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published on 5 February 2005 in water

Icebergs

Vagabonds on the sea
Last January, the media gave great importance to the news of the possible collision of a big iceberg and the floating ice shelf of Ross Bay, in Antarctica. The satellite images provided by NASA and ESA (European Space Agency), that constantly follow the movements of the bigger masses of ice drifting in the polar seas, created great expectations for an event that was imagined would be  highly spectacular. In actual fact, however, after a few days of euphoria, it was understood that the collision would not take place in the immediate future and that, due to many variables, it would not  be easy to predict its exact date.
However, apart from the attraction of the event, the study of the behaviour of these sea vagabonds allows us to collect important information in different fields: how these great masses form and break away from glaciers and floating ice shelves, the sea currents and the winds that push them, the consequences that the formation of icebergs can have on the life of our planet, starting from living organisms close by right up to climatic changes.
In the year 2000 an immense iceberg, called B-15, broke off from the Ross ice shelf and slowly went adrift. The initial size was impressive: 295 km long, 37 km wide, with a surface area of 11,000 km2 and a thickness between 200 and 350m – it was one of the biggest icebergs ever seen. In October 2004, after a violent storm, B-15 calved into about ten smaller masses, the biggest of which was named B-15A.
Even the dimensions of the ‘fragments’ are considerable: B-15A is 160 km long, has a surface area of 3,000 km2,  equal to the Italian Valle d’Aosta region, and weighs about two million tonnes. Initially it seemed that B-15A was getting closer, at great speed (about 2-3 km a day), to the Drygalski ice tongue, the floating part of the David glacier (one of Antarctica’s most important glaciers). The combination of winds and currents, together with the great thickness of the iceberg, have made it ‘run aground’ on the sea bed about 5 km from the glacier and have currently slowed down its movements, so that the much awaited collision seems to be averted.
The area of possible impact is to be found in the Ross Sea, near to Terranova Bay, where even the Italian base of the Programma Nazionale di Ricerche in Antartide (National Programme of Research in Antarctica), the Mario Zucchelli Station can be found. At present, B-15A is anchored to the deep sea bed  between the Beaufort and Franklin islands and since its successive moves depend on the interaction between winds, tides, sea currents and the shape of the sea bed, it is difficult to predict its future.

Real hazards?
There has been much talk about the possible dangers created by the wandering of B-15A, highlighting catastrophic aspects at times: even the possible interruption of the Gulf current  that brings warm water from the Gulf of Mexico towards Northern Europe (making the cold  climate of the European Atlantic coast milder).
But what is true?
True
The presence of a bulky ‘object’ in the Mc Murdo Sound canal makes it difficult for the ice-breakers to reach the American base of Mc Murdo, the first research station in Antarctica, and the Scott base in New Zealand. However this ‘only’ implies some logistical problems:  supplies and the movements of the personnel are guaranteed by aeroplane connections , and in any case the bases are equipped with stocks of food and fuel for over a year: so there is nothing to fear for our researchers!
Some penguin colonies, instead, are suffering severe damage due to B-15A.The penguins of Adelia, the smallest among the different species of Antarctica, live most of the year in the open sea but make their nests along the coast, on the mainland close to the sea where they go to catch fish: when they reach their nest they feed their young with regurgitated food that has usually been stored in their stomach for several days. The presence of B-15A, stuck between Franklin Island and Cape Royds, where about 3,000 penguin couples make their nests, prevents the adults from reaching the sea easily, forcing them to a  ‘deviation’ of over 100 km. The result is that the parents consume the entire amount of food they are carrying and reach the nest with nothing left for their young. Researchers estimate that many offspring (up to 90% of the chicks) could die of hunger. There is another colony, in Cape Bird, where more than 50,000 specimens are in danger. The colonies will most probably survive but an entire generation of penguins will be missing. This is a very serious problem because the number of penguins in Antarctica has been decreasing drastically in these last years.
A danger for man could be the collision of ships with the ‘mountains of ice’: the emotions aroused by the most famous sea tragedy, that of the Titanic, are still alive. From this point of view, Arctic icebergs are more dangerous, because the northern routes  are more frequently used by commercial ships. Nowadays, however, the risk of a collision is greatly diminished, not only because of more sophisticated navigation and bearing instruments but also because of the constant satellite monitoring that keeps these ‘sea vagabonds’ under control.
False
It has been suggested that the collision and subsequent fragmentation of B-15A could cause an interruption in the Gulf current. It is incomprehensible how this alarmist piece of news   originated, because no iceberg has ever been spotted close to the routes of the current: normally Antarctic icebergs do not  cross the  southern 45-55° latitude.
Another fear is that the melting of big icebergs could cause the level of the seas to rise. One must consider, however, that the icebergs present in the sea, due to the fact that they are floating, have already reached a hydrostatic equilibrium with the oceans, so that their melting cannot influence the sea level. This is also true for sea ice and the ice of floating ice shelves. Only icebergs that fall into the sea directly from a terrestrial glacier front can raise the sea level; however, the volume of these types of icebergs is usually so small that the variation would be impossible to record even with the most sophisticated measuring methods. Only the melting of large masses of continental ice can influence the sea level significantly. As one can see, much alarmism isn’t backed up scientifically: so let’s not blame these giants for faults that maybe are mainly ours…

Icebergs, how they are formed
The formation of an iceberg in Antarctica and in the seas surrounding the North Pole, is a very common and not at all exceptional event. What may be exceptional, at times, is the size of the masses of ice that break off, dimensions that can reach several thousand km2.
Icebergs (the word originates from ice and berg(mountain), mountain of ice) form in two conditions:

  • when terrestrial glaciers descend to the sea, the last part of the tongue of ice, on meeting the sea water, begins to float due to a phenomenon called ‘calving’: ice is, in fact, lighter than water (this can easily be seen while having a drink in summer: the ice floats on the surface of the liquid in our glass). This brings about the formation of fractures in the mass of ice and the subsequent  breaking off of portions of varying sizes. The shape of this kind of iceberg is usually irregular, with an indented and uneven surface;
  • when very large glaciers converge before flowing into the sea, the union of the tongues gives origin to flat shelves of floating ice, the so-called ice shelves (that must not be confused with the ice pack, which originates from ice formed due to the freezing of sea water).

Extensive ice shelves surround the Antarctic continent: the Ross ice shelf in the Ross sea and the Ronne ice shelf in the Weddell Sea are the biggest. The movement of the currents and the tides in the underlying waters, together with the constant thrust of the glaciers that supply  the ice shelves, cause their fracturing and fragmentation, and in this way  1,450 to 2,000 km3 of ice are lost annually (equivalent in volume to about half the drinking water consumed in the world in one year). The shape of icebergs of this kind is usually that of a flat shelf with a relatively smooth and regular surface.
These are typical of the Antarctic zone, while the icebergs of the first kind are more easily found in the Arctic seas, where emersed lands are not surrounded by floating ice shelves and the numerous terrestrial glaciers can, therefore, flow directly into the sea: so, with this simple illustration  it is possible to ‘guess’ the origin of a ‘mountain of ice’!
Since ice is less dense than water, icebergs float on the surface of the sea: the part below the waterline is therefore about 7-10 times (depending on the difference in density between water and ice) taller than the one above it. If you consider that some icebergs can reach several tens of metres above  the sea level, you can well understand how the name ‘mountain of ice’ is particularly suitable: an iceberg whose visible part is 30 metres high continues under sea level to a depth of over 200 metres!
The biggest iceberg ever seen was one from Antarctica, observed in 1956, whose measurements were 335 x 97 km, with a surface area of 31,000 km2, equal to that of Belgium. After the calving of B-15, which was as big as the Abruzzo region, the present giant of the seas record goes to the C-19A iceberg, that is bigger than Liguria.
Icebergs are made up mainly of fresh water  having been formed from the ice of glaciers, originated, therefore, from the transformation of snow. Their function as an important supply of drinking water for the populations of the far North, such as the Inuits, has been relevant in the past. Even nowadays projects are periodically submitted for the exploitation of these precious resources; for example, icebergs have been towed close to the coasts of countries with a shortage of drinking water, but for the moment the cost of these operations is far superior to the benefits.

A life spent travelling
Once the icebergs have broken off from the glacier or ice shelf, they drift pushed by winds, currents and tides. Wind and wave erosion together with the progressive melting that occurs while the glaciers are moving towards warmer latitudes reduce their dimensions, together with further calving due to, for example, violent storms or collisions among themselves or with the mainland. An icebergs’ fate, therefore, is that of getting smaller and smaller until it disappears, but its’ life span can be several years long.
Icebergs can be seen only at high latitudes because they melt with the increase of temperature. Antarctic icebergs, for example, do not normally cross the so-called ‘Antarctic convergence zone’  , a belt at the 45-55° latitude,  in the Southern Hemisphere,  however   exceptions are  always possible: the most globetrotting iceberg ever seen, in 1894, drifted up to  the 26°30’ latitude in the Southern Hemisphere, in the Atlantic Ocean.
Iceberg names: their identification card
Big icebergs (more interesting to study, but also more dangerous for navigation) are constantly monitored and kept under control by satellites (now also with the use of GPS). To simplify their identification, the NIC (National Ice Centre of the USA that is controlled by  the Defence Department and the NOAA, National Oceanic and Atmospheric  Administration together with the American coast guards, gives each new iceberg a name.
In Antarctica, iceberg names are formed by a letter, that identifies the area of origin, and a number, that indicates the order in which the iceberg has been sighted, in progressive numbers starting from 1976, the year in which the monitoring service was created; there is yet another letter, that identifies the ‘children’ born from subsequent calving.  The iceberg B-15A, which  is often referred to, is the biggest fragment (A) of B-15, the fifteenth iceberg that was formed in zone B, in the Eastern Ross Ice shelf.
The zones into which Antarctica has been subdivided are, counter-clockwise:

  • A : 0-90W (Weddell Sea/Bellinghausen)
  • B: 90W-180 (Eastern Ross Ice shelf/Amundsen)
  • C: 180-90E (Western Ross Ice shelf/Wilkesland)
  • D: 90E-0 (Eastern Weddell Sea/Amery)

Is the weather changing?
Every year the Antarctic ice shelves ‘lose’ about 1,450-2,000 km3 of ice: what does this imply? Are the ice shelves breaking down  fast and will soon disappear?
Observing the available data, we can establish that from when Antarctic explorations started, the calving of icebergs, even of big dimensions, is a common event, which is not at all ‘catastrophic’. Even the dimensions of the ice shelves vary continuously, expanding and retreating according to a trend that hasn’t yet been established. In May 2002, the Ross ice shelf ‘lost’ an iceberg a little less than 200 km long and within another few months at least other three giants, between 80 and 50 km long, broke off from the same zone: subsequent to these events, the Ross ice shelf has gone back to its measurements of 1911 that had been recorded by the explorer Robert Scott. This would seem to imply an expansion respect to the first years of last century. From 1979, though, it apparently seems that the fragmentation of Antarctic ice shelves has intensified. However, only by observing the icebergs, it is impossible to establish if the ice  of Antarctica is decreasing: to be able to make more precise statements, the amount of ice lost in the form of icebergs and the quantity of new ice formed due to annual snowfall must be calculated and compared. Only if these data are available over a certain number of years is it possible to determine a  balance and therefore try to make a forecast about the future of Antarctic ice .
What is certain is that respect to the Forties  an increase of 2.5°C in the average annual temperature has been observed that should accelerate the melting of ice , but alongside this phenomenon, in some parts of Antarctica, an increase in snowfall has been registered in recent years – this should increase the supply of the ice cap and its glaciers. Which of these tendencies will prevail in the future is still unknown, but it is clear how everything depends on a fragile equilibrium, in which the above-mentioned factors are just two of the many variables involved in the complex game of the Earth’s climate. In conclusion, more years of studies and constant observations are necessary to understand the functioning of the climate of our planet and to be able to present models that foresee the near future: the fundamental importance of field research  can therefore be understood to be able to support, with objective data collected ‘in nature’, scientific theories and forecast models.

The ice factory
The researchers of the bases of Terranova Bay (Italian base) and Mc Murdo (USA base) are studying the behaviour of B-15A ‘live’,  and also the behaviour of its ‘smaller brothers’ and  its very numerous ‘close relatives’ present in the area. This study is particularly important to  better understand the functioning of the ‘motors’ of our climatic system to then be able to formulate hypotheses on the possible evolution of the climate in the near future. The circulation of the Circumantarctic Current and the heat exchange with the warmer waters of the Equatorial belt seem to play a fundamental role in influencing the climate of our planet, at all latitudes. Terranova Bay is a broad inlet  in the Ross Sea that has the singular characteristic of always being free from sea ice , due to the presence of the ice tongue  of the Drygalski Glacier and the strong winds that blow from land towards the sea. The winds, in fact, often blowing at over 200 km/hr, push the forming sea ice towards the open sea, while the Drygalski tongue inhibits the entrance into the bay of sea ice formed elsewhere and pushed along by the currents. This body of water that remains free from ice, allows a great exchange of heat between the masses of cold air that come from the ice cap and the ocean waters and this has important repercussions on the global ocean circulation. Also, due to the fact that the sea ice that has just formed gets pushed away, Terranova Bay produces  10 to 30% of the sea ice produced in the Ross Sea, even though it accounts for only 1% of its surface: this implies that it is one of the planet’s most important ‘ice factories’, and it is clear that its study is of great importance to understand the behaviour of the planet itself.

Written by Paola Tognini

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
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