Future Science Opportunities in Antarctica and the Southern Ocean (2011) / Chapter Skim
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2 Fundamental Questions of Global Change
2 Fundamental Questions of Global Change
Pages 23-74
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From page 23... ...
Antarctica and the Southern Ocean are intimately involved in global processes that provide the key to understanding those changes. Formation of the deepest water in the global ocean circulation occurs in the Southern Ocean, as does upwelling to the sea surface of all the deep waters from other oceans.
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From page 24... ...
. A big question persists: As the world warms, how much will ice loss accelerate, ice sheets shrink, and sea levels rise?
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From page 25... ...
higher than today. Evidence suggests that most of this sea level rise during the Glacial Minimum was from the melting of the West Antarctic Ice Sheet (WAIS)
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From page 26... ...
Although the loss of floating ice shelves does not contribute to sea level rise directly, the ice shelves provide a back pressure against the flow of ice, essentially buttressing the interior ice locked up on land and preventing it from flowing quickly. Once ice shelves are lost, continental ice flows more rapidly into the sea.
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From page 27... ...
Parts of the East Antarctic Ice Sheet and most of the West Antarctic Ice Sheet rest upon ground that is below sea level. The ice that extends above sea level literally weighs down upon the ice underneath, pressing it onto submerged ground.
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. Future sea level rise poses risks to U.S.
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From page 29... ...
Local variations in sea level also depend upon changes in ocean circulation and storm activity. As it happens, loss of ice from West Antarctica would cause about a 15 percent greater sea level rise along the eastern and western United States than the global average, with the largest increase centered approximately at Washington, DC, highlighting how the United States is uniquely exposed to the fate of West Antarctica and the Antarctic ice sheet (Mitrovica et al., 2009)
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From page 30... ...
This is partly because of difficulties inherent in observing and model ing ice flow: it is difficult to make physical measurements deep within and beneath ice sheets and ice shelves; many timescales of ice motion are longer than those afforded by instrumental records; and ice is a non-Newtonian fluid, whose motion depends sensitively upon its interactions with sediment or rock at its bed. As stated, the 2007 IPCC report neglected the possibility of change in the rate at which Antarctic ice is discharged into the ocean because not enough was known (IPCC, 2007)
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From page 31... ...
Teams of collaborators would need to include glaciologists, geologists, oceanographers, atmospheric scientists, and so on, and expansion of existing efforts across federal agen cies and academia. Those components of ice sheets that can change relatively rapidly, especially those associated with ice streams and ice shelves, require particular attention.
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From page 32... ...
Subglacial topography and the composition of the underlying rock are important determinants of glacial flow. Determining which regions are below sea level is important for evaluating instabilities in the ice.
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From page 33... ...
In stark contrast to the rapid warming of the Arctic, Antarctica and the Southern Ocean present a mixed picture of both climate change and climate variability. Significant progress in understanding changes in the southern high-latitude coupled climate system over the next 20 years will require construction and operation of an observing system for the atmosphere, ocean, sea ice, and glacial ice.
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From page 34... ...
BOX 2.3 THE OZONE HOLE AND GREENHOUSE GASES The Antarctic ozone hole is generated by a blend of three factors operating in the stratosphere. First, chlorine gas mediates the destruction of ozone, and its 20th-century increase is due to the rise in atmospheric chlorofluorocarbon levels -- long-lived chemicals produced by the chemical industry and used, for example, as coolants in refrigerators and air conditioners.
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From page 35... ...
The Southern Ocean westerly winds buffer the Antarctic continent from the warmer, wetter, and dustier lower latitude atmosphere. These westerly winds drive the Antarctic Circumpolar Current in the ocean (see Box 2.4)
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From page 36... ...
Locally, circulation and upwelling in the Southern Ocean affect its sea ice and ice shelves. The Southern Ocean circulation is dominated by the eastward flow of the Antarctic Circumpolar Current, which encircles Antarctica, driven by westerly winds.
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From page 37... ...
(ACC, Antarctic Circumpolar Current; F, Front; C, Current; G, Gyre)
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From page 38... ...
. Summer melting of the Antarctic ice sheet for 1979-2009 obtained from satellite observations appears strongly influenced by westerly wind variability associated with the Southern Annular Mode, as well as the El Niño-Southern Oscillation (Tedesco and Monaghan, 2009)
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From page 39... ...
. As the cooling effect of the ozone hole disappears, the increased greenhouse gas forcing may quickly result in enhanced warming of the surface of East Antarctica while continuing to produce stratospheric cooling (Crow, 2011)
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From page 40... ...
Would surface melting rapidly increase and accelerate ice flow into the ocean? Would increases in snow accumulation compensate for mass loss to the ocean?
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From page 41... ...
Nutrient-rich surface waters in the Antarctic Circumpolar Current sink to middepths north of its Subantarctic Front, providing the source for a large fraction of the global ocean's primary productivity, and feeding the world's great fishing grounds along its continental margins. Within and south of the Antarctic Circumpolar Current, seawater upwells from great depth, bringing nutrients and carbon and slightly higher temperatures to the sea surface.
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From page 42... ...
. What Changes Are Occurring in the Southern Ocean?
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From page 43... ...
The shoaling of warming water could be a cause of the increased melting rate of West Antarctic ice shelves. Questions for the Future Climate change in response to greenhouse gas forcing in the Southern Ocean will come about as the ocean and cryosphere warm in direct response to warming of the near-surface atmosphere.
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From page 44... ...
The largest mass losses are from the glaciers from the West Antarctic Ice Sheet where it enters the Bellingshausen and Amundsen seas. SOURCE: Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience (Rignot et al., 2008)
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From page 45... ...
• Will the North Atlantic's deep overturning, far from the Southern Ocean, be affected by changes in Southern Ocean winds, overturning, and stratification? Required Tools and Actions The climate-interacting portions of the Southern Ocean's circulation extend from the top to the bottom of the water column, and from the grounding lines of the ice shelves on the continental shelves for thousands of kilometers to the north, past the northernmost extent of sea ice to several hundred kilometers north of the Antarctic Circumpolar Current.
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From page 46... ...
On balance, the whole of the Southern Ocean is a net sink for excess anthropogenic atmospheric CO2, but there are large, counterbalancing elements, and the relative importance of these elements can shift. The balance is currently dominated by CO2 uptake in the deep mixed layers just north of the Antarctic Circumpolar Current.
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From page 47... ...
These maps highlight the sensitivity of the Southern Ocean under greenhouse gas forcing scenarios. Maps for "pre-industrial" (1765)
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From page 48... ...
The problem is greatest in cold polar waters because the low temperature increases CO2 solubility and decreases the saturation level; the result is that the polar oceans could experience decreases in carbonate saturation to harmful levels by midcentury. Much of the anthropogenic CO2 that has been absorbed by the ocean is in the North Atlantic, but the Southern Ocean north of the polar front zone also shows a large CO2 increase over the past decade.
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From page 49... ...
There is also a depth or pressure dependence of carbonate levels. Cartoon depicting chemical mechanisms involved in ocean acidification.
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From page 50... ...
Questions for the Future A major question is how the rate of CO2 uptake, and the resulting acidification of the Southern Ocean, will change as the overall climate system changes (warming, wind stress, position of frontal systems in the Antarctic Circumpolar Current, rates of ocean upwelling)
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From page 51... ...
Southern Ocean Sea Ice Global Context Sea ice covers most of the Southern Ocean in winter and extends as far north as 55°S in places, almost 1500 km from the continent of Antarctica. In summer the ice melts almost all the way back to the continental and ice shelf margins, cycling between coverage of 15-16 million km2 in winter to 2 million km2 in summer (NSIDC, 2010)
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From page 52... ...
. In the long term, with continuing anthropogenic release of greenhouse gases, the atmosphere will continue to warm and winter Antarctic sea ice extent is expected to be much reduced (in summer it retreats to the continental edge around much of the continent as part of the normal annual cycle)
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From page 53... ...
Sea ice loss is expected at all longitudes and especially in the West Antarctic vicinity. Such reduction in the seasonal cycle of sea ice concentration and volume would lead to a reduction of brine rejection at the margins of Antarctica.
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From page 54... ...
facilitate better future predictions of sea ice cover. This can potentially help scientists understand why current climate models predict a decline in Antarctic sea ice over the 20th century while observations show a small increase.
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From page 55... ...
From this improved understanding the goal of simulating realistic projections of past, current, and future Antarctic sea ice conditions is realizable. The Southern Ocean's Interaction with Glacial Ice Global Context The edges of the continental ice sheet extend into the ocean in massive ice shelves that cover about half of the circumference of Antarctica (Figure 2.6)
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From page 56... ...
. The impact of increased glacial ice loss on sea level rise was discussed previously (Section 2.1)
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From page 57... ...
They include the following: • Long-term measurements of the properties and circulation patterns of the continental shelf waters adjacent to floating ice shelves, including their vari ability and change (hydrographic sections, moorings)
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From page 58... ...
To apply an efficient systems approach in Antarctic studies scientists have to continue to improve the understanding of subsystems that frequently suffer from poor observational records, especially concerning seasonal cycles, and poor characterization of their dynamic features. Simultaneous efforts to increase the density and scope of observations and quantitative description of the subsystems using conceptual, analytical, and numerical models should provide researchers with the information needed to design observing systems and models that can capture the main features and resonance points of Antarctic and the Southern Ocean as a whole.
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From page 59... ...
The most productive regions are the continental shelves and other regions, where the strong currents of the Antarctic Circumpolar Current scour the continental shelves, harvesting iron from the sediments, resulting in much higher produc FIGURE 2.11 Cartoon view of the marine ecosystem of the west Antarctic Peninsula. The system is currently characterized by large predators such as penguins, seals, and whales, sustained by upwelling that supports high productivity and large krill populations.
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From page 60... ...
These variations make Antarctica and the Southern Ocean a unique, living laboratory for the study of ecosystem change. Ecosystems in Antarctica are simple enough that they can be studied for rules on how they respond to change, yet they are sufficiently complex that they are analogous to ecosystems in the rest of the globe.
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From page 61... ...
In the Palmer Station region of the Antarctic Peninsula, the mean winter (June-July-August) air temperature has risen by 6°C since 1950, and, in consequence, the duration of sea ice cover has declined by more than 80 days since 1978 (Figure 1)
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From page 62... ...
. FIGURE 1 Climate variability and ecosys tem response on the western Antarctic Peninsula; as temperatures have warmed and sea ice duration has decreased, Adé lie breeding pairs have decreased while those for Gentoo and Chinstrap pen guins near Palms have increased.
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From page 63... ...
FIGURE 2 Climate variability and ecosystem response in the McMurdo Dry Valleys. Top: midsummer (December-January)
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From page 64... ...
Major questions related to environmental change include the following: • How vulnerable or resilient are marine, freshwater, and terrestrial food webs to changes such as warming, enhanced water availability, habitat disturbance, ocean acidification, pollutant accumulation, and loss of sea ice?
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From page 65... ...
But there is a dearth of observations at other terrestrial, coastal, and interior sites to indicate the future effects of climate change. To place these local changes in a continent-wide context, and predict the future course of change across Antarctica, a comprehensive coordinated observing and prediction system encompassing all the major elements of the Antarctic environment is needed, including the terrestrial ecosystems, permafrost, surrounding ocean, sea ice, ice shelves, ice sheets, and subglacial habitats.
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From page 66... ...
In Antarctica these tectonic processes have driven the uplifting of vast reaches of Earth's surface, producing spectacular mountain ranges such as the Transantarctic Mountains that cut across the continent and the Gamburtsev Mountains that are hidden completely beneath the thick cover of the East Antarctic Ice Sheet (Figures 2.12 and 2.13)
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From page 67... ...
What Is Currently Known About Antarctica's Geologic History? The tectonic opening of key oceanic passageways has controlled global climate and shifted global circulation patterns within the atmosphere and the deep oceans.
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Understanding the opening of the Southern Ocean 68
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From page 69... ...
Researchers have learned much about the processes of past climate change from Antarctic sediment and ice cores. The Antarctic ice cores provide key insights into past changes in the global atmosphere while sediment cores reveal how the ice sheets have waxed and waned in the past (see Section 3.1)
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From page 70... ...
While much emphasis has been placed on the apparent instability of portions of the West Antarctic Ice Sheet, much of East Antarctica remains absolutely unknown yet is critical to the understanding of the continent and the ice sheets. Regions to be sampled include the enigmatic Gamburtsev Mountains, subglacial lakes, and other major subglacial provinces.
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. This type of information is crucial to develop reliable ice sheet models.
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From page 72... ...
Validation activities based upon actual sampling of the ice sheet's properties would improve these models. In 20 years, the committee envisions that there will be an improved understanding of the tectonic evolution of Antarctica, including the formation of the major mountain ranges, the distribution of key geologic terrains beneath the ice sheets, and the opening of major ocean basins surrounding the continent.
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From page 74... ...
Red lights help maintenance workers doing routine repairs on the South Pole Telescope. SOURCE: Daniel Luong-Van/NSF.
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