Die gute Nachricht: Eis der Ostantarktis ist und bleibt stabil

Klimaalarm im März 2015 im Focus:

Klima-Erwärmung
Gigantischer Antarktis-Gletscher schmilzt – Holland in Not: Meeresspiegel droht um drei Meter zu steigen

Vor der Ost-Antarktis haben Forscher zwei unterseeische Täler entdeckt. Sie ermöglichen den Zufluss von warmem Meerwasser unter den größten Gletscher der Ostantarktis. Das könnte dessen ungewöhnlich raschen Eisverlust erklären. Kollabierte der Gletscher endgültig, würde der Meeresspiegel dramatisch ansteigen.

  • Der größte Gletscher der Ost-Antarktis schmilzt extrem rasch.
  • Kollabiert der Gletscher könnte der Meeresspiegel weltweit um 3,3 Meter steigen.
  • Abschwächung der Starkwinde führt zu mehr Hitze-Extremen.

Könnte, würde: Der Konjunktiv ist König. Steht es wirklich so schlimm um den Totten Gletscher? Wir haben uns bereits damals bemüht, den Sachverhalt zu beleuchten (“Antarktischer Totten-Gletscher schrumpft: Ursache könnte die Zunahme des Meereises sein“). Im Mai 2016 kommentierte auch Rud Istvan auf Climate Etc. den besagten Gletscher und ein alarmierendes Nature-Paper von Aitken et al. 2016. Sein Fazit:

The alarming estimates from this new Nature paper, particularly as represented by the media, are grievously wrong both with respect to the amount of and the rate of sea level rise that might be associated with melting of the EIAS Totten glacier. There is unjustified author spin in the press releases and author’s interviews. There are underlying bad assumptions never mentioned except by reference to a previously refuted [here] bad paper by Rignot. A tangled web of deceit, to paraphrase a famous poem.

Vielleicht sollte man auch nicht versuchen, die Welt anhand eines Gletschers erklären zu wollen, so groß die Verlockung auch sein mag. Am 5. Mai 2017 erinnerte die University of Bristol daran, dass das ostantarktische Eis im letzten Jahrzehnt insgesamt gewachsen und nicht etwa geschrumpft ist.  Die Uni verpackt das Ganze natürlich politisch korrekt in die Aussage “nicht so stark gewachsen wie gedacht”. Hier die Pressemitteilung:

New research shows growth of East Antarctic Ice Sheet was less than previously suggested
Scientists have known for over a decade that the West Antarctic Ice Sheet has been losing mass and contributing to sea level rise. Its eastern neighbour is, however, ten times larger and has the potential to raise global sea level by some 50 metres. Despite its huge size and importance, conflicting results have been published on the recent behaviour of the East Antarctic Ice Sheet. A study led by a group of NASA scientists, that was published in 2015, suggested that this part of Antarctica was gaining so much mass that it compensated for the losses in the west. Determining what the largest ice sheet on the planet is doing is vital for our understanding of the factors that are influencing present day, and future, sea level rise.

To address this question, a team of scientists led by the University of Bristol and including the University of Wollongong, Australia have studied the problem by combining different satellite observations within a statistical model that is able to separate the processes related to ice mass changes over the continent. Professor Jonathan Bamber from the Bristol Glaciology Centre which is part of the School of Geographical Sciences, said: “We used similar data sets to the NASA team but added other satellite data from a mission called the Gravity Recovery And Climate Experiment (GRACE) to help solve for mass gains and losses. “We then conducted different experiments, using similar assumptions made in the NASA study but found that in every experiment, mass loss from the west always exceeded gains in the east.” The researchers concluded that over the study period, 2003-2013, Antarctica, as a whole, has been contributing to sea level rise and that the gains in East Antarctica were around three times smaller than suggested in the 2015 study.

Paper: ‘Constraining the mass balance of East Antarctica’ by A. Martin-Espanol, J. Bamber and A. Zammit-Mangion in Geophysical Research Letters. Plain language summary available at: http://www.globalmass.eu/constraining-the-mass-balance-of-east-antarctica/

Weitere Studien aus der Ostantarktis erhärten den Trend zu mehr Eis. Ein Team um Morgane Philippe veröffentlichte 2016 in The Cryosphere eine Arbeit zum Küstenstreifen des Dronning Maud Land. Das Resultat steht bereits im Titel: Eiszuwachs im 20. Jahrhundert:

Ice core evidence for a 20th century increase in surface mass balance in coastal Dronning Maud Land, East Antarctica
Ice cores provide temporal records of surface mass balance (SMB). Coastal areas of Antarctica have relatively high and variable SMB, but are under-represented in records spanning more than 100 years. Here we present SMB reconstruction from a 120 m-long ice core drilled in 2012 on the Derwael Ice Rise, coastal Dronning Maud Land, East Antarctica. Water stable isotope (δ18O and δD) stratigraphy is supplemented by discontinuous major ion profiles and continuous electrical conductivity measurements. The base of the ice core is dated to AD 1759 ± 16, providing a climate proxy for the past  ∼ 250 years. The core’s annual layer thickness history is combined with its gravimetric density profile to reconstruct the site’s SMB history, corrected for the influence of ice deformation. The mean SMB for the core’s entire history is 0.47 ± 0.02 m water equivalent (w.e.) a−1. The time series of reconstructed annual SMB shows high variability, but a general increase beginning in the 20th century. This increase is particularly marked during the last 50 years (1962–2011), which yields mean SMB of 0.61 ± 0.01 m w.e. a−1. This trend is compared with other reported SMB data in Antarctica, generally showing a high spatial variability. Output of the fully coupled Community Earth System Model (CESM) suggests that, although atmospheric circulation is the main factor influencing SMB, variability in sea surface temperatures and sea ice cover in the precipitation source region also explain part of the variability in SMB. Local snow redistribution can also influence interannual variability but is unlikely to influence long-term trends significantly. This is the first record from a coastal ice core in East Antarctica to show an increase in SMB beginning in the early 20th century and particularly marked during the last 50 years.

Die Stabilität des Eises im Dronning Maud Land bekräftigt auch eine Arbeit von Vikram Goel und Kollegen, die Ende Mai 2017 in The Cryosphere als Diskussion erschien:

Glaciological settings and recent mass balance of the Blåskimen Island in Dronning Maud Land, Antarctica
The Dronning Maud Land coast in East Antarctica has numerous ice rises that very likely control the dynamics and mass balance of this region. However, only a few of these ice rises have been investigated in detail. Here, we report field measurements of Blåskimen Island, an isle-type ice rise adjacent to the Fimbul Ice Shelf. Blåskimen Island is largely dome shaped, with a pronounced ridge extending to the southwest from its summit (410 m a.s.l.). Its bed is mostly flat and about 100 m below the current sea level. Shallow radar-detected isochrones dated with a firn core reveal that the surface mass balance is higher on the southeastern slope than the northwestern slope by ~ 37 %, and this pattern has persisted for at least the past decade. Radar stratigraphy shows upward arches underneath the summit, indicating that the summit position has been stable over at least one characteristic time of this ice rise (~ 600 years). Ensemble estimates of the mass balance using the input-output method show that this ice rise has thickened by 0.07–0.35 m ice equivalent per year over the past decade.

Am 16. Juni 2017 kam dann noch eine Publikation von Pittard et al. in den Geophysical Research Letters dazu, die in die gleiche Richtung geht. Die Autoren prognostizieren, dass das Lambert-Amery glacial system in der Ostantarktis auch in den kommenden 500 Jahren stabil bleiben wird und möglicherweise sogar noch an Masse zulegt.

Future sea level change from Antarctica’s Lambert-Amery glacial system
Future global mean sea level (GMSL) change is dependent on the complex response of the Antarctic Ice Sheet to ongoing changes and feedbacks in the climate system. The Lambert-Amery glacial system has been observed to be stable over the recent period yet is potentially at risk of rapid grounding line retreat and ice discharge given a significant volume of its ice is grounded below sea level, making its future contribution to GMSL uncertain. Using a regional ice sheet model of the Lambert-Amery system, we find that under a range of future warming and extreme scenarios, the simulated grounding line remains stable and does not trigger rapid mass loss from grounding line retreat. This allows for increased future accumulation to exceed the mass loss from ice dynamical changes. We suggest the Lambert-Amery glacial system will remain stable, or gain ice mass and mitigate a portion of potential future sea level rise over the next 500 years, with a range of +3.6 to -117.5 mm GMSL-equivalent.