In den vergangenen Wochen haben wir mehrfach über die beiden Eisschilde an Nord- und Südpol berichtet. Eine Übersicht aller Artikel gibt es im thematischen Inhaltsverzeichnis zum Blog (Grönland, Antarktis). Zum Ausklang der Serie hier noch weiteres Wissenswertes zur aktuellen Eisforschung in Grönland.
Im Januar 2015 meldete die University of Texas at Austin, dass sie ein visuelles 3D-Modell zum grönländischen Eis und seiner altersmäßigen Schichtung erstellt habe. In der entsprechenden Pressemitteilung heißt es:
A 3-D View of the Greenland Ice Sheet Opens Window on Ice History
Scientists using ice-penetrating radar data collected by NASA’s Operation IceBridge and earlier airborne campaigns have built the first comprehensive map of layers deep inside the Greenland Ice Sheet, opening a window on past climate conditions and the ice sheet’s potentially perilous future. This new map allows scientists to determine the age of large swaths of the second largest mass of ice on Earth, an area containing enough water to raise ocean levels by about 20 feet. “This new, huge data volume records how the ice sheet evolved and how it’s flowing today,” said Joe MacGregor, the study’s lead author, a glaciologist at The University of Texas at Austin Institute for Geophysics (UTIG), a unit of the Jackson School of Geosciences.
Greenland’s ice sheet has been losing mass during the past two decades, a phenomenon accelerated by warming temperatures. Scientists are studying ice from different climate periods in the past to better understand how the ice sheet might respond in the future. Ice cores offer one way of studying the distant past. These cylinders of ice drilled from the ice sheet hold evidence of past snow accumulation and temperature and contain impurities such as dust and volcanic ash compacted over hundreds of thousands of years. These layers are visible in ice cores and can be detected with ice-penetrating radar. Ice-penetrating radar works by sending radar signals into the ice and recording the strength and return time of reflected signals. From those signals, scientists can detect the ice surface, sub-ice bedrock and layers within the ice.
New techniques used in this study allowed scientists to efficiently pick out these layers in radar data. Prior studies had mapped internal layers, but not at the scale made possible by these newer, faster methods. Another major factor in this study was the scope of Operation IceBridge’s measurements across Greenland, which included flights that covered distances of tens of thousands of kilometers across the ice sheet. “IceBridge surveyed previously unexplored parts of the Greenland Ice Sheet and did it using state-of-the-art CReSIS radars,” said study co-author Mark Fahnestock, an IceBridge science team member and glaciologist from the Geophysical Institute at the University of Alaska Fairbanks (UAF-GI).
CReSIS is the Center for Remote Sensing of Ice Sheets, a National Science Foundation science and technology center headquartered at the University of Kansas in Lawrence, Kansas. IceBridge’s flight lines often intersect ice core sites where other scientists have analyzed the ice’s chemical composition to map and date layers in the ice. These core data provide a reference for radar measurements and provide a way to calculate how much ice from a given climate period exists across the ice sheet, something known as an age volume. Scientists are interested in knowing more about ice from the Eemian period, a time from 115,000 to 130,000 years ago that was about as warm as today. This new age volume provides the first data-driven estimate of where Eemian ice may remain.
Comparing this age volume to simple computer models helped the study’s team better understand the ice sheet’s history. Differences in the mapped and modeled age volumes point to past changes in ice flow or processes such as melting at the ice sheet’s base. This information will be helpful for evaluating the more sophisticated ice sheet models that are crucial for projecting Greenland’s future contribution to sea-level rise. “Prior to this study, a good ice-sheet model was one that got its present thickness and surface speed right. Now, they’ll also be able to work on getting its history right, which is important because ice sheets have very long memories,” said MacGregor.
This study was published online on Jan. 16, 2015, in Journal of Geophysical Research: Earth Surface. It was a collaboration among scientists at UTIG, UAF-GI, CReSIS and the Department of Earth System Science at the University of California, Irvine. It was supported by NASA’s Operation IceBridge and the National Science Foundation’s Arctic Natural Sciences.
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Einige Wochen zuvor, am 16. Dezember 2014 hatte auch Spiegel Online Neues zum Grönlandeis zu berichten:
Anstieg des Meeresspiegels: Grönlands Gletscher schmelzen anders als gedacht
Der riesige Eispanzer Grönlands spielt eine zentrale Rolle beim prognostizierten Anstieg des Meeresspiegels. Daten aus 20 Jahren zeigen nun, wie wenig Forscher bislang über das Abschmelzen der Gletscher wissen.
„Anders als gedacht“, das klingt nicht gut, wenn man an die teuren Eissschmelzmodelle denkt, die offenbar jahrelang die Prozesse falsch berechnet haben. Kritik wurde dabei stets als Angriff auf die Wissenschaft interpretiert. Nun ist es raus: Wir wissen noch immer viel zuwenig über die Vorgänge, die wir angeblich so zuverlässig in den Computern modellieren. Auf spiegel.de beginnt der Artikel:
Das Schmelzen des Grönlandeises verläuft wesentlich komplexer als bekannt. In bislang beispielloser Auflösung hat ein internationales Forscherteam an fast hunderttausend Punkten die Entwicklung der Gletscher dokumentiert. Demnach verlor der Eisschild von 2003 bis 2009 pro Jahr im Mittel 243 Gigatonnen Masse. Das berichten die Wissenschaftler um Beata Csatho von der Universität Buffalo im US-Staat New York in den „Proceedings of the National Academy of Sciences“.
Weiterlesen auf Spiegel Online.
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Etwa um die gleiche Zeit wies der Geologe James Edward Kamis in Climate Change Dispatch auf neue Ergebnisse der NASA und des Postdamer GFZ-Instituts hin, die eine heiße geothermale Zone unter dem Grönlandeis dokumentierten. Zwei große subglaziale Seen und lokal beschleunigtes Gletscherabfließen könnten hiermit zusammenhängen.
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Auch auf Grönland gibt es Jahreszeiten. Das hatte man in der Vergangenheit in den Medienberichten gerne mal ausgespart, denn im Winter stoppt vielerorts der Gletscherschwund und das Eis regeneriert, baut sich stattdessen sogar leicht vor. Verfolgt man die Gletscherstirn über den Verlauf von einigen Jahren, dann tanzt der Eisrand im Takte der Jahreszeiten munter hin- und her. Zudem gibt es Zusammenhänge mit der arktischen Meereisbedeckung. Im Folgenden eine Pressemitteilung der American Geophysical Union (AGU) vom 1. Juni 2015 zu einer neuen Arbeit, die diese Bewegungen schön dokumentiert:
The ebb and flow of Greenland’s glaciers:
New study could improve understanding of Greenland’s contribution to sea-level rise
In northwestern Greenland, glaciers flow from the main ice sheet to the ocean in see-sawing seasonal patterns. The ice generally flows faster in the summer than in winter, and the ends of glaciers, jutting out into the ocean, also advance and retreat with the seasons. Now, a new analysis shows some important connections between these seasonal patterns, sea ice cover and longer-term trends. Glaciologists hope the findings, accepted for publication in the June issue of the American Geophysical Union’s Journal of Geophysical Research-Earth Surface and available online now, will help them better anticipate how a warming Greenland will contribute to sea level rise.
“Rising sea level can be hard on coastal communities, with higher storm surges, greater flooding and saltwater encroachment on freshwater,” said lead author Twila Moon, a researcher at the National Snow and Ice Data Center (NSIDC). NSIDC is part of the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder. “We know that sea level will go up in the future,” Moon said. “The challenge is to understand how quickly it will rise, and one element of that is better understanding how Greenland glaciers behave.” Moon and colleagues from the University of Washington focused on 16 glaciers in northwest Greenland, collecting detailed information on glacier speed, terminus position (the “end” of the glacier in the ocean) and sea ice conditions, during the years 2009-2014.
Sea ice had an important influence on the glaciers: When the waters in front of the glacier were completely covered by sea ice, the ends of the glaciers often advanced out away from land; icebergs that might otherwise have broken off and floated away stayed attached. When sea ice broke up in the spring, the ends of the glaciers usually quickly retreated back toward land as icebergs broke away. By contrast, seasonal swings in glacier speed had little to do with sea ice conditions or glacier terminus location. Rather, the speed (velocity) of ice flow is likely responding to changes in the surface melt on top of the ice sheet and the movement of meltwater through and under the ice sheet.
Over the longer-term, however, Moon and her colleagues found a tight relationship between the speed of glaciers and terminus location. When sea ice levels were especially low and glaciers’ toes (termini) retreated more than normal and then didn’t re-advance, the glaciers sped up, moving ice toward the sea more quickly. While low sea ice is likely not the full cause of the changes, it may be a visible indication of other processes, such as subsurface ice melt, that also affect terminus retreat, Moon said. It’s important to recognize that the mechanisms driving seasonal glacier changes—in northwestern Greenland and around the world—are not necessarily the same ones driving longer-term trends, Moon said. Knowing the differences may help researchers better anticipate the impact of anomalously low sea ice years, for example.
“We do know we’re going to see sea ice reduction in this area, and it’s possible we can begin to estimate how that may affect glacier velocities,” Moon said. It’s also possible, she said, that researchers and communities interested in long-term glacial changes—the kind that affect sea levels—may not need to focus as much on seasonal advance and retreat of the rivers of ice. “It may be that we need to instead pay more attention to these out-of-bounds events, these anomalous years of very low sea ice or very high melt that likely have the greatest influence on longer-term trends.”
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Brandheiß aus der Presse dann noch eine aufregende Meldung der Königlichen Universität Leuven vom 16. Januar 2016 über eine sensationelle Entdeckung: Das grönländische Eis schmilzt umso stärker, je wolkiger es ist! Wer hätte das gedacht.
Greenland ice sheet melts more when it’s cloudy
Clouds play a bigger role in the melting of the Greenland ice sheet than was previously assumed. Compared to clear skies, clouds enhance the meltwater runoff by a third. Those are the findings of an international study that was coordinated by KU Leuven and published in Nature Communications.
Greenland’s ice sheet is the second largest ice mass in the world – the largest is Antarctica. The ice sheet is losing mass at a high speed and increasingly contributes to the sea level rise on our planet. The role of clouds in this loss of snow and ice has never been calculated before, nor can it be deduced from theoretical climate models. For lack of observations, the different models do not agree on the importance of clouds over the ice sheet.
“Clouds always have several effects”, lead author Kristof Van Tricht from the Department of Earth and Environmental Sciences explains. “On the one hand, they help add mass to the ice sheet when it snows. On the other, they have an indirect effect on the ice sheet as well: they have an impact on the temperature, and snow and ice react to these changes by melting and refreezing. That works both ways. Clouds block the sunlight, which lowers the temperature. At the same time, they form a blanket that keeps the surface warm, especially at night. In this study, we examine the net result of these two indirect effects on the entire Greenland ice sheet.”
The researchers used specific satellite observations to detect clouds over the Greenland ice sheet from 2007 to 2010. They compared the results with ground-based observations. The researchers combined these observations with snow model simulations and climate model data to map the net effect of clouds.
“Over the entire Greenland ice sheet, clouds raise the temperature, which triggers additional meltwater runoff: 56 billion tons per year – a third more than clear skies. Contrary to what you would expect, this effect is not so much visible during the daytime melting process, but rather during the following night. A snowpack is like a frozen sponge that melts during the day. At night, clear skies make a large amount of meltwater in the sponge refreeze. When the sky is overcast, by contrast, the temperature remains too high and only some of the water refreezes. As a result, the sponge is saturated more quickly and excess meltwater drains away.”
The study highlights the need for accurate cloud representations in climate models that aim to estimate the amount of meltwater. “With climate change at the back of our minds, and the disastrous consequences of a global sea level rise, we need to understand these processes to make more reliable projections for the future. Clouds are more important for that purpose than we used to think.”
Full text
The study “Clouds enhance Greenland ice sheet meltwater runoff” was published in the open access journal Nature Communications (www.nature.com/ncomms/2016/160112/ncomms10266/full/ncomms10266.html).
More information
Kristof Van Tricht is preparing a PhD in geography under the supervision of Professor Nicole van Lipzig, Stef Lhermitte, and Irina Gorodetskaya from the KU Leuven Department of Earth and Environmental Sciences.
