Rising sea levels and vulnerable Antarctic Ice Shelves

In climate science news this week, a new study shows that Sea levels are rising at faster clip as polar melt accelerates:

Global sea levels are climbing at a faster rate than previously thought, according to a new analysis that underscores scientists’ concerns about the impact of melting glaciers and ice sheets near the Earth’s poles. [Photo: A Weddell Sea tabular, a remnant of the Larsen B Ice Shelf.]

The new research published Monday in the journal Nature Climate Change shows that the rate of sea-level rise appears to have accelerated over the past 15 years, a period in which scientists elsewhere documented a surprisingly rapidly retreat of some of Earth’s great ice masses, from Greenland to West Antarctica.

The findings appear to contradict earlier studies suggesting that the rate of sea-level rise had actually slowed slightly in recent years.

Australian scientists detected the increase in a study that analyzed decades of records from tidal gauges around the world, together with satellite data that show changes in water levels as well as subtle shifts in land formations.

Using these more precise measurements, the researchers discovered that scientists had slightly overstated sea-level rise that occurred in the 1990s, and underestimated the rate of increase since 1999, said Christopher Watson, a University of Tasmania geodesist who co-authored the study along with colleagues from the university and from Australia’s national science agency, the Commonwealth Scientific and Industrial Research Organization.

[A Harvard study also documented faster sea-level rise]

The adjusted figures showed ocean levels rising over the past two decades at a rate of between 2.6 and 2.9 millimeters a year — or just over a tenth of an inch, he said. That rate is consistent with the projections of the Intergovernmental Panel on Climate Change, the U.N.-sponsored scientific body regarded as the internationally accepted authority on global warming.

“The acceleration is also consistent with what we expect, given the increasing contributions from the West Antarctic and Greenland ice sheets,” Watson said in an e-mail.

Watson noted that sea levels can fluctuate naturally as water is exchanged between the sea and land, and for that reason, the higher rate of increase described in the study is too small to be regarded as statistically significant. What is clear, he said, is that sea levels are rising at roughly double the average rate observed in the last century, with significant implications for coastal cities around the globe.

The IPCC projects that, at current rates of warming, global sea levels could rise by as much as three feet by the end of the century.

“Accelerating sea level is a massive issue for the coastal zone — the once-in-a-lifetime inundation events will become far more frequent, and adaptation will need to occur,” Watson said. “Agencies need to fully consider the impact of accelerating sea level and plan accordingly.”

The melting of polar ice caps is particularly acute in the Antarctic.  Think Progress reports, Massive Antarctic Ice Shelf Will Disappear Completely In A Few Years, According To NASA:

An Antarctic ice shelf roughly half the size of Rhode Island will disintegrate completely within the next few years, according to a NASA study released Thursday.

In 2002, two-thirds of the Larsen B Ice Shelf — which had been intact for more than 10,000 years — broke up in less than six weeks. The remaining portion of the ice shelf covers about 625 square miles along the Antarctic Peninsula, extending toward the southern tip of South America.

Using data collected from airborne surveys and radar, a team led by Ala Khazendar at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, found that the remaining portion of Larsen B is weakening very quickly, which is causing the shelf to become increasingly fragmented. Two of its tributary glaciers are also flowing faster and thinning more rapidly, according to NASA.

This was the first study to look comprehensively at the health of the Larsen B remnant and its tributary glaciers, and analysis of the data puts the remnant shelf’s future in question. An increasingly widening rift will eventually split along the entire shelf, the study found, shattering the remnant sheet into hundreds of icebergs that will drift away from the continent’s edge. According to Khazendar, the Larsen B remnant will completely disintegrate by 2020, allowing Antarctic glaciers to flow unimpeded into the ocean.

* * *

Antarctica’s ice shelves are like cliffs of ice that extend from the shelf of the continent out into the ocean. Without ice shelves to impede their movement, Antarctic glaciers flow into the ocean at much faster rates, accelerating global sea level rise. Antarctica has several ice shelves of varying size that hang over the edge of the continent — the Ross Ice Shelf, which is the largest, is about the size of France.

The NASA study supports previous research suggesting that Antarctica’s ice shelves are melting at a rate much faster than previously anticipated. In March, research published in Science highlighted the accelerating loss of ice from most of Antarctica’s ice shelves. The melting was most pronounced in the West Antarctic, where losses increased by nearly 70 percent in the last decade. If all the ice that sits on the West Antarctic bedrock is allowed to flow into the ocean, global sea level could rise by nine feet — something that scientists don’t think is likely to happen, though they also aren’t sure how much grounded ice will eventually melt. That will be determined, they say, not only by how much the Earth warms, but by local conditions in Antarctica, including how wind patterns divert warm or cold water to various parts of the continent.

What is clear, however, is that changes to the Antarctic’s ice shelves are happening at an increasingly quick pace.

The Larsen C Ice Shelf is also vulnerable. Chris Mooney of the Washington Post reports,  This ice shelf is nearly the size of Scotland — and scientists are worried about its stability:

In early 2002, scientists peering in on Antarctica from satellites saw something they found very alarming.

The Larsen B ice shelf, bigger than Rhode Island and 720 feet thick, underwent a sudden collapse in only 35 days (video here). It was following in the footsteps of the 1,500-square-kilometer Larsen A ice shelf, which collapsed in 1995, but Larsen B was more than two times larger.

The ice shelf, according to the National Snow and Ice Data Center, had “likely existed” for 12,000 years before it collapsed and ceased to be a feature of the Earth.

Larsen A and Larsen B left behind Larsen C, a dramatically bigger mass floating outward into icy waters off the Antarctic peninsula. It’s the biggest ice shelf in this region of the continent, encompassing an area “two and a half times the size of Wales and slightly smaller than Scotland,” according to the British Antarctic Survey. It’s also now the most north-lying — which, in this case, means in the direction of warmer South America — of the Antarctic peninsula’s great ice shelves that remain.

* * *

And Larsen C, too, is starting to look vulnerable, suggests a newly published study by a team of researchers with the British Antarctic Survey, the U.S. Geological Survey and several U.S. universities. The research notes several mechanisms that “could pose an imminent risk” to the ice shelf.

A study earlier this year had already found a growing rift in the ice shelf that is “likely in the near future to generate the largest calving event since the 1980s,” as the paper put it. And researchers also know that the ice shelf has been sinking — in other words, becoming lower in the water. Because ice shelves float atop the water with the vast majority of their mass below the surface and only a small percentage above, such sinking suggests a loss of overall mass.

The sinking has been about a meter in the last 15 years, which may not sound like much for an ice shelf that rises about 50 meters out of the water and is some 350 meters thick. But it’s well within scientists’ ability to measure — and it worries them.

Now, in the new paper in the journal The Cryosphere, scientists report on the result of 15 years’ worth of aerial and ground-based radar surveys of the Larsen C ice shelf, seeking to understand reasons for its dip into the water. There are two principal possibilities: “compaction” — in which the ice atop the ice shelf loses some of the air that is contained within it, perhaps due to warmer temperatures and surface melting — and loss of mass from beneath, presumably due to warmer seas melting the ice shelf from below.

The upshot of the study is that both factors seem to be involved. “The ice shelf lost 4 meters of ice, and it lost 60 centimeters of air, and that caused the upper surface to go down by 1 meter,” explains Paul Holland of the British Antarctic Survey, the study’s lead author. “Effectively, of the lowering, about half was due to air loss, and the other half was due to ice loss.”

In other words, it appears that the warming of the Antarctic Peninsula — where a possible new record high temperature for the entire Antarctic continent of 63.5 degrees Fahrenheit was just set — may be contributing to ice melt at the surface of Larsen C. Meanwhile, warmer seas may also be taking a toll from beneath.

All of which, well, brings up bad memories. “We know with Larsen B, it did this lowering trick before it collapsed,” says Holland. “We also know that its snowpack was completely depleted of air before it collapsed.”

Still, Larsen C remains a truly gigantic ice shelf, and these processes alone — if they were the only things happening — would not necessarily lead to a collapse this century, the new study estimates. There’s just too much ice to lose. The study estimates that a collapse due to air loss and subsequent meltwater seeping through the ice shelf could take as many as 250 years to unfold, and one due to melting ice from below might take 170 to 500 years.

But these also may not be the only processes afoot. And as past research suggested in the wake of the 1995 Larsen A ice shelf collapse, “after retreat beyond a critical limit, ice shelves may disintegrate rapidly.”

As mentioned before, a large rift in the southern part of the Larsen C ice shelf has “propagated rapidly beyond a band of marine ice that has stabilised all such rifts during the observational era,” notes the new paper. The study found that this could threaten the stability of the ice shelf “within a few years.” It is one reason the new study says the ice shelf could face an “imminent risk.”

“In the southern end of Larsen C, there’s a big sort of rift, a crack that goes all the way through the ice shelf,” says Holland. “If this rift propagates further,” he continues, “and causes an iceberg calving, it’s possible this could cause a destabilization of the ice shelf. This is one of the mechanisms that could be one of the first signs of a collapse.”

A prior study reached a similar conclusion[.]

* * *

Another potential vulnerability, meanwhile, involves a feature called Bawden Ice Rise, which is literally what it sounds like — a region on the Antarctic Peninsula where the ice rises, presumably because of a rock beneath it. Another “imminent risk” could occur from the “ungrounding” of the Larsen C ice shelf from the rise.

However, there is much uncertainty about each of these possibilities, so it is important to emphasize that the researchers are far from certain that Larsen C will collapse any time soon. Officially, the researchers who conducted the work now think that “a collapse could occur within a century, although maybe sooner and with little warning,” according to the British Antarctic Survey.

The good news is that unlike other Antarctic ice shelves that have been observed to be losing mass — such as in West Antarctica and the East Antarctic Totten glacier region — Larsen C does not hold back a very large amount of land-based ice that would, upon sliding into the ocean, contribute to sea level rise. Holland estimates that the contribution to sea level rise from that ice — currently hemmed in by the ice shelf — would be about 10 centimeters.

* * *

So in sum, scientists don’t really know when we might lose the Larsen C ice shelf, but they’re definitely concerned. And combined with other recent news regarding the ice shelves of West Antarctica and East Antarctica’s massive Totten Glacier, it’s enough to make you think — yet again — that ice giant at the bottom of the Earth is stirring.