Grönländisches Inlandeis besaß vor 5000 Jahren deutlich weniger Eismasse als heute

Hätten Sie es gewusst? In der Zeit von 5000-3000 Jahren vor heute war die grönländische Eiskappe viel kleiner als heute! Das fand eine Studie der University at Buffalo heraus. Interessant: Die höchsten Atmosphären-Temperaturen der letzten 10.000 Jaren herrschten davor, nämlich vor 9000-5000 Jahren. Es dauerte also einige tausend Jahre, bis das Eis voll darauf reagierte. Auch die Ozeane zeigten eine starke Verzögerung. Ihr Temperaturhöhepunkt der Nacheiszeit ereignete sich zeitgleich zum Grönland-Eisminimum, also 5000-3000 Jahre vor heute. Im Folgenden die entsprechende Pressemitteilung der University at Buffalo vom 22. November 2013:

Greenland’s shrunken ice sheet: We’ve been here before
Clues in the Arctic fossil record suggest that 3-5,000 years ago, the ice sheet was the smallest it has been in the past 10,000 years

Think Greenland’s ice sheet is small today? It was smaller — as small as it has ever been in recent history — from 3-5,000 years ago, according to scientists who studied the ice sheet’s history using a new technique they developed for interpreting the Arctic fossil record. “What’s really interesting about this is that on land, the atmosphere was warmest between 9,000 and 5,000 years ago, maybe as late as 4,000 years ago. The oceans, on the other hand, were warmest between 5-3,000 years ago,” said Jason Briner, PhD, University at Buffalo associate professor of geology, who led the study. “What it tells us is that the ice sheets might really respond to ocean temperatures,” he said. “It’s a clue to what might happen in the future as the Earth continues to warm.”

The findings appeared online on Nov. 22 in the journal Geology. Briner’s team included Darrell Kaufman, an organic geochemist from Northern Arizona University; Ole Bennike, a clam taxonomist from the Geological Survey of Denmark and Greenland; and Matthew Kosnik, a statistician from Australia’s Macquarie University. The study is important not only for illuminating the history of Greenland’s ice sheet, but for providing geologists with an important new tool: A method of using Arctic fossils to deduce when glaciers were smaller than they are today.

Scientists have many techniques for figuring out when ice sheets were larger, but few for the opposite scenario. “Traditional approaches have a difficult time identifying when ice sheets were smaller,” Briner said. “The outcome of our work is that we now have a tool that allows us to see how the ice sheet responded to past times that were as warm or warmer than present — times analogous to today and the near future.” The technique the scientists developed involves dating fossils in piles of debris found at the edge of glaciers. To elaborate: Growing ice sheets are like bulldozers, pushing rocks, boulders and other detritus into heaps of rubble called moraines. Because glaciers only do this plowing when they’re getting bigger, logic dictates that rocks or fossils found in a moraine must have been scooped up at a time when the associated glacier was older and smaller. So if a moraine contains fossils from 3,000 years ago, that means the glacier was growing — and smaller than it is today — 3,000 years ago.

This is exactly what the scientists saw in Greenland: They looked at 250 ancient clams from moraines in three western regions, and discovered that most of the fossils were between 3-5,000 years old. The finding suggests that this was the period when the ice sheet’s western extent was at its smallest in recent history, Briner said. “Because we see the most shells dating to the 5-3000-year period, we think that this is when the most land was ice-free, when large layers of mud and fossils were allowed to accumulate before the glacier came and bulldozed them up,” he said.

Because radiocarbon dating is expensive, Briner and his colleagues found another way to trace the age of their fossils. Their solution was to look at the structure of amino acids — the building blocks of proteins — in the fossils of ancient clams. Amino acids come in two orientations that are mirror images of each other, known as D and L, and living organisms generally keep their amino acids in an L configuration. When organisms die, however, the amino acids begin to flip. In dead clams, for example, D forms of aspartic acid start turning to L’s. Because this shift takes place slowly over time, the ratio of D’s to L’s in a fossil is a giveaway of its age. Knowing this, Briner’s research team matched D and L ratios in 20 Arctic clamshells to their radiocarbon-dated ages to generate a scale showing which ratios corresponded with which ages. The researchers then looked at the D and L ratios of aspartic acid in the 250 Greenland clamshells to come up with the fossils’ ages. Amino acid dating is not new, but applying it to the study of glaciers could help scientists better understand the history of ice — and climate change — on Earth.

Carlson et al. fassten im August 2014 in den Geophysical Research Letters die Entwicklung des grönländischen Eises gut zusammen. Nach Ende der letzten Eiszeit ging das Grönlandeis dramatisch zurück. Nach Beendigung des mittelholozänen Klimaoptimums legte das Eis dann wieder an Masse zu, mit einem Maximum während der Kleinen Eiszeit. Das heutige Schmelzen knabbert an diesem kürzlichen Maximum. Hier die Kurzfassung:

Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent
Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.

Das Abschmelzen des grönländischen Inlandeises während des mittelholozänen Klimaoptimums war auch Thema einer Arbeit von Lena Håkansson und Kollegen, die im August 2014 in den Quaternary Science Reviews erschien. Die Autoren untersuchten Seen in Westgrönland. Vor 6500 Jahren war das Eis soweit abgeschmolzen, dass eine Eisausbreitung erreicht wurde, die dem heutigen Stand entspricht. Erstaunlicherweise blieb der Eisrand in den folgenden gut 1000 Jahren relativ stabil, obwohl damals Temperaturen herrschten die mehr als 2°C wärmer waren als die aktuell dort gemessenen. Vor 5400 Jahren war die Position jedoch nicht mehr zu halten, und der Eisrand zog sich 1,5 km weit ins Landinnere zurück, wobei Flächen plötzlich eisfrei wurden, die heute wieder vom Eis bedeckt sind. Während der Kleinen Eiszeit um 1750 dehnte sich das Eis dann wieder aus, wobei gegen Ende der Kleinen Eiszeit, vielleicht um 1850, das Ausdehnungmaximum erreicht wurde. Irgendwann zwischen 1750-1850 muss der Eisrand die heutige Position überschritten haben. Seit Ende der Kleinen Eiszeit schrumpft das Eis jetzt wieder, wobei schließlich die heutige Eisrandlage erreicht wurde. Hier die Kurzfassung der wichtigen Arbeit:

Slow retreat of a land based sector of the West Greenland Ice Sheet during the Holocene Thermal Maximum: evidence from threshold lakes at Paakitsoq
Records from two connected proglacial threshold lakes at Paakitsoq, west Greenland have been analyzed in order to investigate the response of a land-based ice sheet margin to Holocene climate change. The results are used to test whether or not the land terminating margin at Paakitsoq behaved synchronously with the nearby marine terminating Jakobshavn Isbræ during the Holocene. The radiocarbon dated lake sediment cores indicate that the ice margin retreated to its present position ∼6.5 ka ago and thereafter maintained a relatively stable configuration similar to the present for >1000 years, despite summer temperatures >2 °C higher than today. The lakes became non-glacial after 5.4 cal. ka BP, when the ice margin retreated behind a drainage divide situated ∼1.5 km inland of the present margin. By this time, Jakobshavn Isbræ had already reached its minimum configuration. The Paakitsoq ice margin remained >1.5 km inland of its present margin until after 240 ± 20 cal. yr BP; after this time, both Paakitsoq ice margin and Jakobshavn Isbræ reached their Holocene maxima during the later stage of the Little Ice Age. Our results suggest that the present ice margin position at Paakitsoq is relatively stable in a warming climate but after a total retreat of ∼1.5 km behind the present margin position it may become marine based and more unstable due to marine melting and calving processes.

Auch das Naturkundemuseum von Dänemark beschäftigte sich in einer Pressemitteilung vom 20. Februar 2015 mit der Eiskappenschmelze in Grönland 8000 bis 5000 Jahre vor heute. Damals war es laut Studie 2-4°C wärmer als heute und der grönländische Eisschild schrumpfte auf eine Größe ab, die deutlich kleiner war als die heutige Ausdehnung. Die Forscher berechneten, dass damals über einen Zeitraum von dreitausend Jahren das Eisvolumen pro Jahr um 100 Gigatonnen pro Jahr abnahm.  Hier die Pressemitteilung im englischen Original:

Greenland is melting – the past might tell what the future holds

A team of scientists lead by Danish geologist Nicolaj Krog Larsen have managed to quantify how the Greenland Ice Sheet reacted to a warm period 8,000-5,000 years ago. Back then temperatures were 2-4 degrees C warmer than present. Their results have just been published in the scientific journal Geology, and are important as we are rapidly closing in on similar temperatures.

While the world is preparing for a rising global sea-level, a group of scientists led by Dr. Nicolaj Krog Larsen, Aarhus University in Denmark and Professor Kurt Kjær, Natural History Museum of Denmark ventured off to Greenland to investigate how fast the Greenland Ice Sheet reacted to past warming. With hard work and high spirits the scientists spent six summers coring lakes in the ice free land surrounding the ice sheet. The lakes act as a valuable archive as they store glacial meltwater sediments in periods where the ice is advanced. That way is possible to study and precisely date periods in time when the ice was smaller than present. It has been hard work getting all these lake cores home, but is has definitely been worth the effort. Finally we are able to describe the ice sheet’s response to earlier warm periods, says Dr. Nicolaj Krog Larsen of Aarhus University, Denmark.

Evidence has disappeared

The size of the Greenland Ice Sheet has varied since the Ice Age ended 11,500 years ago, and scientists have long sought to investigate the response to the warmest period 8,000-5,000 years ago where the temperatures were 2-4 °C warmer than present. The glaciers always leave evidence about their presence in the landscape. So far the problem has just been that the evidence is removed by new glacial advances. That is why it is unique that we are now able to quantify the mass loss during past warming by combining the lake sediment records with state-of-the-art modelling, says Professor Kurt Kjær, Natural History Museum of Denmark.

16 cm of global sea-level rise from Greenland

Their results show that the ice had its smallest extent exactly during the warming 8,000-5,000 years ago – with that knowledge in hand they were able to review all available ice sheet models and choose the ones that best reproduced the reality of the past warming. The best models show that during this period the ice sheet was losing mass at a rate of 100 Gigaton pr. year for several thousand years, and delivered the equivalent of 16 cm of global sea-level rise when temperatures were 2-4 °C warmer. For comparison, the mass loss in the last 25 years has varied between 0-400 Gigaton pr. year, and it is expected that the Arctic will warm 2-7 °C by the year 2100.

 

Am 4. September 2014 berichtete das dänische Niels-Bohr-Institut über eine neue Temperatur-Rekonstruktion, die ein lange bestehendes Problem auflöst. Lange ging man davon aus, dass es vor 12.000 Jahren eine Kälteperiode in Grönland gegeben hat, die aber unerklärlicher Weise mit einer hohen Sonnenaktvität und einer erhöhten CO2-Konzentration zusammenfiel. Man konnte sich nicht erklären, wie es dazu kam. In einer neuen Studie überprüfte man nun die Temperaturkurve mit einer neuen Methode auf Basis von Stickstoffisotopen und wurde fündig: Die Kältephase war nämlich keine Kältephase, sondern eine Warmphase. Nun passte es wieder: Wärmephase bei hoher Sonnenaktivität und erhöhtem CO2. Die Autoren nennen diese beiden Klimafaktoren als wichtigste Klimatreiber. Hier die Pressemitteilung im vollen Wortlaut:

Past temperature in Greenland adjusted

New temperature curve

One of the common perceptions about the climate is that the amount of carbon dioxide in the atmosphere, solar radiation and temperature follow each other – the more solar radiation and the more carbon dioxide, the hotter the temperature. This correlation is also seen in the Greenland ice cores that are drilled through the approximately three kilometer thick ice sheet. But during a period of several thousand years up until the last ice age ended approximately 12,000 years ago, this pattern did not fit and this was a mystery to researchers. Now researchers from the Niels Bohr Institute among others have solved this mystery using new analytical techniques. The results are published in the prestigious scientific journal Science.

The Greenland ice sheet is an archive of knowledge about the Earth’s climate more than 125,000 years back in time. The ice was formed by the precipitation that fell as snow from the clouds and remained year after year, gradually being compressed into ice. By drilling down through the approximately three kilometer thick ice sheet, the researchers retrieve ice cores, which provide detailed knowledge of the climate of the past annual layer after annual layer. By measuring the content of the special oxygen isotope O18 in the ice cores, you can get information about the temperature in the past climate, year by year. But something didn’t fit. In Greenland, the end of the Ice Age started 15,000 years ago and the temperature rose quickly. Then it became colder again until 12,000 years ago, when there was again a rapid rise in temperature. The first rise in temperature is called the Bølling-Allerød interstadial and the second is called the Holocene interglacial. 

Temperatures contrary to expectations

“We could see that the concentration of carbon dioxide and solar radiation was higher during the cold period between the two warm periods compared with the cold period before the first warming 15,000 years ago. But the temperature measurements based on the oxygen isotope O18 showed that the period between the two warm periods was colder than the cold period before the first warming 15,000 years ago. This was the exact opposite of what you would expect,” explains Postdoc Vasileios Gkinis, Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen.

The researchers investigated ice cores from three different Greenland ice cores: the NEEM project, the NGRIP project and the GISP 2 project. But amount of the oxygen isotope O18 was not enough to reconstruct period temperatures in detail or their geographic distribution. To get more detailed temperature data, the researchers used two relatively new methods of investigation, both of which examine the layer of compressed granular snow that is formed between the top layer of soft and fluffy snow and the layer deeper down in the ice sheet, where the compressed snow has been turned into ice. This process of transforming the fluffy snow into hard ice is physical and both the thickness and the movement of the water molecules are dependent on the temperature.

“With the first method, we measured the nitrogen content and by measuring the relationship between the two isotopes of nitrogen, N15 and N14, we could reconstruct the thickness of the compressed snow 19,000 years back in time,” explains Vasileios Gkinis. The second method involved measuring the diffusion of air with water molecules with different isotope composition in the layers with the compressed snow. This process of smoothing the original water isotope variations from precipitation is dependent on the temperature, as the water molecules in vapour form are more mobile at warmer temperatures.

Data for the diffusion of the water molecules in the individual annual layers in the Greenland ice cores has thus made it possible to calculate the temperature in the layers with compressed snow 19,000 years back in time. “What we discovered was that the previous temperature curve, which was only based on the measurements of the oxygen isotope O18, was inaccurate. The oxygen temperature curve said that the climate in central Greenland was colder around 12,000 years ago than around 15,000 years ago, despite the fact that two key climate drivers – carbon dioxide in the atmosphere and solar radiation – would suggest the opposite. With our new, more direct reconstruction, we have been able to show that the climate in central Greenland was actually warmer around 12,000 years ago compared to 15,000 years ago. So the temperatures actually follow the solar radiation and the amount of carbon dioxide in the atmosphere. We estimate that the temperature difference was 2-6 degrees,” says Bo Vinther, Associate Professor at the Centre for Ice and Climate at the Niels Bohr Institute, University of Copenhagen.