In der Antarktis lagern riesige Mengen an Wasser in Form von Eis. Was macht der Klimawandel mit der Antarktis? Wir schauen heute, was es Neues zum Thema gibt.
Die US-amerikanische National Science Foundation hatte am 1.11.2019 gute Nachrichten: Die hohen Eisklippen am Rand des antarktischen Eisschildes sind wohl stabiler als gedacht:
Antarctic ice cliffs may not contribute to ice-sheet instability as much as predicted
Even the tallest ice cliffs should support their own weight rather than collapsing catastrophically
Antarctica’s ice sheet spans close to twice the area of the contiguous United States. Its land boundary is buttressed by massive, floating ice shelves extending hundreds of miles out over the frigid waters of the Southern Ocean. When these ice shelves collapse into the sea, they expose towering cliffs of ice along Antarctica’s edge.
Scientists have assumed that ice cliffs taller than 90 meters, or almost 100 yards (about the height of the Statue of Liberty), would rapidly collapse under their own weight. Release of that land-ice, it was thought, could contribute more than 6 feet of sea level rise by the end of the century — enough to completely flood Boston and other coastal cities. Now NSF-funded researchers at MIT have found that these rates of collapse may be overestimated.
In a paper published in Geophysical Research Letters, the team reports that for a 90-meter ice cliff to collapse entirely, the ice shelves supporting the cliff would have to break apart extremely quickly, within a matter of hours — a rate of ice loss that has not been observed in the modern record.
If a supporting ice shelf were to melt away over a longer period of days or weeks, the scientists found, the remaining ice cliff wouldn’t suddenly crack and collapse under its own weight, but instead would slowly flow out, like a mass of cold honey that’s been released from a dam.
„The current worst-case scenario of sea level rise from Antarctica is based on the idea that cliffs higher than 90 meters would fail catastrophically,“ said Brent Minchew of MIT, a co-author of the paper. „That scenario is probably not going to play out.“
„Ice-cliff failure has rightly become a focus of significant investigations by the glaciological community,“ said Paul Cutler, a program director in NSF’s Office of Polar Programs, which funded the study. „This project adds further key insights to the mix.“
Ähnliche Pressemitteilung des MIT hier. Hier das Paper von Clerc et al. 2019:
Marine Ice Cliff Instability Mitigated by Slow Removal of Ice Shelves
The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice cliff instability hypothesis posits that cliffs taller than a critical height (~90 m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from preexisting, static ice cliffs. Here we show how the critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales greater than days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate the marine ice cliff instability, implying less ice mass loss.
Der El Nino 2015/16 hat dem Eis der Antarktis gut getan. Zusätzlicher Schneefall hat die Eismassen in der Antarktischen Halbinsel und in der Westantarktis wachsen lassen, wie Bodart & Bingham 2019 berichteten:
The Impact of the Extreme 2015–2016 El Niño on the Mass Balance of the Antarctic Ice Sheet
Interannual variations associated with El Niño‐Southern Oscillation can alter the surface‐pressure distribution and moisture transport over Antarctica, potentially affecting the contribution of the Antarctic ice sheet to sea level. Here, we combine satellite gravimetry with auxiliary atmospheric data sets to investigate interannual ice‐mass changes during the extreme 2015–2016 El Niño. Enhanced precipitation during this event contributed positively to the mass of the Antarctic Peninsula and West Antarctic ice sheets, with the mass gain on the peninsula being unprecedented within GRACE’s observational record. Over the coastal basins of East Antarctica, the precipitation‐driven mass loss observed in recent years was arrested, with pronounced accumulation over Terre Adélie dominating this response. Little change was observed over Central Antarctica where, after a brief pause, enhanced mass‐loss due to weakened precipitation continued. Overall, precipitation changes over this period were sufficient to temporarily offset Antarctica’s usual (approximately 0.4 mm yr−1) contribution to global mean sea level rise.
Die Oberflächen-Massenbilanz des Eises der Antarktischen Halbinsel zeigt in den letzten dreieinhalb Jahrzehnten keinen Trend, wie van Wessem et al. 2016 dokumentierten. Aus dem Abstract:
There are no significant trends in any of the modelled Antarctic Peninsula SMB components, except for snowmelt that shows a significant decrease over the last 36 years (−0.36 Gt yr−2).
Und wie sieht es mit der Oberflächen-Massenbilanz des Eises in der Westantarktis aus? Sie ist positiv während der letzten 110 Jahre, wie Wang et al. 2019 dokumentierten:
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.
Eine Erwärmung der Antarktis würde aufgrund gesteigerten Schneefalls zu einem Wachstum der antarktischen Eismassen führen, fanden Lenaerts et al. 2016:
Present-day and future Antarctic ice sheet climate and surface mass balance in the Community Earth System Model
We present climate and surface mass balance (SMB) of the Antarctic ice sheet (AIS) as simulated by the global, coupled ocean–atmosphere–land Community Earth System Model (CESM) with a horizontal resolution of ∼1∘ in the past, present and future (1850–2100). CESM correctly simulates present-day Antarctic sea ice extent, large-scale atmospheric circulation and near-surface climate, but fails to simulate the recent expansion of Antarctic sea ice. The present-day Antarctic ice sheet SMB equals 2280±131 Gtyear−1, which concurs with existing independent estimates of AIS SMB. When forced by two CMIP5 climate change scenarios (high mitigation scenario RCP2.6 and high-emission scenario RCP8.5), CESM projects an increase of Antarctic ice sheet SMB of about 70 Gtyear−1 per degree warming. This increase is driven by enhanced snowfall, which is partially counteracted by more surface melt and runoff along the ice sheet’s edges. This intensifying hydrological cycle is predominantly driven by atmospheric warming, which increases (1) the moisture-carrying capacity of the atmosphere, (2) oceanic source region evaporation, and (3) summer AIS cloud liquid water content.
Am 24.8.2016 brachte die Columbia University eine Pressemitteilung mit ähnlichem Inhalt:
By Mid-Century, More Antarctic Snowfall May Help Offset Sea-Level Rise
Increasing Precipitation Masked by Natural Variability—For Now
Snowfall is expected to increase over Antarctica as temperatures warm. Photo: Michael Stukel
When Antarctica’s air temperature rises, moisture in the atmosphere increases. That should mean more snowfall on the frozen continent. So why hasn’t that trend become evident in Antarctica’s surface mass balance as climate models predict?
In a new study, scientists used historical records and climate simulations to examine that question. They found that the effect of rising temperatures on snowfall has so far been overshadowed by Antarctica’s large natural climate variability, which comes from random, chaotic variations in the polar weather. By mid-century, however, as temperatures continue to rise, the study shows how the effect of human-induced warming on Antarctica’s net snow accumulation should emerge above the noise.
The expectation of more snowfall is something of a silver lining as temperatures rise. Global warming is already increasing sea level through melting ice and thermal expansion. The increase in snowfall over Antarctica could help reduce the amount of global sea level rise by 51 to 79 millimeters, or about 2 to 3 inches, by the year 2100, according to the study. That would be a small but important benefit: the Intergovernmental Panel on Climate Change estimates global sea level rise will be at least 10 times that by 2100 under the same high-emissions scenario used in the new study.
“Increased snowfall over Antarctica is the sole process connected to global warming that is thought to have a significant mitigating effect on global sea level rise,” said lead author Michael Previdi, a professor at Columbia University’s Lamont-Doherty Earth Observatory. “While the magnitude of this effect is uncertain, it is likely that the balance of different processes determining Antarctica’s net contribution to global sea level rise will be decidedly different in the future than it has been in the recent past.”
On a continental scale, surface mass balance is the difference between the amount of snowfall that accumulates and the amount of snow lost to sublimation. It affects global sea level because the amount of water on earth is essentially constant, so when more water is stored as snow or ice on land, less water is available to contribute to rising seas.
Antarctica’s annual mean surface mass balance estimated using CMIP5 climate models. Future snowfall increases will also likely be largest around the edges of the continent, where storms blow in and temperatures tend to be warmer. Image: Previdi and Polvani, 2016.
For the study, published this week in the journal Environmental Research Letters, Previdi and co-author Lorenzo Polvani of Lamont-Doherty Earth Observatory evaluated surface mass balance simulations from 35 coupled atmosphere-ocean climate models, which simulate the physical forces that affect Antarctica.
The models allow scientists to quantify both the human influence on surface mass balance and the influence of natural variability. The scientists found that from 1961 to 2005, global warming increased Antarctica’s surface mass balance by 124 billion tons per year, smaller in magnitude than natural year-to-year variability, which was found to be plus or minus 126 billion tons per year.
When the scientists looked at all 35 models, 46 percent of the individual simulations showed a statistically significant trend in surface mass balance from 1961 to 2005, the year that most of the models’ historical simulations end. The likelihood of seeing a statistically significant trend in surface mass balance rises as the models forecast ahead in time. After 2015, the models cross a threshold where it becomes “likely,” with a 66 percent chance, that evidence of anthropogenic climate change will emerge in Antarctica’s surface mass balance. By 2040, it becomes „very likely,“ with a higher than 90 percent chance.
Previdi and Polvani repeated their analysis with different emissions scenarios and also considered surface mass balance trends starting in 1979, at the dawn of the satellite era. The analyses showed similar results, with the global warming signal “very likely” to emerge by mid-century.
“The apparent discrepancy between models and observations can be easily reconciled by considering the large surface mass balance variations generated naturally within the Antarctic climate system,” they write.
Previous studies also found no significant change in the total Antarctic surface mass balance in recent decades, though a 2013 ice core study found a 10 percent increase in surface mass balance in coastal regions since the 1960s. All temperature records, meanwhile, indicate that Antarctica warmed from 1961 to 2005. Ice cores also show a strong relationship between the continent’s surface mass balance and temperature changes through history, including the end of the last ice age when temperatures rose dramatically.
Hier das dazugehörige Paper von Previdi & Polvani 2016:
Anthropogenic impact on Antarctic surface mass balance, currently masked by natural variability, to emerge by mid-century
Global and regional climate models robustly simulate increases in Antarctic surface mass balance (SMB) during the twentieth and twenty-first centuries in response to anthropogenic global warming. Despite these robust model projections, however, observations indicate that there has been no significant change in Antarctic SMB in recent decades. We show that this apparent discrepancy between models and observations can be explained by the fact that the anthropogenic climate change signal during the second half of the twentieth century is small compared to the noise associated with natural climate variability. Using an ensemble of 35 global coupled climate models to separate signal and noise, we find that the forced SMB increase due to global warming in recent decades is unlikely to be detectable as a result of large natural SMB variability. However, our analysis reveals that the anthropogenic impact on Antarctic SMB is very likely to emerge from natural variability by the middle of the current century, thus mitigating future increases in global sea level.
Eine gute Strategie: Der anthropogene Klimawandel wird in der Antarktis wohl erst Mitte dieses Jahrhunderts spürbar werden. Dann werden hoffentlich alle in Rente sein, so dass man sich wegen Fehlprognosen keine Sorgen machen muss.
Wie stark würde der Meeresspiegel ansteigen, wenn der Larsen C Eisschelf vollständig kollabieren würde? Die European Geosciences Union brachte in einer Pressemitteilung von 2018 die Antwort: Nur ein paar Millimeter:
New study puts a figure on sea-level rise following Antarctic ice shelves’ collapse
An international team of scientists has shown how much sea level would rise if Larsen C and George VI, two Antarctic ice shelves at risk of collapse, were to break up. While Larsen C has received much attention due to the break-away of a trillion-tonne iceberg from it last summer, its collapse would contribute only a few millimetres to sea-level rise. The break-up of the smaller George VI Ice Shelf would have a much larger impact. The research is published today in the European Geosciences Union journal The Cryosphere.
Recent, rapid warming in the Antarctic Peninsula is a threat to ice shelves in the region, with Larsen C and George VI considered to have the highest risk of collapse. Because these large ice platforms hold back inland glaciers, the ice carried by these glaciers can flow faster into the sea when the ice shelves collapse, which contributes to sea-level rise. The new study shows that a collapse of Larsen C would result in inland ice discharging about 4 mm to sea level, while the response of glaciers to George VI collapse could contribute over five times more to global sea levels, around 22 mm.
“These numbers, while not enormous in themselves, are only one part of a larger sea-level budget including loss from other glaciers around the world and from the Greenland, East and West Antarctic ice sheets. Taken together with these other sources, the impacts could be significant to island nations and coastal populations,” explains study-author Nicholas Barrand, a glaciologist at the University of Birmingham in the UK. He adds: “The Antarctic Peninsula may be seen as a bellwether for changes in the much larger East and West Antarctic ice sheets as climate warming extends south.”
Warming in the Antarctic Peninsula led, in 2002, to the dramatic collapse of Larsen B, an ice shelf just north of Larsen C. Unprecedented in its size, almost the entire ice shelf broke up in just over two weeks after being stable for the last 10,000 years.
“Larsen C is the most northerly remaining large ice shelf, therefore subject to the warmest temperatures, and the likeliest candidate for future collapse. George VI is further west and south, in a slightly cooler climate, but is still vulnerable to a warming atmosphere and ocean,” says lead-author Clemens Schannwell, who conducted the work while at the University of Birmingham and the British Antarctic Survey.
Last summer, an iceberg twice the size of Luxembourg broke away from Larsen C. But despite the recent attention on this ice shelf, the team found its future collapse would have a modest effect on global sea level. By using computer models to simulate the interactions between the Antarctic Peninsula ice sheet and the ice shelves, the team found that the glacier response to collapse of Larsen C would add up to 2.5 mm to sea level by 2100 and 4.2 mm by 2300.
“The vulnerability to change at George VI Ice Shelf and the possible sea level implications from these changes, are far greater,” says Schannwell. Sandwiched between the Antarctic Peninsula and Alexander Island, George VI Ice Shelf is, at 24,000 square kilometres, around half the size of Larsen C. But it would contribute far more to sea-level rise because it is fed by larger glaciers and is very effective at holding back the ice that drains into it from these glaciers. According to the simulations presented in the new The Cryosphere study, adjustment of the glaciers flowing into it following a collapse could contribute up to 8 mm to global sea levels by 2100 and 22 mm by 2300.
“Prior to our work, we didn’t know what would happen to the upstream ice in the Antarctic Peninsula if these shelves were to be lost. This could have important implications for the local environment and for global sea levels, information that is essential for climate-change mitigation planning and policy,” says Schannwell, who is now at the University of Tübingen in Germany.
“In light of the increasing temperatures projected for the coming century, the Antarctic Peninsula provides an ideal laboratory to research changes in the integrity of floating ice shelves. This region can tell us about ice shelf processes and allow us to observe the response of inland ice to ice-shelf changes. We should view these dramatic changes in the Antarctic Peninsula as a warning signal for the much larger ice sheet-ice shelf systems elsewhere in Antarctica with even greater potential for global sea-level rise.” concludes Barrand.
Paper: Schannwell et al. 2018
Der Kollaps eines Eisschelfs ist nicht außergewöhnlich, wie 2018 die Louisiana State University mitteilte. Bereits vor 12.000 Jahren ist in der Antarktis ein Eisschelf in die Brüche gegangen. Pressemitteilung hier, Paper von Bart et al. 2018 hier.