Bewegung des grönländischen Inlandeises verlangsamt sich: Presse sprachlos und vergisst vor lauter Überraschung, darüber zu berichten

Während sich die Diskussion um die Abschmelzgefahr des antarktischen Inlandeises wohl ersteinmal erledigt hat, muss man sich um die grönländische Eiskappe noch immer Sorgen machen. Denn der große nordpolare Eisschild taut derzeit in der Tat ab, was durch eine Vielzahl von Daten belegt ist. Richtig überraschend ist die Schmelze nicht, befinden wir uns doch derzeit in der Modernen Wärmeperiode. Bereits vor 1000 Jahren zur Zeit der Mittelalterlichen Wärmeperiode stiegen die Temperaturen in Grönland kräftig an und ließen das Eis dahinschmelzen. So richtig neu ist die Situation daher nicht. Alles eine Frage der historischen Perspektive.

Studien zeigen, dass das Grönland-Schmelzwasser zwischen 1992 und 2012 etwa 7 mm zum globalen Meeresspiegelanstieg beigetragen hat. Besonders in den grönländischen Küstenzonen hat das Eis gelitten. Da wundert es nicht, dass einige Berichterstatter die Situation in dramatischen Tönen darstellen. So titelte Der Standard am 19. Dezember 2014:

Grönlands Gletscher schwinden noch schneller als befürchtet

Hatte Der Standard neue Messdaten, die einen noch schnelleren Eisschwund belegen würden? Nein, es war eher etwas Konzeptionelles:

Das Schmelzwasser schrumpfender Gletscher sammelt sich in Form von Seen in Senken auf der Oberfläche der Eismassen. Bisher hielt man diese Wasseransammlungen harmlos – ein Irrtum, wie sich nun zeigt: Die Bildung der neuen Seen infolge des Klimawandels droht in Grönland einer aktuellen Studie zufolge das Verschwinden der Gletscher zusätzlich zu beschleunigen, was wiederum den Meeresspiegel schneller ansteigen lässt. 

Tja, zu blöd. Denn nur ein halbes Jahr später wurde der See-Alarm wieder abgeblasen. Am 3. Juni 2015 meldete das Massachusetts Institute of Technology per Pressemitteilung:

Draining lakes unlikely to worsen Greenland’s contribution to sea levels

 Each summer, Greenland’s ice sheet — the world’s second-largest expanse of ice, measuring three times the size of Texas — begins to melt. Pockets of melting ice form hundreds of large, ‘supraglacial’ lakes on the surface of the ice. Many of these lakes drain through cracks and crevasses in the ice sheet, creating a liquid layer over which massive chunks of ice can slide. This natural conveyor belt can speed ice toward the coast, where it eventually falls off into the sea. Now researchers have found that while warming temperatures are creating more inland lakes, these lakes cannot drain their water locally, as lakes along the coast do, and are not likely to change the amount of water reaching the ground in inland regions.

Each summer, Greenland’s ice sheet — the world’s second-largest expanse of ice, measuring three times the size of Texas — begins to melt. Pockets of melting ice form hundreds of large, ‘supraglacial’ lakes on the surface of the ice. Many of these lakes drain through cracks and crevasses in the ice sheet, creating a liquid layer over which massive chunks of ice can slide. This natural conveyor belt can speed ice toward the coast, where it eventually falls off into the sea.

In recent years, scientists have observed more lakes forming toward the center of the ice sheet — a region that had been previously too cold to melt enough ice for lakes to form. The expanding range of lakes has led scientists to wonder whether Greenland will ultimately raise global sea levels higher than previously predicted.

Now researchers at MIT, Woods Hole Oceanographic Institution (WHOI), and elsewhere have found that while warming temperatures are creating more inland lakes, these lakes cannot drain their water locally, as lakes along the coast do, and are not likely to change the amount of water reaching the ground in inland regions.

‘It’s essentially a check on the inner ice starting to move along this fast conveyor belt,’ says Laura Stevens, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences. ‘One of the big questions about the Greenland ice sheet is how much of the ice sheet [travels towards the coast] during the summer, and how much is entering into the ocean. Our hypothesis that inland lakes are less likely to drain locally suggests the ice sheet in that region won’t speed up. That’s good news, at least for the time being.’

Stevens and her colleagues, including Thomas Herring, a professor of geophysics at MIT, have published their results today in the journal Nature.

A trickle and a trigger

In summer 2006, Sarah Das, a glaciologist at WHOI, led a team to document the drainage of North Lake, a 10-meter-deep, 2-kilometer-wide lake on the western side of Greenland. The group observed that each summer, the lake, like many others, drained quickly, completely emptying in just a couple of hours.

‘You can hear the water rushing down in the distance, and even if you’re a couple kilometers away, you see all these microcracks running along the ground around you,’ Stevens says.

The researchers set up one GPS station near the lake to record the surface of the ice during its draining, and later identified a large fracture in the basin through which the water drained. However, it wasn’t clear what triggered the fracture that caused the lake to drain so quickly.

Das returned to Greenland in summer 2011, along with Stevens and others, to get a more detailed picture of the lake’s seasonal draining. The team set up 16 GPS stations in two rings around the lake, and recorded the movement of the ice as the lake drained once each summer over three consecutive summers.

From the GPS data, they observed a period of six to 12 hours, just before the lake drained, in which some water from the lake trickled to the bottom of the ice sheet through ‘moulins’ — narrow vertical channels in the ice. During this brief period, the researchers observed water collecting at the bottom of the ice sheet, pushing up on the surface ice. This initial pooling of water seemed to trigger the rest of the lake to drain.

‘That water will cause the ice above it to be jacked up like a dome, and then you’ve created tension at the surface that allows the ice sheet to start to fracture,’ Stevens says. ‘Once a fracture gets beneath the lake, then water just starts to pour into that fracture, and the whole thing goes.’

A check on runaway lake drainage

North Lake is located within the coastal region of Greenland, where the ice sheet is thinner, and more moulins route water at the surface of the ice sheet to its base. In contrast, lakes further inland are higher in elevation and form over thicker ice. Stevens says it’s unlikely that inland lakes would drain, as there are fewer moulins near inland lakes, which prevents water from getting to the ground locally. Without these trigger channels, larger fractures would not form in the lake basin, and lakes would stay intact, simply refreezing in the winter or overflowing into a surface stream.

‘It is critical to understand how and why these lakes drain in order to predict how much mass the ice sheet will contribute to sea-level rise in our warming climate,’ Stevens says. ‘We find that while lakes are forming inland, they probably won’t drain by this…mechanism. The inland lakes will more likely drain their water via surface stream runoff, which transfers the water to the bed in more coastal areas of the ice sheet. So, while we see inland ice beginning to speed up as more melt happens inland, the draining of inland lakes likely won’t exacerbate the situation.’

Bereits am 16. März 2014 hatte Der Standard eine Grönlandeis-Attacke im Programm:

Massiver Eisverlust auch bei bisher stabilen Grönland-Gletschern
Auch die bisher als stabil geltenden Gletscher im Nordosten Grönlands verlieren einer aktuellen Studie zufolge riesige Eismassen. Jedes Jahr schwindet der nordöstliche Eisstrom demnach um zehn Gigatonnen (zehn Milliarden Tonnen), berichtet ein internationales Team um Shfaqat Abbas Khan von der Technischen Universität Dänemark in Kopenhagen im Fachjournal “Nature Climate Change”. Eigentlich galt der untersuchte Teil des grönländischen Eisschilds bisher als stabil. Der künftige Anstieg des Meeresspiegels sei daher gravierend unterschätzt worden, so die Forscher.

Einen Tag später stieg auch Spiegel Online auf das Thema ein:

Klimawandel: Grönlands Nordosten beginnt zu tauen
Der Nordosten Grönlands galt als tiefgefroren und stabil trotz Erderwärmung. Nun aber haben Wissenschaftler eine erschreckende Entdeckung gemacht: Gletscher der Region schrumpfen. Was bedeutet das für den Anstieg der Ozeane?

Eine genauere Analyse der entsprechenden Arbeit zeigt jedoch, dass es sich bei dem schnell schmelzenden Studiengebiet um ein kleines, “briefmarkengroßes” Gebiet handelt und es gleich in der Nachbarschaft ein anderes Gebiet gibt in dem eine gegenteilige Entwicklung gefunden wurde, also eine Abkühlung. Wie repräsentativ ist daher die Alarmmeldung für das riesengroße Grönland wirklich?

Seltsamerweise wurden andere hochinteressante Pressemitteilungen in der deutschsprachigen Presse mit keiner Silbe erwähnt. So fanden Forscher vom NASA Goddard Space Flight Center heraus, dass sich die Eisbewegung im Südwesten des grönländischen Eisschildes nun deutlich verlangsamt hat, wie die NASA am 28. Oktober 2015 in einer wenig beachteten Pressemitteilung bekanntgab:

Land-Facing, Southwest Greenland Ice Sheet Movement Decreasing

In the face of decades of increasing temperatures and surface melting, the movement of the southwest portion of the Greenland Ice Sheet that terminates on land has been slowing down, according to a new study being published by the journal Nature on Oct. 29. Researchers derived their results by tracking ice sheet movement through Landsat satellite images taken from 1985 to 2014 across a roughly 3,088-square-mile (8000-square-kilometer) region in southwest Greenland. They found that, between 2007 and 2014, ice movement slowed in 84 percent of the study area, during a period of high surface melt, compared to the years between 1985 and 1994. The average slowdown was 12 percent, or 32.8 feet (10 meters) per year.

The finding is contrary to the widely held view that a greater amount of surface melting will result in faster-moving ice sheets, as the movement of both ocean- and land-terminating ice sheets is caused in part by surface meltwater, which makes its way to the bedrock through openings in the ice and acts as a lubricant. The amount of meltwater draining from the ice sheet in four out of the five years between 2007 and 2012 has been the most substantial of the last 50 years.

Researchers found that while the larger summertime meltwater volume of recent years has led to greater lubrication of the ice sheet base, speeding up its flow as expected, by the end of summer the meltwater has also established channels at the base that act as efficient drainage systems to lessen the water under the ice sheet, slowing it down by winter. “This suggests that further increases in melting will not cause these land-terminating margins of the ice sheet to speed up,” said lead author Andrew Tedstone, a glaciologist at the University of Edinburgh, Scotland. “Nevertheless, it is unclear how much more slowdown we will see under the current and future melting conditions,” said co-author Noel Gourmelen, University of Edinburgh. “More research and observation are needed to determine this.”

While these results may be viewed as good news for the Greenland ice sheet, they are offset by the fact that it is not the change in movement of the land- but rather the ocean-terminating portion of the ice sheet that is contributing to sea level rise. “The ongoing acceleration of both glacier surface melt volumes and the ice motion of ocean-terminating glaciers ensures that Greenland’s contribution to sea level rise will likely increase in our warming world,” said co-author Peter Nienow, University of Edinburgh. The Greenland Ice Sheet is the second largest mass of ice on Earth, containing enough water that if it all melted, ocean levels would rise by about 20 feet. Greenland has shed on average 303 gigatons of ice per year since 2004, and with every successive year the loss has increased by 31 gigatons. (Each gigaton equals one billion metric tons.) Recent estimates suggest that surface melting is responsible for 60 percent of Greenland’s ice sheet losses, while the remainder is caused by ice sheet discharge into the ocean.    

Thomas Neumann, a cryospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland who was not involved in the study, said the finding highlights the importance of having access to a long time series of remote sensing data, such as the Landsat record. “By analyzing velocity estimates extracted from 30 years of Landsat data, this study highlights the complex, and sometimes counterintuitive, interplay between surface meltwater and ice motion.” 

Um die gleiche Zeit berichtete auch die Presseabteilung der University of Edinburgh über ermutigende Ergebnisse, die es aber offenbar in keine deutsche Zeitung schafften. Ergebnis: Die Fließgeschwindigkeit des Eises hat sich verlangsamt seit 2001 um ca. 12% trotz stärkerer (jedoch seit 2005 stabilisierter) Schmelzrate:

Satellites shed light on Greenland Ice Sheet response to warming

Parts of Greenland’s ice sheet have been found to be less vulnerable to climate warming than was thought – a discovery that could have a small but beneficial impact on sea level forecasts.

Satellite images have revealed that despite dramatic increases in ice melt across Greenland in recent years, the speed of ice movement in some areas has slowed down rather than accelerated. The finding, observed on a sector of the ice sheet that terminates on land rather than in the ocean, will help scientists improve predictions of how quickly Greenland’s ice will be lost in a warming climate. Until recently, scientists thought that the increased volumes of meltwater from Greenland’s ice in response to climate warming would speed up the motion of all parts of the ice sheet by helping the ice slide more rapidly. However, their latest study shows that in recent decades, ice movement in some areas that terminate on land has slowed down rather than accelerated. The discovery suggests that further increases in ice melting, fuelled by climate change, may further slow movement of these sectors of the ice sheet.

A team of researchers from the University of Edinburgh used satellite data to track the shift of ice features such as crevasses in an 8000km2 area of Greenland over three decades. They found that, despite a 50 per cent rise in meltwater from the ice surface in recent years, overall movement in the past 10 years was slower than in previous decades. They found that this was caused by large amounts of meltwater produced in summer producing channels at the base of the ice sheet, which drain away water efficiently, slowing the glacier’s movement the subsequent winter. Scientists say more research is needed to understand the movement of other parts of the ice sheet, which terminate in the ocean and which have seen acceleration in recent decades. The study, published in Nature, was carried out in collaboration with the Université Savoie Mont-Blanc in France and the University of Sheffield.

Lesenswert der letzte Satz der Kurzfassung in der besprochenen Originalarbeit von Tedstone et al. 2015:

Bereits Ende 2013 hatte es Hinweise auf die Entwicklung geben, wie The Australian am 19.11.2013 meldete:

Heat ‘not undermining Greenland’s ice sheet’
A study reported today in the Proceedings of the National Academy of Sciences has found the ice sheet moved more slowly than average in 2012, despite “unbelievably warm” temperatures that triggered the most extreme melting in 123 years. The findings alleviate concerns that lubrication caused by meltwaters which sink to the base of the ice sheet could accelerate its flow into the ocean.

Wiederum war es die Gruppe um Andrew Tedstone, die diese Entwicklungen in PNAS öffentlich machte:

Greenland ice sheet motion insensitive to exceptional meltwater forcing
Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt–induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt–induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ∼3.9σ above the 1958–2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt–induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.

Das plötzliche, katastrophale Abgleiten à la Kippunkt wird daher immer unwahrscheinlich. Die Klimaalarmfahnen in Potsdam wehen entsprechend auf Halbmast. Die Fachwelt konzentriert sich daher nun auf den wahrscheinlichsten Einflussfaktor: Den Schneefall und das Abschmelzen/Sublimieren, also die oberflächennahe Massenbilanz (‘surface mass balance’, SMB). Im Februar 2014 stellte ein Forscherteam um Ellyn Enderlin in den Geophysical Research Letters klar:

These observations support recent model projections that surface mass balance, rather than ice dynamics, will dominate the ice sheet’s contribution to 21st century sea level rise.

Tja, und die SMB des grönländischen Inlandeises (ohne Gletscherabfluss) sieht laut Erhebung des Danish Meteorological Institute wie folgt undramatisch aus (Daten abgefragt am 7.1.2016):

Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt) and the season 2011-12 (red) which had very high summer melt in Greenland. For comparison, the mean curve from the period 1990-2013 is shown (dark grey). The same calendar day in each of the 24 years (in the period 1990-2013) will have its own value. These differences from year to year are illustrated by the light grey band. For each calendar day, however, the lowest and highest values of the 24 years have been left out.

 

Khan et al. warnten im August 2014 in einer Arbeit in The Cryosphere davor, sich bei der Zukunftsprognose des grönländischen Eises auf zu kurze Beobachtungsreihen zu stützen. In einer Fallstudie für die letzten 80 Jahre konnten die Wissenschaftler zeigen, dass kurzfristige Schmelzphasen stets mit Phasen von Eiszuwachs oder Stabilität wechselten. Ihre Empfehlung: Man solle sich nicht durch die hochfrequenten Ereignisse irreleiten lassen und diese einfach in die Zukunft extrapolieren. Dies würde zu falschen Ergebnissen führen. Hier die Kurzfassung der Arbeit:

Glacier dynamics at Helheim and Kangerdlugssuaq glaciers, southeast Greenland, since the Little Ice Age
Observations over the past decade show significant ice loss associated with the speed-up of glaciers in southeast Greenland from 2003, followed by a deceleration from 2006. These short-term, episodic, dynamic perturbations have a major impact on the mass balance on the decadal scale. To improve the projection of future sea level rise, a long-term data record that reveals the mass balance beyond such episodic events is required. Here, we extend the observational record of marginal thinning of Helheim and Kangerdlugssuaq glaciers from 10 to more than 80 years. We show that, although the frontal portion of Helheim Glacier thinned by more than 100 m between 2003 and 2006, it thickened by more than 50 m during the previous two decades. In contrast, Kangerdlugssuaq Glacier underwent minor thinning of 40–50 m from 1981 to 1998 and major thinning of more than 100 m after 2003. Extending the record back to the end of the Little Ice Age (prior to 1930) shows no thinning of Helheim Glacier from its maximum extent during the Little Ice Age to 1981, while Kangerdlugssuaq Glacier underwent substantial thinning of 230 to 265 m. Comparison of sub-surface water temperature anomalies and variations in air temperature to records of thickness and velocity change suggest that both glaciers are highly sensitive to short-term atmospheric and ocean forcing, and respond very quickly to small fluctuations. On century timescales, however, multiple external parameters (e.g. outlet glacier shape) may dominate the mass change. These findings suggest that special care must be taken in the projection of future dynamic ice loss.

Prozentual hat sich sich die grönländische Eismasse in den letzten 10 Jahren übrigens kaum geändert, wie Calvin Beisner and J.C. Keister am 6.6.2014 auf WUWT darstellten (die rote Linie ist der Trend):

 

Zum guten Schluß noch die Auflösung eines Rätsels: Im Sommer 2012 ging es dem grönländischen Eis kräftig an den Kragen und schmolz dramatisch dahin. Das Handelsblatt berichtete damals (16.8.2012):

Grönland: Eis schmilzt so rasant wie nie
Seit 1979 untersuchen Forscher die Eisschmelze in Grönland. Doch so schnell wie 2012 ist das Eis noch nie geschmolzen. Die Forscher rechnen damit, dass bis zum Ende des Jahres alle alten Rekorde überschritten werden.[...] Tedesco betonte, die Veränderungen stimmten mit den Modellen zur Vorhersage in etwa überein. Überraschend sei aber die Geschwindigkeit des Schmelzens.

Im Juni 2014 lösten Kaitlin Keegan und Kollegen das Rätsel in PNAS auf. Es war wohl der Ruß von Waldbränden, der der grönländischen Eisoberfläche mächtig zusetzte. Die Forscher fanden ein Ereignis aus dem Jahr 1889, wo es eine ähnliche Situation mit Waldbränden und grönländischer Oberflächen-Schmelzepisode gegeben hat. Hier der Abstract, der Studie, die es natürlich ebenfalls nicht in die deutsche Presse geschafft hat:

Climate change and forest fires synergistically drive widespread melt events of the Greenland Ice Sheet
In July 2012, over 97% of the Greenland Ice Sheet experienced surface melt, the first widespread melt during the era of satellite remote sensing. Analysis of six Greenland shallow firn cores from the dry snow region confirms that the most recent prior widespread melt occurred in 1889. A firn core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from Northern Hemisphere forest fires reduced albedo below a critical threshold in the dry snow region, and caused the melting events in both 1889 and 2012. We use these data to project the frequency of widespread melt into the year 2100. Since Arctic temperatures and the frequency of forest fires are both expected to rise with climate change, our results suggest that widespread melt events on the Greenland Ice Sheet may begin to occur almost annually by the end of century. These events are likely to alter the surface mass balance of the ice sheet, leaving the surface susceptible to further melting.

Die politisch filternde und selektiv berichtende Presse wird zunehmend zum Problem, nicht erst seit den Vorkommnissen in der Kölner Silvesternacht.