Icebergs

Introduction

An iceberg is a large piece of ice that has broken, or calved, from the front of a glacier into a body of water. Icebergs form mostly during the spring and summer, when warmer weather increases the rate of calving at the boundaries of ice sheets and smaller outlying glaciers. Because the density of ice is slightly less than that of sea water, icebergs float with approximately 90% of their mass found below the ocean surface. Due to increases in global temperatures, the increased calving of icebergs from large glaciers and ice shelves may eventually have considerable impact on sea level and thus on coastal ecosystems.

Historical Background and Scientific Foundations

After an iceberg sank the ocean liner RMS Titanic in 1912, iceberg tracking began in earnest with the formation of the

International Ice Patrol (IIP) in 1914. This group, with the financial support of thirteen member nations including the United States, began following icebergs with ships and later with airborne radar as they moved along “Iceberg Alley” from west Greenland down the Labrador current. Because Iceberg Alley follows the Grand Banks shipping lanes located southeast of Newfoundland, Canada, the monitoring of large bergs by the IIP is imperative to the safety of cargo, fishing, whaling, and cruise vessels in the North Atlantic.

The true pioneers of iceberg science, however, were the Soviet researchers who set up 31 ice camps on sea ice floes or glacial bergs in the Arctic Ocean from 1937 until the collapse of the Soviet Union in 1991. The data collected in these camps included long-term measurements of arctic weather and studies of how sea-ice forms. The Americans followed suit in 1952 by establishing Fletcher's Ice Island, a research site on a large, drifting ice-shelf berg used until it migrated into the Atlantic in the 1970s. These Soviet and American research camps allowed scientists to collect hundreds of thousands of daily measurements, including a chronological atlas of arctic sea ice distribution. Since the loss of the drift stations, satellites and automated buoys have been used for data collection. However, modern technology has been unable to replace the daily, on-site measurements made by researchers living on the ice itself.

Even with the extensive data collected by arctic researchers over six decades, much about icebergs remains unknown, including how currents, winds, and tides can affect their movements. It is difficult to track the movements of icebergs of a kilometer square or less, which comprise half the total mass of Antarctic bergs. As these small icebergs are prone to capsizing and are filled with crevasses, placing global positioning trackers on them has proved to be a daunting task.

Icebergs have been helpful in the study of certain ocean currents. It has been assumed that the deep keels and low profiles of icebergs cause them to reflect primarily the movement of subsurface currents rather than of winds. The estimated 1,500 cubic km of icebergs breaking off Antarctic glaciers yearly do appear to follow the counter-clockwise motion of the Antarctic coastal current. Nevertheless, satellites show that many icebergs spin out of the coastal current into seas farther north, possibly carried on regional gyres. Fragments originally close together may then move in opposite directions, finally ending up hundreds of kilometers apart.

Icebergs are categorized by the IIP according to their size and shape. The smallest size category, growlers, are less than 3 ft (1 m) in height and 16 ft (5 m) in length. This name comes from the growling sound sailors hear as these icebergs bob in the water. Size classes then increase from “bergy bits” to “small,” “medium,” “large,” and finally “very large” icebergs, the latter measuring greater than 246 ft (75 m) in height and 699 ft (213 m) in length.

WORDS TO KNOW

CALVING, ICEBERGS: Separation of an iceberg from a glacier at the point where the glacier flows into the sea. When an iceberg calves, it breaks loose and floats free in the water: this immediately raises sea level slightly, with no further change as the iceberg melts.

GLACIAL BERGS: Icebergs that have formed by breaking off the edge of a glacier that is flowing to the sea.

GLACIER: A multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.04 mi² (0.1 km²) in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of freshwater on Earth and is second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.

ICE FLOES: Plates of floating sea ice, that is, ice formed by freezing of the top layer of the ocean; distinct from icebergs, which are thicker and are produced by the breakup of glaciers flowing into the sea. Any floating plate of ice wider than 6 mi (10 km) is termed an ice field, not an ice floe.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

TABULAR BERG: Flat-topped (i.e., like a table, hence “tabular”) iceberg, usually formed by breaking off from the edge of a floating ice shelf.

TIDEWATER GLACIER: Glacier that flows into the ocean, as opposed to melting on dry land. Tidewater glaciers in Greenland and the West Antarctica Peninsula have significantly speeded up their flow to the sea in recent years, contributing to accelerated sea-level rise.

In addition to this categorization based on size, icebergs are classified according to their shapes. The two main categories are tabular (an iceberg with steep sides and a flat top with a length-to-height ratio of greater than 5:1) and non-tabular (all icebergs that do not exhibit the tabular shape). These non-tabular bergs are then further subdivided into dome (an iceberg with a rounded top), pinnacle (an iceberg with one or more spires), wedge (an iceberg having a steep vertical side on one end and sloping on the other), dry-dock (an iceberg that has eroded so that a slot or channel is formed), and blocky (an iceberg with a flat top and steep vertical sides).

The largest iceberg ever reliably measured was B–15, a tabular berg that calved off the Ross Ice Shelf in Antarctica in March, 2000. At 4,250 square mi (11,000 square km), an area larger than the state of Rhode Island, B–15 subsequently broke into smaller bergs, some still large enough to support their own weather systems of high winds, blowing snow, and fog.

Impacts and Issues

Interest in icebergs recently intensified with record increases in the calving of bergs from continental ice shelves in outlying parts of Antarctica and tidewater glaciers in the Northern Hemisphere. While the calving of B–15 in Antarctica was preceded by years of cracking and is considered to be part of a long-time, natural calving process in the Ross Ice Shelf, in 1995—without prior warning—approximately 500 square mi (1,300 square km) of the Larsen A Ice Shelf fell apart into thousands of icebergs over the course of just a few days.

The Larsen A Ice Shelf is situated on the Antarctic Peninsula, where average temperatures over the past 55 years have risen more than 4.5°F (2.5°C). Scientists were unsure whether this breakup was caused by global climate change, but an even more dramatic ice-shelf breakup occurred in 2002, when the Larsen B ice shelf, 1,254 square mi (3,250 square km), broke apart in only 35 days. In 2006, scientists with the British Antarctic Survey reported that the Larsen B breakup was definitely due to human-caused global warming, not natural processes. Other scientists have reported evidence that global warming is accelerating the movement of glaciers to the sea in both Antarctica and Greenland. The glaciers dump their mass into the oceans in the form of icebergs, raising sea level.

As global temperatures have risen, the rates of snowfall, ice melting, and glacier flow have risen as well. These opposing processes tend to offset each other on a regional scale, but data show that the Antarctica and Greenland ice sheets are both losing mass overall. Scientists estimate that these two ice sheets are presently losing 125 billion metric tons of ice per year. The current rate of ice loss from these sources increases sea level rise over background levels by only 1.4 in (0.35 mm) per year; however, ice loss has accelerated over the last decade and will likely continue to accelerate over the course of the twenty-first century. Scientists are concerned as to how the resultant sea level rise may impact human populations living in coastal areas.

The loss of large icebergs from ice shelves in the Antarctic due to climate warming can also have wide-ranging impacts on coastal ecosystems. When the B–15 iceberg restricted the northwestward drift of pack ice in the southwestern Ross Sea off Antarctica, large areas of what would normally be open sea were covered with ice. Thus, one of the most biologically productive regions in the Antarctic saw a decrease in phytoplankton productivity of more than 40%. This had cascading effects on those organisms, such as krill, that depend on phytoplankton growth, and on higher-level predators such as the Adélie penguins that feed on krill.

See Also Antarctica: Melting; Glacier; Heinrich Events; Melting; Polar Ice.

BIBLIOGRAPHY

Periodicals

Krajick, Kevin. “Tracking Icebergs for Clues to Climate Change.” Science 292 (2001): 2244–2245.

Marshall, Gareth J., et al. “The Impact of a Changing Southern Hemisphere Annular Mode on Antarctic Peninsula Summer Temperatures.” Journal of Climate 19 (2006): 5388–5404.

Shepherd, Andrew, and Duncan Wingham. “Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets.” Science 315 (2007): 1529–1532.

Smith, H. Jesse. “Climate Science: The Iceberg Cometh.” Science 296 (2002): 619.

Wuethrich, Bernice. “Lack of Icebergs Another Sign of Global Warming?” Science 285 (1999): 37.

Web Sites

“International Ice Patrol (IIP): Frequently Asked Questions.” International Ice Patrol. <http://www.uscg.mil/lantarea/iip/FAQ/FAQ_Icebergs.shtml> (accessed October 19, 2007).

“Questions and Answers about Icebergs.” National Snow and Ice Data Center. <http://nsidc.org/icebergs/questions.html> (accessed October 16, 2007).

Michele Chapman