The Ice - Stephen J. Pyne Weyerhaueser Cycle of Fire

Скачать книгу

of the clouds; clouds glide over the icescape like disembodied grey shadows. All these perspectives of the pack, moreover, have in common that they are pictures of surfaces, patterns of colors on a flat canvas of sea and ice. They lack depth, borders. Yet this variety of perspectives is nowhere else approached by the ice terranes of Antarctica.

      The pack is most interesting when it exhibits, within a single scene, a good mixture of all its potential effects. Such an ensemble is rare. More typical is the modulation of a given scene by variations in the intensity and the distribution of light. The result is a subtle permutation of color or of apparent composition. There is a darkening or lightening of hues or a reconstitution of ice and sea amid a somber twilight. The scene simplifies into a vast duotone of grey, dull ivory, or pale blue. In dense, overcast pack, sky and sea take on the same color, sheen, and texture. In the absence of motion and color the horizon vanishes into a common smudge of grey. The sparseness of strong light blurs shape as well as color into an immense spectral monotone. Gneisslike bands of slightly lighter or darker greys differentiate the elements of the scene—if anything can. This oppressive uniformity is broken only by the irregular arrangement of ice. White on white, grey on grey, opaque cloud on opaque floe, flat ice on a flat horizon of floe and fog, a collage of white particles on a white surface. Only icebergs, mountainous white shadows, manage to interrupt the scene. Occasionally, a berg of blue ice, still enveloped in mist, captures a shaft of sunlight and gleams like sapphire, an effect both stunning and enchanting—the setting of a blue sun amid a grey twilight.

      As autumn deepens and the pack stiffens near the coast, the terrane sheds the variability of light, motion, and objects that makes it attractive. Instead, by a process of subtraction, the scene simplifies and intensifies. More and more is removed. The scene is compressed, like the Antarctic sea and atmosphere, into a shallow surface—a linearity of banded low clouds, floes, and tabular bergs. Perspective lapses into indeterminacy. Composition blurs as borders proceed to infinity and objects lose their mass and structure. Color is erased into whiteout and greyout. Even motion slows. The pack, welding into a continuous sheet, dampens ocean swells. The only movement is the slow freezing of open leads, the grappling of frazil ice to floe, the muted impact of floe upon floe, the stately, imperceptible tread of the bergs—luminous shadows, ice sphinxes, full of grey inscrutability. The pack as a whole may move under the impress of tides and deep ocean swells, but it does so with ponderous undulations, like an earthquake in slow motion. Near the shore, as winter approaches, motion ceases. Ice fragments weld into a rigid mosaic of inertness, a still life painted in white and grey. Then, amidst the polar night, all discriminations are lost in a frozen entropic darkness.

      These are the common esthetics of all The Ice. What makes the pack special among the icescapes of Antarctica is its relative abundance and variability, its dramatic mixing of Earth and Ice. Compared to normal landscapes, the pack is esthetically impoverished. What dramatic spectacles it contains appear episodically and mechanically. Compared, however, to the interior icescape—monolithic and seasonally invariant—the pack offers a bewildering ensemble of effects and scenery. Its matrix of fluids—the air and the sea—brings far more mobility and uncertainty to the scenery of the pack than is possible where an ice mass is wholly embedded in solids. There is an element of randomness. Surprise is possible. Accordingly, the pack is most spectacular where it mixes floes, sea, bergs, and broken sky; when the ice grows and moves and storms reshuffle; when the ice is not complete, the fog not total, the sky neither wholly obscured nor utterly open; when there is a certain proportion of light and dark in vivid contrast, not homogenized into a uniform twilight; when some ice surfaces reflect incident light and some ice prisms refract them. White ice abuts black sea, with no gradation between them other than the muted light that diffuses through mixed clouds.

      It is an ensemble that mimics the ebb and flow of the polar day, that instantly encapsulates the seasonal progradation and retrogradation of the pack. The contrast—the proportions—accounts for the effectiveness of the scene. No single process or esthetic dominates completely. In the scene’s finest expressions, all are mixed and in motion. Each, by its contrast, heightens the other.

      RETROGRADATION

      The floes hesitate, suspended in an instantaneous equilibrium of seasons, like the globular clusters of an expanding universe caught at their maximum extent before gravity induces a slow collapse.

      Then the retrogradation of the pack begins. Maximum progradation is reached sometime in October, just after the vernal equinox. Now comes the recession. It is not an implosion so much as an erosion of the ragged perimeter, a steady exposure of sea at the expense of ice. The processes that directed the progradation of the pack now guide its retrogradation. The same principles of positive feedback accelerate the trend. What has changed is the general climate under which these processes operate. The polar night favors ice production; the polar day, ice erosion. What makes the recession possible is not simply the removal of old ice but the failure to regenerate new ice. The net balance between ice growth and ice decay tilts toward loss. The areal extent of the Antarctic ice field is halved as the pack retreats inward, drawing back the veil, a disintegrating vortex of ice floes and bergs.

      The individual floes ablate in several ways, but surface meltwater, so integral to the destruction of sea ice in the Arctic, has little role in the Antarctic. Winds from the interior polar desert sublimate the surface snow off floes and evaporate any standing meltwater. The katabatics of Antarctica are much drier and 60–100 percent stronger than Arctic winds. Only thin films of water develop over the surface, although melting does occur internally, concentrated around the black bodies of inclusions like microorganisms trapped within the rapidly assembled skeleton of the floe. Instead, open leads between floes take the place of surface meltponds. Winds drive apart those floes along the outer edge, but instead of refreezing, the interstitial leads remain open. The growth of open water reduces the albedo of the pack, encouraging solar heating of open water; warmer water upwells into the pools, promoting further melting of floes; waves penetrate the pack, leading to more thermal decay and mechanical disintegration; icebergs, increasingly mobile in the more fluid matrix of the collapsing pack, plow through floes, widening leads and crushing smaller ice masses. Since storms tend to hover over the circumpolar ring of maximum thermal upwelling, which generally coincides with the perimeter of the pack, the seasonal temperature change brought on by the waxing polar day can initiate the process of retrogradation. The storm belt, in turn, moves inward with the receding pack. Curiously, the pace of the recession outstrips that of the progression.

      By February sea ice is at a minimum. Over half the total decay occurs from mid-November to mid-January. For the most part, the pack contracts steadily southward toward the coast. The Weddell Sea is again an exception. Here the pack persists, although it retreats along an axis from east to west. But the collapse is never complete. Some floes linger, mingling with bergs and other ice breccia. Some are trapped in the Weddell Sea gyre, adding another year to their life cycle. Some are caught as fast ice in protected areas of the shore, part of a coastal ice terrane that, unlike the pack, rings the continent with a nearly immobile, quasi-permanent shield. A hybrid of sea ice, snow, and firn, fast ice connects the ice terrane of the pack to the ice terrane of the continent. It is itself a partial metamorphosis of sea ice into land ice, floe to glacial tongue, pack ice to ice shelf.

      The processes that shape sea ice and land ice are also responsible for fast ice, with the addition that both sets of processes here interact and their ices intercalate into a unique stratigraphy. The ice begins under the same conditions that generate sea ice. A protected embayment, perhaps sprinkled with grounded bergs; light winds, currents, and tides; and open water (a polynya is ideal) to generate fresh crystals—all lead to an ice matrix and the creation of a sheet of sea ice frozen to coastal ice or land. Anchor ice and frazil ice add to the expansion of the lower stratum. Crystal growth by congelation ice and the incorporation of other ice fragments expands the floe along its margins. Snow, recrystallizing meltwater, and (when the snow weights the floe so that the ice layer falls beneath sea level) percolating seawater all add ice strata to the surface. The saturation of surface snow (and sometimes firn) by seawater, which then freezes, can account for 25–90 percent of the total ice mass. Some seawater represents simple wash, trapped on the surface; but some enters the floe interior from beneath, along fissures that then freeze to form lenses and veins of infiltration ice. Grounded bergs promote fast ice by creating a breakwater that

Скачать книгу