A crucial crack in an Antarctic ice sheet grew 11 miles in only 6 days

The widening rift in the Larsen C ice shelf in West Antarctica has reached a (relative, for ice) fever pitch in the past few months, stretching for more than 110 miles and gaping more than 1,000 feet across. In just the last couple of days, the crack grew another 11 miles and took a sharp right turn toward the sea. It now ends a mere eight miles from totally breaking through, according to observations from ice-monitoring mission Project Midas. Floating ice shelves like Larsen C hold back the flow of ice sheets and glaciers into the sea, meaning the entire West Antarctic Ice Sheet is about to get more unstable.

— source grist.org

Antarctic ice rift spreads

The main rift in Larsen C, which is likely to lead to one of the largest icebergs ever recorded, is currently 180 km long. The new branch of the rift is 15 km long. Last year, researchers from the UK’s Project Midas, led by Swansea University, reported that the rift was growing fast. Now, just 20km of ice is keeping the 5,000 sq km piece from floating away.

— source swansea.ac.uk

Giant crack in Antarctic ice shelf spotlights advances in glaciology

A giant rift in the Larsen C ice shelf (shown here dividing the purple and pink areas) is expected to reach the ocean soon. NASA/GSFC/JPL

A massive crack in Antarctica’s fourth-biggest ice shelf has surged forward by at least 10 kilometres since early January. Scientists who have been monitoring the 175-kilometre rift in the Larsen C ice shelf say that it could reach the ocean within weeks or months, releasing an iceberg twice the size of Luxembourg into the Weddell Sea.

The plight of Larsen C is another sign that global warming is destabilizing ice along the eastern Antarctic Peninsula and raising sea levels. But scientists’ studies of the rift also illuminate how far glaciology has come since the collapse of the ice shelf’s northern siblings: Larsen A in 1995 and Larsen B in 2002, which occupied separate embayments further out along the peninsula.

“Larsen B was a turning point in our understanding,” says Ala Khazendar, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “It was the biggest collapse of its kind up to that point, and it served to demonstrate how ice shelves regulate the movement of ice from the interior of the ice sheet to the ocean.”

For decades beforehand, researchers had debated the extent to which ice shelves buttress glaciers on land — acting like corks that slow the land ice’s inevitable march to the sea. The late Bob Thomas, a NASA glaciologist who helped to popularize the idea, went so far as to uncork a bottle of wine and pour some out to demonstrate the effect during his talks.

Satellite data collected after Larsen B collapsed largely settled the debate1, 2. The speed at which glaciers connected to Larsen A and B flowed to the sea increased — by up to a factor of eight — after those ice shelves disintegrated, says Eric Rignot, a glaciologist at the University of California, Irvine. “Some of [the glaciers] have slowed down a little bit, but they are still flowing five times faster than before,” he notes.

Source: NASA Earth Observatory/Jessie Allan/US Geological Survey

Since Larsen B’s collapse, ice-sheet modellers have tweaked their simulations to better reflect the forces driving glacial flow and to help quantify this corking effect — bolstering confidence that limited observations from the Larsen ice shelves could be applied more broadly.

Researchers are now looking back to the history of Larsen A and B (see ‘Cracking up’) to understand what the future might hold for Larsen C, which covers 50,000 square kilometres with ice up to 350 metres thick. Many fear that the expanding crack is a sign that Larsen C has begun a long decline that will inevitably end in its total collapse. How soon that could come after the iceberg breaks off is an open question.

Shock waves

The effects of a collapse could be felt far beyond Antarctica. The glaciers that flow into Larsen C contain enough water to raise the global sea level by about a centimetre — and they are likely to flow faster to the ocean in the absence of an ice shelf. In comparison, global sea levels are rising by about 3 millimetres a year, and a recent study estimated that one-third of that comes from ice loss in Antarctica and Greenland3.

Satellite images show that Larsen C has been receding since the 1980s, and radar measurements suggest that its ice is also thinning, Rignot says. Scientists have also seen meltwater ponds forming on the ice shelf’s surface 4; the same sort of ponds probably hastened the disintegration of Larsen B by carving holes in the ice and expanding cracks.

The ice sheet is protected, to some degree, from rapid collapse by favourable seafloor geometry. A pair of underwater ridges that surround Larsen C create friction that slows the flow of ice to the ocean. Nonetheless, Khazendar and his colleagues say that two of the glaciers flowing into Larsen C have already begun to accelerate as the ice shelf has weakened.

The parallels with the decline of Larsen B are striking, says Adrian Luckman, a glaciologist at Swansea University, UK, who heads a team that has monitored the Larsen C ice crack for several years. Larsen B experienced a major iceberg-calving event in 1995, followed by gradual retreat and then complete collapse seven years later. Larsen C may follow a similar pattern, he says, although it’s not clear how soon collapse might follow the imminent calving event.

For now, researchers are anxiously watching the expanding ice rift. Chris Borstad, a geophysicist at the University Centre in Svalbard, Norway, is particularly interested in Larsen C’s ‘suture zones’ — areas where glacial ice flows off land and merges. The ice is softer in these areas, which are often held together by ice that freezes from below.

Dozens of significant cracks run into one of these zones on Larsen C, and then stop, he says. The current crack was among them, but it somehow broke through in 2014 and has continued to expand ever since. It’s not clear why the crack made it through the soft ice, and whether other rifts will follow suit in the coming years.

“We don’t know why, but there’s something very effective about these boundaries for stopping cracks, and that may be the key,” Borstad says. “To answer that question, we really need to get out there into the field.”

— source nature.com by Jeff Tollefson

Antarctica Just Shed a Manhattan-Sized Chunk of Ice

The Pine Island Glacier on the coast of West Antarctica is a case in point. A massive iceberg roughly 225 square miles in size — or in more familiar terms, 10 times the size of Manhattan — broke off in July 2015. Scientists subsequently spotted cracks in the glacier on a November 2016 flyover. And in January, another iceberg cleaved off the glacier.

The ocean under Pine Island Glacier’s ice shelf has warmed about 1°F since the 1990s. That’s causing the ice shelf to melt and pushing the grounding line — the point where the ice begins to float — back toward land, creating further instability.

— source climatecentral.org

Hidden lakes drain below West Antarctica’s Thwaites Glacier

Thwaites Glacier on the edge of West Antarctica is one of the planet’s fastest-moving glaciers. Research shows that it is sliding unstoppably into the ocean, mainly due to warmer seawater lapping at its underside.

Researchers at the University of Washington and the University of Edinburgh used data from the European Space Agency’s CryoSat-2 to identify a sudden drainage of large pools below Thwaites Glacier, one of two fast-moving glaciers at the edge of the ice sheet. The study published Feb. 8 in The Cryosphere finds four interconnected lakes drained in the eight months from June 2013 and January 2014. The glacier sped up by about 10 percent during that time, showing that the glacier’s long-term movement is fairly oblivious to trickles at its underside.

— source washington.edu

Polar Sea Ice the Size of India Reportedly Vanishes in Record Heat

Sea ice off Antarctica and in the Arctic is at record lows for this time of year after declining by twice the size of Alaska in a sign of rising global temperatures, climate scientists say. “There are some really crazy things going on,” said Mark Serreze, director of the U.S. National Snow and Ice Data Center (NSIDC) in Boulder, Colorado, saying temperatures in parts of the Arctic were 20 degrees Celsius (36°F) above normal some days in November. Worldwide, this year is on track to be the warmest on record. Combined, the extent of polar sea ice on Dec. 4 was about 3.84 million square kilometers (1.48 million square miles) below the 1981-2010 average, according to NSIDC satellite measurements. That is roughly the size of India, or two Alaskas.

— source scientificamerican.com

Don’t be fooled by a cooling Antarctica

For most of the latter half of the 20th century, the Antarctic Peninsula was one of the fastest-warming places on the planet, with serious repercussions for the local environment, including the spectacular disintegration of a millennia-old ice shelf, and global sea level rise.

But a new study detailed Thursday in the journal Nature suggests temperatures on the peninsula have dropped slightly since the late 1990s. This relative cooling is partly driven by the recovery of the ozone hole, and is, along with the warming that preceded it, within the realm of the wild shifts in climate the region has naturally experienced in the past, they found.

The cooling is relatively minor — less than 2 degrees F (1 degree C) since the 1990s — and it doesn’t negate the background warming that is happening because of the steady rise of heat-trapping greenhouse gases in the Earth’s atmosphere, the researchers said. It is simply masking it for the time being. Eventually, human-driven warming will overwhelm the influence of the ozone hole recovery and natural climate drivers, and temperatures will once again rise.

Up and down

The Antarctic Peninsula is the arm of land that juts up from the continent. Between 1951 and 2000, the temperature at one weather station on its west coast rose by about 5 degrees F (2.8 degrees C), compared to a little more than 1 degree F (0.5 degrees C) globally.

That warming was accompanied by significant decreases in sea ice, changes to plant and animal communities, and the melt of land-bound glaciers and the floating ice shelves that buttress them.

In 2002, the Larsen B ice shelf spectacularly collapsed, stunning Antarctic researchers. A 2014 study pointed to the warm air that bathed the peninsula in the preceding summers as the precipitating cause of the ice shelf’s demise.

But since the 1990s, the average temperature of the peninsula has declined by 0.9°F (0.5°C) per decade, Turner and his colleagues found when they looked at weather station data. That is about the same rate that it had warmed in the previous five decades.

There had been signs that the warming had abated, such as a slowing of the expansion of plants that linked the higher temperatures, and a smaller proportion of glaciers in retreat. Turner and his team wanted to know what was causing the relative downturn.

“Nowhere on Earth does climate change for a single reason,” he said.

Cooling causes

The researchers found that changes in regional wind patterns — from warm-inducing westerlies to cold-bringing easterlies — were linked to both the warming and cooling periods the peninsula has undergone.

The development and then recovery of the ozone hole was a key driver, as it helped to alter those winds, as did the prevailing climate patterns in the tropical Pacific Ocean.

“The ozone hole really had a big impact, especially in the summer,” Turner said.

The period of warming was due to some combination of the ozone hole, greenhouse warming, and a tendency toward El Niño-like conditions in the Pacific, all favoring warmer westerlies. The cooling period has happened as the ozone hole has begun to heal and the Pacific Ocean favored La Niña-like conditions, ushering in more easterlies. (Those easterly winds also blew sea ice toward the peninsula and helped to reinforce the cooling, as the ice blocked the transfer of heat from the ocean to the atmosphere.)

“This is an excellent study by some of the international leaders in Antarctic climate — what it does is provide a climate context for a trend that’s been reported in a couple of ways previously,” Ted Scambos, a glaciologist at the National Snow & Ice Data Center in Boulder, Colo., said in an email. Scambos was not involved in the study.

Warming will win out

To put the warming and cooling periods into context, the team included analyses of ice core records. These cylinders of ice drilled from Antarctic glaciers can reveal temperature patterns and showed that episodes like these had occurred in the past as the result of natural variations in climate. The warming period, while unusual, wasn’t unprecedented. Nor is the cooling period.

That doesn’t mean that greenhouse gas-driven warming isn’t playing a role now, either on the peninsula or on Antarctica as a whole (of which the peninsula only makes up 1 percent). Instead, it means that the large ups and downs driven by the ozone hole and natural variability are overwhelming the long-term warming signal for the time being.

If emissions of carbon dioxide and other gases continue, that signal will eventually emerge, likely sometime in the next two to three decades, Turner said. Then the impacts — from disrupting the peninsula’s resident Adélie penguins to causing further ice melt and sea level rise — will resume.

“The effect of this cooling/flattening of the warming trend in the northern Peninsula is to put the region into a giant holding pattern,” Scambos said, “hanging right at the tipping point of further massive retreat and glacier speed-up.”

— source grist.org By Andrea Thompson