Icebergs once drifted to Florida, new climate model suggests

This is a map showing the pathway taken by icebergs from Hudson Bay, Canada, to Florida. The blue colors (behind the arrows) are an actual snapshot from the authors' high resolution model showing how much less salty the water is than normal. The more blue the color the less salty it is than normal. In this case, blue all the way along the coast shows that very fresh, cold waters are flowing along the entire east coast from Hudson Bay to Florida. -  UMass Amherst
This is a map showing the pathway taken by icebergs from Hudson Bay, Canada, to Florida. The blue colors (behind the arrows) are an actual snapshot from the authors’ high resolution model showing how much less salty the water is than normal. The more blue the color the less salty it is than normal. In this case, blue all the way along the coast shows that very fresh, cold waters are flowing along the entire east coast from Hudson Bay to Florida. – UMass Amherst

Using a first-of-its-kind, high-resolution numerical model to describe ocean circulation during the last ice age about 21,000 year ago, oceanographer Alan Condron of the University of Massachusetts Amherst has shown that icebergs and meltwater from the North American ice sheet would have regularly reached South Carolina and even southern Florida. The models are supported by the discovery of iceberg scour marks on the sea floor along the entire continental shelf.

Such a view of past meltwater and iceberg movement implies that the mechanisms of abrupt climate change are more complex than previously thought, Condron says. “Our study is the first to show that when the large ice sheet over North America known as the Laurentide ice sheet began to melt, icebergs calved into the sea around Hudson Bay and would have periodically drifted along the east coast of the United States as far south as Miami and the Bahamas in the Caribbean, a distance of more than 3,100 miles, about 5,000 kilometers.”

His work, conducted with Jenna Hill of Coastal Carolina University, is described in the current advance online issue of Nature Geosciences. “Determining how far south of the subpolar gyre icebergs and meltwater penetrated is vital for understanding the sensitivity of North Atlantic Deep Water formation and climate to past changes in high-latitude freshwater runoff,” the authors say.

Hill analyzed high-resolution images of the sea floor from Cape Hatteras to Florida and identified about 400 scour marks on the seabed that were formed by enormous icebergs plowing through mud on the sea floor. These characteristic grooves and pits were formed as icebergs moved into shallower water and their keels bumped and scraped along the ocean floor.

“The depth of the scours tells us that icebergs drifting to southern Florida were at least 1,000 feet, or 300 meters thick,” says Condron. “This is enormous. Such icebergs are only found off the coast of Greenland today.”

To investigate how icebergs might have drifted as far south as Florida, Condron simulated the release of a series of glacial meltwater floods in his high-resolution ocean circulation model at four different levels for two locations, Hudson Bay and the Gulf of St. Lawrence.

Condron reports, “In order for icebergs to drift to Florida, our glacial ocean circulation model tells us that enormous volumes of meltwater, similar to a catastrophic glacial lake outburst flood, must have been discharging into the ocean from the Laurentide ice sheet, from either Hudson Bay or the Gulf of St. Lawrence.”

Further, during these large meltwater flood events, the surface ocean current off the coast of Florida would have undergone a complete, 180-degree flip in direction, so that the warm, northward flowing Gulf Stream would have been replaced by a cold, southward flowing current, he adds.

As a result, waters off the coast of Florida would have been only a few degrees above freezing. Such events would have led to the sudden appearance of massive icebergs along the east coast of the United States all the way to Florida Keys, Condron points out. These events would have been abrupt and short-lived, probably less than a year, he notes.

“This new research shows that much of the meltwater from the Greenland ice sheet may be redistributed by narrow coastal currents and circulate through subtropical regions prior to reaching the subpolar ocean. It’s a more complicated picture than we believed before,” Condron says. He and Hill say that future research on mechanisms of abrupt climate change should take into account coastal boundary currents in redistributing ice sheet runoff and subpolar fresh water.

New study finds Antarctic Ice Sheet unstable at end of last ice age

This is one of many icebergs that sheared off the continent and ended up in the Scotia Sea. -  Photo courtesy of Michael Weber, University of Cologne
This is one of many icebergs that sheared off the continent and ended up in the Scotia Sea. – Photo courtesy of Michael Weber, University of Cologne

A new study has found that the Antarctic Ice Sheet began melting about 5,000 years earlier than previously thought coming out of the last ice age – and that shrinkage of the vast ice sheet accelerated during eight distinct episodes, causing rapid sea level rise.

The international study, funded in part by the National Science Foundation, is particularly important coming on the heels of recent studies that suggest destabilization of part of the West Antarctic Ice Sheet has begun.

Results of this latest study are being published this week in the journal Nature. It was conducted by researchers at University of Cologne, Oregon State University, the Alfred-Wegener-Institute, University of Hawaii at Manoa, University of Lapland, University of New South Wales, and University of Bonn.

The researchers examined two sediment cores from the Scotia Sea between Antarctica and South America that contained “iceberg-rafted debris” that had been scraped off Antarctica by moving ice and deposited via icebergs into the sea. As the icebergs melted, they dropped the minerals into the seafloor sediments, giving scientists a glimpse at the past behavior of the Antarctic Ice Sheet.

Periods of rapid increases in iceberg-rafted debris suggest that more icebergs were being released by the Antarctic Ice Sheet. The researchers discovered increased amounts of debris during eight separate episodes beginning as early as 20,000 years ago, and continuing until 9,000 years ago.

The melting of the Antarctic Ice Sheet wasn’t thought to have started, however, until 14,000 years ago.

“Conventional thinking based on past research is that the Antarctic Ice Sheet has been relatively stable since the last ice age, that it began to melt relatively late during the deglaciation process, and that its decline was slow and steady until it reached its present size,” said lead author Michael Weber, a scientist from the University of Cologne in Germany.

“The sediment record suggests a different pattern – one that is more episodic and suggests that parts of the ice sheet repeatedly became unstable during the last deglaciation,” Weber added.

The research also provides the first solid evidence that the Antarctic Ice Sheet contributed to what is known as meltwater pulse 1A, a period of very rapid sea level rise that began some 14,500 years ago, according to Peter Clark, an Oregon State University paleoclimatologist and co-author on the study.

The largest of the eight episodic pulses outlined in the new Nature study coincides with meltwater pulse 1A.

“During that time, the sea level on a global basis rose about 50 feet in just 350 years – or about 20 times faster than sea level rise over the last century,” noted Clark, a professor in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences. “We don’t yet know what triggered these eight episodes or pulses, but it appears that once the melting of the ice sheet began it was amplified by physical processes.”

The researchers suspect that a feedback mechanism may have accelerated the melting, possibly by changing ocean circulation that brought warmer water to the Antarctic subsurface, according to co-author Axel Timmermann, a climate researcher at the University of Hawaii at Manoa.

“This positive feedback is a perfect recipe for rapid sea level rise,” Timmermann said.

Some 9,000 years ago, the episodic pulses of melting stopped, the researchers say.

“Just as we are unsure of what triggered these eight pulses,” Clark said, “we don’t know why they stopped. Perhaps the sheet ran out of ice that was vulnerable to the physical changes that were taking place. However, our new results suggest that the Antarctic Ice Sheet is more unstable than previously considered.”

Today, the annual calving of icebergs from Antarctic represents more than half of the annual loss of mass of the Antarctic Ice Sheet – an estimated 1,300 to 2,000 gigatons (a gigaton is a billion tons). Some of these giant icebergs are longer than 18 kilometers.

Sea level rise: New iceberg theory points to areas at risk of rapid disintegration

In events that could exacerbate sea level rise over the coming decades, stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean, according to new iceberg calving simulations from the University of Michigan.

“If this starts to happen and we’re right, we might be closer to the higher end of sea level rise estimates for the next 100 years,” said Jeremy Bassis, assistant professor of atmospheric, oceanic and space sciences at the U-M College of Engineering, and first author of a paper on the new model published in the current issue of Nature Geoscience.

Iceberg calving, or the formation of icebergs, occurs when ice chunks break off larger shelves or glaciers and float away, eventually melting in warmer waters. Although iceberg calving accounts for roughly half of the mass lost from ice sheets, it isn’t reflected in any models of how climate change affects the ice sheets and could lead to additional sea level rise, Bassis said.

“Fifty percent of the total mass loss from the ice sheets, we just don’t understand. We essentially haven’t been able to predict that, so events such as rapid disintegration aren’t included in those estimates,” Bassis said. “Our new model helps us understand the different parameters, and that gives us hope that we can better predict how things will change in the future.”

The researchers have found the physics at the heart of iceberg calving, and their model is the first that can simulate the different processes that occur on both ends of the Earth. It can show why in northern latitudes-where glaciers rest on solid ground-icebergs tend to form in relatively small, vertical slivers that rotate onto their sides as they dislodge. It can also illustrate why in the southernmost places-where vast ice shelves float in the Antarctic Ocean-icebergs form in larger, more horizontal plank shapes.

The model treats ice sheets-both floating shelves and grounded glaciers-like loosely cemented collections of boulders. Such a description reflects how scientists in the field have described what iceberg calving actually looks like. The model allows those loose bonds to break when the boulders are pulled apart or rub against one another.

The simulations showed that calving is a two-step process driven primarily by the thickness of the ice.

“Essentially, everything is driven by gravity,” Bassis said. “We identified a critical threshold of one kilometer where it seems like everything should break up. You can think of it in terms of a kid building a tower. The taller the tower is, the more unstable it gets.”

Icebergs do have a tendency to form before that threshold though, Bassis suspects, due to cracks that are already there-either formed when capsizing bergs crash into the water and send shockwaves through the surrounding ice, or when melted water on the surface cuts through. The former is believed to have led to the Helheim Glacier collapse in 2003. The glacier had begun to retreat slowly in 2002, but suddenly gave way the following year when the thinner ice had broken away, exposing a thicker ice coast.

The latter-melted water pools-are occurring more frequently due to climate change, and they’re believed to have played a role in the rapid disintegration of the Antarctica’s Larsen B ice shelf, which crumbled over about six weeks in 2002.

When the researchers added random cracks to their model, it could mirror both Helheim and Larsen B.

A third feature is also required for the most dramatic ice collapses to occur. Icebergs can’t float away and make room for more icebergs to break off the main sheet unless the system has access to open water. So areas that border deep, unobstructed ocean rather than fjords or other waterways are at greater risk of rapid ice loss. The researchers point to the Thwaites and Pine Island glaciers in Antarctica and the Jakobshavn Glacier in Greenland, which is already retreating rapidly, as places vulnerable to “catastrophic disintegration” because they have all three components.

“The ice in those places gets thicker as you go back. If our threshold is right, then if these places start to retreat as you expose the thicker calving font, they’re susceptible to catastrophic breakup,” Bassis said.

Retreat of the current ice coasts in these places areas could occur via melting or iceberg calving.

The contribution of the Greenland ice sheet to sea-level rise will continue to increase

New research has shown surface ice melt will be the dominant process controlling ice-loss from Greenland. As outlet glaciers retreat inland the other process, iceberg production, remains important but will not grow as rapidly.

The Greenland ice sheet is often considered an important potential contributor to future global sea-level rise over the next century or longer. In total, it contains an amount of ice that would lead to a rise of global sea level by more than seven metres, if completely melted.

Changes in its total mass are governed by two main processes – fluctuations in melting and snowfall on its surface, and changes to the number of icebergs released from a large number of outlet glaciers into the ocean.

The ice loss from the ice sheet has been increasing over the last decade, with half of it attributed to changes in surface conditions with the remainder due to increased iceberg calving – the process by which ice detaches from the glacier to become an iceberg.

Researchers from the Vrije Universiteit Brussel, funded by ice2sea, a European Union project, tackled the question of how both processes will evolve and interact in the future. This was done with a computer model, which projects the future ice sheet evolution with high accuracy using the latest available techniques and input data.

They devised a method to generalize projections made in earlier research which concerned just four of Greenland’s outlet glaciers. By doing so they could apply the earlier findings to all calving glaciers around the Greenland ice sheet. Their results indicate a total sea-level contribution from the Greenland ice sheet for an average warming scenario after 100 and 200 years of 7 and 21 cm, respectively.

The balance between the two processes by which ice is lost is, however, changing considerably in the future so that iceberg calving may only account for between 6 % and 18 % of the sea-level contribution after 200 years. This is important, because variations in outlet glacier dynamics have often been suspected to have the potential for very large sea-level contributions.

Lead author Dr Heiko Goelzer, of the Vrije Universiteit Brussel, says,

“Our research has shown that the balance between the two most important mass loss processes will change considerably in the future so that changes in iceberg calving only account for a small percentage of the sea-level contribution after 200 years with the large remainder due to changes in surface conditions.”

The limited importance of outlet glacier dynamics in the future is the result of their retreat back onto land and of strongly increasing surface melting under global warming, which removes ice before it can reach the marine margin.

Ice2sea coordinator Professor David Vaughan, of the British Antarctic Survey says,

“This scenario is no reason to be complacent. The reason the significance of calving glaciers reduces compared to surface melting is, so much ice will be lost in coming decades that many glaciers currently sitting in fjords will retreat inland to where they are no longer affected by warming seas around Greenland.”

Earth from space: Giant iceberg enters Nares Strait

ESA's Envisat satellite has been tracking the progression of the giant iceberg that calved from Greenland's Petermann glacier on August 4, 2010. This animation, generated from 24 Envisat Advanced Synthetic Aperture Radar images acquired from July 31 to September 1, shows that the iceberg, the largest in the northern hemisphere, is now entering Nares Strait -- a stretch of water that connects the Lincoln Sea and Arctic Ocean with Baffin Bay. -  ESA
ESA’s Envisat satellite has been tracking the progression of the giant iceberg that calved from Greenland’s Petermann glacier on August 4, 2010. This animation, generated from 24 Envisat Advanced Synthetic Aperture Radar images acquired from July 31 to September 1, shows that the iceberg, the largest in the northern hemisphere, is now entering Nares Strait — a stretch of water that connects the Lincoln Sea and Arctic Ocean with Baffin Bay. – ESA

ESA’s Envisat satellite has been tracking the progression of the giant iceberg that calved from Greenland’s Petermann glacier on 4 August 2010. This animation shows that the iceberg, the largest in the northern hemisphere, is now entering Nares Strait – a stretch of water that connects the Lincoln Sea and Arctic Ocean with Baffin Bay.

The Petermann glacier in northern Greenland is one of the largest of the country’s glaciers – and until August it had a 70 km tongue of floating ice extending out into the sea. The glacier regularly advances towards the sea at about 1 km per year.

Earlier this year, satellite images revealed that several cracks had appeared. Envisat radar images showed that the ice tongue was still intact on 3 August but, on 4 August, a huge chunk had detached.

Calvings from the Petermann glacier are quite common, but one of this magnitude is rare. Less significant events took place in 2001, in 2008 when a 27 sq km iceberg made its way south to Davis Strait, and in 2009.

This iceberg is about 30 km long and 15 km wide at its foot and almost 7 km wide at its head, covering an area of around 245 sq km. By 22 August this giant mass of ice had been carried about 22 km from its birth place.

On 1 September imagery showed that the iceberg had travelled almost another 6 km from the edge of the glacier and rotated westward (about 39°), just tipping into Nares Strait. The animation also shows that the iceberg hit a small island, which may delay further progression for a short while and may also cause the iceberg to break.

It is expected that the iceberg will soon be fully in Nares Strait, but its course depends on winds blowing off the glacier and currents in the strait, as well as sea ice that could block its path.

The animation was generated from 21 Envisat Advanced Synthetic Aperture Radar (ASAR) Wide Swath Mode (spatial resolution 150 m × 150 m) and three ASAR Image Mode (spatial resolution 30 m × 30 m) images.