A mere 1,000 feet above the frigid waters of the North Atlantic the debate began in earnest. The pilot of the US Coast Guard's sturdy C130 plane believed the object which had appeared on both of the plane's radars was an iceberg. One of two young but experienced ice observers on board disagreed.

To definitively identify the target, the plane started to descend to a mettle-testing 400 feet. This was part of the mission, and what is demanded of the staff of the International Ice Patrol (IIP) by the hundreds of ships that traverse this relatively small part of the ocean and rely on its findings for their safety.

Ever since the Titanic struck what was actually one of more than 350 icebergs drifting amid the northern Atlantic shipping lanes in April, 1912, the US Coast Guard has undertaken annual iceberg patrols to help protect passenger and freight vessels that sail through the congested waters east of Canada and down the east coast of America.

"Before we started there were 113 recorded sinkings caused by icebergs," says Michael Hicks, the present Commander of the International Ice Patrol. "There have only been 19 since (omega) the Titanic sank, and all of those were vessels that chose to ignore our warnings."

In the past, in a single year, more than 2,000 icebergs have been spotted, tracked and on occasion ineffectually bombed by aircraft, in order to prevent calamitous disasters at sea. Yet in other years, including this one, few if any bergs manage to migrate south from the Arctic Circle. If the unidentified floating object below the approaching plane is in fact an iceberg, it will be the first one seen in the shipping lanes since May 2005 ­ a situation perplexing to oceanographers but emboldening to those shouting loudly about the effects of climate change.

"I've been trying to understand the variability for years," says Don Murphy the IIP's veteran oceanographer. "And every year that goes by I get another year of experience and realise how little we really know."

After a century of study, there are still many unknowns regarding the movement of icebergs and the reasons for the wide variability in the number that mischievously make it into the 300-mile-long, 60-mile-wide area of the North Atlantic that mariners refer to as "iceberg alley" due to the number of bergs pushed through this part of the ocean by currents and underwater topographical features.

What is certain is that any that do make it this far south will be nearing both the end of a three-year journey and their existence.

"When the bergs get this far south, their days are numbered," Hicks says, explaining that the warmer waters of the Gulf Stream cause all icebergs to ultimately melt, ensuring one less threat to ships and one less iceberg for the IIP to monitor. What formed from 1,000-year-old ice atop the majestic ice cap in Greenland ends up becoming an indistinguishable part of the ocean and a sterile statistic in the Coast Guard's table of meddlesome bergs.

The International Ice Patrol is a unique organisation. A division of the US Coast Guard, it is funded by 17 countries including the UK, and is the only world body that constantly monitors icebergs that stray into the Atlantic shipping lanes parallel to and south of Newfoundland. While the Canadian Ice Service is dedicated to monitoring sea ice and icebergs in Canadian waters, and the Danish Meteorological Service is concerned with bergs around its territory of Greenland, the IIP has, since its inception in 1912, been charged with alerting cross-Atlantic traffic to any iceberg threats.

"Almost immediately after the Titanic sank the US Navy assigned two cruisers to the Grand Banks to patrol for icebergs and to let the ships know where they were," says Hicks. "The following year the US Navy could no longer spare the ships to do that so the Revenue Cutter Service, which was the predecessor to the US Coast Guard, stepped up. The UK actually asked the US government, since we had started doing this, to continue and, with the exception of the World War years, we have been doing it ever since."

The International Convention for the Safety of Life at Sea (Solas), a major treaty governing merchant ships, mandates that all vessels crossing the North Atlantic during the months when there is a threat from icebergs ­ usually February to July ­ receive and read the IIP's notifications. However, they are not compelled to act on them, hence the 19 sinkings that have occurred since 1913.

An iceberg (technically a piece of ice longer than 45 feet at the waterline) has to make it past 48 degrees north ­ a line of latitude that passes through northern France in Europe ­ before the IIP count it as having the potential to interfere with shipping (the Titanic sank just south of 42 degrees north, and icebergs have been spotted in waters as far south as those parallel to Washington DC).

Surprisingly it wasn't until 1946 that the Coast Guard started using aircraft to supplement its ships in the search for these monstrous and potentially lethal pieces of ice.

"Up until that time it was just cutters," says Hicks. "Generally they would assign two or three cutters and they would take turns going out from Halifax or St John's, finding the southernmost iceberg and staying with it and radioing their position to ships coming across the Atlantic."

The IIP stopped using ships in 1973, and has relied on aeroplanes ever since.

"The C130 aircraft can cover a much larger area in a lot less time," says Hicks. "They also have the capacity to carry side-looking airborne radar which is an old but very effective radar system. It was designed to detect oil spills but it works pretty well for icebergs."

The planes, which conduct 12 or more missions each month, normally fly for six to eight hours at an altitude of between 5,500 and 8,000 feet, yet because of the weather, even at that height, the sea is visible only 30 per cent of the time.

"We will descend as low as 400 feet if we detect a target on the radar that we can't identify and the cloud base is low," Hicks explains. "Hopefully we will see the surface by that point."

Once an iceberg has been detected, its size and location are plotted on to a map which is marked with the limits of all known ice. The map is then disseminated to all ships crossing the Atlantic (more than a dozen a day) and is posted on the IIP's website. To ensure a margin of safety, the line depicting the limits of all known ice is usually drawn 30 miles south of the actual last known position of an iceberg.

When the planes aren't flying, the maps are still updated every 24 hours.

"We forecast where we think the bergs are going using a computer model," Hicks says. "That model uses ocean currents, winds, water temperature and waves to predict where the iceberg is going to drift and how long it is going to take to melt."

Hicks admits the model is not flawless.

"In the short term it does a pretty good job," he says. "But as you go beyond six or seven days it becomes less reliable, just like a weather forecast, so that's why we patrol so often." Studies (omega) show that icebergs generally drift distances of no greater than 12 miles in a day.

With three fifths of this year's ice season already over, not a single berg has been spotted within 350 miles of the shipping lanes. Only once before, in 1966, has the IIP recorded zero icebergs in a season. There is no question the low number of bergs is unusual.

"On average we expect to see about 450 icebergs a year," says Don Murphy, who explains that all of the icebergs that appear in the North Atlantic are produced by ice flowing slowly but steadily from glaciers in Greenland into the sea and breaking away. The floating chunk of frozen water is then at the mercy of ocean currents which initially carry it northwards and then westwards across to Canada's Baffin Bay before the Labrador Current pushes it southwards.

"The currents are weak and variable along Baffin island so every iceberg's southward track is characterised by long periods of no motion," says Murphy. "Most will go aground on a pinnacle or submerged mountain around the edge of the continental shelf and you have to wait for them to deteriorate until finally they can float off the bottom and can continue their path. Then boom! They get stuck again or get driven so far into a bay that they never make it out and waves destroy them."

Murphy says that the ones that do make it ­ less than one per cent of all the bergs produced annually ­ are "the ones with a shape that ensures they aren't as sensitive to the wind's effects so they stay offshore or have enough mass that even if they are grounded for a while, when they float again, they are large enough that they still bring a substantial amount of ice with them."

Those icebergs that survive all the way from the glaciers to the Grand Banks take up to three years to make the 1,500-mile journey. Once there they will be tracked by the IIP and left to disintegrate in the ocean. However, from shortly after the Titanic sank and up until the 1960s, there was a belief that icebergs could, and should, be destroyed.

"It was thought that if you shot a shell at an iceberg that the ice was so brittle that it would just disintegrate," Murphy says. "The US Navy attempted it but all they did was loosen a basketful of ice. Then they decided to float mines against the sides of the bergs and blow them up but that didn't work either."

Further attempts were made after the Second World War using larger bombs. "They arranged to drop 1,000lb bombs on an iceberg. They hit it 17 times over a six-day period but the end result was an insignificant change in the size of the berg so they gave up. It was a silly idea and it was abandoned; the ocean does the job perfectly."

Icebergs always succumb to warm water and warm weather but now there is considerable debate as to whether climate change is playing any part in the fate or future of these floating photogenic white sculptures.

"There is no doubt in my mind that major climate change is happening," says Murphy who has been a professional oceanographer for 22 years. "Studies in Greenland show that the glaciers are moving twice as fast as before. That means a lot of production of ice. My expectation has always been if the Greenland glaciers started moving faster there would be increased production [of icebergs] for decades and there should be an increase in the number of icebergs into the shipping lanes. That was my model. But the last couple of years that hasn't happened, and I'm having a hard time understanding what is going on except that there are complicating factors having to do with increased storms. Maybe the destruction processes dominate over the production processes."

The main destruction process is wave action. Icebergs that run aground are the most vulnerable to sustained wave attack. In past years large concentrations of sea ice have been thought to help icebergs remain afloat and prevent erosion from waves.

In 2005, according to the US National Snow and Ice Data Centre, sea-ice cover was at its lowest extent since satellite monitoring began in 1979, and this year the IIP have noticed "very little ­ although not an absolute minimum" level of sea-ice conditions. Yet a computer model linking sea-ice levels to the number of icebergs making it into the shipping lanes has performed "horribly" according to Murphy.

So while climate change could be expected to bring about an increase in the number of icebergs being forced into the ocean, its effect in reducing the level of sea ice through increased sea temperatures could equally mean that those icebergs liquefy long before they reach any areas of concern.

Yet, Murphy points out, that does not explain the huge discrepancy in the number of icebergs recorded in years before climate change was an issue: e.g., 15 icebergs in 1952; 1,500 in 1972. After thoroughly studying and analysing data from as far back as 1900, Murphy can find no significant or consistent pattern in the number of icebergs making it into the shipping lanes.

"It's a very complicated system and there are a lot of moving parts," he says, but he claims some people are eager to ascribe meaning to the figures.

"Back in the mid-1990s, when we had thousands of icebergs, I got a call from Japanese TV who wanted to do a story on us because they believed the large number of icebergs was indicative of global warming," he says. "Then, in 1999, we had only 22 icebergs and I got a call from a European TV company who wanted to do a story because they were certain that the fact that there were only 22 bergs in the shipping lanes was a clear indication of global warming."

Murphy himself is reluctant to draw any conclusions from the changing number of icebergs. Commander Hicks also believes the century-long variability precludes any simple answers.

"There would have to be a decade of consistently light or consistently heavy years to say something is happening here and we haven't seen that," he says. Nevertheless, only 11 icebergs were spotted in the shipping lanes last year and they remain even more elusive this year.

This makes the debate on board the low-flying C130 all the more intense. As the plane continues its descent through the clouds, seagulls are spotted atop the floating white object. At 400 feet above the churning ocean, when the flight crew and the ice observers can all see the object in detail through thick Plexiglas windows, they realise it is, disappointingly, an inverted, dead Northern Right whale.

"We'll keep looking," says Hicks. "We know there are icebergs out there and maybe some will make it south before July." If they do, the International Ice Patrol will certainly count them but they will happily leave it to others to decide upon the significance of the number.

Chilling facts The truth about ice loss and global warming

140 cubic miles The amount of overall ice mass lost from the Greenland ice sheet in 2005, enough to fill Loch Ness 70 times over.

5 degrees The average air temperature increase in the Arctic over the last 100 years.

60% The increase in ice loss from the Greenland ice sheet since 1996.

22ft The rise in sea levels if the Greenland ice sheet melted, which would deluge Florida, the Netherlands, Bangladesh and most of London.

14% The drop in Arctic sea ice since the 1970s.

20% The decrease in the polar bear population in Canada's western Hudson Bay in the last 17 years, due to loss of sea ice.

10 The number of years since 1988 that have been the hottest on record.

30 How many times faster greenhouse gases are being released compared to the last period of extreme global warming.

$150bn The cost to the world's economies of fully implementing the Kyoto agreement ­ the benefits of which would not begin to be felt for 100 years.

0 tons The amount of Arctic ice that will be left during the summer months, within 80 years.