduction of the Pacific plate under the western part of the North American continent. In Chapter 9, Hay and Leslie discuss possible changes in the volume of liquid water stored as groundwater as a cause of some of these Cretaceous sea-level changes. These and many other problems reviewed in Chapters 7 and 8 need to be solved before these curves can be read as eustatic changes. If stratigraphic sequence boundaries do turn out to be related to eustatic sea-level change, the record from the Triassic (about 250 Ma) through to the present becomes rich with events that need to be considered in the light of possible causal processes. From 250 to 2500 Ma Although there is some direct stratigraphic record of sea-level change that extends back into the Paleozoic and Precambrian (Christie-Brick et al., 1988; Bond et al., 1988), most of the data are derived from continental freeboard consideration. Geologic evidence on the area of the continents covered with marine sediments indicates that continental freeboard, the average elevation of the continents above sea level, has been approximately constant since the Archean, 2500 Ma. Continental freeboard today, taking the mean of the conti- nents, is about 750 m, and changes of more than a few hundred meters are apparently ruled out by the geologic evidence. Provided that the mass of seawater (plus water on land) has not varied significantly, an approximately constant continental freeboard since the Archean, combined with a 50 percent decrease of heat flow from the mantle, has been shown by Schubert and Reymer (1985) to require a net growth of the continents of about 25 percent (1 km3/yr) over the past 2500 m.y. They believe that post-Archean changes in the mean thickness of the continen

OVERVIEW AND RECOMMENDATIONS 25 tat crust are highly unlikely, hence the continents must have grown in area. If freeboard has been within +200 m of its present value since the end of the Archean, the continents have grown in area by anywhere from 10 to 40 percent. On the other hand, if the mass of seawater has increased significantly since the Archean, isostasy requires that the thickness of the continental crust should have likewise increased. For example, if the flux of water to the ocean from the Earth's interior has been constant during the Earth's lifetime, the entire growth of continental volume estimated by Schubert and Reymer can be accounted for by an increase in continental thickness rather than continental area. Continental growth is required as long as freeboard at the end of the Archean was not more than 400 m smaller than it is today. Continental ice ages, which must have caused a fall in sea level, occurred at intervals during the Proterozoic and the Paleozoic. According to Crowell (1982), evidence from what is now southern Ontario shows that there were large-scale glaciations between 2500 and 2100 Ma. One ice sheet may have reached southern Wyoming. A similar glaciation probably occurred in South Africa sometime between 2700 and 2200 Ma, and possible tillites are reported from Western Australia. During the span of 600 m.y. between 2700 and 2100 Ma, a period as long as the entire time in which multicelled animals and plants have existed, several widespread glaciations, and presumably concomitant falls in sea level, apparently occurred, separated by ice-free eras. There is no record of glaciation anywhere on Earth during the ensuing 1100 m.y. until the Late Precambrian, when continental glaciation flourished intermittently on all the continents, except possibly Antarctica, from about 950 to 560 Ma. Apparently there were three glaciation peaks, one about 940 Ma, another around 770 Ma, and a third about 615 Ma, in regions around the present North Atlantic, in central and southwestern Africa, and in Brazil, western North America, and Australia. Each of these extreme glacial times must have been a period of markedly lower sea level in comparison with earlier or later periods. These Late Precambrian glaciations apparently occurred at low latitudes, distant from the Earth's poles, for which no satisfactory explanation has come forth. Perhaps, there was a reduced greenhouse effect or the patterns of oceanic and atmospheric circulation were markedly different during Precambrian times from those of today, or of the past half billion years. The next widespread glacial epoch began in the supercontinent Gondwana near the end of the Ordovician lasting into the Silurian, from about 450 to 400 Ma. The record extends in scattered localities from northern Europe to South Africa and from the Sahara region to Bolivia and Peru. For 90 m.y., from 330 Ma to 240 Ma, a Late Paleozoic ice age existed in large areas of the Gondwana supercontinent, beginning in what is now South America, reaching a climax in South America, Africa, and Antarctica, and ending in Australia. There were apparently several moderately sized ice caps instead of one huge one, because it does not appear that sea level was drastically lowered. The center of the glaciated regions was near the South Pole, as the supercontinent of Gondwana drifted across it. There was no continental glaciation between Late Permian and Paleogene time, even though Antarctica lay near the South Pole. This may have been the result of a greenhouse effect caused by a high atmospheric CO2 content, related to high rates of seafloor spreading and undersea volcanism (Berner et al., 19831. DO CHANGES IN SEA LEVEL CAUSE CHANGES IN CLIMATE? Climate variability and change cause local or regional variations in sea level over months, years, or decades. Quantitatively the seasonal oscillation is the most important of these, followed by interannual variations of 5 to 10 yr, related to the Southern Oscillation in atmospheric pressure between the two sides of the Pacific Ocean, and manifested in E1 Nino in the eastern Pacific and in many other phenomena elsewhere.

26 OVERVIEW AND RECOMMENDATIONS Longer-term global climatic changes can be reflected in eustatic sea-level change through three processes: the waxing and waning of continental ice caps and alpine glaciers; variations in the quantity of liquid water stored on land in lakes, rivers, and underground aquifers; and steric changes in the volume of seawater resulting from warming or cooling. It is also often suggested that a change in sea level can produce a change in climate. As Barron and Thompson (Chapter 11) point out, this suggestion is consistent with the apparent correlation of climate and sea level over geologic time. Several lines of evidence show that during the past 70 m.y. globally averaged surface temperatures have declined by 6° to 12°C, global sea level has fallen by perhaps 200 m, and the total land area above sea level has increased by about one-third. There is a variety of physical mechanisms by which sea-level changes could directly affect global climate: changes in albedo, regional changes in atmosphere-surface coupling, changes in ocean circulation, changes at ice-sh~.~.t ocean marring ~nr1 rho in Not_ mosphere chemical composition. ~ =~ A ~ A ~ ~ ~ _ _ A ^ ~ _ ~^ ~ ^ ~ ~4/ ~ ~ Virtually every aspect of the Earth's climate is affected by the exchange of heat, moisture, and momentum between the atmosphere and the underlying surface. Changes in the surface energy balance are determined by changes in surface albedo and surface wetness. A rise of sea level will increase the area covered by ocean water, which has a much lower albedo and a much higher wetness than the land. A change in global albedo by 0.01 will produce about a 1°C change in surface temperature. Such an albedo change would require a major change in the land/sea ratio, caused by a rise or fall of 100 m or more in sea level. The change in wetness resulting from this assumed change in relative areas of sea and land would lead to marked changes in evaporation, and therefore in precipitation, although not necessarily in the same region, and also to a significant change in average summer surface temperatures because of the much larger thermal inertia of the ocean compared to the land. However, model calculations indicate that when both surface albedo and mois- ture availability are altered simultaneously, they produce nearly complete compensating effects. For example, modeled deforestation of the Amazon Basin, which would produce the same effects on albedo and wetness as a fall in sea level, produced only a small net surface temperature change in the deforested area and no rletect~hl~. aloha ~.~1~. climate effects (Henderson-Sellers and Gornitz, 1985~. In general, surface roughness is an order of magnitude higher over the land than over the ocean. ConsequentlY a chance in ocean/land proportions caused bv a rise or fall of ~n _ =~ ~_ ~ A ~ ~ ~ ~ ~ _ ~1 1 ~ ~ ~ ~ . _ ~ _ . . _ . ~ _ _ _ level may result In a marked redistribution of the surface areas of momentum exchange between the land and ocean and the atmosphere. This could have a marked effect on atmospheric circulation systems. In some regions, for example, the Blake Plateau in the Western North Atlantic, it can be shown from studies of seafloor erosion that the position of the Gulf Stream has varied repeatedly by hundreds of kilometers with changes of sea level during the past 20 m.y. But the extent of possible climatic change caused by such movements in current position is unknown. The same lack of knowledge about climatic effects afflicts examples of seafloor subsidence such as that of the Greenland-Iceland-Faroe Ridge between the Arctic and the Atlantic oceans, and the Walvis Ridge off South Africa. Sea-level change can also isolate or reconnect small basins along the ocean margins. Enhanced evaporation in partially isolated basins can produce water masses of markedly different density than those of the main ocean, and thereby affect deep water and mid-water formation. A marked increase in atmospheric CO2 accompanied the deglaciation of the Northern Hemisphere ice sheets about 10,000 yrBP; it probably had a significant warming effect on the lower atmosphere. One hypothesis to explain at least part of this increase in atmos- pheric CO2 involves the submergence of the continental shelves by the rise in sea level. In tropical waters this resulted in a large increase in the growth of coral reefs and precipitation of other calcareous sediments with a corresponding release of CO2 to the subsurface ocean layers and the atmosphere.

OVERVIEW AND RECOMMENDATIONS 27 If bottom water temperatures were unchanged, the fall of the sea level by 20 to 130 m during the last glaciation should also have resulted in the release of methane (a potent greenhouse gas) from methane ices (clathrates) in the upper layers of continental slope sediments. These sediments are believed to contain 2.2 mg/cm3 of methane, or a total of 3400 gigatons of methane (Revelle, 1983a). A fall of 100 m in sea level without a change in ocean-bottom temperature should release 425 gigatons of methane. If the sea-level change took 5000 yr, 0.085 gigatons would have been released each year. Methane in the atmosphere has a half life of about 8 yr, implying that the release of methane from continental slope sediments should have increased the preindustrial methane level of less than 2 gigatons by about 50 percent. In fact, evidence from ice cores indicates that the methane content of the atmosphere decreased by nearly 50 percent during the latter half of the last glacial epoch and was at no time higher than the postglacial, preindustrial level of 650 parts per billion (Stauffer, 1988~. This information suggests that during the glacial period the temperature of the ocean waters bathing the continental slope sediments de- creased by more than 1°C than it is at present. This would prevent destabilization of the sedimentary clathrates by the release of pressure that resulted from the drop in sea level. As far as the apparent correlation between the 70-m.y. fall in eustatic sea level and the drop in temperature over the same period are concerned, it now seems most reasonable to ascribe both phenomena to the same set of processes within the Earth and not to attempt to forge a causal relation between them. The high sea levels of late Cretaceous time were most probably caused by intense oceanic lithosphere formation and seafloor spreading from the mid-ocean ridges (see Harrison, Chapter 8~. This same process resulted in perhaps a tenfold higher level of atmospheric CO2 and a correspondingly warmer climate, perhaps 10°C warmer (Berner et al., 1983~. FORECASTING CHANGES IN SEA LEVEL RELATED TO GREENHOUSE GASES On a time scale of 102 to 105 yr, global changes in sea level (eustatic changes) can result mainly from the buildup or decay of alpine or continental glaciers, and from long-term ocean volume or steric changes caused by temperature or salinity changes in waters below the thermocline. The concern here is with forecasting eustatic changes related to the rise of CO2 and other greenhouse gas concentrations in the atmosphere. The past few thousand years have been a time of a high and relatively stable stand of sea level after 100 millennia of rapidly varying levels during the last ice age. Regional steric and other irregular variations of 10 to 20 cm from year to year, or from decade to decade, have been common, but there has been at most only a small unequivocally detected long- term trend in eustatic sea level. This situation can be expected to change with the advent of greenhouse-gas-induced climate change. Sundquist (Chapter 12) examined the probable future course of atmospheric CO2 con- centrations over the next 1000 yr. It might be expected that oceanic biogeochemical processes related to the dissolution of calcium carbonate sediments on the deep-sea floor would, within a few centuries, reduce the content of free CO2 in seawater, and hence the atmospheric content. This turns out not to be so, provided sufficient CO2 has been generated by fossil fuel combustion. The total remaining reserves of coal, oil, natural gas, oil shale, and oil sands that are ultimately recoverable for human use are believed to correspond to about 7500 billion tons of carbon. Of this amount approximately 3500 billion tons have already been identified. Sundquist assumes that 2500 billion tons will ultimately be consumed in human activities, with a peak rate of production in the middle of the next century of 16.8 billion tons/yr com- pared to the annual carbon emissions in 1985 of 6 billion tons. He assumes that the production rate will decline to 6 billion tons by about A.D. 2150 and go nearly to zero by A.D. 2350. With this assumed history of hydrocarbon combustion, Sundquist finds that the

28 OVERVIEW AND RECOMMENDATIONS CO2 content of the air rises from 350 parts per million by volume (ppmv)(700 billion tons) in 1985 to 800 ppmv (1600 billion tons) around A.D. 2160, and slowly declines thereafter to 550 ppmv (1100 billion tons) by A.D. 2700. The peak concentration is approximately 2.9 times the base concentration of 280 ppmv in 1880, from which increases of atmo- spheric CO2 content are usually calculated. Also to be taken into account are the increasing concentrations of methane, nitrous oxide, tropospheric ozone, and other minor greenhouse gases that can be expected 150 yr from now. General circulation models of the atmosphere constructed by the National Center for Atmospheric Research, the NASA Goddard Institute of Space Sciences, the Geophysical Fluid Dynamics Laboratory of NOAA, and others indicate that the estimated increased concentrations of greenhouse gases should result in an average global temperature rise of 3° to 6°C in the atmosphere near the Earth's surface in the next 100 yr. [Recent modeling studies show that more sophisticated parameterization of clouds, taking into account both ice and liquid droplets, greatly reduces the sensitivity of the climate models to increased CO2 (Cess et al., 1989; Mitchell et al., 19894. These newer models indicate a smaller temperature increase than the 3° to 6°C range given above.] The next question to ask is: How will this atmospheric temperature change affect the , ~ world's oceans. This question has recently been studied by Frei et al. (1988~. They use two kinds of models: a "pure diffusion" (PD) model in which heat is carried downward by eddy diffusion, assuming vertical diffusion coefficients of 1.3 and 2 cm2/s and a modified diffusion model in which cold, polar water sinks to the bottom of the ocean and mass is conserved by assuming a slow, global upwelling. Frei et al. (1988) call this model an upwelling-diffusion (UD) model; they assume that the coefficient of vertical eddy diffu- sion is about 0.65 cm2/s and that the global average upwelling rate to balance deep and bottom water formation is about 4 m/yr. A considerably smaller rate of upwelling and, cor- respondingly, a higher rate of downward vertical diffusion would correspond to estimates by Whitehead (1989) that cold deep and bottom water is formed at a rate of only 5 million to 10 million m3/s instead of the rate of 40 million m3/s assumed by Frei et al. (19883. Measurements of tritium distribution in the North Atlantic made by the "GEOSECS" Expedition in 1972 (Ostlund et al., 1974) and 10 yr later by the "Transient Tracers in the Ocean" Expedition (PCODF, 1981; Ostlund, 1983) indicate that the tritium "front" in the Atlantic deep water between depths of 2500 and 5000 m moved about 800 km south during this 10-yr period, indicating that the mass of deep water sinking in the Norwegian Sea and cascading downward through the Denmark Strait, is 10 million to 20 million m3/s. Hence, the rate of upwelling is probably smaller and the rate of downward diffusion greater than assumed by Frei et al. (1988~. Basically, a PD model transports heat relatively rapidly into the oceans, which slows the atmospheric temperature response to the rising CO2 concentration but increases the rate of sea-level rise. The UD model reduces heat penetration into the ocean, allowing the climate to warm relatively rapidly but reducing the sea-level response. Frei et al. estimate that the rise in sea level during the past 100 yr caused by thermal (steric) expansion was between 3 and 8 cm, with the lower value corresponding to the UD model. They project a rise of 10 to 50 cm during the next century resulting from thermal expansion, the range arising from uncertainties in CO2 and trace gas concentrations and in estimates of climate sensitivity to greenhouse warming. To this estimate of steric sea-level rise in the next century must be added the NRC (1985) Committee on Glaciology's estimate of the contribution to sea-level rise by ice wastage in a CO2-enhanced environment. This would come from three sources: glaciers and small ice caps, the Greenland Ice Sheet, and the Antarctic Ice Sheet. This NRC committee estimates a sea-level rise by the year 2100 (the assumed time for a doubling of atmospheric CO2) from ice wastage of 0.1 to 1.6 m with "most likely" values of 0.2 to 0.9 m; the "most likely" scenario can be expressed as 0.55 + 0.21 m if one assumes that this

OVERVIEW AND RECOMMENDATIONS 29 range expresses a standard deviation from the mean and if the "errors" are considered to be independent. Revelle (1983b), using a two-dimensional vertical diffusion model for ocean thermal expansion, estimated a total sea-level rise of about 0.7 m. Also, in 1983, Hoffman et al. (1983) forecast a larger global rise, between 1.44 m and 2.17 m by A.D. 2100. This was estimated to result from thermal expansion of ocean waters and from ablation and partial melting of alpine glaciers and the ice caps of Antarctica and Greenland. Of the total rise, an average 0.72 m was estimated to result from ocean thermal expansion, and 0.72 to 1.45 m were added from ice discharge to maintain the relative contributions thought to have contributed to sea-level rise over the past 100 yr. Because the range of rise related solely to ocean thermal expansion was calculated to be 0.28 to 1.15 m, the ratio approach led to an extreme upper limit of more than 3 m (of this amount, mountain glacier melting could contribute, at moss, 0.3 to 0.5 m). Robin (1987) forecast a rise of 0.80 m by A.D.2100 with a range of 0.20 to 1.65 m. Gornitz et al. (1982) calculated the component of sea-level rise resulting from ocean thermal expansion between 1980 and 2050 as 0.20 to 0.30 m. MacCracken et al. (1989), incorporating the Frei et al. (1988) model results and, with some adjustments, the findings of Revelle (1983b), estimate a total sea-level change by the year 2100 of less than 0.5 to 1 m. In all these estimates, the possible change, largely a result of human activities, in the quantity of water stored in lakes, man-made reservoirs, and underground aquifers has been neglected. Robin (1987) estimated the net effect of these three sources together at the present time as causing an annual rise in sea level of 0.08 mm, or 8 mm/century. Most calculations indicate that sea level will continue to rise for at least several hundred years at average rates of centimeters per year or less. Of course, a considerably more rapid rate would ensue if the West Antarctic ice cap should disintegrate during the next 1000 yr. As we have seen, such a dissolution is probably impossible during the next several hundred years because, as Bryan et al. (1988) have shown, deep ocean convection on the southern side of the great circumpolar Antarctic Current may markedly delay much Antarctic temperature change in the upper ocean layers. The assumed rate of carbon combustion is highly uncertain. In order to reach an atmospheric level of 800 ppmv by A.D. 2180, an average rate of CO2 production corre- sponding to 9 billion tons of carbon per year during the next 200 yr is assumed. This is at least 50 percent higher than the present rate of 6 billion tons/yr, which is estimated to include tropical deforestation of greater than about 1 billion tons/yr (Machta, 1983~. On the one hand, economic and social development of the now developing countries, which make up the vast majority of mankind, may well require a considerable increase above present levels of carbon combustion, and consequently increase the worldwide rate, per- haps by a factor of 3 or more. On the other hand, as Goldemberg et al. (1987), Mintzer (1987), and Bach (1988) have persuasively argued, it may be possible, through increases . . , . . . . . . . . , , - . . ~ .~ , , , In energy use ettlclency and substitution of other energy sources tor lOSSll fuels, lo reduce considerably the influx of CO2 to the atmosphere and ultimately to the oceans. With these alternative energy sources in use, the atmospheric burden of CO2 might remain at all times much below our estimated figure of 800 ppmv, and the consequent rise in air and sea tem- perature and steric sea level would be considerably reduced. In these calculations it has been tacitly assumed that the circulation of the deep water of the world ocean will continue relatively unchanged, except that the deep water will be somewhat warmer because of vertical and lateral mixing. In other words, the future ocean circulation will be "surprise free." However, the warming of the atmosphere in high latitudes, and the corresponding warming of the subsurface ocean waters, may greatly reduce the volume of water sinking to the depths. The projected rise in sea level from steric expansion would then be intermediate between the range estimated from the UD model (10 to 50 cm), and the range computed from the PD model (20 to 110 cm).

30 OVERVIEW AND RECOMMENDATIONS HOW CAN WE IMPROVE THE MEASUREMENT OF SEA-LEVEL CHANGE? The apparent trend of sea level at a particular place as measured by a tide gauge is the sum of trends in motion of the gauge itself as the land on which it is mounted moves vertically, the trend of change in steric sea level, and the trend of change in water mass under the tide gauge. To understand what is happening one needs to be able to make measurements that will separate these three components of the observed sea level. This problem is addressed by Munk et al. (Chapter 141. Each component of the observed sea level is considered separately. The vertical motion of the tide gauge can be measured with fair accuracy in four different ways: by measuring changes in the acceleration of gravity at the sea surface under the gauge, by very-long-baseline interferometry (VLBI), by satellite laser ranging, and by use of the satellite signals of the global positioning system (GPS'. Without repeated measurements over several years, none of these methods is sufficiently accurate to deter- mine the vertical position of the tide gauge. The acceleration of the Earth's gravity, g, can be measured with an uncertainty of about part in 108 by the methods described in Chapter 14. This accuracy corresponds to a sensitivity to height changes of the tide gauge of about 30 mm. VLBI observing stations yield estimates of intercontinental baselines with an rms scattering of 20 to 30 mm. Up to the present, the scatter of the vertical components has been 3 or 4 times larger than this. One of the major sources of error is atmospheric refraction caused by water vapor. Improved water vapor radiometers are being developed and placed in operation. The rms scatter of all components of the baseline should then be reduced to the 10- to 20-mm level. However, reducing the scatter below the 10-mm level may be very difficult (Carter et al., 19861. Global Positioning System surveys of benchmarks separated by 8 to 50 km agree in the height components by +10 to 30 mm. Carter et al. (1986) believe that these results are about as good as can be expected reliably for the foreseeable future. However, there are plans for increasing the accuracy of space-based geodetic techniques. It is possible that satellite laser ranging (SLR) can improve position accuracy from the present 1-cm value to 1 mm in the next decade. If vertical positions can achieve this accuracy, we should be in a position to improve our knowledge of eustatic change considerably. The Geodynamic Laser Ranging System (GLRS) of the Earth Observing System (EOS) will allow a large number of tide gauges to be included in the network. At present, the single measurement errors in all four methods of measuring the changes in the elevation of the tide gauge are comparable at +10 to 30 mm, and it seems likely that they will remain for some time to come. In contrast, it is desirable that the motion of the tide gauge be determined to a fraction of 1 mm/yr. Hence observational strategies will have to rely on repetitive measurements spanning intervals of several years and even then the desired accuracy can only be achieved by GLRS. The change in the steric height of sea level can perhaps best be monitored with bottom- mounted upward-looking fathometers plus tide gauges. (The alternative of measuring bottom pressure with sufficient accuracy presents many difficulties, because of seemingly inevitable unpredictable drift of the pressure gauges at pressures of a few tens of atmos- pheres.) Because of the marked variation in the velocity of sound in water with changes in temperature (some 23 times larger than the changes of specific volume or density with temperatures, it should be quite practical to estimate the integrated changes in temperature over the water column above the fathometer. The main problems in measuring steric changes, as Roemmich (Chapter 13) points out, are that they tend to extend over great depths and are not confined to or concentrated in the upper ocean. In the subtropical North Atlantic, steric changes extend to at least 3000 m with maxima in the thermocline at depths of 300 to 700 m and below the thermocline at about 1800 m. Hence the inverted fathometer is likely to be most useful in the depths of the open ocean far from land. The

OVERVIEW AND RECOMMENDATIONS 31 logistical problems of maintaining operating fathometers in such depths and locations and combining their measurements with surface mounted tide gauges may be difficult to solve. In principle, however, the combination of inverted echo sounders plus one or more of the four methods for measuring the vertical motions of the tide gauge, plus tide-gauge measurements at the sea surface should allow us to separate the 3 major components of changes in RSL (volume of water, mass of water, and changes in the elevation of the tide gauge). A further difficulty arises, as Roemmich shows in Chapter 13: steric height variations occur over such a range of space and time scales that possible trends cannot be identified, even in a 30-yr time series at a single station, no matter how dense the sampling. However, the major steric changes seem to be relatively coherent over distances of several thousands of kilometers. Munk et al. (Chapter 14) therefore suggest that 10 spatially independent stations, each forming 5 independent samples over a 25-yr period, could be combined to give a +7-mm standard deviation in a long-term trend of 10 to 25 mm in steric level. It should be possible within the next few years to measure change in sea level with the precision of 1 cm or less from satellite altimeter measurements such as those planned for the TOPEX/POSEIDON experiment (B. Tapley, University of Texas, personal communi- cation, 19891. Born et al. (1986) listed the magnitude of different sources of error in such measurements. The accuracy of the measurements will be limited by the calibration of the altimeter and of the estimated heights of fixed points on land. The effects of ionospheric and tropospheric refraction can be eliminated through use of two-frequency altimeters and a water vapor radiometer in the satellite. The drift of the altimeter, and not the absolute calibration, is important for measurement in changes of sea level during the life of the instrument. RECOMMENDATIONS 1. Long-term sea-level measurements of sufficient accuracy over the world's oceans could provide one of the most significant data sets for understanding global change, particularly climatic change resulting from the greenhouse effect. It is for this reason that the planning committees for the World Climate Research Program and the Intergovern- mental Oceanographic Commission of UNESCO have given a very high priority to extend- ing the global sea-level network in the Indian, South Atlantic, and South Pacific oceans. This effort is being supported, insofar as available funds will allow, by the U.S. National Oceanic and Atmospheric Administration (NOAA). We strongly recommend that the national oceanographic and meteorological communities lend moral and intellectual support to this sea-level program and to develop satellite altimeter methods for changes in sea level. 2. A polar orbiting satellite equipped with a radar, preferably a laser, altimeter should be operated on a continuing basis to measure changes in volume of the Antarctic and Greenland ice sheets. These ice sheets may be the principal sources of variations in sea level during the next century. As reviewed by the Topographic Science Working Group (1988), detailed and repeated height measurements by near polar-orbiting satellites are required to study the mass balance and dynamics of ice sheets. Repeat surveys of the ice sheets at 1- to 5-yr intervals with a vertical resolution of 10 cm are required for determination of elevation changes indicative of changes in ice volume, thus providing a measurement of net mass balance. Only refined radar altimeters or laser altimeter systems are capable of global coverage with the requisite accuracy. The output of ice in the mass balance equation occurs through iceberg discharge, surface melting near the margin, melting at the bottom of ice shelves, evaporation, and ablation. A 1-m difference in surface elevation of the ice shelves reflects a nearly 10-m

32 OVERVIEW AND RECOMMENDATIONS difference in ice shelf thickness (Topographic Science Working Group, 19881. The position of the grounding line can also be observed in elevation data because of a marked change in slope. The output of the ice sheets is not known better than 30 to 100 percent of the total snow accumulation, thus its measurement is critical in an assessment of the mass balance of the ice sheets. 3. A geological record of sea-level change is well preserved in numerous basins for at least the past 200 m.y., a span that includes the nearly ice-free Cretaceous Period and the present ice age. Comparison of the record between glacial and nonglacial times will provide an improved understanding of how depositional systems respond to sea-level change, as well as insights about nonglacial mechanisms of sea-level change. Support of programs, both national and international, that address the questions of the sea-level record during the past 250 m.y. should be vigorously pursued. One such program, the Global Sedimentary Geology Program (of the International Union of Geological Sciences) on Cretaceous Resources, Events and Rhythms, addresses a variety of questions that we have raised, viz., (a) Is there a global correlation of sequences? (b) Are sequences caused by eustatic fluctuations and/or global tectonic variations, or are sequences developed as a result of regional and local tectonic adjustments? (c) What are the relationships between subsidence, sea level, sediment supply, erosion, and other factors in mid-Cretaceous sedimentary basins? 4. To improve estimates of future steric changes in ocean volume caused by greenhouse warming of the ocean water, coupled ocean-atmospheric general circulation models should be improved and used to trace probable changes in ocean and atmospheric temperature as the greenhouse gas concentrations in the atmosphere gradually in crease. 5. To measure absolute elevation of tide gauges, measurements of position using satellite laser ranging, the global positioning system, and very-long-baseline interfer- ometry techniques and absolute gravity should be started. REFERENCES Bach, W. (19881. Modelling the climatic effects of trace gases: Reduction strategy and options for a low risk policy, Paper prepared for the World Congress "Climate and Develc~nment " Hamh~r~ November 7-10, 1988. ~rim ~7 ~~~~~~~ - -2~ Barnett, T. P. (1984~. The estimation of "global" sea level change: A problem of uniqueness, J. Geophys. Res. 89, 7980-7988. Baumgartner, A., and E. Reichel (1975). The World Water Balance, Elsevier, Amsterdam. Bentley, C. R. (1985~. Glaciological evidence: The Ross Sea sector, in Glaciers, Ice Sheets, and Sea Level: Effects of a CO2-Induced Climatic Change, Committee on Glaciology, National Research Council, National Academy Press, Washington, D.C., pp. 178-196. Berner, R. A., A. C. Lasaga, and R. M. Garrets (1983~. The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years, Am. J. Sci. 283, 641-683. Bond, G. C., M. A. Kominz, and J. P. Grotzinger (19881. Cambro-Ordovician eustasy: Evidence from modelling of subsidence in Cordilleran and Appalachian passive margins, Frontiers in Sedimentary Geology. New Perspectives in Basin Analysis K. L. KleinsDehn and C. Paola eds Sprin~er-Verla~. New York. Do. 129-160. , ~r _ ~ ~ A ~_~^ ~ ~ ~ _. V ~ ~ - r - -= - - ~ - - ~~o 7 ~ ~ ~ ~~7 r r - ~ - ~ ~ ~ ~ Born, C. H., B. D. Tapley, J. C. Ries, and R. H. Stewart (1986~. Accurate measurement of mean sea level change by altimeter satellites, J. Geophys. Res. 91(C16), 1 1~778-1 19782. Broecker, W. S. (1987~. Unpleasant surprises in the greenhouse?, Nature 328, 123-126. 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, J. Phys. Oceanogr. 18(6), 851-867. Carter, W. E., D. S. Robertson, T. E. Pyle, and J. Diamante (1986). The application of geodetic radio interferometric surveying to the monitoring of sea-level, Geophys. J. R. Astron. Soc. 87, 3-13.

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