Sea-Level Change (1990) / Chapter Skim
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Overview and Recommendations
Overview and Recommendations
Pages 3-34
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From page 3... ...
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
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From page 4... ...
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)
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From page 5... ...
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.
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From page 6... ...
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.
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From page 7... ...
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 Pleistocene (Chapters 4 and 5~.
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From page 8... ...
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.
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From page 9... ...
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.
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From page 10... ...
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.
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From page 11... ...
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 corresponding 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.
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From page 12... ...
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 )
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From page 13... ...
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.
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From page 14... ...
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~.
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From page 15... ...
Roemmich (Chapter 13) shows that at mid-latitudes in the North Atlantic the thermocline fluctuations in steric height exhibit greater variance in the time domain than the deeper changes and also more variability around the subtropical gyro.
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From page 16... ...
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)
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From page 17... ...
2. Tectonic uplift/subsidence of the land mass on which the tide gauge sits will induce
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From page 18... ...
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.
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From page 19... ...
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.
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From page 20... ...
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)
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From page 21... ...
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.
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From page 22... ...
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.
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From page 24... ...
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.
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From page 25... ...
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.
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From page 26... ...
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 atmospheric CO2 involves the submergence of the continental shelves by the rise in sea level.
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From page 27... ...
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.
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From page 28... ...
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)
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From page 29... ...
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.
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From page 30... ...
The change in the steric height of sea level can perhaps best be monitored with bottommounted 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 atmospheres.)
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From page 31... ...
, 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.
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From page 32... ...
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.
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From page 33... ...
NRC (19851. 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., 330 pp.
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From page 34... ...
A model for Holocene retreat of the West Antarctic ice sheet, Quat.
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