An iceberg about six times the size of Auckland is about to break off Antarctica

Only 4.8 remaining kilometres of ice continue to connect the impending iceberg to the Larsen C ice shelf. Photo / AP

By Chris Mooney

An enormous iceberg, more than 5180 square kilometres in area - almost six times the size of Auckland - is poised to detach from one of the largest floating ice shelves in Antarctica and float off in the Weddell Sea, south of the tip of South America.

Scientists have been expecting the break from the Larsen C ice shelf, monitoring the progress of a crack that extended to more than 160km long in recent months. The latest update from scientists with Nasa and the University of California Irvine found that only 4.8km of ice connect the impending iceberg to the larger shelf.

Those parts of the iceberg that have already detached have begun to move rapidly seaward, widening the rift in recent days and leaving the remaining ice "strained near to breaking point," according to Adrian Luckman, a scientist monitoring Larsen C at Swansea University in Wales.

The expected calving will not affect global sea level, because the ice that has detached was already afloat in the ocean. But some scientists fear that it could hasten the destabilisation of the larger Larsen C ice shelf.

The iceberg will be one of the most massive ever seen from Antarctica. It will be more than 182m thick and contain roughly 1 trillion tonnes of ice, according to an analysis by the European Space Agency and Noel Gourmelen, a scientist at the University of Edinburgh.

Scientists are divided about the impact the break will have on Antarctica's ice shelf.

Some have contended there's little proof that the break, which will reduce the size of the Larsen C more than scientists have observed previously, reflects the advance of climate change. Ice shelves do, after all, break off sometimes.

"We do not need to press the panic button for Larsen C. Large calving events such as this are normal processes of a healthy ice sheet, ones that have occurred for decades, centuries, millennia - on cycles that are much longer than a human or satellite lifetime," Helen Amana Fricker, an Antarctic scientist at the Scripps Institution of Oceanography, wrote recently.

But others disagree.

"Of course this is due to climate warming in the peninsula," Eric Rignot, a Nasa and University of California Irvine expert on Antarctica, said.

Antarctica has seen an increase in breaks in its ice shelves in recent years.

This November 10, 2016 aerial photo released by NASA, shows a rift in the Antarctic Peninsula's Larsen C ice shelf. Photo / AP

The Larsen A ice shelf, far closer to the northern tip of the Antarctic peninsula - and therefore, warmer latitudes - collapsed in 1995. In 2002, the same thing happened with Larsen B, its southern cousin, slightly closer to the South Pole.

Now, Larsen C, still closer to the south pole and subject to cooler temperatures, has seen a major break.

But there are big gaps in scientists' knowledge about what might have disturbed the Larsen C ice shelf.

Recent studies have suggested that the ice of Larsen C has begun to flow more quickly to the sea through the shelf in recent years. The ice shelf has also been thinning and its surface has been getting lower in the water, suggesting that it might be melting from below.

But Fricker presented data to suggest that the ice shelf has since begun to thicken again.

"Yes, I agree Larsen C is 'next in line' southwards after Larsen A and B," Fricker said. "However, there is actually no research showing that Larsen C is getting thinner and flowing faster. In fact, in recent years, it is the opposite."

There is a similar debate over whether this individual break will destabilise the ice shelf and lead to further disintegration.

According to Rignot, Larsen C holds back around 1cm of global sea level rise in the form of glaciers feeding into the remaining ice shelf. If the ice shelf were to continue to disintegrate, the ice might flow more rapidly into the sea.

An even larger fear is the southward and poleward progression of ice shelf collapse, Rignot said, pointing out that farther south are ice shelves that, by stabilising glaciers, are preventing vastly more sea level rise than Larsen C does.

Larsen C is among Antarctica's largest ice shelves but still pales in comparison to the Ross Ice Shelf and Filchner-Ronne Ice Shelf. Scientists this month reported a major melt event that occurred several years ago atop the surface of the Ross Ice Shelf accompanied by at least some rainfall, which also gave them concern.

Scientists will be watching the break closely and trying to glean lessons about what to expect from other potentially vulnerable ice shelves in Antarctica.

"While it might not be caused by global warming, it's at least a natural laboratory to study how breakups will occur at other ice shelves to improve the theoretical basis for our projections of future sea level rise," said Nasa's Tom Wagner, who directs the agency's polar programmes.