Robert Schribbler: Warm Water Rising From the Depths: Much of Antarctica Now Under Threat of Melt

by Robert Schribbler, Robert Schribbler's blog, December 5, 2014

Antarctica. A seemingly impregnable fortress of cold. Ice mountains rising 2,100 meters high. Circumpolar winds raging out from this mass of chill frost walling the warm air out. And a curtain of sea ice insulating the surface air and mainland ice sheets from an increasingly warm world. A world that is now on track to experience one of its hottest years on record.

Antarctica, the coldest place on Earth, may well seem impregnable to this warming. But like any other fortress, it has its vulnerable spots. In this case, a weak underbelly. For in study after study, we keep finding evidence that warm waters are rising up from the abyss surrounding the chill and frozen continent. And the impact and risk to Antarctica’s glacial ice mountains is significant and growing.

(Collapse of ice structure at the leading edge of the Filchner-Ronne Ice Shelf adjacent to a rapidly warming Weddell Sea during January of 2010. A new study has found warm water upwelling from the Circumpolar Deep Water is rapidly approaching this massive ice shelf. Loss of Filchner-Ronne and its inland buttressed glaciers would result in 10 feet of sea level rise. Image source: Commons.)

For a study this week confirmed that Antarctica is now seeing a yearly loss of ice equal to one half the volume of Mt Everest every single year. A rate of loss triple that seen just ten years ago. An acceleration that, should it continue, means a much more immediate threat to coastal regions from sea level rise than current IPCC projections now estimate.

Shoaling of the Circumpolar Deep Water

The source of this warm water comes from a deep-running current that encircles all of Antarctica. Called the Circumpolar Deep Water, this current runs along the outside margin of the continental shelf. Lately, the current has been both warming and rising up the boundaries of the continental zone. And this combined action is rapidly bringing Antarctica’s great ice sheets under increasing threat of more rapid melt.

According to a new study led by Sunke Schmidtko, this deep water current has been warming at a rate of 0.1 degrees Celsius per decade since 1975. Even before this period of more rapid deep water warming, the current was already warmer than the continental shelf waters near Antarctica’s great glaciers. With the added warming, the Circumpolar Deep Water boasts temperatures in the range of 33 to 35 degrees Fahrenheit — enough heat to melt any glacier it contacts quite rapidly.

Out in the deep ocean waters beyond the continental shelf zone surrounding Antarctica, the now warmer waters of this current can do little to effect the great ice sheets. Here Sunke’s study identifies the crux of the problem — the waters of the Circumpolar Deep Water are surging up over the continental shelf margins to contact Antarctica’s sea fronting glaciers and ice shelves with increasing frequency.

In some cases, these warm waters have risen by more than 300 feet up the continental shelf margins and come into direct contact with Antarctic ice — causing it to rapidly melt. This process is most visible in the Amundsen Sea where an entire flank of West Antarctica is now found to be undergoing irreversible collapse. The great Pine Island Glacier, the Thwaites Glacier and many of its tributaries altogether composing enough ice to raise sea levels by 4 feet are now at the start of their last days. All due to an encroachment of warm water rising up from the abyss.

(Antarctic rivers of ice. Rising and warming waters from the Circumpolar Deep Water along continental margins have been increasingly coming into contact with ice shelf and glacier fronts that float upon or face the surrounding seas. The result has been much higher volumes of melt water contributions than expected from Antarctica. Image source: University of California.)

But the warm water rise is not just isolated to the Amundsen Sea. For Sunke also found that the warm water margin in the Weddell Sea on the opposite flank of West Antarctica was also rapidly on the rise. From 1980 to 2010, this warm water zone had risen from a depth of about 2100 feet to less than 1100 feet. A rapid advance toward another massive concentration of West Antarctic ice.

The impacts of a continued rise of this kind can best be described as chilling.

Sunke notes in an interview with National Geographic:

If this shoaling rate continues, there is a very high likelihood that the warm water will reach the Filchner Ronne Ice Shelf, with consequences which are huge.

Filchner Ronne, like the great Pine Island Glacier, has been calving larger and larger ice bergs during recent years. Should warm waters also destabilize this vast ice shelf another 1.5 feet of sea level rise would be locked in due to its direct loss. Including the massive inland glaciers that Filchner Ronne buttresses against a seaward surge, much larger than the ones near the Amundsen sea, would add a total of 10 feet worth of additional sea level rise.

Together, these destabilized zones would unleash much of West Antartica and some of Central Antartica, resulting in as much as 14 feet of sea level rise over a 100 to 200 year timeframe. This does not include Greenland, which is also undergoing rapid destabilization, nor does it include East Antarctica — which may also soon come under threat due to the encroachment of warm waters rising from the depths.

Are IPCC Projected Rates of Sea Level Rise Too Conservative?

The destabilization of glaciers along the Amundsen sea, the imminent threat to the Filchner Ronne Ice Shelf, and the less immediate but still troubling threat to East Antarctica’s glaciers, together with a rapidly destabilizing Greenland Ice Sheet, calls into question whether current IPCC predictions for sea level rise before 2100 are still valid.

IPCC projects a rise in seas of 1-3 feet by the end of this Century. But much of that rise is projected to come from thermal expansion of the world’s oceans — not from ice sheet melt in Antarctica and Greenland. Current rates of sea level rise of 3.3 milimeters each year would be enough to hit 1 foot of sea level rise by the end of this Century. However, just adding in the melting of the Filchner Ronne — a single large ice shelf — over the same period would add 4.4 milimeters a year. Add in a two century loss of the Amundsen glaciers — Pine Island and Thwaites — and we easily exceed the three foot mark by 2100.

Notably, this does not include the also increasingly rapid loss of ice coming from Greenland, the potential for mid century additions from East Antarctica, or lesser but still important additions from the world’s other melting glaciers.

Such more rapid losses to ice sheets may well reflect the realities of previous climates. At current CO2e levels of 481 ppm (400 ppm CO2 + methane and other human greenhouse gas additions) global sea levels were as much as 75-120 feet higher than they are today. Predicted greenhouse gas levels of 550 to 600 ppm CO2e by the middle of this century (Breaking 550 ppm CO2 alone by 2050 to 2060) are enough to set in place conditions that would eventually melt all the ice on Earth and raise sea levels by more than 200 feet. For there was no time in the past 55 million years when large ice sheets existed under atmospheric CO2 concentrations exceeding 550 parts per million.

Glaciologist Eric Rignot has been warning for years that the IPCC sea level rise estimates may well be too conservative. And it seems that recent trends may well bear his warnings out. If so, the consequences to millions of people living along the world’s coastlines are stark and significant. For the world, it appears we face the increasing likelihood of a near-term inland mass-migration of people and property. A stunning set of losses and tragedy starting now and ongoing through many decades and centuries to come.

Links:

Warming Seas Drive Rapid Acceleration of Melting Antarctic Ice

Mass Loss of the Amundsen Sea Embayment of West Antarctica

Multidecadal Warming of Antarctic Waters

Research Casts Alarming Light on Decline of West Antarctic Glaciers

Antarctic Ice Shelf Being Eaten Away by Sea

http://robertscribbler.wordpress.com/2014/12/05/warm-water-rising-from-the-depths-much-of-antarctica-now-under-threat-of-melt/

Dr. Mauri Pelto discusses the Pine Island Glacier and the two recent papers on the Western Antarctic Ice Sheet, Part I

interview by Peter Sinclair, Climate Crocks, May 17, 2014


Link:  http://climatecrocks.com/2014/05/17/the-weekend-wonk-dr-mauri-pelto-on-antarctic-melt-part-1/

"Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya," by K. I. Ohshima et al., Nature Geosci., (2013); doi:10.1038/ngeo1738

Nature Geoscience, (2013); doi:10.1038/ngeo1738

Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya

• Kay I. Ohshima,
• Yasushi Fukamachi,
• Guy D. Williams,
• Sohey Nihashi,
• Fabien Roquet,
• Yujiro Kitade,
• Takeshi Tamura,
• Daisuke Hirano,
• Laura Herraiz-Borreguero,
• Iain Field,
• Mark Hindell,
• Shigeru Aoki and
•  Masaaki Wakatsuchi

Abstract

The formation of Antarctic Bottom Water—the cold, dense water that occupies the abyssal layer of the global ocean—is a key process in global ocean circulation. This water mass is formed as dense shelf water sinks to depth. Three regions around Antarctica where this process takes place have been previously documented. The presence of another source has been identified in hydrographic and tracer data, although the site of formation is not well constrained. Here we document the formation of dense shelf water in the Cape Darnley polynya (65°–69° E) and its subsequent transformation into bottom water using data from moorings and instrumented elephant seals (Mirounga leonina). Unlike the previously identified sources of Antarctic Bottom Water, which require the presence of an ice shelf or a large storage volume, bottom water production at the Cape Darnley polynya is driven primarily by the flux of salt released by sea-ice formation. We estimate that about 0.3–0.7×10⁶ m³ s⁻¹ of dense shelf water produced by the Cape Darnley polynya is transformed into Antarctic Bottom Water. The transformation of this water mass, which we term Cape Darnley Bottom Water, accounts for 6–13% of the circumpolar total.

At a glance

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http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1738.html

4th source of Antarctic Bottom Water pinpointed with help of tagged southern elephant seals

Tagged seals help find missing piece in global climate puzzle

Researchers pinpoint fourth known source of bottom water, a crucial oceanic heat-sink.

• by Richard A. Lovett, Nature News,

24 February 2013

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Southern elephant seals fitted with satellite-linked instruments similar to the one above helped oceanographers map deep currents off Antarctica.

MARTIN BIUW

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By tracking the voyages of elephant seals off Antarctica, and with the help of satellite imaging and undersea sensors, researchers have discovered a long-elusive source for the deep-ocean streams of cold water that help to regulate the Earth's climate.

Antarctic bottom water (AABW) is cold, highly saline water that forms near the shores of Antarctica. Being denser than typical seawater, it sinks to the depths and then moves north insluggish currents that spread across the globe.

Three sources of AABW were known until now. The first, in the Weddell Sea, was found in 1940; two others were found in the Ross Sea and along the Adélie Coast of East Antarctica in the 1960s and ‘70s. But for years, researchers have suggested that these were not the only ones. In particular, water samples from an area called the Weddell Gyre contain atmospheric pollutants known as chlorofluorocarbons (CFCs), indicating that the deep water came into contact with the air far too recently to have been carried there from one of the known AABW sinks.

Now, Kay Ohshima, a physical oceanographer at Hokkaido University in Sapporo, Japan, and his colleagues have traced that water to a fourth AABW source, in the Cape Darnley polynya. Their results are published today in Nature Geoscience¹.

Polynyas are regions of open water near sea ice that are kept from freezing by wind and currents that sweep newly formed ice away. Polynyas have relatively high salinity, because most of the salt in sea water is expelled as it freezes.

Armed with the hypothesis that the missing source might be such a polynya, the researchers used satellite sensors to hunt for polynya regions where ice formed particularly rapidly. When satellite data suggested that Cape Darnley might be a candidate, the researchers moored instruments on the seabed, hoping to spot the descending current. In addition, they relied on data from elephant seals (Mirounga leonina) tagged with instruments that monitor ocean conditions.

“The seals went to an area of the coastline that no ship was ever going to get to, particularly in the middle of winter,” says Guy Williams, a physical oceanographer at the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, Australia, and a co-author of the study.

The elephant seals confirmed the researchers' hunch. “Several of the seals foraged on the continental slope as far down as 1,800 metres,” he says, “punching through into a layer of this dense water cascading down to the abyss. They gave us very rare and valuable wintertime measurements of this process.”

The new finding fills a gap in researchers’ understanding of the Southern Ocean’s role in global climate, “including carbon dioxide, temperature, the stability of the Antarctic ice sheet and changes in sea level", says Richard Alley, a geophysicist at Pennsylvania State University in University Park, who was not part of the study.

Still, Williams and Ohshima say that the Cape Darnley polynya represents, at most, about one-eighth of the world’s AABW, and that other, similar sources might remain to be discovered.

Michael Meredith, a polar oceanographer at the British Antarctic Survey in Cambridge, UK, who wrote an accompanying commentary on the study, says that if the total rate of AABW formation declines, the resulting changes in cold-water circulation could have important effects on global climate, letting the ocean depths warm and thereby changing the rate of heat exchange between Antarctica and the tropics. Moreover, he says, sea levels could rise — owing to the fact that water expands as it warms — and temperature changes could affect deep-sea ecosystems.

Nature

 

doi:10.1038/nature.2013.12488

References

1. Ohshima, K. I. et al. Nature Geosci. http://dx.doi.org/10.1038/ngeo1738 (2013).
   
   
   http://www.nature.com/news/tagged-seals-help-find-missing-piece-in-global-climate-puzzle-1.12488

Stop-and-go deglaciation, Nature Geoscience editorial, doi: 10.1038/ngeo1574

Nature 

Geoscience

 

5

(2012) 585; doi: 10.1038/ngeo1574; published online August 31, 2012

Stop-and-go deglaciation

Past transitions from glacial to interglacial climates have not been smooth. It would be wise to prepare for similarly sudden episodes of ice loss in future climate changes.

Only when climate records at centennial to millennial scales became available did the complexity of glacial terminations become apparent. Early low-resolution palaeoclimate reconstructions suggested a rapid and relentless meltdown. But at a closer look, an interhemispheric asynchrony emerged. Warming began in Antarctica around 18,000 years ago, which was interrupted by an abrupt reversal about 14,500 years ago. However, this southern cooling was countered by warming in the Northern Hemisphere until it too was interrupted by 1,000 years of cooling. Full interglacial conditions in both hemispheres were only reached after 11,700 years ago.

The long and complicated transition from full glacial conditions to early interglacial conditions (and beyond) are the subject of a Progress Article and a Review on pages 601 and607 of this issue. Of particular interest is the transfer of water from vast continental ice sheets to the world's oceans, which led to a 120-metre rise in sea level.

Post-glacial sea-level rise followed an uneven trajectory. Against a background of a gradually rising waters, several distinct meltwater pulses, presumably from sudden partial ice-sheet collapses, pushed sea level up tens of metres within a few centuries. Synthesizing work on the past two glacial terminations, Carlson and Winsor (607) argue that rapid disintegration events are a hallmark of ice sheets that reach the ocean, whereas ice sheets that terminate on land have melted more steadily as more solar radiation reached them.

Törnqvist and Hijma (601) review studies of later, decimetre-scale rises that have become possible thanks to increasingly precise measurements. About 8,500 and 8,200 years ago, two rapid jumps in sea level have been linked to the final drainage of Lake Agassiz in northern North America, a lake of glacial meltwater on the margin of one of the largest ice sheets. The analysis suggests that such injections of fresh water into the ocean were not only the consequence of warming, but also a necessary condition for abrupt climate change associated with changes in the ocean circulation.

It is tempting, in the face of rising temperatures, to draw comparisons between the warming at the end of glacial periods and the projected melting of the remaining ice sheets on Greenland and Antarctica. The underlying causes of the deglaciation — subtle changes in incoming solar radiation as the Earth's orbit around the Sun evolved — were far slower than the changes in radiative forcing that are attributable to anthropogenic greenhouse gas emissions, and direct comparisons cannot be made. But in view of the climatic complications during the last deglaciation, the response to future climate change might be far from gradual.

http://www.nature.com/ngeo/journal/v5/n9/full/ngeo1574.html

Climate scientists discover new weak point of the Antarctic ice sheet

Climate scientists discover new weak point of the Antarctic ice sheet

Edge of the Filchner-Ronne Ice Shelf in the Weddell Sea, Photo: Ralph Timmermann, Alfred Wegener Institute

 by R&D Daily, May 15, 2012

The results of the climate modelers from the Alfred Wegener Institute will come as a surprise to the professional world with the majority of experts assuming that the consequences of global warming for Antarctica would be noticeable primarily in the Amundsen Sea and therefore in the western part of Antarctica.

“The Weddell Sea was not really on the screen because we all thought that unlike the Amundsen Sea its warm waters would not be able to reach the ice shelves. But we found a mechanism which drives warm water towards the coast with an enormous impact on the Fichner-Ronne Ice Shelf in the coming decades,' says Dr. Hartmut Hellmer, oceanographer at the Alfred Wegener Institute and lead author of the study.

Using different model calculations, he and his colleagues Dr. Frank Kauker, Dr. Ralph Timmermann and Dr. Jürgen Determann as well as Dr. Jamie Rae from Met Office, Hadley Centre, U.K.,  demonstrate that as a result of a chain reaction large ice masses could presumably slide into the ocean within the next six decades.

This chain reaction is triggered by rising air temperatures above the southeastern Weddell Sea.

“Our models show that the warmer air will lead to the currently solid sea ice in the southern Weddell Sea becoming thinner and therefore more fragile and mobile in a few decades," says  Frank Kauker. If this happens, fundamental transport processes will change.

click to enlargeIllustration of the present conditions of the circulation in the Southern Weddell Sea: A consolidated sea-ice cover forms high-salinity shelf water (blue arrow) that prevents the warm coastal current (red arrow) from passing the coastal shelf and moving into the shelf ice cavity. Graphic: Alfred Wegener Institute

“This will mean that a hydrographic front in the southern Weddell Sea will disappear which has so far prevented warm water from getting under the ice shelf. According to our calculations, this protective barrier will disintegrate by the end of this century," explains Hellmer.

An inflow of warmer water beneath the Filchner-Ronne Ice Shelf will melt the ice from below.

“We expect the greatest melting rates near the so-called grounding line, the zone in which the ice shelf settles on the sea floor at the transition to the glacier. At this point the Filchner-Ronne Ice Shelf is melting today at a rate of around 5 m per year. By the turn of the next century the melt rates will rise to up to 50 m per year," says Hellmer’s colleague Jürgen Determann.

How the ice streams behind will react in the event of a melt of such enormous proportions is currently being investigated by Jürgen Determann. One thing is obvious, however: “Ice shelves are like corks in the bottles for the  ice streams behind them. They reduce the ice flow because they lodge in bays everywhere and rest on islands. If, however, the ice shelves melt from below, they become so thin that the  dragging surfaces become smaller and the ice behind them starts to move," explains Hellmer.

"If the high melting rates are completely compensated by  inland ice flow, this loss in mass would correspond to an additional rise in global sea level of 4.4 mm per year," adds Jürgen Determann. According to the latest estimates based on remote sensing data, global sea level rose for the period 2003-2010 at a rate of 1.5 mm per year due to melting of glaciers and ice shelves. This occurs in addition to the 1.7 mm per year due to thermal expansion of the oceans.

The forecasts of the current study are based on independent calculations of the ocean models BRIOS (Bremerhaven Regional Ice Ocean Simulations) and FESOM (Finite Element Sea Ice Ocean Model). The scientists used the atmospheric projections of the British Met Office Hadley Centre in Exeter as  forcing data. These included, for example, information on the future development of the wind and of the temperature in Antarctica.

click to enlargeIllustration of simulated conditions for the year 2070: Due to thinning of the sea-ice cover less high-salinity shelf water is formed. This amount of shelf water is too small to prevent the warm coastal current from passing the continental slope. It fills the deeper part of the Filchner Ice Shelf cavitiy, bringing lot of warmth to the bottom side and the grounding line of the ice shelf, which starts to melt from below. Graphic: Alfred Wegener Institute

Hellmer and his colleagues have thoroughly checked  the model results for being realistic: “We started the BRIOS model in 1860 to see whether its results also  represent the current situation. We found that this condition was satisfied. For example, the water temperatures for the Weddell Sea predicted by BRIOS are close to those we have actually measured  in the recent past," says Ralph Timmermann, who adds, “The BRIOS model has been verified on many occasions in the past. It correctly predicts sea ice thickness, concentration, and drift as well as circulation patterns. And FESOM is well on the way to attaining BRIOS status. However, it has a far higher resolution, which is why we have to wait a long time until the computer has calculated several decades and more. BRIOS only needs less than a week for a century."

The study was conducted as part of the EU-funded research programme “Ice2sea." This project brings together scientists from 24 leading research institutions of the European Union and from Chile, Norway and Iceland. Together, the scientists aim for decoding the interactions between ice and climate and in this way facilitate more precise predictions about the effects of melting ice on sea level.

Ice2sea project

Twenty-first-century warming of a large Antarctic ice shelf cavity by a redirected coastal current.

Source: Alfred Wegener Institute

http://rdmag.com/News/2012/05/Environment-Oceanography-Climate-scientists-discover-new-weak-point-of-the-Antarctic-ice-sheet/

60% reduction in 40 years in the volume of Antarctic Bottom Water, the cold dense water that drives global ocean currents

Latest Southern Ocean research shows continuing deep ocean change

by John Hartz, Skeptical Science, May 21, 2012

This is a reprint of a press release posted by the Australian Commonwealth Science and Industrial Research Organization (CSIRO)  May 4, 2012.

New research by teams of Australian and US scientists has found there has been a massive reduction in the amount of Antarctic Bottom Water found off the coast of Antarctica.

 

Deploying a mooring carrying a suite of monitoring sensors into the sea ice. Credit: Steve Rintoul

Comparing detailed measurements taken during the Australian Antarctic program's 2012 Southern Ocean marine science voyage to historical data dating back to 1970, scientists estimate there has been as much as a 60% reduction in the volume of Antarctic Bottom Water, the cold dense water that drives global ocean currents.

In an intensive and arduous 25-day observing program, temperature and salinity samples were collected at 77 sites between Antarctica and Fremantle. Such ship transects provide the only means to detect changes in the deep ocean.

The new measurements, which have not yet been published, suggest the densest waters in the world ocean are gradually disappearing and being replaced by less dense waters.

 "The amount of dense Antarctic Bottom Water has contracted each time we've measured it since the 1970s," said Dr Steve Rintoul, of CSIRO and the Antarctic Climate and Ecosystems CRC. "There is now only about 40% as much dense water present as observed in 1970."

The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970.

"It's a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important," Dr Rintoul said.

The research was carried out by more than 50 scientists on the Australian Antarctic Division's research and resupply vessel Aurora Australis, which sailed to Commonwealth Bay, west along the Antarctic coast, and returned into Fremantle.

The Australian Antarctic Division's Chief Scientist, Dr Nick Gales, said the findings of the oceanographic study are profoundly important.

"Not only will this research improve our understanding of ocean currents, but will also feed into our knowledge of how the Southern Ocean and the Antarctic continent drives the world's climate processes," Dr Gales said.

Dr Rintoul was Chief Scientist on the recent voyage and has made a dozen voyages to the Southern Ocean. "When we speak of global warming, we really mean ocean warming: more than 90% of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean.

The Southern Ocean is particularly important because it stores more heat and carbon dioxide released by human activities than any other region, and so helps to slow the rate of climate change" Dr Rintoul said. "A key goal of our work is to determine if the Southern Ocean will continue to play this role in the future."

The causes of the observed changes in the Southern Ocean are not yet fully understood. Changes in winds, sea ice, precipitation, or melt of floating glacial ice around the edge of Antarctica may be responsible. Data collected on the latest voyage will help unravel this mystery.

A major challenge is the lack of observations at high latitude, where much of the ocean is covered by sea ice in winter. During the voyage scientists deployed nine drifting profilers, called Argo floats, which will transmit profiles of temperature and salinity every 10 days for the next five years. These ice-capable floats in the seasonal ice zone in the Australian sector of the Southern Ocean are funded through Australia's Integrated Marine Observing System.

"The Argo floats have revolutionised our ability to measure the ocean, particularly in winter when ship observations are very rare," said Dr Rintoul. "On this voyage, we deployed a new kind of float designed to survive encounters with the sea ice. These floats will allow us to see how dense water forms in winter for the first time."

The Aurora Australis visited Commonwealth Bay as part of a celebration of the centenary of Sir Douglas Mawson's Australian Antarctic Expedition. Dr Rintoul's team had the opportunity to repeat oceanographic measurements made by Mawson's team 100 years ago, obtaining one of the few century-long records obtained anywhere in the ocean.

"Our measurements collected in 2012 are quite different to those collected by Mawson in 1912," Dr Rintoul said. "This is an indication of a change in the ocean currents that may be related to a reduction in the amount of dense water formed near Antarctica."

"Mawson's expedition really marked the transition from the "Heroic Age" of Antarctic exploration to a period where science was the primary motivation for Antarctic expeditions. I think he would have gotten a real kick out of the idea that measurements made by his team a century ago are still useful and that Australian scientists are continuing his legacy by studying Antarctica and its connection to the rest of the globe."

http://www.skepticalscience.com/Southern-Ocean-research-shows-continuing-deep-ocean-change_CSIRO.html

Increased Antarctic Ice Loss caused by Warm Ocean Currents [Hamish Pritchard, Helen Amanda Fricker, David Vaughan]

Increased Antarctic Ice Loss by Warm Ocean Currents

New study coauthored by a Scripps researcher differentiates the causes of thinning ice shelves

Scripps Institution of Oceanography, University of California, San Diego, April 25, 2012

Reporting in this week's journal Nature, an international team of scientists, including Helen Amanda Fricker of Scripps Institution of Oceanography at UC San Diego, has established that warm ocean currents are the dominant cause of recent ice loss in Antarctica. New measurement techniques have been used to differentiate, for the first time, between the two causes of thinning ice shelves -- warm ocean currents melting the underside, and warm air melting from above. This finding brings scientists a step closer to providing reliable projections of future sea-level rise. The work was initiated during a late 2009 visit to Scripps by British Antarctic Survey (BAS) scientist Hamish Pritchard, lead author of the study. 

Working with Fricker, Pritchard used measurements made by a laser instrument mounted on NASA's  Ice, Cloud and Land Elevation Satellite (ICESat) to estimate the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt around the continent. Of the 54 ice shelves studied, warm ocean currents are melting 20, most of which are in West Antarctica. In every case, the inland glaciers that flow down to the coast and feed into these thinning ice shelves are also draining more ice into the sea, contributing to sea-level rise. Only the Larsen Ice Shelf, on the eastern side of the Antarctic Peninsula (the long stretch of land pointing towards South America), is thinning because of warm air above it instead of melting from ocean currents.

"In most places in Antarctica, we can't explain the ice-shelf thinning through melting of snow at the surface, so it has to be driven by warm ocean currents melting them from below," said Pritchard.

Ice shelves are where the ice sheet meets the ocean, and basal melting from ice shelves in itself is not unusual -- it is a standard 'background' process for the Antarctic ice sheet to lose some of the mass it gains through snowfall so that it remains more or less in balance. The other way ice shelves lose mass is by iceberg calving. This study has confirmed that, under some ice shelves, basal melting is large enough that they are currently thinning, sometimes up to several meters per year. 

The study has also shown that when an ice shelf is thinning, the inland glaciers feeding it are also losing ice into the ocean, which will contribute to sea-level rise.

"We've looked all around the Antarctic coast and we see a clear pattern: in all the cases where ice shelves are being melted by the ocean, the inland glaciers are speeding up. It's this glacier acceleration that's responsible for most of the increase in mass loss from the continent and this is contributing to sea-level rise" said Pritchard.

Helen Amanda Fricker

"In the last decade or so we have seen some inland glaciers accelerate and thin dramatically when an ice shelf completely disappears," said Fricker, an associate professor in Scripps' Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics. "In this study we also see that glaciers are losing mass to the ocean even while the ice shelves are still there, but getting thinner. This suggests that ice loss is very sensitive to quite small changes in the ocean circulation around Antarctica." 

"What's really interesting is just how sensitive these glaciers seem to be," added Pritchard. "Some ice shelves are thinning by a few metres a year and, in response, the glaciers drain billions of tons of ice into the sea. This supports the idea that ice shelves are important in slowing down the glaciers that feed them, controlling the loss of ice from the Antarctic ice sheet. It means that we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt -- the oceans can do all the work from below."

Credit: British Antarctic Survey

Other recent studies have shown that the rate of ice-shelf thinning is sensitive to quite small changes in the flow of warm ocean currents around Antarctica. Circulation changes are mostly driven by changes in the pattern of winds around Antarctica which, in turn, are related to global changes in climate. The new research shows that understanding how the ocean responds to climate change is critical to forecasting future Antarctic ice loss.

David Vaughan, a co-author of the paper and the leader of ice2sea -- a major EU-funded program, summed up the research: "This study shows very clearly why the Antarctic ice sheet is currently losing ice, which is a major advance. But the real significance is that it also shows the key to predicting how the ice sheet will change in the future is in understanding the oceans."

The study was carried out by an international team from British Antarctic Survey, Utrecht University, Scripps Institution of Oceanography and Earth & Space Research. NASA's ICESat collected measurements during the period 2003-2009. 

British Antarctic Survey (BAS), a component of the Natural Environment Research Council, delivers and enables world-leading interdisciplinary research in the Polar Regions. Its skilled science and support staff based in Cambridge, Antarctica and the Arctic, work together to deliver research that uses the Polar Regions to advance our understanding of Earth as a sustainable planet. Through its extensive logistic capability and know-how BAS facilitates access for the British and international science community to the UK polar research operation. Numerous national and international collaborations, combined with an excellent infrastructure help sustain a world leading position for the UK in Antarctic affairs. For more information visit www.antarctica.ac.uk

Ice2sea brings together the EU's scientific and operational expertise from 24 leading institutions across Europe and beyond. Improved projections of the contribution of ice to sea-level rise produced by this major European-funded programme will inform the fifth IPCC report (due in 2013). In 2007, the fourth Intergovernmental Panel on Climate Change (IPCC) report highlighted ice-sheets as the most significant remaining uncertainty in projections of sea-level rise. Understanding about the crucial ice-sheet effects was "too limited to assess their likelihood or provide a best estimate of an upper bound for sea-level rise." Ice2sea is funded by the European Union's Seventh Framework Programme (FP7). www.ice2sea.eu

http://scrippsnews.ucsd.edu/Releases/?releaseID=1264

Seth Borenstein, AP: Antarctica's floating ice shelves are melting from beneath due to warmer ocean waters being pushed closer by changing winds

Study: Antarctic Ice Melting From Warm Water Below 


by Seth Borenstein, Associated Press, April 25, 2012


Antarctica's massive ice shelves are shrinking because they are being eaten away from below by warm water, a new study finds. That suggests that future sea levels could rise faster than many scientists have been predicting. 


The western chunk of Antarctica is losing 23 feet of its floating ice sheet each year. Until now, scientists weren't exactly sure how it was happening and whether or how man-made global warming might be a factor. The answer, according to a study published Wednesday in the journal Nature, is that climate change plays an indirect role — but one that has larger repercussions than if Antarctic ice were merely melting from warmer air. 


Hamish Pritchard, a glaciologist at the British Antarctic Survey, said research using an ice-gazing NASA satellite showed that warmer air alone couldn't explain what was happening to Antarctica. A more detailed examination found a chain of events that explained the shrinking ice shelves. Twenty ice shelves showed signs that they were melting from warm water below. Changes in wind currents pushed that relatively warmer water closer to and beneath the floating ice shelves. The wind change is likely caused by a combination of factors, including natural weather variation, the ozone hole and man-made greenhouse gases, Pritchard said in a phone interview. 


As the floating ice shelves melt and thin, that in turn triggers snow and ice on land glaciers to slide down to the floating shelves and eventually into the sea, causing sea level rise, Pritchard said. Thicker floating ice shelves usually keep much of the land snow and ice from shedding to sea, but that's not happening now. That whole process causes larger and faster sea level rise than simply warmer air melting snow on land-locked glaciers, Pritchard said. 


"It means the ice sheets are highly sensitive to relatively subtle changes in climate through the effects of the wind," he said. What's happening in Antarctica "may have already triggered a period of unstable glacier retreat," the study concludes. 


If the entire Western Antarctic Ice Sheet were to melt — something that would take many decades if not centuries — scientists have estimated it would lift global sea levels by about 16 feet. 


NASA chief scientist Waleed Abdalati, an expert in Earth's ice systems who wasn't involved in the research, said Pritchard's study "makes an important advance" and provides key information about how Antarctica will contribute to global sea level rise. 


Another outside expert, Ted Scambos of the National Snow and Ice Data Center, said the paper will change the way scientists think about melt in Antarctica. Seeing more warm water encircling the continent, he worries that with "a further push from the wind" newer areas could start shrinking.


http://abcnews.go.com/Technology/wireStory/study-antarctic-ice-melting-warm-water-16212616#

Different magnitudes of projected subsurface ocean warming around Greenland and Antarctica by Yin et al., Nature Geoscience (2011)

Nature Geoscience (2011); doi:10.1038/ngeo1189

Different magnitudes of projected subsurface ocean warming around Greenland and Antarctica

Jianjun Yin,

Jonathan T. Overpeck,

Stephen M. Griffies,

Aixue Hu,

Joellen L. Russell and

 

Ronald J. Stouffer

Abstract

The observed acceleration of outlet glaciers and ice flows in Greenland and Antarctica is closely linked to ocean warming, especially in the subsurface layer. Accurate projections of ice-sheet dynamics and global sea-level rise therefore require information of future ocean warming in the vicinity of the large ice sheets. Here we use a set of 19 state-of-the-art climate models to quantify this ocean warming in the next two centuries. We find that in response to a mid-range increase in atmospheric greenhouse-gas concentrations, the subsurface oceans surrounding the two polar ice sheets at depths of 200–500 m warm substantially compared with the observed changes thus far. Model projections suggest that over the course of the twenty-first century, the maximum ocean warming around Greenland will be almost double the global mean, with a magnitude of 1.7–2.0 °C. By contrast, ocean warming around Antarctica will be only about half as large as global mean warming, with a magnitude of 0.5–0.6 °C. A more detailed evaluation indicates that ocean warming is controlled by different mechanisms around Greenland and Antarctica. We conclude that projected subsurface ocean warming could drive significant increases in ice-mass loss, and heighten the risk of future large sea-level rise.” 

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1189.html