NASA-Studie mit überraschendem Befund: Antarktische Eiskappe wächst derzeit und bremst globalen Meeresspiegelanstieg

Die Meldung schlug ein wie ein Blitz: Die Antarktis verliert derzeit nicht etwa an Eismasse, sondern wächst sogar. Dies berichtete am 30. Oktober 2015 die NASA per Pressemitteilung. Damit steuert die Antarktis momentan nichts zum Meeresspiegelanstieg bei. Im Gegenteil, das Eiswachstum bremst den Anstieg sogar mit 0,23 mm pro Jahr. Die NASA widerspricht damit dem letzten IPCC-Bericht von 2013 (AR5), in dem noch von einem positiven Meeresspiegel-Beitrag der Antarktis von 0,27 mm pro Jahr ausgegangen worden war.

Im Folgenden die Pressemitteilung der NASA im Original:

—————————-

NASA Study: Mass Gains of Antarctic Ice Sheet Greater than Losses

A new NASA study says that an increase in Antarctic snow accumulation that began 10,000 years ago is currently adding enough ice to the continent to outweigh the increased losses from its thinning glaciers. The research challenges the conclusions of other studies, including the Intergovernmental Panel on Climate Change’s (IPCC) 2013 report, which says that Antarctica is overall losing land ice. According to the new analysis of satellite data, the Antarctic ice sheet showed a net gain of 112 billion tons of ice a year from 1992 to 2001. That net gain slowed to 82 billion tons of ice per year between 2003 and 2008.

“We’re essentially in agreement with other studies that show an increase in ice discharge in the Antarctic Peninsula and the Thwaites and Pine Island region of West Antarctica,” said Jay Zwally, a glaciologist with NASA Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study, which was published on Oct. 30 [2015] in the Journal of Glaciology. “Our main disagreement is for East Antarctica and the interior of West Antarctica – there, we see an ice gain that exceeds the losses in the other areas.”  Zwally added that his team “measured small height changes over large areas, as well as the large changes observed over smaller areas.”

Scientists calculate how much the ice sheet is growing or shrinking from the changes in surface height that are measured by the satellite altimeters. In locations where the amount of new snowfall accumulating on an ice sheet is not equal to the ice flow downward and outward to the ocean, the surface height changes and the ice-sheet mass grows or shrinks. But it might only take a few decades for Antarctica’s growth to reverse, according to Zwally. “If the losses of the Antarctic Peninsula and parts of West Antarctica continue to increase at the same rate they’ve been increasing for the last two decades, the losses will catch up with the long-term gain in East Antarctica in 20 or 30 years — I don’t think there will be enough snowfall increase to offset these losses.”

The study analyzed changes in the surface height of the Antarctic ice sheet measured by radar altimeters on two European Space Agency European Remote Sensing (ERS) satellites, spanning from 1992 to 2001, and by the laser altimeter on NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) from 2003 to 2008. Zwally said that while other scientists have assumed that the gains in elevation seen in East Antarctica are due to recent increases in snow accumulation, his team used meteorological data beginning in 1979 to show that the snowfall in East Antarctica actually decreased by 11 billion tons per year during both the ERS and ICESat periods. They also used information on snow accumulation for tens of thousands of years, derived by other scientists from ice cores, to conclude that East Antarctica has been thickening for a very long time.  “At the end of the last Ice Age, the air became warmer and carried more moisture across the continent, doubling the amount of snow dropped on the ice sheet,” Zwally said.

The extra snowfall that began 10,000 years ago has been slowly accumulating on the ice sheet and compacting into solid ice over millennia, thickening the ice in East Antarctica and the interior of West Antarctica by an average of 0.7 inches (1.7 centimeters) per year. This small thickening, sustained over thousands of years and spread over the vast expanse of these sectors of Antarctica, corresponds to a very large gain of ice – enough to outweigh the losses from fast-flowing glaciers in other parts of the continent and reduce global sea level rise. Zwally’s team calculated that the mass gain from the thickening of East Antarctica remained steady from 1992 to 2008 at 200 billion tons per year, while the ice losses from the coastal regions of West Antarctica and the Antarctic Peninsula increased by 65 billion tons per year.

“The good news is that Antarctica is not currently contributing to sea level rise, but is taking 0.23 millimeters per year away,” Zwally said. “But this is also bad news. If the 0.27 millimeters per year of sea level rise attributed to Antarctica in the IPCC report is not really coming from Antarctica, there must be some other contribution to sea level rise that is not accounted for.” “The new study highlights the difficulties of measuring the small changes in ice height happening in East Antarctica,” said Ben Smith, a glaciologist with the University of Washington in Seattle who was not involved in Zwally’s study. “Doing altimetry accurately for very large areas is extraordinarily difficult, and there are measurements of snow accumulation that need to be done independently to understand what’s happening in these places,” Smith said.

To help accurately measure changes in Antarctica, NASA is developing the successor to the ICESat mission, ICESat-2, which is scheduled to launch in 2018. “ICESat-2 will measure changes in the ice sheet within the thickness of a No. 2 pencil,” said Tom Neumann, a glaciologist at Goddard and deputy project scientist for ICESat-2. “It will contribute to solving the problem of Antarctica’s mass balance by providing a long-term record of elevation changes.”

—————————-

Natürlich gefielen die neuen Ergebnisse den IPCC-nahen Klimakollegen überhaupt nicht. Sie führten andere Satellitenwerte (GRACE) ins Feld, die sie jedoch zuvor kräftig “korrigiert” hatten, um auf ihr Ergebnis zu kommen. Wer sich für die Details und Hintergründe der Diskussion interessiert, dem sei die ausführliche Analyse von Steve McIntyre auf Climate Audit ans Herz gelegt.

Unterdessen reduzierte eine andere Studie von britischen und französischen Wissenschaftlern auch den zukünftigen Meeresspiegelbeitrag der Antarktis. In einer Pressemitteilung des British Antarctic Survey vom 18. November 2015 teilten die Forscher mit, dass die Antarktis bis 2100 im schlimmsten Fall 30 cm beisteuern würde, was drastisch unter jenen Werten anderer Prognostiker liegt, die wahre Sintfluten heraufziehen sahen. Natürlich fragt man sich, was wohl der wahrscheinlichste Wert ist, der weder in der Pressemiteilung, noch im Abstract der Arbeit genannt wird. Dazu schauen wir in die Originalarbeit und lesen den Wert (50%-Quantil) direkt in der Graphik ab (Abbildung 1). Das Ergebnis: Die Antarktis wird bis 2100 im wahrscheinlichsten Fall 15 cm zum Meeresspiegelanstieg beitragen.

Abbildung 1: Beitrag der Antarktis zum globalen Meeresspiegelanstieg. Kurven geben verschiedene Wahrscheinlichkeiten an, wobei 95% das Szenario “im schlimmsten Fall” und 5% das Szenario “mindestens diesen Betrag” darstellen. Quelle: Ritz et al. 2015

 

Im Folgenden die Pressemitteilung in voller Länge:

—————————-

Sea-level rise from Antarctic collapse may be slower than suggested

A new study by scientists in the UK and France has found that Antarctic ice sheet collapse will have serious consequences for sea level rise over the next two hundred years, though not as much as some have suggested.

This study, published this week in the journal Nature, uses an ice-sheet model to predict the consequences of unstable retreat of the ice, which recent studies suggest has begun in West Antarctica.

An international team of researchers, including a scientist from British Antarctic Survey (BAS), predict that the contribution is most likely to be 10 cm of sea-level rise this century under a mid to high climate scenario, but is extremely unlikely to be higher than 30 cm. When combined with other contributions, that’s a significant challenge for adapting to future sea level rise. But it’s also far lower than some previous estimates, which were as high as one metre from Antarctica alone.

The study’s central estimate raises the Intergovernmental Panel on Climate Change (IPCC) central prediction of 60 cm global sea-level rise by just a few centimetres under the mid to high scenario they used. But the team’s method allowed them to assess the likelihood of sea-level rise from substantial parts of the ice sheet collapsing, which the IPCC could not due to a lack of evidence. They predict there is a one in twenty chance that Antarctic collapse could contribute more than 30 cm sea-level rise by the end of the century and more than 72 cm by 2200. This does not rule out larger contributions on longer time scales.

Dr Tamsin Edwards, Lecturer in Environmental Sciences at the OU, says:

“Our method is more comprehensive than previous estimates, because it has more exploration of uncertainty than previous model predictions and more physics than those based on extrapolation or expert judgment.”

Dr Dominic Hodgson, a glaciologist at British Antarctic Survey, says:

“This study is significant. Advances in modelling are needed to reduce the error in global sea level predictions so that mitigation strategies (and Government expenditure) are focussed on the most likely sea level scenarios. This study takes us one step closer to understanding Antarctica’s likely contribution to future sea level rise.”

The paper ‘Potential sea-level rise from Antarctic ice sheet instability constrained by observations’ is authored by Catherine Ritz (Centre national de la recherche scientifique and Université Grenoble Alpes, France), Tamsin L. Edwards (The Open University, University of Bristol), Gaël Durand (Centre national de la recherche scientifique and Université Grenoble Alpes, France), Antony J. Payne (The University of Bristol), Vincent Peyaud (Centre national de la recherche scientifique and Université Grenoble Alpes, France) and Richard C.A. Hindmarsh (British Antarctic Survey). It was published on Wednesday 18 November 2015 in the academic journal Nature – Read here.

This study is a research outcome of the Ice2Sea Programme, a major EU funded research programme to understand the likely contribution of Antarctica’s ice to sea level rise.

—————————-

Auch eine Arbeit aus dem August 2015 im Fachblatt The Cryosphere sieht nur einen begrenzten Beitrag der Antarktis zum Meeresspiegelanstieg bis 2100. Cornford und Kollegen schreiben in ihrer Kurzfassung:

Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate
We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet, deploying sub-kilometer resolution around the grounding line since coarser resolution results in substantial underestimation of the response. Each of the simulations begins with a geometry and velocity close to present-day observations, and evolves according to variation in meteoric ice accumulation rates and oceanic ice shelf melt rates. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rate anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions and ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Within the Amundsen Sea Embayment the largest single source of variability is the onset of sustained retreat in Thwaites Glacier, which can triple the rate of eustatic sea level rise.

Der Anteil des westantarktischen Schmelzens der dortigen Gletscher überschreitet nicht 20 cm (SSH) bis 2100. Die simulierten Größen hängen stark von den Startbedingungen ab, die Simulationen sind also sehr unsicher.

Und schließlich ist da noch eine Arbeit eines Teams um Natalya Gomez, die am 10. November 2015 in Nature Communications erschien. Auch diese Studie widerspricht Meeresspiegel-Alarmsszenarien. Die Forscher fanden Mechanismen, die den Eisverlust in der Antarktis im Zuge einer Klimaerwärmung beschränken. Konkret vergrößert sich die Stabilität der antarktischen Gletscher, sobald sie den Kontakt mit dem Meerwasser verlieren. Dies geschieht durch ein initiales Zurückweichen der Gletscher und Küstenhebung durch Entlastung der geschmolzenen Eismassen.

Gomez und Kollege beschrieben ihre Ergebnisse am 10. November 2015 in einer Pressemitteilung der kanadischen McGill University:

—————————-

Geophysics could slow Antarctic ice retreat

Gravitational effects, variations in Earth structure could damp rise in global sea levels

The anticipated melting of the massive West Antarctic Ice Sheet could be slowed by two big factors that are largely overlooked in current computer models, according to a new study.

The findings, published online in Nature Communications, suggest that the impact on global sea levels from the retreating ice sheet could be less drastic — or at least more gradual — than recent computer simulations have indicated.

Over the past year, numerous studies have warned that parts of the West Antarctic Ice Sheet are on the verge of a runaway retreat. Just last week a high-profile research paper forecast that this could lead eventually to a rise in global sea levels of as much as three metres.

The authors of the new Nature Communications paper, however, focus on two geophysical elements that they say aren’t adequately reflected in computer simulations for this region: the surprisingly powerful gravitational pull of the immense ice sheet on surrounding water, and the unusually fluid nature of the mantle beneath the bedrock that the ice sits on.

“The fate of the polar ice sheets in a warming world is a major concern for policy makers – and attention is rightly focused on the importance of restraining CO2 emissions and preparing for rising sea levels,” says lead author Natalya Gomez, an assistant professor of Earth and Planetary Sciences at McGill University in Montreal. “But our study shows that for Antarctica, in particular, computer models also need to take into account how gravitational effects and variations in Earth structure could affect the pace of future ice-sheet loss.”

The gravity effect

Most people think of gravity as the force that keeps our feet on the ground. But any large body – such as a massive expanse of ice — exerts a gravitational pull on other bodies, including water.

As the West Antarctic Ice Sheet melts, the researchers project, the reduction in its mass would reduce the gravitational pull to such an extent that it would lower sharply the sea level near the ice. This, in turn, would slow the projected pace of retreat of the ice sheet.

The elasticity effect

Gomez and co-authors David Pollard of Pennsylvania State University and David Holland of New York University also factor another important variable into their simulations. When an ice sheet retreats, the solid Earth beneath it, freed from the load of the ice, rebounds upward.  This rebound occurs in two parts: an elastic component that happens right away, and a viscous component that happens over hundreds to thousands of years. (The Earth’s interior — or mantle — flows like a fluid but very slowly because it is very viscous).

The West Antarctic sits atop a region where the mantle flows more easily than in other parts of the Earth.  So the land there will pop up faster than scientists – and their computer models — would expect based on the average viscosity of the Earth’s mantle.

“Our simulations show that when we assume a structure for the Earth’s interior that resembles the structure underneath the West Antarctic, the Earth’s surface rebounds higher and more quickly near the edge of the retreating ice sheet,” says co-author Holland of NYU. “This makes the water along that edge shallower, which slows the retreat of the ice sheet.”

CO2 emissions a crucial factor

The researchers’ simulations also confirm that the levels of future CO2 emissions will be a crucial factor in the pace of retreat for the region’s ice. “The lower the levels of CO2 in the atmosphere, the more the geophysical factors will be able to help stem the ice’s retreat,” Gomez says. “The greater the emissions, the more the geophysical forces risk being overwhelmed by the strength of warming.”