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Islands of ice

Jan 1, 2003

An irony of geography and meteorology conspire to bring sailors and icebergs together. The great circle route between New York and London intersects the Labrador current at the Grand Banks. Ships follow the great circle route, icebergs follow the Labrador current: their meeting place is, in truth, no place for ships. Instead, ships must stay well to the south of Newfoundland and the Grand Banks, giving berth to icebergs calved from the mountain-thick glacial ice caps of Greenland and Baffin Island.

The Ice Age lingers on in these places. Although the Earth is warming (more than 1°C in the last century), and the effects of El Niño are debated by Kansas wheat farmers, our climate today is but a brief respite from a prolonged period of ice-covered continents and the dry, frigid winters of the Ice Ages. Twenty thousand years ago, ice sheets a mile thick and a continent wide covered the northern half of North America and a good portion of Europe. The most recent Ice Age ended suddenly about 12,000 years ago, and the ice caps covering Greenland, Antarctica, and portions of the Canadian Arctic are the stubborn, tiny remnants of once great ice sheets. Snow continues to fall there, and this keeps the ice sufficiently thick to flow downhill and toward the sea. Icebergs split off from the glaciers, collapsing into the glacier-carved fjords.

Carried by tide and current, pushed by winds, icebergs in the Arctic Atlantic drift past Greenland, through Baffin Bay, and into the area of the Grand Banks. Now a nuisance, they are tracked by aircraft, spacecraft, and surface vessels, their progress through the shipping lanes monitored by a multinational force of specialists that constitutes the International Ice Patrol (IIP).

The simple beauty of icebergs (the Irish explorer-monk St. Brendan called them "floating crystal castles") clearly contributes to our interest in them. But they are an unique part of the oceans. Most directly, icebergs are the children of continental ice caps in the polar regions. As objects carried by the sea, their fate reflects the combined actions of the ocean and the atmosphere. As potential vessel destroyers, they play a role in everything from the contemporary cinema to the unemployment rate in Newfoundland. And, as remnants of a temporarily suspended Ice Age, they reflect the long term climatic historyand futureof Earth.

The sources of icebergs

The oceans contain two types of ice: icebergs and sea ice. Icebergs are pieces of glacial ice, the ice itself formed from compressed snow in a continental glacier or ice cap. Sea ice is frozen sea water. The difference is important. Sea ice participates in a short-term cycle (one to a few years) of freezing and thawing, while the ice in an iceberg formed as much as 50,000 to 80,000 years ago. Large numbers and sizes of icebergs can be produced only by large ice sheets, relegating iceberg production to Antarctic and Greenlandic ice caps. Antarctica has about 10 times the volume of ice as Greenland, which has, in turn, about 10 times the volume of ice as other ice caps in the world.

Unsurprisingly then, Antarctica produces the majority of icebergs in the world, perhaps on the order of 90% of the annual total. Antarctica is a stunning land of extremes, and, true to its nature, the icebergs produced there are extreme as well. Most of these bergs form at the edges of ice shelves, the huge, flat expanses of ice at the edge of Antarctica. Ice shelves sit for the most part on solid ground, but the weight of the ice is sufficient to push that ground below sea level. At their outer edges, the ice shelves float on the sea. Portions of the floating areas occasionally break off from the shelf (by mechanisms yet poorly understood), and the resulting icebergs can be extremely big. A recent example: B-9, shorn from the edge of the Ross Ice shelf in October of 1987. At 95 miles long, 22 miles wide, and 1,000 feet thick, this berg was roughly the size of Long Island. Despite their size, Antarctic bergs are not a significant threat to commercial shipping. The Circumpolar Current, which circulates clockwise around Antarctica at 60° to 65° S, traps most of these icebergs in waters infrequently traveled by vessels. The truly threatening icebergs are born a world away, in Disko Bay of the West Greenland coast.

Although glaciers on Ellesmere Island, Iceland, Spitzbergen, and Novaya Zemlya contribute a thousand-odd icebergs to the Atlantic Ocean each year, the glaciers of Greenland spawn the majority of Arctic icebergs. Fully 90% of the 10,000 to 16,000 icebergs dropped annually into the North Atlantic are generated from Greenland, and most of these from a few glaciers in Western Greenland. The champion of these is the Jakobshavn Glacier, which empties into Disko Bay. This glacier alone produces annually as many Arctic icebergs as are produced outside of Greenland.

The mechanisms of iceberg calving are poorly understood. On land, a glacier typically excavates a deep, U-shaped valley, through which it flows to the sea. As a glacier enters the sea, it maintains (for the most part) its shape and thickness, until the bottom of the glacier extends into open water. The bottom of the glacier, the land, and the sea meet along the grounding line, and most icebergs form seaward of this. The glacier is stressed by tides, wind, solar insulation and even harmonic oscillations generated by waves. These forces, singly or in combination, are thought to contribute to breaking off pieces of the glacier, which enter the sea as icebergs.

Shape, size, and stability

Icebergs calve from two distinctly different sources: glaciers and ice shelves. This dichotomy is reflected in the two classes of icebergs shapes. Tabular bergs are prismatic: flat-topped and -bottomed; with steep, typically vertical, sides. Tabular bergs are the offspring of ice shelves, and are typical of Antarctic seas. (One small ice shelf is found on Ellesmere Island in the Canadian Arctic, and it too produces tabular icebergs, called ice islands.) Non-tabular bergs include just about everything else.

Non-tabular icebergs have a variety of shapes and sizes, and these terms are useful in that they provide some insights into the way icebergs float and degrade. Arched, blocky, domal, pinnacled, and valley are all used to describe shape. Valley, or dry-dock, shapes are thought, for example, to represent bergs that have capsized due to preferential melting of the submerged ice. Sizes are characterized by more prosaic terms, from "very large" (longer than 200 meters, taller than 75 meters) to "small." The smallest blocks of ice in the seas are apparently too small to warrant the status of true icebergs: they get their own names. Growlers are less than five meters across and one meter high, while bergy bits range up to 14 meters long and four meters high. Don't be fooled by the diminutive termseven growlers can weigh hundreds of tons, 90% of which lies underwater.

That feature, the relative amount of ice above versus below water, is a subject of surprising complexity. The same rules that govern ship stability pertain to icebergs as well, the most basic being that the mass of sea water displaced by a berg must equal the mass of the berg itself. Roughly speaking, the exposed portion is but one-sixth or -seventh of the total volume of the iceberg. This does not mean that the deepest part of the berg is six to seven times as high as the highest exposed bit. That assumption may be safely made for a tabular berg, but non-tabular bergs can have maximum depth-to-height ratios of anywhere from 3:1 to 7:1. It is also possible, and not uncommon, for differential melting between the exposed and submerged portions to change the static stability of an iceberg. Thus, they capsize, bringing the submerged portions into the air. The rare green coloring of some icebergs, long a mystery, may be due to marine algae, once submerged, hoisted into sight by a capsize.

Berg migration

The dynamics of icebergs are also analogous, to a degree, to those of boats. Despite their size and mass, their deep draft and large sail area make them sensitive to currents, tides and winds. Once calved from the edge of a glacier and dumped into the sea, an iceberg is moved by a combination of these forces, modified perhaps by the presence of land, other icebergs, and sea ice. The forces pushing icebergs, and their effects, are still poorly understood.

The primary forces moving icebergs are currents and tides. Like ships (and not boats), the large draft of icebergs and their relatively small sail area make them sensitive to the force of water on their hulls. Unlike ships, large bergs, with drafts of hundreds of meters, respond more to forces on their bottoms than their sides. This difference becomes smaller with the size of the berg. Despite their size, icebergs respond rapidly to changes in current speed and direction. A study by oceanographers from Dalhousie University in Nova Scotia used radar ranges and bearings observed from oil drilling ships to show that icebergs move with the tides, closely following tidal currents on time scales as short as a few hours.

Winds have their influence as well, but the degree and even direction of the influence is not well known. It seems clear that bergs should be driven by the wind, but the relationship between wind speed and resulting iceberg speed is a difficult one. The spinning Earth adds another (potential) complexity: an object driven by winds should bend to the right due to the Coriolis effect. Enter the Dalhousie study, which indicated that icebergs are driven directly downwind, at 1% to 2.5% of wind speed. This is all well and good, but the IIP wind push model assumes speeds of 2.5% to 4.8% of wind speed and has the iceberg moving 50° to the right of downwind.

In a gross sense, icebergs should act as good current probes, following the gyrations of currents in the oceans. The majority of Greenlandic bergs, calved from the west coast glaciers, end up in the cold Labrador Current of Baffin Bay. Like an atmospheric low, cold ocean currents circulate counterclockwise, and so icebergs formed on the east side of Baffin Bay are carried first northward, then westward, and finally southward along the shores of Baffin Island, Labrador, and, ultimately, Newfoundland to the Grand Banks.

The confluence of the cold waters of the Labrador current and the warmer waters of the Gulf Stream conspire to produce the fogs for which the Grand Banks are justifiably famous. This same confluence brings the ice to the shipping lanes, and all the dangers that follow. The IIP, formed in the aftermath of the Titanic sinking, is charged with informing mariners of "the ice conditions and the extent of the dangerous regions" around the transatlantic shipping lanes in the Western Atlantic. The U.S. Coast Guard conducts the IIP for the U.S., from offices at Avery Point in Groton, Conn. Using radar-equipped Hercules aircraft, reports from private vessels, and commercial aircraft, the IIP strives (and achieves) to locate the "southeastern, southern, and southwestern limits of the region of icebergs in the vicinity of the Grand Banks." The IIP is quite good at what it does; since its inception, reports MST first class Laurie Valliere of the IIP, not a single life has been lost to an iceberg collision. The IIP's impressive record is accomplished not by locating every iceberg in their patrol area (40° to 52° N and 57° to 39° W), but just the limit of all known ice around the Newfoundland coast. The IIP makes few, if any, claims to the locations of individual icebergs within the limit of all known ice. They concentrate their efforts on the limit, and hope mariners respect those limits.

The IIP considers icebergs south of the 48th parallel a potential risk to shipping, and does endeavor to locate as many of these icebergs as possible. This list of monthly totals of icebergs south of 48° N isn't a record of every iceberg in the area and, because the methods used to find them have changed over the years, these counts are only approximate indicators of the number of bergs carried into the shipping lanes by the Labrador current.

There is phenomenal variation in icebergs from year to year. An average year has about 475 sighted bergs. Compare, however, the 1981 count (63) with that in 1993 (1,765). That's a nearly 30-fold differencean enormous variationand Don Murphy, chief scientist for the IIP, admits (with a combination of glee and wonder that suggests he loves his job) that he "can't explain the variation." Much of the variation has to do with sea ice, the annual ice produced from freezing sea water. Sea ice entraps icebergs and then protects them from the destructive forces of waves. As the sea ice moves south through Baffin Bay, so too do the bergs trapped within it. Finally, near the Davis Straight, the sea ice breaks up, leaving the bergs to travel, unprotected, southward along the coast. The majority end their lives stranded along the shores of Baffin Bay, where they melt after a two- or three-year life span.

As part of its work, the IIP tracks those icebergs it finds. Part of tracking them is to predict where the bergs are going, but prediction, as we have seen, is still a tough job. While computer models may seem a foolproof technique, the output of a computer program is only as accurate as the equations and data coded within them, and these are rough approximations at best. This is one of the reasons the IIP doesn't predict the locations of individual targets: the models aren't accurate enough. The predicted positions are insufficient to allow the mariner to program a GPS with their coordinates, an action everyone involved with the IIP greatly fears.

The vast majority of icebergs that make it into the Grand Banks degrade and melt into the ocean there. The temperature of the sea water plays an essential role in the preservation and destruction of ice. The lifetime of a berg is inversely proportional to the temperature difference between the berg and the ocean; with a temperature of minus 1° C, icebergs finding their way out of the cold Labrador current are doomed. In water of moderate temperatures (greater than 10° C, or 50° F), an iceberg has a lifetime of a few days. Thus the reason Don Murphy starts each of his work days at the web site of the University of Rhode Island's Graduate School of Oceanography, looking at their sea surface temperature (SST) page. Water above 10° C or so is very unlikely to have any ice, so the SST map allows Murphy to limit those portions of their operational area that needs to be searched. The occasional iceberg will cross the Gulf Stream, however. Famous examples include an iceberg sighted, in 1926, at 30° 20' N, 62° 32' W, 250 nautical miles southeast of Bermuda.

Harbingers of catastrophe

The 1984 iceberg season was an exceptional one. The 2,000 icebergs south of 48° N remain a remarkably high total. Judging from the marks left on the sea floor, that is a pittance compared to episodes in the recent past. Good evidence suggests periods in the recent past, about 8,000 years apart, during which 10 to 100 times as many icebergs sailed the North Atlantic. Imagine not the 2,000 scant bergs found in 1984, but 20,000, even 200,000 bergs floating in the Atlantic from Baffin Bay all the way to Portugal, every year for a century.

As glaciers move, they erode the rock beneath them, and incorporate bits of it into the ice. A wide variety of particles are entrained in the ice, from boulders as big as houses to rock so fine it has the consistency of finely ground and sifted flour. As the icebergs melt, the freeloading earth drops to the ocean floor in a characteristic jumble of boulders, cobbles, and sand. These deposits have been found and identified in cores collected from the sea floor by dozens of researchers. The debris' composition is a handy way of identifying the source of the debris, and most appear to come from Arctic Canada west of Baffin Island. Now known as Heinrich layers, these iceberg-rafted debris are found in sediments of 12, 21, 26, and 34, 50, and 66 thousand years old. Heinrich layers mark the times at which the Earth's climate was at its coldest, and was about to change rapidly, from 8,000-year periods of intense cold, to briefer periods of relative warmth, such as we enjoy now. Icebergs, as we have seen, are sensitive to ocean surface temperatures. This relationship works in both directions: icebergs may affect sea temperatures, and climate, the world over.

All those melting Heinrich icebergs introduced an enormous flux of cold, fresh water into the saline waters of Baffin Bay and the northern Atlantic Ocean. A little fresh water may not seem important, but the northern Atlantic Ocean plays a pivotal role in maintaining Earth's climate, via the thermohaline circulation. The Gulf Stream brings warm, moderately salty water into the North Atlantic. The heat in this water is enough to grant Europe an unusually warm climate. This heat is provided by evaporation of ocean water into the atmosphere, which cools the remaining sea water, making it more saline and denser. Cold, salty, and dense, this water sinks to the sea floor, and then circulates toward the equator, around Africa, and into the Indian and Pacific Oceans, where it rises back to the surface and is reheated. During ice ages, the thermohaline circulation slows down dramatically. What has this to do with icebergs? The cold, fresh water from melting Heinrichian icebergs may decelerate the thermohaline circulation. The temperature of North America, Europe, and indeed the globe are tied to that circulation, and they may change as much as 7°C (14° F) in a few years or a decade when this happens. The rate of temperature change measured in the past century (1° C) is only about a percent of that during the Heinrich events.

Don Murphy of the IIP knows about icebergs, and they worry him. Murphy is paid to understand how icebergs form, migrate, and decay during their three-year journey from Arctic Greenland to the shipping lanes around the Grand Banks. His very knowledge makes him nervous while at sea in the IIP's "Operational Area" around Newfoundland: "It's hard for me to even relax out there," says Murphy. He occasionally takes USCG 180-foot buoy tenders out to the IIP operational area of offshore Newfoundland. Despite the skill and experience of the crew, rarely have any navigated within ice-prone waters. Often, Murphy reports, he finds the officer of the deck with his or her head buried in the hood of the surface search radar. A mistake, Murphy insists. A growler weighing nearly 1,000 tons is "near impossible to see" on radar in a 10-foot sea. On each cruise, he talks to the OODs and the commanding officer about the dangers of icebergs and navigating in their waters. "I would encourage mariners to be especially vigilant in the Operational Area. Mariners have to slow down." The most important tool on board isn't the radar, he notes, but the bow watch. Trust that bow watch, Murphy adds. Always remember that "if the bow watchman sees something white ahead, it's not a wave crest, it's an iceberg."

For further information, the IIP web site can be reached via the USCG home page at: www.uscg.mil. The national geophysical data center maintains files of iceberg positions and similar data. It can be found at: www.ngdc.noaa.gov. The University of Rhode Island's Graduate School of Oceanography Sea Surface Temp. page is at www.rs.gso.uri.edu. n

Larry McKenna is a navigation instructor, freelance writer, and sailor who lives in Overland Park, Kan.


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