Das Eis in der Antarktis, wie geht es ihm? Schmilzt es, wächst es? Im Folgenden wollen wir die neueste Literatur zum Thema vorstellen. Es gibt viel zu berichten. Schnallen Sie sich am besten an. Beginnen wollen wir mit der Temperaturentwicklung, denn die ist neben dem Schneefall der wichtigste Steuerungsfaktor für das antarktische Inlandeis.
Auf Notrickszone zeigte Kirye zehn Küstenstationen der Antarktis, keine davon erwärmte sich während der letzten 10 Jahre. Hier ein Beispiel:
Und hier die Temperaturentwicklung der ganzen Antarktis laut UAH- und RSS-Satellitenmessungen (von Climate4You via Notrickszone):
Laut Clem et al. 2018 kühlte die Ostantarktis in den letzten 60 Jahren ab, während sich die Westantarktis erwärmte. Die Autoren stellen einen Zusammenhang mit dem SAM-Ozeanzyklus her, der Southern Annular Mode. Auch Euan Mearns beschäftigte sich mit der Temperaturentwicklung der Antarktis während der letzten Jahrzehnte.
Passend dazu ein Video von Akademie Raddy:
Anhand von Höhen- und Schwerefeldmessungen per Satellit sowie GPS-Messungen am Boden stellten Martin-Español et al. 2017 für das Intervall 2003-2013 einen Eismassenzuwachs in der Ostantarktis und eine Eismassenreduktion in der (viel kleineren) Westantarktis fest. Auch der NASA-Forscher Jay Zwally interpretiert eine Zunahme des ostantarktischen Eises. Ein Mitte 2018 angekündigtes Paper scheint jedoch noch immer in den Review-Mühlen festzuhängen…
Wird das Eis der Ostantarktis von der schmelzenden Westantarktis irgendwann in den Schmelzstrudel mitgezogen? Nein, das passiert nicht, sagte die Indiana University in einer Pressemitteilung von 2017:
New study validates East Antarctic ice sheet should remain stable even if western ice sheet melts
A new study from Indiana University-Purdue University Indianapolis validates that the central core of the East Antarctic ice sheet should remain stable even if the West Antarctic ice sheet melts. The study’s findings are significant, given that some predict the West Antarctic ice sheet could melt quickly due to global warming.
If the East Antarctic ice sheet, which is 10 times larger than the western ice sheet, melted completely, it would cause sea levels worldwide to rise almost 200 feet, according to Kathy Licht, an associate professor in the Department of Earth Sciences in the School of Science at IUPUI. Licht led a research team into the Transarctic Mountains in search of physical evidence that would verify whether a long-standing idea was still true: The East Antarctic ice sheet is stable.
The East Antarctic ice sheet has long been considered relatively stable because most of the ice sheet was thought to rest on bedrock above sea level, making it less susceptible to changes in climate. However, recent studies show widespread water beneath it and higher melt potential from impinging ocean water. The West Antarctic ice sheet is a marine-based ice sheet that is mostly grounded below sea level, which makes it much more susceptible to changes in sea level and variations in ocean temperature. „Some people have recently found that the East Antarctic ice sheet isn’t as stable as once thought, particularly near some parts of the coast,“ Licht said.
Recent studies have determined that the perimeter of the East Antarctic ice sheet is potentially more sensitive and that the ice may have retreated and advanced much more dynamically than was thought, Licht said. „We believed this was a good time to look to the interior of the ice sheet. We didn’t really know what had happened there,“ Licht said. The research team found the evidence confirming the stability of the East Antarctic ice sheet at an altitude of 6,200 feet, about 400 miles from the South Pole at the edge of what’s called the polar plateau, a flat, high surface of the ice sheet covering much of East Antarctica.
To understand how an ice sheet changes through time, a continuous historical record of those changes is needed, according to Licht. The team found layers of sediment and rocks that built up over time, recording the flow of the ice sheet and reflecting climate change. Finding that record was a challenge because glaciers moving on land tend to wipe out and cover up previous movements of the glacier, Licht said.
The big question the team wanted to answer was how sensitive the East Antarctic sheet might be to climate change. „There are models that predict that the interior of the East Antarctic ice sheet wouldn’t change very much, even if the West Antarctic ice sheet was taken away,“ Licht said. According to these models, even if the ice sheet’s perimeter retreats, its core remains stable. „It turns out that our data supports those models,“ she said. „It’s nice to have that validation.“
The team’s research findings are presented in a paper, “Middle to Late Pleistocene stability of the central East Antarctic Ice Sheet at the head of Law Glacier,” that was published today online in the journal Geology. The research presented is in collaboration with Mike Kaplan, Gisela Winckler, Joerg Schaefer and Roseanne Schwartz at Lamont-Doherty Earth Observatory in New York.
Auch ein Nature Editorial hatte das aktuelle Wachstum des ostantarktischen Eises im Januar 2018 zum Thema. Natürlich ging es dem Eis in dieser Region auch schon einmal schlechter, heiß es weiter. Allerdings müsse man wohl bis ins warme Pliozän (5,3-2.6 Million Jahre vor heute) zurückgehen:
A history of instability
The East Antarctic ice sheet may be gaining mass in the current, warming climate. The palaeoclimate record shows, however, that it has retreated during previous episodes of prolonged warmth.
The phrase “at a glacial pace” once invoked a sense of slow and unchangeable movement, an almost imperceptible motion. But decades of remote sensing and seafloor observations have shown that glaciers and ice sheets can respond to disturbances much more dynamically than once thought. But as satellites captured the surges and retreat of Greenland’s maritime glaciers in the past decades the Antarctic ice sheets — east and west of the Trans-Antarctic mountains — were at least assumed to be stable. But this, too, turned out to be wrong. First came sediment1 and model2 evidence that the West Antarctic ice sheet collapsed during previous interglacial periods and under Pliocene warmth. Then came erosional data showing that several regions of the East Antarctic ice sheet also retreated and advanced throughout the Pliocene3. An extended record4 of ice-sheet extent from elsewhere on the East Antarctic coast now paints a more complicated picture of the sensitivity of this ice sheet to warming.
Kurioserweise wendete sich das Blatt eine halbes Jahr später als ebenfalls in Nature ein Paper von Shakun et al. 2018 auch im Pliozän keine großen Probleme für das Eis in der Antarktis sahen:
Minimal East Antarctic Ice Sheet retreat onto land during the past eight million years
The East Antarctic Ice Sheet (EAIS) is the largest potential contributor to sea-level rise. However, efforts to predict the future evolution of the EAIS are hindered by uncertainty in how it responded to past warm periods, for example, during the Pliocene epoch (5.3 to 2.6 million years ago), when atmospheric carbon dioxide concentrations were last higher than 400 parts per million. Geological evidence indicates that some marine-based portions of the EAIS and the West Antarctic Ice Sheet retreated during parts of the Pliocene1,2, but it remains unclear whether ice grounded above sea level also experienced retreat. This uncertainty persists because global sea-level estimates for the Pliocene have large uncertainties and cannot be used to rule out substantial terrestrial ice loss3, and also because direct geological evidence bearing on past ice retreat on land is lacking. Here we show that land-based sectors of the EAIS that drain into the Ross Sea have been stable throughout the past eight million years. We base this conclusion on the extremely low concentrations of cosmogenic 10Be and 26Al isotopes found in quartz sand extracted from a land-proximal marine sediment core. This sediment had been eroded from the continent, and its low levels of cosmogenic nuclides indicate that it experienced only minimal exposure to cosmic radiation, suggesting that the sediment source regions were covered in ice. These findings indicate that atmospheric warming during the past eight million years was insufficient to cause widespread or long-lasting meltback of the EAIS margin onto land. We suggest that variations in Antarctic ice volume in response to the range of global temperatures experienced over this period—up to 2–3 degrees Celsius above preindustrial temperatures4, corresponding to future scenarios involving carbon dioxide concentrations of between 400 and 500 parts per million—were instead driven mostly by the retreat of marine ice margins, in agreement with the latest models5,6.
Siehe auch Beitrag hierzu auf cato.org.
Vor 8 Millionen Jahren hatte die Erdatmosphäre einen ähnlichen CO2-Gehalt wie heute. Untersuchungen zeigten nun, dass das sich der Antarktische Eisschild damals kaum zurückgezogen hatte. Das Eis ist offenbar stabiler als gedacht. Hier gehts zur Pressemitteilung der National Science Foundation. Lesen Sie hierzu auch einen Beitrag in Popular Mechanics.
Pressemitteilung der University of Edinburgh von 2017:
Central parts of Antarctica’s ice sheet have been stable for millions of years, from a time when conditions were considerably warmer than now, research suggests.
The study of mountains in West Antarctica will help scientists improve their predictions of how the region might respond to continuing climate change. Its findings could also show how ice loss might contribute to sea level rise.
Although the discovery demonstrates the long-term stability of some parts of Antarctica’s ice sheet, scientists remain concerned that ice at its coastline is vulnerable to rising temperatures. Researchers from the Universities of Edinburgh and Northumbria studied rocks on slopes of the Ellsworth Mountains, whose peaks protrude through the ice sheet. By mapping and analysing surface rocks — including measuring their exposure to cosmic rays — researchers calculated that the mountains have been shaped by an ice sheet over a million-year period, beginning in a climate some 20C warmer than at present.
The last time such climates existed in the mountains of Antarctica was 14 million years ago when vegetation grew in the mountains and beetles thrived. Antarctica’s climate at the time would be similar to that of modern day Patagonia or Greenland. This time marked the start of a period of cooling and the growth of a large ice sheet that extended offshore around the Antarctic continent. Glaciers have subsequently cut deep into the landscape, leaving a high-tide mark — known as a trimline — in the exposed peaks of the Ellsworth range.
The extended ice sheet cooled the oceans and atmosphere, helping form the world of today, researchers say. Their study is among the first to find evidence for this period in West Antarctica. The research, published in Earth and Planetary Science Letters, was done in collaboration with the Scottish Universities Environmental Research Centre. It was funded by the UK Natural Environment Research Council and supported by British Antarctic Survey.
Professor David Sugden, of the University of Edinburgh’s School of GeoSciences, said: „These findings help us understand how the Antarctic Ice Sheet has evolved, and to fine-tune our models and predict its future. The preservation of old rock surfaces is testimony to the stability of at least the central parts of the Antarctic Ice Sheet — but we are still very concerned over other parts of Antarctica amid climate change.“
Durch das Schmelzen des Eises in der Westantarktis steigt diese isostatisch auf, was das darüberliegende Eis wiederum stabilisiert, fand ein Forscherteam aus Dänemark und Colorado.
Immer wieder gibt es reißerische Klima-Geschichte über den Tottengletscher im ostantarktischen Wilkesland. Gwyther et al. 2018 konnten jetzt zeigen, dass das basale Abschmelzen des Gletschers starken natürlichen Schwankungen unterliegt (Pressemitteilung des NSIDC hier). Einen Langzeitschmelztrend gibt es nicht. Auch im westlichen Rossmeer sind die Gletscher stabil (Fountain et al. 2017, Pressemitteilung hier). Der schnell schmelzende Pine-Island-Gletscher in der Westantarktis hat ein heißes Geheimnis, das jetzt gelüftet wurde: Unter dem Gletscher liegt eine vormals unbekannte vulkanische Hitzequelle. Pressemitteilung der University of Rhode Island aus dem Juni 2018 (via EurekAlert!):
Researchers discover volcanic heat source under glacier
Plays critical role in movement, melting
A researcher from the University of Rhode Island’s Graduate School of Oceanography and five other scientists have discovered an active volcanic heat source beneath the Pine Island Glacier in Antarctica. The discovery and other findings, which are critical to understanding the stability of the West Antarctic Ice Sheet, of which the Pine Island Glacier is a part, are published in the paper, „Evidence of an active volcanic heat source beneath the Pine Island Glacier,“ in the latest edition of Nature Communications.
Assistant Professor Brice Loose of Newport, a chemical oceanographer at GSO and the lead author, said the paper is based on research conducted during a major expedition in 2014 to Antarctica led by scientists from the United Kingdom. They worked aboard an icebreaker, the RRS James Clark Ross, from January to March, Antarctica’s summer. „We were looking to better understand the role of the ocean in melting the ice shelf,“ Loose said. „I was sampling the water for five different noble gases, including helium and xenon. I use these noble gases to trace ice melt as well as heat transport. Helium-3, the gas that indicates volcanism, is one of the suite of gases that we obtain from this tracing method. „We weren’t looking for volcanism, we were using these gases to trace other actions,“ he said. „When we first started seeing high concentrations of helium-3, we thought we had a cluster of bad or suspicious data.“
The West Antarctic Ice Sheet lies atop a major volcanic rift system, but there had been no evidence of current magmatic activity, the URI scientist said. The last such activity was 2,200 years ago, Loose said. And while volcanic heat can be traced to dormant volcanoes, what the scientists found at Pine Island was new. In the paper, Loose said that the volcanic rift system makes it difficult to measure heat flow to the West Antarctic Ice Sheet. „You can’t directly measure normal indicators of volcanism — heat and smoke — because the volcanic rift is below many kilometers of ice,“ Loose said
But as the team conducted its research, it found high quantities of an isotope of helium, which comes almost exclusively from mantle, Loose said. „When you find helium-3, it’s like a fingerprint for volcanism. We found that it is relatively abundant in the seawater at the Pine Island shelf. „The volcanic heat sources were found beneath the fastest moving and the fastest melting glacier in Antarctica, the Pine Island Glacier,“ Loose said. „It is losing mass the fastest.“ He said the amount of ice sliding into the ocean is measured in gigatons. A gigaton equals 1 billion metric tons.
However, Loose cautions, this does not imply that volcanism is the major source of mass loss from Pine Island. On the contrary, „there are several decades of research documenting the heat from ocean currents is destabilizing Pine Island Glacier, which in turn appears to be related to a change in the climatological winds around Antarctica,“ Loose said. Instead, this evidence of volcanism is a new factor to consider when monitoring the stability of the ice sheet.
The scientists report in the paper that „helium isotope and noble gas measurements provide geochemical evidence of sub-glacial meltwater production that is subsequently transported to the cavity of the Pine Island Ice Shelf.“ They say that heat energy released by the volcanoes and hydrothermal vents suggests that the heat source beneath Pine Island is about 25 times greater than the bulk of heat flux from an individual dormant volcano.
Professor Karen Heywood, from the University of East Anglia in Norwich, the United Kingdom, and chief scientist for the expedition, said: „The discovery of volcanoes beneath the Antarctic ice sheet means that there is an additional source of heat to melt the ice, lubricate its passage toward the sea, and add to the melting from warm ocean waters. It will be important to include this in our efforts to estimate whether the Antarctic ice sheet might become unstable and further increase sea level rise.“
Does that mean that global climate change is not a factor in the stability of the Pine Island Glacier? No, said Loose. „Climate change is causing the bulk of glacial melt that we observe, and this newly discovered source of heat is having an as-yet undetermined effect, because we do not know how this heat is distributed beneath the ice sheet.“
He said other studies have shown that melting caused by climate change is reducing the size and weight of the glacier, which reduces the pressure on the mantle, allowing greater heat from the volcanic source to escape and then warm the ocean water. „Predicting the rate of sea level rise is going to be a key role for science over the next 100 years, and we are doing that. We are monitoring and modeling these glaciers,“ Loose said.
The scientists conclude by writing: „The magnitude and the variations in the rate of the volcanic heat supplied to the Pine Island Glacier, either by internal magma migration, or by an increase in volcanism as a consequence of ice sheet thinning, may impact the future dynamics of the Pine Island Glacier, during the contemporary period of climate-driven glacial retreat.“
In addition to Heywood, Loose worked with Alberto C. Naveira Garabato, of the National Oceanography Centre at the University of Southampton, United Kingdom; Peter Schlosser of Arizona State University’s School of Earth and Space Exploration and the Lamont-Doherty Earth Observatory at Columbia University; William Jenkins of the Woods Hole Oceanographic Institution in Massachusetts; and David Vaughn of the British Antarctic Survey, Cambridge, United Kingdom.
Study finds surprisingly high geothermal heating beneath West Antarctic Ice Sheet
UC Santa Cruz team reports first direct measurement of heat flow from deep within the Earth to the bottom of the West Antarctic ice sheet
Artikel auf Spiegel.de 2017:
Forscher entdecken 91 Vulkane unterm Eis
Überraschung in der Antarktis: Versteckt unter kilometerhohem Eis haben Forscher Dutzende bislang unbekannte Vulkane gefunden. Bei Eruptionen droht eine starke Schmelze – die Meeresspiegel könnten ansteigen.
Der westantarktische Kamb Ice Stream gab den Forscher stets Rätsel auf, denn hier verdickte sich das Eis, ganz im Gegensatz zum allgemeinen Schmelztrend in der Westantarktis. Was könnte die Ursache sein? Ebenfalls ein Vulkan, berichtete die University of Washington 2018:
Volcano under ice sheet suggests thickening of West Antarctic ice is short-term
A region of West Antarctica is behaving differently from most of the continent’s ice: A large patch of ice there is thickening, unlike other parts of West Antarctica that are losing ice. Whether this thickening trend will continue affects the overall amount that melting or collapsing glaciers could raise the level of the world’s oceans.
A study led by the University of Washington has discovered a new clue to this region’s behavior: A volcano under the ice sheet has left an almost 6,000-year record of the glacier’s motion. The track hidden in the middle of the ice sheet suggests that the current thickening is just a short-term feature that may not affect the glacier over the long term. It also suggests that similar clues to the past may be hiding deep inside the ice sheet itself. “What’s exciting about this study is that we show how the structure of the ice sheet acts as a powerful record of what has happened in the past,” said Nicholas Holschuh, a UW postdoctoral researcher in Earth and space sciences. He is first author of the paper published Sept. 4 in The Cryosphere.
The data come from the ice above Mount Resnik, a 1.6-kilometer (mile-high) inactive volcano that currently sits under 300 meters (0.19 miles) of ice. The volcano lies just upstream of the thickening Kamb Ice Stream, part of a dynamic coastal region of ice that drains into Antarctica’s Ross Sea. Studies show Kamb Ice Stream has flowed quickly in the past but stalled more than a century ago, leaving the region’s ice to drain via the four other major ice streams, a switch that glaciologists think happens every few hundred years. Meanwhile the ice inland of Kamb Ice Stream is beginning to bulge, and it is unclear what will happen next. “The shutdown of Kamb Ice Stream started long before the satellite era,” Holschuh said. “We need some longer-term indicators for its behavior to understand how important this shutdown is for the future of the region’s ice.”
The paper analyzes two radar surveys of the area’s ice. One was collected in 2002 by co-authors Robert Jacobel and Brian Welch, using the ice-penetrating radar system at St. Olaf College in Minnesota, and the other in 2004 by co-author Howard Conway, a UW research professor of Earth and space sciences. Conway noticed the missing layers and asked his colleagues to investigate. “It wasn’t until we had spent probably six months with this data set that we started to piece together the fact that this thing that we could see within the ice sheet was forming in response to the subglacial volcano,” Holschuh said.
The study shows that the mysterious feature originates at the ice covering Mount Resnik. The authors believe that the volcano’s height pushes the relatively thin ice sheet up so much that it changes the local wind fields, and affects depositing of snow. So as the ice sheet passes over the volcano a section missed out on a few annual layers of snow. “These missing layers are common in East Antarctica, where there is less precipitation and strong winds can strip away the surface snow,” Holschuh said. “But this is really one of the first times we’ve seen these missing layers in West Antarctica. It’s also the first time an unconformity has been used to reconstruct ice sheet motion of the past.”
Over time, the glacial record shows that this feature followed a straight path toward the sea. During the 5,700-year record, the five major coastal ice streams are thought to have sped up and slowed down several times, as water on the base lubricates the glacier’s flow and then periodically gets diverted, stalling one of the ice streams. “Despite the fact that there are all these dramatic changes at the coast, the ice flowing in the interior was not really affected,” Holschuh said.
What the feature does show is that a change occurred a few thousand years ago. Previous UW research shows rapid retreat at the edge of the ice sheet until about 3,400 years ago, part of the recovery from the most recent ice age. The volcano track also shows a thinning of the ice at about this time. “It means that the interior of the ice sheet is responding to the large-scale climate forcing from the last glacial maximum to today,” Holschuh said. “So the long-timescale climatic forcing is very consistent between the interior and the coast, but the shorter-timescale processes are really apparent in the coastal record but aren’t visible in the interior.”
Holschuh cautions that this is only a single data point and needs confirmation from other observations. He is part of an international team of Antarctic scientists looking at combining the hundreds of radar scans of Antarctic and Greenland glaciers that were originally done to measure ice thickness. Those data may also contain unique details of the glacier’s internal structure that can be used to recreate the history of the ice sheet’s motion.
“These persistent tracers of historic ice flow are probably all over the place,” Holschuh said. “The more we can tease apart the stories of past motion told by the structure of the ice sheet, the more realistic we can be in our predictions of how it will respond to future climate change.” The research was funded by the National Science Foundation and NASA. The other co-author is Knut Christianson, a UW assistant professor of Earth and space sciences.
Medley & Thomas 2019 dokumentierten eine Zunahme des Schneefalls in der Antarktis, was dem Eisschild zugute kam (NASA-Pressemitteilung dazu). Die Autoren stellen einen Zusammenhang mit dem SAM-Ozeanzyklus her, der Southern Annular Mode. Die Universität von Colorado in Boulder lastet die Schneezunahme hingegen dem Ozonloch an (Pressemitteilung, Paper von Lenaerts et al. 2018).
Jenkins et al. 2018 wiesen auf dekadische Zyklen im Schmelzen des Westantarktischen Eisschildes am Rand der Amundsensee hin. Der Zusammenhang des Schmelzens mit der Ozeantemperatur ist nichtlinear:
West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability
Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The nonlinear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high.
Und hier noch mehr zeitlich variable Zusammenhänge. Wang et al. 2019: berichteten über zeitlich variable Zusammenhänge der Oberflächen Eis-Massenbilanz in der Westantarktis mit dem SAM-Zyklus sowie ENSO:
A New 200‐Year Spatial Reconstruction of West Antarctic Surface Mass Balance
High‐spatial resolution surface mass balance (SMB) over the West Antarctic Ice Sheet (WAIS) spanning 1800–2010 is reconstructed by means of ice core records combined with the outputs of the European Centre for Medium‐Range Weather Forecasts “Interim” reanalysis (ERA‐Interim) and the latest polar version of the Regional Atmospheric Climate Model (RACMO2.3p2). The reconstruction reveals a significant negative trend (−1.9 ± 2.2 Gt/year·per decade) in the SMB over the entire WAIS during the nineteenth century, but a statistically significant positive trend of 5.4 ± 2.9 Gt/year·per decade between 1900 and 2010, in contrast to insignificant WAIS SMB changes during the twentieth century reported earlier. At regional scales, the Antarctic Peninsula and western WAIS show opposite SMB trends, with different signs in the nineteenth and twentieth centuries. The annual resolution reconstruction allows us to examine the relationships between SMB and large‐scale atmospheric oscillations. Although SMB over the Antarctic Peninsula and western WAIS correlates significantly with the Southern Annular Mode due to the influence of the Amundsen Sea Low, and El Niño/Southern Oscillation during 1800–2010, the significant correlations are temporally unstable, associated with the phase of Southern Annular Mode, El Niño/Southern Oscillation and the Pacific decadal oscillation. In addition, the two climate modes seem to contribute little to variability in SMB over the whole WAIS on decadal‐centennial time scales. This new reconstruction also serves to identify unreliable precipitation trends in ERA‐Interim and thus has potential for assessing the skill of other reanalyses or climate models to capture precipitation trends and variability.