Saturday, January 8, 2011

Last chance to hold Greenland back from tipping point

05 January 2011 by Anil Ananthaswamy 

New data and models show that Greenland's ice cap, the world's second largest, is on track to hit a point of no return in 2040

ON 4 AUGUST 2010, the Petermann glacier in Greenland sounded a warning. A gigantic slab of ice broke off and the glacier retreated 15 kilometres, leaving it further inland than it has been since observations began a century ago.

That warning went unheeded at the UN climate talks in Cancún, 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 metres. Ice melting at the surface and breaking off at the margins of the ice sheet is already adding up to about 300 gigatonnes 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 kilometres (see map).

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 mobilise 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 glacierflows has more than doubled over the past 10 years. In July, its margin withdrew by about 1.5 kilometres, 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 metres 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 kilometres inland.

"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 metres 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. From photographs going back to 1931, the team calculated that the glacier has retreated by 17 metres per year on average, but in 2010 it lost ground by 35 metres. "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 modelling, 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 (CryosphereDOI: 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 modelled 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 Cancún 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 HydrometeorologyDOI: 10.1175/2009JHM1140.1).

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

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