Monday, January 10, 2011

Last chance to hold Greenland back from tipping point

On 4 August 2010, the Petermann glacier in Greenland sounded a very big warning. A gigantic slab of ice, the size of Manhattan, broke off. The 100 square mile ice island, which fell away from the Petermann Glacier in northern Greenland, is the largest piece of ice to break from the Arctic icecap since 1962, causing the glacier to retreat 15 kilometers, leaving it further inland than it has been since observations began a century ago.

That warning went unheeded at the UN climate talks in CancĂșn, Mexico, last month. Delegates left without agreeing drastic cuts in greenhouse gas emissions, leaving the planet on course for 3.2 °C of global warming, and Greenland - the world's second largest ice cap - heading for a point of no return.

The suggestion is that Greenland will reach a tipping point in the early 2040s. After that no amount of action on our part can save the ice sheet. Unless governments dramatically up their game, the only thing that will change that date is natural variations in the climate, which might either hasten or delay the tipping point.

Greenland's ice sheet holds enough ice to raise global sea levels by 7 meters. Ice melting at the surface and breaking off at the margins of the ice sheet is already adding up to about 300 gigatons each year. That accounts for about 25 per cent of the annual, global rise in sea levels

Last month's meeting of the American Geophysical Union in San Francisco highlighted the situation. Jason Box of Ohio State University in Columbus and colleagues listed Greenland's "biggest losers": the five glaciers and ice streams that lost the greatest area of ice in the past decade. The Petermann glacier topped the chart, with 500 square kilometers. (see map below left)

But not all ice is created equal. Glaciers in the north like Petermann and Humboldt lost a lot of thin, floating ice that does not impede the outward flow of ice behind. That means the glaciers did not immediately surge seaward. But thicker ice was exposed to the ocean.

Thicker ice acts like a cork in a bottle: take it away and the glaciers accelerate. "If we continue to lose ice, we'll start losing important ice," says team member Ian Howat, also at Ohio State University. "If these glaciers were to accelerate and mobilize the large amount of ice up in northern Greenland, it has the potential for a huge change."

It is the kind of change that has already been seen in Greenland, south of 70 degrees latitude. For instance, the speed at which the Jakobshavn glacier flows has more than doubled over the past 10 years. The Jakobshavn Glacier already flows so fast you can stand on a ridge above it and watch it move. It is the fastest moving glacier on Earth, flowing from the land to the sea at more than 14 kilometers per year. In July 2010, its margin withdrew by about 1.5 kilometers, bringing its grounding line - where the glacier lifts off the bedrock and begins floating - to a knife edge, where bits can break off to form icebergs.

Beneath the ice, Greenland is built like a soup dish: the bedrock slopes down towards the interior and in the case of Jakobshavn bottoms out some 1600 meters below sea level. Jakobshavn's margin is now perched on the edge of that dish. If it breaks up further, it would end up on a downward slope, with nothing to stop it slipping 80 kilometers inland.

The image below is a kind of topographic map showing the elevation of the bedrock below and around the Jakobshavn Glacier — as if Greenland’s overlying ice had been removed. Warm colors show areas of the bedrock that are above sea level. The green and blue colors show areas where it is below sea level. The fjord through which Jakobshavn flows out to sea on Greenland’s western coast is visible as the green and blue trough at the left side of the image. As is evident from the map, the fjord actually continues inland for another 50 miles, and to a depth of 1.6 kilometers below sea level. Why is this significant?

Calving of icebergs from Jakobshavn

The calving front of the glacier marked on the map shows the current edge of the glacier. This is where ice bergs (right) calve from Jakobshaven and
float out to sea. Since 2001, the calving front has retreated about 6 miles (10 kilometers). During July 6 and 7 this past summer, it pulled back an entire mile. Now, the calving front is poised right at the western edge of the fjord’s continuation into the ice sheet.

“If the calving front retreats from where it is now, it can draw out a lot of ice,” says Konrad Steffen, director of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.

The effect will be like pulling the plug from the drain of a bathtub. It will mean that much more ice will be able to drain from the Greenland Ice Sheet through Jakobshavn into the sea.

“You can actually pull out a lot of ice with a fjord below sea level like this,” Steffen says. “And there are about four or five others like this.”

"It would cause a huge embayment into the ice sheet, something that
we have never seen before," says Howat. Jakobshavn is one of many glaciers perched on similar topography. "Once a glacier hits this point, the dynamics of the ice take over. No matter what climate does, whether it gets warmer or colder, that glacier is going to keep [retreating]," says Howat.

One-way ticket

Other factors could also put glaciers on a one-way ticket to extinction.
Kristin Poinar of the University of Washington, Seattle, and colleagues have been studying the bottom of Jakobshavn. The ice there is slushy due to the enormous friction and pressure at those depths: friction within the glacier and against the bedrock generates enormous amounts of heat. Studies show this "temperate ice" layer is about 270 meters thick and acts like a conveyor belt, helping the ice slip faster into the sea.

Not only that, it could give glaciers some form of "memory" of past warm events, says Poinar.In the 1990s, warm ocean waters caused Jakobshavn to speed up dramatically, creating more temperate ice, which could stick around for decades. That means the consequences of sudden changes like Jakobshavn's increase in speed in the 1990s could be felt for tens or hundreds of years, says Poinar.

Thousands of smaller glaciers are also showing dramatic declines. Sebastian Mernild of the Los Alamos National Laboratory in New
Mexico and colleagues have been studying one in south-east Greenland, close to the Sermilik fjord (see right). From photographs going back to 1931, the team calculated that the glacier has retreated by 17 meters per year on average, but in 2010 it lost ground by 35 meters. "The same trend is happening to all the glaciers in east Greenland," says Mernild.

Last year was also a bad one for the ice sheet as a whole. By combining observations and modeling, Mernild's team calculated that 52 per cent
of the ice sheet experienced surface melting. Natural annual variability can't be ruled out, says Mernild, "but if you check the trends, surface melt has been increasing since 1972, all the way up to 2010, and 2010 was a record year". South-west Greenland saw a dramatic increase in the number of melting days, about 50 days more than the average for the past 50 years. And three decades of measurements from the Watson river drainage basin in west Greenland show that surface runoff in 2010 was 30 to 40 per cent higher than average (Cryosphere, DOI: 10.5194/tc-4-231-2010).

More melting is in store, warns Mernild. His team's models show that
Greenland's glaciers haven't fully responded to the temperature rises. In other parts of the world, including Antarctica and the Himalayas, glaciers are about 25 per cent out of equilibrium, meaning that even if warming were to stop today, the glaciers would continue to melt further before stopping. But temperatures in and around Greenland have been increasing faster than elsewhere, and the glaciers there are 70 per cent out of equilibrium, says Mernild.

This worries Mernild. His team modeled the fate of Greenland, using a scenario for future human development outlined by the Intergovernmental Panel on Climate Change. The scenario assumes rapid economic growth, a global population that peaks in 2050, and rapid
adoption of new, efficient technologies for energy use and generation. Given the outcome in CancĂșn last month, it is a likely scenario for the future.

Mernild's models show that if it does play out, Greenland will reach a tipping point in about 30 years. After that nothing will prevent the ice cap from eventually vanishing entirely (Journal of Hydrometeorology, DOI: 10.1175/2009JHM1140.1).

"We can see which way the trend is going," says Mernild. "It doesn't look nice".

New Scientist,"Last chance to hold Greenland back from tipping point", by Anil Ananthaswamy, accessed January 8, 2011
Seismologik, "Climate Change: Glacier In Greenland The Size Of Manhattan Falls Away", accessed January 8, 2011

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