The Ice. Stephen J. Pyne

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СКАЧАТЬ crosses the summit to form foehn winds. Of the winds greater than 10 meters per second in the region, 79 percent are cold barrier-winds from the south and 20 percent are foehn winds from the west. The climate on the east side of the mountains, which is consequently severe, helps account for the prevalence of the pack in the Weddell Sea and the otherwise anomalous presence of the Larsen Ice Shelf.

      The barrier winds sustain a geophysical “conveyor belt” to transport ice, cold air, and cold water north within the Weddell gyre. The great outward swelling of the convergence, the string of bergs and broken pack that flare north from the peninsula, the persistent pack ice and shelf ice so influential in the formation of Antarctic bottom water—all depend on this peculiar wind regime. In no other sector of the Antarctic has a similar pattern so fully developed, and only the Ross Sea offers even a mild analogue. The interruption of the polar easterlies exposes the upper portions of the west side of the peninsula to marine influences that make it a distinctive climatic region of the Antarctic. It alone is spared a wind regime connected directly to the polar plateau. In this balmier state rain rather than snow is a frequent occurrence, and some influence from South America is manifest across the Drake Passage.

      The katabatic winds have many effects. They scour some snow off the surface, sublimate other snows, and redeposit still more. This erosion helps to maintain the steep topographic gradients that, in turn, encourage katabatic flow. Where the cold katabatic wind slides over warmer seas, snowspouts—whirls of entrained snow—may dance along the coast, the product of violent mixing. On a larger scale, the interaction of katabatic outflow and marine air helps account for the belt of shallow cyclones that encircles the continent. The effects are limited to a few kilometers beyond the coast, but they can be dramatic—a nearly perpetual veil of clouds and blowing snow, torn only by occasional outbreaks of wind associated with frontal passages. Unstable lapse rates, associated with katabatics, promote the transfer of heat and moisture from the ocean to the atmosphere. Offshore winds interact with the pack ice in important ways, too. They sublimate and redeposit snow, remove surface meltwater, and drive floes outward. During pack progradation, floe separation is an important mechanism for promoting interstitial freezing and frazil-ice formation. During storms, when pack coverage is not total, floe separation exposes seawater, whose released heat and moisture may intensify the storm. The katabatics extend the influence of the interior ice outward.

      Everywhere the presence of ice is felt. Sea ice is a filter, intervening between air and sea, land and ocean; a lever, amplifying small changes in environmental conditions into larger effects; and a matrix, for a mixture of ice masses and for life. The explosive growth of pack ice—in effect, an extension of the Antarctic land mass—is one of the fundamental facts of Antarctic and Earth weather. The pack severs the connection between ocean and atmosphere around the continent, expanding and intensifying the polar heat sink. It reflects incident radiation, cools air and water in contact with it, and breaks the exchange of heat from ocean to atmosphere. Compared to the atmosphere, the oceans have higher heat capacity (1,600: 1), greater mass (400:1), and larger momentum (4:1). The atmosphere drives ocean currents, mainly by an exchange of momentum from wind to wave; the ocean, in turn, drives atmospheric processes, primarily by a transfer to heat. Open water transmits nearly one hundred times as much heat to the atmosphere as ice does. Regionally, ice amplifies the conditions that generate the ice; several mechanisms unite ice cover, temperature, and albedo in positive feedback. Globally, The Ice can amplify small changes of atmospheric conditions into larger effects, perhaps even full-blown ice ages.

      Biotic Barrier

      The pack ice is one of the great biotic boundaries on the planet. It divides the biotic from the abiotic environments of Antarctica, and it marks the limits of life on Earth. Except along the pack, Antarctica constitutes an enormous abiotic oasis segregated from the planetary biosphere. The pack is both a matrix for indigenous life and a biological filter against migration—the only ice terrane with indigenous life. Its biota contributes immensely to the complexity and attractiveness of the terrane.

      The biology of Antarctica is almost wholly a marine biology. There are several hundred species of mosses and lichens at selected land sites, a few insects and a spider, a handful of higher plants, but these are confined to subantarctic islands, the milder west coast of the peninsula, and far-flung deglaciated oases. Other than a few cryoalgae, who colonize melting snowfields, no organisms live on land ice exclusively; there is no terrestrial cryo-ecosphere. The biota of Antarctica—proverbially productive and exotic—is confined to the Southern Ocean. A much impoverished terrestrial biota thrives best in areas subject to maritime influences. For geologic eons, the continent has been isolated from any land connection, and its ice sheets have been unable to support an indigenous biota. For a while in the Tertiary Period, a land bridge joined the peninsula to South America, much as the Panamanian isthmus now joins the Americas; but this was severed. Unlike Australia, which was also isolated, Antarctica could not sustain terrestrial life on its own. Unlike the Arctic, where seasonally exposed land supports a terrestrial population that can inhabit sea ice or amphibious mammals who can occupy sea and land, the Antarctic lacks organisms who can live off the icescape or who can occupy the interior from the sea. And unlike the Arctic pack, the Antarctic pack is not continuous enough in space and time to weld land mass to land mass. Its biological connections are wholly maritime, sustained by upwelling from the nutrient-rich circumpolar deep waters. Biologically, the continent is a vast, cold, desert island, surrounded by a formidable moat of frigid surface waters.

      Milieu of terrestrial and Martian biomes. Note the intermediary status of Antarctic biomes, especially away from the peninsula. Redrawn, original courtesy Smithsonian Institution.

      With the exception of far-ranging migratory species, such as some birds and whales, the great proportion of Antarctic species are unique and endemic, confined to the continent or within the convergence. The geography of krill, for example, conforms to the frontal systems of the Southern Ocean. Most krill live within the Antarctic divergence, and the Antarctic convergence segregates the Antarctic from subantarctic species. Similarly, 86 percent of Antarctic coastal fishes are found nowhere else. The isolation and cold of Antarctica—both long-standing environmental parameters—have greatly simplified the Antarctic ecosystem. Marine life in The Ice shows the same traits as the Southern Ocean and the pack. It is mobile and migratory, strongly seasonal, with powerful circumpolar mixing superimposed over regional diversity. Terrestrial life shows an even stronger tendency toward reductionism. There are few species; they experience simple life histories, form discrete populations, and occupy circumscribed sites; their interactions are few and direct. It has been said that Antarctica has the most diminutive continental flora and fauna on the planet. For all this The Ice is responsible. Once ice has claimed an area, there is little opportunity for organisms to recolonize. The species that exist are survivors from the last glaciations.

      The pack, however, does provide a matrix for life. It exerts an indirect influence through its effects on the Antarctic atmosphere and the Southern Ocean. But its biotic services are direct, too, and seasonal productivity closely parallels the annual cycle of sea ice. The pack furnishes a necessary platform for many marine mammals and birds. The heaviest concentrations of krill, squid, and fish are somewhat away from the ice-abraded shoreline; the pack furnishes rafts to carry seals and penguins to primary feeding grounds, small islands upon which they may rest, sleep, and even mate. Paradoxically, unlike the Arctic, where the continuous pack provides a platform for humans, the Antarctic pack has been a primary barrier for human movement into and around the Antarctic. In this respect, the sea ice again functions as an insulator, a filter, not between air and sea but between civilization and terra nunquam cognita.

      Perhaps most spectacularly, the pack features a special biota of microorganisms. Unlike land ice, sea ice is not a desert. Two communities of organisms actually exist within it. A snow community forms from sea wash collected on the snowy surface of floes. More productive by several orders of magnitude is an epontic community of microorganisms that thrives on the bottom and interior of floes. The sudden freezing of congelation СКАЧАТЬ