Monday, April 4, 2016

The alarming science driving much higher sea level projections for this century

The calving front of Helheim Glacier in Greenland.
(Knut Christianson)

From WashingtonPost by Chris Mooney

For many scientists studying Antarctica, and particularly the vulnerable West Antarctic ice sheet, a major new study significantly increasing expectations for sea-level rise is the culmination of a large body of prior research — combined with alarming recent observations.
The study, just published in Nature, is based on an improved understanding of past warm eras in Earth’s history that featured much higher seas.
By creating advanced computer simulations of how Antarctica’s ice melts and flows — ones that can accurately capture sea level during these eras — the current study was also able to project considerable sea-level rise in the relatively near future.
In effect, it found that we’re about to start repeating the past.
The research stated that for a very high-carbon-emissions scenario, melting ice from Antarctica alone could cause seas to rise 1.14 meters (3.74 feet), give or take 36 centimeters, by 2100 — and vastly more, more than 15 meters (over 50 feet), by 2500.
The world has recently taken steps to try to avoid such a high-emissions pathway — steps whose effectiveness remains to be seen — but even in a more moderate emissions scenario, the study found the Antarctic contribution could be 58 centimeters (nearly two feet), give or take 28 centimeters, and close to six meters (nearly 20 feet) by 2500.

 According to a new study, high levels of greenhouse gas emissions could cause oceans to rise by close to two meters in total (over six feet) by the end of the century, and more than 13 meters (42 feet) from Antarctica alone by 2500.
(Nature, Rob DeConto, David Pollard)

That would be in addition to contributions from melting mountain glaciers, swelling seas caused by warming itself, and losses from Greenland.
The smaller ice sheet in Greenland is melting even faster than Antarctica and contains up to six meters (20 feet) of potential sea-level rise, including about two meters in vulnerable areas where marine-based ice is in contact with the ocean.
So how did scientists reach this conclusion?
Through a series of key steps that weave together modern observations and a better understanding of the ancient Earth.

 The Thwaites Glacier in West Antarctica. (AFP via NASA)

Alarm about West Antarctica.

 The first major sign that consensus projections about sea-level rise might be too conservative came in 2014, when two groups of scientists published research suggesting that the marine-based glaciers of West Antarctica had begun an irreversible retreat, as a result of warm water reaching the bases of glaciers such as the enormous Thwaites (which is larger than Pennsylvania, and over a mile thick in places) and melting them from below.
Thwaites and other West Antarctic glaciers not only are perched deep below the sea surface but also rest on seabeds that get deeper as you move inland, meaning that as they move downhill, they will present taller and thicker fronts to the ocean, and ice could flow out of them still faster.
Researchers have dubbed this condition a “Marine Ice Sheet Instability,” and other recent research has suggested that similar alignments exist in the far-larger ice sheet of East Antarctica, as well.

In light of these observations, the community of West Antarctic scientists has recently called for an urgent international research mission to the remote region to gather new data about the Thwaites Glacier, in particular.
The reason is that there is still a lack of understanding of much about Thwaites, including the precise topography of the seafloor beneath it.
Better mapping of this terrain could validate, or potentially refute, some of the results of the most recent computer modeling study, in which a major retreat of Thwaites drives a key part of the result.
The new study, said Robin Bell, an Antarctic expert at Columbia University’s Lamont-Doherty Earth Observatory, “points a laser pointer at where we need to go” to do more research.
“Right now we do not know if the topography under the Thwaites Glacier is a smooth ramp sloping back to some of the thickest ice in West Antarctica or if it is a lumpy terrain,” Bell said.
“We just do not know, and it matters for models like this.”

 The giant boulders of Eleuthera in the Bahamas have sparked debate
among scientists about their origin.
(Charles Ommanney/The Washington Post)

The troubling message from the Earth’s past.

 At the same time, eyeing places such as Thwaites and fearing the potential for far greater sea-level rise than currently forecast by groups such as the United Nations’ Intergovernmental Panel on Climate Change, scientists have been closely examining the ancient past for clues about what Earth, and its ice sheets, are actually capable of.

For instance, the last interglacial period (sometimes called the Eemian), between 130,000 and 115,000 years ago, featured a sea-level high stand that is believed to have been 6 to 9 meters above current levels.
The Eemian has drawn major attention lately because events at that time, including claims of enormous waves moving very large boulders in the Bahamas, have fueled the prominent climate researcher James Hansen’s particularly dire ideas about polar melt, rising seas, the shutdown of ocean circulations and worsening storms.
In other words, scientists are looking more and more at this particular period, not all that long ago in geologic time, as a possible analogue for our own.
The problem has been that computer models that simulate Antarctica in different climates have, until lately, been unable to capture these past sea-level high stands.
“We were not [able] to get more than a meter of sea-level rise out of the ice sheet in the last interglacial,” said the University of Massachusetts at Amherst’s Rob DeConto, who authored the new study with David Pollard of Pennsylvania State University.
But what about the way ice moves and changes was missing from models?

The calving front of Jakobshavn Glacier in Greenland. (Brennan Linsley/AP)

The key message from Greenland.

Even as the scientists were struggling to get the equations to work, real world events were hinting at a possible solution.

Take, for instance, the Jakobshavn Glacier of Greenland, which may be the fastest retreating major glacier in the world and one that holds the potential for 0.6 meters (nearly two feet) of sea-level rise, according to University of California at Irvine glaciologist Eric Rignot. Jakobshavn’s flow speed increased greatly after the loss of its floating ice shelf, an extension of the glacier out over open water in the fjord where it lies, in the early 2000s.

Ice shelves provide stability because they tend to freeze onto landscape features, such as islands, the sides of fjords or the seafloor — and thus provide back-pressure on glaciers, holding them in place.
However, at Jakobshavn, and at the Larsen B ice shelf on the Antarctic peninsula — part of which dramatically disintegrated in 2002 — ice shelves are vulnerable.
Warm waters can eat into them from below, and at the same time, surface melting can lead to pools of water that fill crevasses on these shelves and weaken them, widening surface cracks.
After losing its ice shelf, Jakobshavn now terminates in a tall, sheer ice cliff extending above the surface of the water, with the vast bulk of the glacier below the water.
There is no stabilizing shelf any longer. Another Greenland glacier, Helheim, is in a similar state (see the image at the top of this post).
And Antarctica’s Crane Glacier, which was previously held back by the Larsen B ice shelf, is as well.

But this presents a problem.
What these glaciers have in common is that they have quite tall cliffs above the water, approaching 100 meters in height.
In each case, below the water is much more ice — potentially nine times as much.
But that ice is partly held in place by the ocean itself.
It’s the part above the water that matters, because there are reasons to believe that ice, not being very strong as a material, can’t maintain a cliff above around 100 meters in height, about 330 feet, without collapse.
“With the fantastic observations of Jakobshavn, Helheim and other Greenland cliffs, the whole picture started coming together,” Richard Alley, a glaciologist at Penn State who has published with DeConto and Pollard, said of the revelations regarding ice-cliff collapse.
Indeed, one key implication of the research is that, as more major glaciers lose ice shelves, we could be moving into a new world where the dynamics of ice cliffs matter more and more.
Another major Greenland glacier, Zachariae Isstrom in the northeast, has also now lost the vast majority of its ice shelf and features a 75-meter-high cliff, scientists reported last year.
As the glacier retreats further, “the height of the calving cliff will increase from its current 75 m to enhance the risk of ice fracture,” the researchers noted.
West Antarctica’s glaciers could create much bigger cliffs.

 Greenland may seem vast, but it’s actually far smaller than Antarctica and contains much less ice. The reason this matters is that in Antarctica, and most immediately West Antarctica, there is the possibility to create ice cliffs much taller than 100 meters high (above sea level), because we are talking about marine-based glaciers with far more than 1,000 meters (or one kilometer) of thickness overall in their bulkiest parts.
“For this effect to be important, the ice would have to be about a kilometer thick,” said Knut Christianson, a glaciologist at the University of Washington in Seattle.
“In some places, it gets to be significantly deeper than that. The interior of West Antarctica, some areas are more than two kilometers below sea level, more than three kilometers thick.”

Thus, in the new ice-sheet model, two new processes are included — “hydrofracture,” in which ice shelves collapse because of meltwater on their surfaces, and “cliff collapse,” in which too-tall ice cliffs also fail and crumble.
Meanwhile, the model also runs “ensembles,” in which large numbers of simulations with slightly altered physics are tested out — but only model runs capable of accurately simulating sea-level rise for past eras such as the Eemian are ultimately included. 

Add it all together — the past, and the alarming present — and sure enough, scientific understanding produces the possibility of major ice retreat from Antarctica, in this century and beyond.
And it’s important to emphasize that there are reasons to think that the real world could possibly outstrip even this study.
“Their model is not a worst case, as they do place a limit on the rate of retreat in the model that does not come directly from the physics,” Alley said.
In an interview, DeConto explained that the model does not allow Antarctica’s ice to retreat as quickly as Jakobshavn’s ice currently is retreating, for instance.

A feedback with the ocean?

 And there’s another potentially missing factor. Large ice loss from Antarctica would pour enormous amounts of fresh water into the Southern Ocean, in the form of water and also vast icebergs that will begin to melt.
A much noted but controversial climate-change catastrophe study by former NASA researcher James Hansen used the concept of large meltwater injections like this to outline a feedback mechanism that leads to still more ice loss — meltwater stratifies the polar ocean with cold water on top and warmer water below.
The warmer water then causes more melting of ice sheets.

The two studies intersect: The new research finds that the ocean would be gaining, in some extreme scenarios, more than a sverdrup of fresh water as melting really advances, which translates into a flow of about 84 billion tons daily.
“The amount of meltwater going into the Southern Ocean in these scenarios is more than ocean modelers even think about when they think about doing a crazy freshwater forcing experiment,” DeConto said.
In this scenario, not only would seas rise rapidly, but there could be other major effects.
No wonder, then, that the new research, by squaring the past with the present, raises deep and troubling new questions.
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