The Polar Regions and Climatic Change Appendix (1984) / Chapter Skim
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'APPENDIX A: THE ROLE OF POLAR REGIONS IN CLIMATE DYNAMICS'
'APPENDIX A: THE ROLE OF POLAR REGIONS IN CLIMATE DYNAMICS'
Pages 1-114
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From page 1... ...
For the atmosphere the largest changes in the thermal forcing pattern probably arise from changes in sea-surface temperature and surface heat exchange, which in high latitudes are strongly affected by the presence of sea ice. The ocean and the atmosphere thus affect each other, but the response times of the two fluids are very different.
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with the ocean-atmosphere system occurs on the intermediate reaction time scale of sea ice (months or years)
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WINTER SOLSTICE OEC VERNAL SUMMER AUTUMNAL WINTER EQUINOX SOLSTICE EOUINOX SOLSTICE FEB MARCH MAY JUNE AUG SEPT NOV DEC » It FCB MARCH MAY JUNE AUG SEPT NOV DEC 4 21 6 22 B 23 8 22 0" 43" to*
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From page 4... ...
For the purposes of this discussion, the "solar constant" is considered constant. The distribution of solar radiation with latitude and with time of year is a different matter and changes slowly and systematically with time due to variations in the earth's orbit and in the inclination of the earth's axis of rotation to the orbital plane.
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From page 5... ...
. Finally, the surface exchanges related to formation and decay of sea ice are mechanisms draining COa from the atmosphere into the bottom layer of the polar oceans (Budd l980)
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From page 6... ...
To such a change, the tropics contribute primarily variations in precipitation, cloudiness, and ocean surface temperature, whereas polar effects include also changes in boundary-layer structure, in the extent of the highly reflective snow cover, and in the fluxes of heat, moisture, and momentum through sea ice, a major dynamic barrier between the polar oceans and the atmosphere. Climatic anomalies are associated with changes in the global pattern of thermal forcing.
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From page 7... ...
. albedo, which accompanies collapse of the snow cover and puddling on the ice.
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and ultimately affect the rate of melting at the surface. 3.2 Antarctic In contrast, the Antarctic contains a high ice plateau surrounded by an enormous area of seasonal sea ice.
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sensible heat flux heat flux downward from surface by conduction Fm"tiMt Mu» due to melting Solar radiation absorbed at the surface. Surface radiation balance Fr •iO*
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From page 10... ...
Further, the winds play an important role in the production of open-water "polynyas" favoring the formation of sea ice and of Antarctic bottom water near the continent. These processes allow the circumpolar ocean to exchange heat with the atmosphere -l0
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From page 11... ...
. much more actively than in the Arctic, where the ocean is ice covered all year, is very stable in its upper layers, and is severely restricted by geography in exchanging water with other areas.
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Thus, a general global-scale warming would tend to be amplified in the polar regions, and this polar amplification would then be likely to have a feedback effect on the global climate system. However, at all stages of this line of reasoning, associated changes in cloudiness and precipitation that have not been allowed for could lead to a different outcome.
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5. SEA ICE AND CLIMATE The largest variations in the intensity of the polar heat sinks are associated with variations of ice extent on the ocean.
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. In the Northern Hemisphere, the annual maximum extent of sea ice is about 6 percent of the hemispheric area, and the annual variation is about one half of this.
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During the warming of the early part of this century, variations in the mean area of Arctic sea ice were roughly from l0 to l5 percent. Changes in mean thickness may have been as much as one third.
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From page 16... ...
Their influence should be in the sense that greater ice extent corresponds to stronger thermal forcing and more vigorous global circulation. Atmospheric blocking highs and their associated distortion of the normal storm-track patterns have been linked to sea ice as both a direct and indirect source of climatic unrest.
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From page 17... ...
. Major climatic effects can be expected to arise from variations of the Antarctic sea ice.
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From page 18... ...
The role of oceanic heat and mass transports in the polar energy balance is, nevertheless, different for the two polar regions. The differences are of three kinds: o In the Arctic, heat and mass exchange with adjacent seas occurs mainly between Greenland and Spitzbergen.
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In the Antarctic, the deeper ocean waters are regions of vertical thermohaline convection. This convection creates the Antarctic bottom water, which may surface centuries later at the rim of the Arctic Ocean (cf.
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From page 20... ...
In such regions the ocean releases enormous amounts of heat and moisture to the atmosphere, quickly transforming the atmosphere to a much warmer and moister state. The heat released to the atmosphere in these regions constitutes a substantial fraction of the total poleward heat transport by the ocean, which, on a global scale, is roughly comparable to the heat transport by the atmosphere (Trenberth l979)
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From page 21... ...
Snow has already been mentioned as the heart of the so-called "albedo feedback" mechanism. At its maximum extent in the Northern Hemisphere, snow cover extends over an area even larger than that of sea ice (cf.
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have demonstrated that the waxing and waning of the Pleistocene ice sheets in lower polar latitudes of the Northern Hemisphere could be explained in this way, allowing also for interactions -22
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Linked with the Milankovitch effects through the ice sheets are secular changes in sea level. For the Antarctic ice sheet in particular, sea-level changes could modify the dominant ice-flow mechanism on the continental shelves and near the edges of the large bedrock depressions beneath the ice sheet.
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From page 24... ...
This discussion emphasizes the fact that the ice sheets take part in a complex system of glacial-climate feedbacks, involving bedrock isostasy and sea-level changes, as well as temperature and precipitation changes. A realistic assessment of the role of the polar ice sheets in long-term climatic unrest must be based on the further development of combined ice-sheet/climate models.
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From page 25... ...
could substantially accelerate the flow of ice sheets, especially in the West Antarctic, requires quantitative checking with adequate ice-sheet models. For the longer-term discussion these models need to be run in conjunction with realistic models of the polar climate to anticipate the course of events as the planet slowly chills toward the next in the long succession of glacial periods.
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From page 26... ...
The answer requires comparative observations at other locations, but these need not be continuous nor simultaneous. In general, these requirements apply to measurements of present conditions and also to interpretation of past climatic conditions from ice cores.
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From page 27... ...
. Since the differential effects on the time scale of centuries are linked with the integrated effects of the decadal time scale, it should be feasible to use model simulation studies for anticipating some of the changes in the ice shelves and for deducing the likely consequences for the grounded ice sheet upstream.
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From page 28... ...
F., l975. Antarctic sea ice from satellite sensing in relation to climate.
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From page 29... ...
Fox, l973. Numerical simulation of the influence of Arctic sea ice on climate.
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From page 30... ...
T., l980. Climate and sea level in isotope stage 5; on East Antarctic ice surge at 95,000 BP?
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From page 31... ...
Schwerdtfeger, W., l970. The climate of the Antarctic ice.
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Gloersen, l983. Arctic Sea Ice Cover l973-76; Satellite Passive Microwave Observations.NASA-SP-459 (206 pp.)
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From page 33... ...
Since the climate system is global, it is best studied on a global scale. Nevertheless, present information clearly suggests that the polar regions play a major, perhaps pivotal, role in long-term climatic change.
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From page 34... ...
Third, lakes and bogs in polar regions contain detailed records of past physical and biotic changes that may be interpreted in terms of climatic events. Although the Pleistocene ice sheets of both America and Europe obliterated the record beyond the past l0,000 to l5,000 years in some areas, longer records are available from unglaciated areas in Alaska, Arctic Canada, and Siberia.
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the ocean -- its surface temperature, ice cover, and circulation patterns -- and (b) sea level which, to a good first approximation, is inversely related to the total volume of water in ice sheets on land.
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-RC11-120% C.dmiiittM OF TOTAL RADIOLARIAN FAUNA 1S 10 9 I So" Figure 3. An example of a core analyzed for both l8O/l6O and the fraction of a particular radiolarian fauna in the total population, both as a function of depth or age.
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From page 39... ...
Although estimates of the extent of sea ice are on a less firm quantitative footing than estimates of temperature at the sea surface, a number of lines of evidence are consistent with substantially increased sea-ice cover during the last glacial maximum in both hemispheres. Since sea ice is known to inhibit primary productivity if it is present during the summer months, strongly reduced biogenic remains, combined with slower accumulation rates, suggest areas where summer sea ice occurred during the last glacial maximum.
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From page 40... ...
. This calculation rests on the assumption that there were not large floating ice shelves, which would alter somewhat the isotopic composition of the ocean while having no effect on sea level.
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From page 41... ...
The development and utilization of techniques to measure the past distribution of sea ice and to determine the frequencies of its fluctuations are of prime importance. Comparison of these fluctuations between the two hemispheres would provide insight into the role of these variations as amplifiers of global climatic change.
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Table l. Glaciological and Climatologlcal Investigations of Polar Ice Cores (From Langvay et al.
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Time horizons; longterm climatic variations. Measurement of longlived isotopes.
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From page 44... ...
Flow behavior. Interface and Sub-Ice Material Sedimentary petrology Interstitial ice chemistry and gas composition Rock fragments and bedrock Geophysics Temperature Centuries to millenia History of glaciation, maximum extent of ice sheet, subaerial weathering, age of till.
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Figure 4. Map of Greenland showing logistic bases and sites of recent Greenland Ice Sheet Project (GISP)
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Intermediatedepth ice cores provide data that are not only distinctive for the region from which they were obtained but also reflect general trends of global significance. Intermediate and deeper ice cores are also invaluable in the investigation of secular variations that must be understood for proper interpretation of a much longer temporal study of deeper ice cores (to bedrock)
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From page 47... ...
As we go deeper into the polar ice sheets, we not only measure all of the parameters listed for the shallow ice cores but also, through necessity, turn to other indirect and remote investigative techniques. For example, glacial/interglacial boundaries are suggested by airborne radar-sounding techniques.
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From page 48... ...
The resulting stratigraphic sequences not only consist of polycrystalline aggregates of snow, firn, or ice crystals but also contain atmospheric air, entrapped when the pore spaces close off, together with all of the organic, inorganic, soluble, and insoluble foreign material that fell with the precipitation or as dry fallout on the snow surface. Thus, ice sheets preserve a record of a considerable variety of paleoenvironmental variables and of their changes over time.
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From page 49... ...
. It is especially useful in polar regions for measurements of accumulation rates, dating of ice cores, and recording of past temperature changes (Dansgaard l977)
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From page 50... ...
Data sufficient to estimate time lags and coupling coefficients between climatic changes within the Northern and Southern Hemispheres and between hemispheres; 3. Comparisons with particle profile data to determine whether correlations exist between atmospheric turbidity and global climate change; 4.
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, and dry fallout. Ice sheets are generally remote from sources of natural and artificial aerosols; therefore, the chemical composition -5l
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From page 52... ...
The ice sheets contain only atmospherically transported material, and because of the low temperatures, chemical substances remain more or less as they were at the time of deposition. Chemical analysis of deep ice-core profiles is an ideal way to investigate the nature and composition of the baseline global aerosol and its changes, to model the interaction of aerosols and climate, and to quantitatively assess the impact on air quality of human activities throughout historic time (Hammer l977, l979; Hammer et al.
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From page 53... ...
more comprehensive laboratory investigation on ice cores by experienced and competent scientists. In regard to equipment, there is immediate need for, and much international interest in, "off-the-shelf" shallow (l00 m)
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From page 54... ...
It has been used infrequently in polar regions of the Southern Hemisphere, primarily because of the general rarity on the Antarctic continent of organic deposits with useful microfossil diversity. In the Northern Hemisphere, interpretation of "pollen diagrams" (plots of relative species abundances as functions of time)
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From page 55... ...
Baselines, global circulation, pollution Volcanic index, dating, global circulation Fluctuations in meteoric deposition Pesticides, ozone layer Greenhouse effect, its change with time Bulk and trace analyses Pollen & plant fragments as indices of biotic events and changes, elemental carbon Environmental conditions of past Fluctuations in particle deposition, surface melt features of past History of deposition and flow Flow characteristics related to time scale -55
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From page 56... ...
Pollen diagrams from Northern Hemisphere polar regions are available from northern Alaska, Arctic Canada, Greenland, Spitsbergen, and the USSR Arctic. Pollen identification from tundra areas is more difficult than that from sites within the forest limit because of less generic diversity and the lack of accepted criteria for distinguishing pollen of different species in a genus (e.g., Carex)
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From page 57... ...
Figure 6. Boundaries of the four proposed Arctic floristic zones.
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From page 58... ...
Pollen analyses have been performed on sediments of six major kinds: lake bottoms, marine muds, ice cores, peat of bogs, frozen ground, and surface soils. The first two are of special interest here.
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From page 59... ...
Peaty sediments have not always given reliable dates in polar regions. Bomb carbon appears to be preferentially deposited in Arctic regions, and in some peats, •^C activity greater than recent levels has been reported at depths of 50 cm and for sites that, on archeological grounds, should date from 2800 years B.P.
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study of lake cores in East Greenland indicates the substantial level of paleoclimatic information that can be obtained through a detailed analysis.
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From page 61... ...
More and longer lake cores are required at strategic sites throughout the polar regions. In the Northern Hemisphere, workers should seek sites outside of the area covered by the late Wisconsin ice sheets so that the history of vegetation and climate prior to the Holocene can be evaluated.
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Pingos are of two distinct generic types: the closed system and the open system. Most closedsystem pingos occur under Arctic climate conditions and a mean annual air temperature (MAAT)
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From page 63... ...
They form below the snowline and appear to require a mean annual air temperature colder than -l0°C to -l2°C. The mean summer temperature was probably 2°C to 6°C (Reger & Pewe" l976)
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From page 64... ...
The mean annual surface temperature at Barrow l00 years ago was about -l2°C, and with the recent warming, it is now about -9°C. 2.4.4 Geochemical indicators.
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From page 65... ...
Rather, the data give general trends and relative magnitudes of climatic changes. Also, the data on glacier distribution, when analyzed on a global scale, afford the basis for reconstructing the areal and vertical extent of past ice sheets, ice caps, and mountain glaciers.
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From page 66... ...
Least reliable under most circumstances because the origin of the channels is commonly debatable. Weathering zones have been widely employed to delimit late Wisconsin ice extent along the eastern seaboard of Canada.
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From page 67... ...
The samples turn out to be composed of mixed shells of different ages, redistributed driftwood, or lacustrine sediments contaminated with old carbon from various sources. This method is of questionable reliability, particularly when it is realized that available dates of this nature could be used to eliminate most Northern Hemisphere late Wisconsin ice sheets.
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From page 68... ...
of Past Climates Reliability Parameters Measured Climatic Inferences Length of Record Inferences of Climate Dating Dating Methods Geomorphic Indicators of Past Climatic Regimes Pingos (ice present) Closed system Open system Pingo scars Closed system Open system Ice wedges Active Dormant Ice and sand wedge casts and polygons Good Good Good Good Good Good Good Good Good Good MAAT*
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From page 69... ...
Reliability Parameters Climatic Length of Inferences Measured Inferences Record of Climate Dating Dating Methods Cryoplanation Required MAAT* Illinoian Good Good terraces at least to present Geologic stratigraphic methods Mean summer temperature +2-6-C Permafrost present Rock glaciers < 0°C Wisconsin Good Good Permafrost present Geologic stratigraphic methods Stratigraphic Indicators of Past Climatic Conditions Permafrost < 0°C Wisconsin Good Good relics (exist at depth)
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From page 70... ...
Past atmospheric composition and climate, gas parameters measured on ice cores. Nature, 276, 3-55.
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From page 71... ...
Gow, l970. Antarctic Ice Sheet: stable isotope analyses of Byrd Station cores and interhemispheric climatic implications.
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From page 72... ...
Establishment of time horizons in polar ice sheets by means of radio-echo sounding. Lyngby, Electromagnetic Institute, Technical University of Denmark.
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From page 73... ...
Oxygen isotope profiles through the Antarctic and Greenland Ice Sheets. Nature, 235 (5339)
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From page 74... ...
Summary of work by the Physics Institute, Bern, on alpine and polar ice. IUGG International Symposium on Isotopes and Impurities in Snow and Ice, Grenoble, France, August 28-30, l975.
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From page 75... ...
Oeschger, H., B Stauffer, P
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From page 76... ...
Goldberg, l97lb. Selenium and sulfur in a Greenland Ice Sheet.
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From page 77... ...
(The effects of surges of the polar ice sheets on climate are reviewed in Appendix A.)
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From page 78... ...
Furthermore, the boundary conditions themselves may be dependent on the system response; for example, change in surface elevation of an ice sheet can change the surface temperature and mass balance. A further complication and possible source of confusion arises from the consideration of numerical modeling calculations in the course of which the imposed boundary conditions are changed.
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From page 79... ...
For example, two simplified models of the Arctic sea ice suggest that 5 years after a 5°C warming a steady state would be reached in which the Arctic Ocean is ice-free in the summer, with the ice -79
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From page 80... ...
. It is not possible to isolate the dynamic and thermodynamic effects of the atmosphere on sea ice, or vice versa (Allison l982)
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From page 81... ...
and its low thermal conductivity, isolating the underlying surface, cause temperatures to be low over snow cover. Snow exerts a local cooling effect on air crossing it, and an extensive snow cover modifies the atmospheric circulation by establishing or reinforcing an upper trough pattern in the westerly wind circulation.
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From page 82... ...
l.3 Ice Sheets and Glaciers In considering the response of glaciers and ice sheets to climatic change, one must first define the terrestrial ice bodies (including Arctic glaciers and large parts of the Greenland and East Antarctic Ice Sheets) , which rest on a substrate close to or above sea level, and marine ice sheets (principally the major part of the West Antarctic Ice Sheet)
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From page 83... ...
The particular importance of marine ice sheets is suggested by a theoretical analysis showing that a simple two-dimensional model ice sheet resting on an initially flat bed below sea level would be inherently unstable (Weertman l974)
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From page 84... ...
Such shoals, currently found along the edge of the continental shelf in the Ross Sea, may have marked one margin of the late Pleistocene West Antarctic Ice Sheet. If, instead of a simple two-dimensional ice sheet, one considers a three-dimensional model, then ice streams (fast-flowing "glaciers" within ice sheets)
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From page 85... ...
That the deglaciation of a marine ice sheet can proceed rapidly is suggested by one reconstruction of the "gutting" of the remnant of the North American Laurentide Ice Sheet that depicts its central portion, grounded below sea level in Hudson Bay, as disappearing in no more than about 200 years. The analogy cannot be pushed too far, however, because of fundamental differences between the late-glacial Hudson Bay ice and the present West Antarctic Ice Sheet.
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From page 86... ...
. 2.2 Snow Cover There is little evidence specifically on snow cover during the Pleistocene and Holocene, except as it relates to changes in the altitude of the permanent snowline and the extents of ice sheets and other glaciers.
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From page 87... ...
The history of the West Antarctic Ice Sheet, however, may have been very different. It must have formed by some combination of thickening sea ice and coalescence and thickening of initial ice shelves growing out from glaciers in the mountainous regions of Marie Byrd and Ellsworth Lands, and from the Ellsworth and Transantarctic Mountains, across the sea that lay between.
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From page 88... ...
Although smaller than the West Antarctic Ice Sheet, its melting would produce about the same rise in sea level because virtually none of it is displacing ocean water. Little is known about the past history of the Greenland Ice Sheet, neither when it first formed, nor the extent of its depletion during subsequent warm intervals.
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From page 89... ...
Ocean cores provide information on ocean paleotemperatures and on the volume of terrestrially stored ice. Fossil coral reefs, especially on uplifted coasts, provide information about paleo-sea levels, which give an independent check on the inferred ice-volume signal from ocean cores; furthermore, suitable coral specimens are amenable to radiometric dating techniques, which form the basis for the estimated age of the last interglacial.
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From page 90... ...
The additional water was about what would have been added by the melting of either the entire West Antarctic or Greenland Ice Sheets, or by contributions from all the ice sheets. If the West Antarctic Ice Sheet was absent for a few thousand years during the warmest part of the last interglacial, the most likely reason is that Antarctic temperatures, especially of the near-surface ocean, had risen too high for its survival on the low-level bedrock between the Ellsworth and Marie Byrd Land mountains.
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From page 91... ...
Figure l. Northern Hemisphere seasonal sea-ice zone (from Barry l980)
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From page 92... ...
There is similar antiphase behavior between the Ross Sea and Weddell Sea ice in Antarctica. Longer (decadal)
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From page 93... ...
. Even less is known abut the thickness of Antarctic sea ice, but, since most of the ice is less than l year old, the mean thickness is surely less than 2 m.
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From page 94... ...
The small montane areas depicted in the southern continents have annual snow covers of varying duration. The stippled areas are ice sheets with snow cover.
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From page 95... ...
In any case, as in Greenland so a fortiori in East Antarctica, the percentage change in the total mass is very slow -- at most one part in l0^ yr . It should be noted, however, that a substantial fraction of the East Antarctic Ice Sheet lies on bedrock far below sea level and could behave like marine ice sheets where a sufficiently deep connection to the ocean exists.
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From page 96... ...
, part or all of which may be attributable to the slow penetration of a surface warming at the end of the last glacial epoch. On the other hand, calculations using the latest data continue to indicate a strongly positive mass balance for the whole part of the West Antarctic Ice Sheet that flows into the Ross Ice Shelf.
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From page 97... ...
In this scenario of what might happen during a polar warming trend, since there is now no ice shelf in the Amundsen Sea to buttress these ice streams, the grounding lines would irreversibly retreat into the vast Byrd Subglacial Basin that underlies most of the present grounded West Antarctic Ice Sheet. Grounding-line retreat would be accompanied by lowering surface elevations in the drainage basins for Thwaites and Pine Island Glaciers.
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From page 98... ...
The ice shelves would then acquire a strongly negative mass balance, and their grounding lines would begin to retreat up the ice streams. In this sequence of events, shrinkage of the Amundsen Sea sector leads to shrinkage of the Ross Sea and Weddell Sea sectors as well -- the end result is the elimination of the entire marine portion of the inland West Antarctic Ice Sheet, in perhaps a few hundred years.
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From page 99... ...
It may also lower the surface albedo of snow on the Arctic pack ice if soot particles are deposited on the surface. As possible man-made climatic modifiers the most important gases after CO2 are the chlorofluorocarbons, -99
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From page 100... ...
If all fossil fuel that is accessible by present techniques were burnt, the atmospheric CO2 level would rise between fivefold and eightfold. This concentration of CO2 would probably not be completely removed from the atmosphere for from 5,000 to l0,000 years, due to the slow rate of mixing of the deep-ocean water and slow removal mechanisms, and the concentration would be likely to remain at more than twice the preindustrial level for several centuries at least.
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From page 101... ...
4.2.l Sea ice. If the increasing amount of CO2 in the atmosphere were to increase temperatures by several degrees in the polar regions, the next concern would be the resulting effect on the thickness and extent of the sea ice.
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From page 102... ...
In the Antarctic, most of the sea ice is less than l year old, is thin by Arctic standards, and undergoes large seasonal changes in area. Therefore, it seems likely that climatic warming would have a relatively greater effect on Antarctic than on Arctic sea ice.
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From page 103... ...
The West Antarctic Ice Sheet probably would be little affected until or unless its ice shelves, exposed to higher ocean temperatures and less protected by sea ice, were destroyed. As discussed in Section 3.2.3, what would happen next is a matter of controversy.
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From page 104... ...
4.3 Global Implications of a Polar Warming The consequences of a CO2-induced warming on the global cryosphere are numerous, involving decreases in winter snow-cover extent and duration, thinning and recession of glaciers, ground ice, and sea ice, and the possible rapid disappearance of marine-based ice sheets causing a 5 m or so rise of global sea level (Hollin & Barry l979)
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From page 105... ...
Model experiments and empirical evidence suggest that an ice-free Arctic Ocean would cause higher winter temperatures and more precipitation over the Arctic Basin and possibly lower temperatures and less precipitation over the continents, but there are no good analogs in postglacial time for these conditions. Warmer polar regions would weaken the equator-pole temperature gradients and tend to reduce the intensity of the atmospheric circulation.
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From page 106... ...
2. Studies of atmospheric effects on snow cover and sea ice, and vice versa, are needed on synpotic and seasonal time scales.
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From page 107... ...
No foreseeable CO2-induced warming will turn the polar ice sheets into "banana belts" in winter, but since their elevation changes may have some effect on the global circulation, the katabatic boundary layer needs improved representation in global circulation models. A theoretical derivation of the ice-sheet surface-pressure distribution will be needed for this, and is being developed.
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From page 108... ...
Evidence should continue to be sought on whether the West Antarctic Ice Sheet was present or absent during the last interglacial. A definite answer to that question would not only shed light on the past behavior of that marine ice sheet but would help test predictions concerning the relative magnitude of climatic warming in the Arctic, the Antarctic, and lower latitudes.
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From page 109... ...
G., l980. Meteorology and climatology of the seasonal sea ice zone.
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From page 110... ...
T., l980. Climate and sea level in isotope stage 5: an East Antarctic ice surge at 95,000 BP?
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From page 111... ...
West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster. Nature, 27l, 32l-325.
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From page 112... ...
An analysis of Arctic sea ice fluctuations, l953-77. Journal of Physical Oceanography, j)
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From page 113... ...
Antarctic Sea Ice Cover, l973-78. Satellite Passive Microwave Observations.
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